We derive two SEDs from the two light curve fits, both of which are very broad (www2.uvm2.uw1ubggURgRCiGiCzGJG HgKg and ubggRgRCiGiCzGJgHgKg). Filters bluer than gG are influenced by Lyman-damping and are not included, with the UV filters being upper limits only. Without dust, we find β₁ = 0.429 ± 0.026 (χ²/d.o.f. = 1.97), and β₂ = 0.326 ± 0.030 (χ²/d.o.f. = 1.41), respectively. This is in qualitative agreement with Filgas et al. (2012), who also find a very blue spectral slope and color evolution. Both SEDs show some scatter and no curvature. Any fits with dust models yield either slightly negative extinction or very low values that are in agreement with 0 within errors. We therefore use the fits without extinction.

For the first of these fits (as we are interested in the early luminosity), we derive $dR_c = -2.335 \pm 0.018$ mag. The early afterglow peak reaches only a moderate luminosity, $R_c = 15.0$ mag.

### Appendix B.3.17. GRB 091127, $z = 0.49044 \pm 0.00008$

Data are taken from Cobb et al. (2010); Filgas et al. (2011a); Vergani et al. (2011); Troja et al. (2012); Gorbovskoy et al. (2013); Coward et al. (2017), and from GCN Circulars (Xu et al. 2009c,d; Klotz et al. 2009; Andreev et al. 2009b; Haislip et al. 2009a,b,c; Kinugasa et al. 2009b). The redshift is given by Vergani et al. (2011). The associated SN 2009nz was spectroscopically confirmed (Berger et al. 2011). This was a well-observed GRB afterglow at moderately low redshift with an associated SN. The early afterglow showed a very flat spectral slope and continuous color evolution (Filgas et al. 2011a). Correcting for a small amount of SMC extinction, we find $dR_c = 1.484^{+0.187}_{-0.205}$ mag. The early afterglow is only moderately luminous, $r_c = 16.4$ mag.

### Appendix B.3.18. GRB 091208B, $z = 1.0633 \pm 0.0003$

Data are taken from Uehara et al. (2012), GCN Circulars (Nakajima et al. 2009; Xu et al. 2009b; Updike et al. 2009c; Andreev et al. 2009a; Kinugasa et al. 2009c; Xin et al. 2009), and our own Swift UVOT (first reported in De Pasquale & Pagani 2009), and FTN, FTS, LT data (first reported in Cano et al. 2009). The redshift is given by Perley et al. (2009).

This GRB was also detected by an improved analysis of Fermi/LAT data (Akerlof et al. 2011). It showed early optical polarization (Uehara et al. 2012). The afterglow, initially discovered by de Ugarte Postigo et al. (2009a, who give no magnitude in their report), shows a simple light curve evolution well fitted by a broken power-law, we find $\alpha_1 = 0.621 \pm 0.008$, $\alpha_2 = 1.554 \pm 0.185$, $t_b = 0.286 \pm 0.037$ d, $n = 10$ fixed, and no host galaxy contribution. Hereby, we excluded our last two detections each in $R_c$ and $I_c$ as they show what seems to be a short-lived rebrightening or flare. The prebreak decay is somewhat shallower than that found by Uehara et al. (2012), who, however, included later, more steeply decaying data, and had significantly less early data to measure the slope.

The SED we derive is very broad (www2.uvm2.uw1ubgg'vrGRcCiGICzGJgHG), red, and shows clear curvature. The www2.uvm2.uw1 bands are affected by Lyman dropout and are not included. For no dust, we find $\beta = 1.991 \pm 0.077$ (χ²/d.o.f. = 3.25). Fits with MW and LMC dust are unable to incorporate the curvature, finding negative extinction and no extinction, respectively. The SMC fit is clearly the best, finding $\beta = 0.24 \pm 0.40$, $A_V = 0.85 \pm 0.19$ mag, and is fully acceptable (χ²/d.o.f. = 0.98).

For this fit, we find $dR_c = -1.757^{+0.365}_{-0.367}$ mag. The early afterglow is moderately luminous, peaking at $R_c = 14.2$ mag.

## Appendix B.3.19. GRB 100219A, $z = 4.66723 \pm 0.00037$

Data are taken from Mao et al. (2012); Thöne et al. (2013); Bolmer et al. (2018), and GCN Circulars (Kuroda et al. 2010f; Kinugasa et al. 2010), as well as our own Swift UVOT data, first reported in Holland & Rowlinson (2010). The redshift is given by Thöne et al. (2013). For this GRB, a source was detected (Holland et al. 2010), nearby but at a clear offset from the GRB afterglow (Jakobsson et al. 2010), which was originally suggested to be the host galaxy (Bloom & Nugent 2010), but was found to be an unrelated foreground galaxy at $z = 0.217$ (Cenko et al. 2010). This source contaminates the Swift UVOT analysis. As a result of the high redshift, the very large $g_G - r_G \approx 3$ mag color, and the low depth of UVOT observations in v, we assume that the actual afterglow is undetected in all Swift UVOT filters except white. In white, we use a high S/N detection of this galaxy from the final epoch as a “host galaxy” and subtract the value in flux space from the other detections, leaving five actual afterglow detections from the early phase. We give upper limits otherwise.

This GRB afterglow is double peaked (Mao et al. 2012) and we fit the two peaks separately. For the first peak, we find $\alpha_{rise,1} = -0.49 \pm 0.18$, $\alpha_{decay,1} = 1.29 \pm 0.06$, $t_{b,1} = 0.0126 \pm 0.0007$ d, $n = 10$ fixed. We back extrapolate this rising slope to derive flux densities for the shifts to high-z. The second peak gives $\alpha_{rise,2} = -0.58 \pm 0.43$, $\alpha_{decay,2} = 1.36 \pm 0.03$, $t_{b,2} = 0.203 \pm 0.021$ d, $n = 10$ fixed. The post peak decay slope is fully in agreement with Thöne et al. (2013). The host galaxy is detected in i' (Thöne et al. 2013), and we find the late $r_G$ data exhibit a decreasing decay rate (also remarked upon by Thöne et al. 2013) indicative of an underlying host, our fit finds $r_G^{host} = 26.19 \pm 0.46$ mag (AB magnitude, corrected for Galactic extinction). The high z and large $r_G - i_G$ color (see below) make this detection somewhat doubtful, however. The similarity of the two post peak decay rates indicates this could be the result of a strong energy injection, as suggested also by Mao et al. (2012). The decay slope also indicates no jet break has taken place, however, this is in strong contrast to the X-ray data, which decay steeply with $\alpha_X \gtrsim 3$ after $3.5 \times 10^4$ s (Mao et al. 2012). This may indicate a similar case to GRB 070110 (Troja et al. 2007) and GRB 130831A (De Pasquale et al. 2016a).

The SED ($g_GRcCiGzGJgHG_Kg$) shows clear signs of a Lyman dropout with $g_G - R_c = 2.92$ mag and also $R_c - i_G = 1.65$ mag. The data redward of Lyman-$\alpha$ can be well fit ($\chi^2$/d.o.f. = 0.049) with an SPL with no dust and $\beta_0 = 1.33 \pm 0.18$. However, the X-ray slope is significantly harder, indicating some dust must be reddening the afterglow (Mao et al. 2012; Japelj et al. 2015). Fixing the spectral slope to the value derived from the XRT repository late time spectrum ($\beta = 0.68$), we find good fits with LMC dust and $A_V = 0.30 \pm 0.09$ mag ($\chi^2$/d.o.f. = 0.20), as well as SMC dust and $A_V = 0.20 \pm 0.06$ mag ($\chi^2$/d.o.f. = 0.17). The 2175 Å bump lies in the J band, and we find no indication of a flux depression, so we prefer the SMC solution. From this fit, we derive a correction for Lyman damping to the $R_c$ composite light curve of 1.19 mag (a factor of almost exactly 3).

Our result agrees very well with that of Thöne et al. (2013), who also used the flux-calibrated X-shooter spectrum and find $\beta = 0.66 \pm 0.13$ $A_V = 0.13 \pm 0.05$ mag. Japelj et al. (2015) find $\beta = 0.73 \pm 0.02$ and $A_V = 0.23 \pm 0.02$ mag for LMC dust (and $\beta \approx 0.66$, $A_V \approx 0.26$ mag after applying a correction to the X-ray
flux), in full agreement with our result. Zafar et al. (2018b), fitting a combination of photometric and X-shooter spectroscopic data with a more general model, find a lower spectral slope $\beta = 0.55_{-0.07}^{+0.06}$, and a lower extinction of $A_V = 0.14 \pm 0.03$ mag for a blue $R_V = 2.65 \pm 0.09$.

## Appendix B.3.20. GRB 100418A, $z = 0.6239 \pm 0.0002$

Data are taken from Marshall et al. (2011); Niino et al. (2012); Laskar et al. (2015); Jelínek et al. (2016); de Ugarte Postigo et al. (2018b), GROND data from the PhD thesis of K. Varela<fnref n="10" />, and from GCN circulars (Updike et al. 2010c,a; Siegel & Marshall 2010; Pearson et al. 2010; Klein et al. 2010; Rumyantsev et al. 2010; Rumyantsev & Pozanenko 2010a,b; Morgan et al. 2010; Moody et al. 2010; Andreev et al. 2010c; Volnova et al. 2010a). The redshift is given by de Ugarte Postigo et al. (2018b). This was a faint, soft GRB (technically an X-ray Flash, XRF) with a complex light curve evolution. The early afterglow shows a long plateau phase at a faint magnitude level with a strong flare superposed which is only detected in Swift UVOT data. This is followed by a very strong rebrightening and a further long plateau phase which goes over into a decay only about 1d after the GRB. The associated SN is likely the faintest found so far for a GRB (Niino et al. 2012; de Ugarte Postigo et al. 2018b). A full analysis of the light curve is given in de Ugarte Postigo et al. (2018b). We also use their X-ray t optical SED which finds $\beta = 1.061_{-0.023}^{+0.024}$, $A_V = 0.086 \pm 0.039$ for SMC dust. Japelj et al. (2015) find a somewhat bluer intrinsic slope and slightly higher extinction, $\beta = 0.73_{-0.08}^{+0.07}$, $A_V = 0.20_{-0.02}^{+0.03}$ mag, for SMC dust. Zafar et al. (2018b) find very similar values to the ones we use here, $\beta = 1.01_{-0.10}^{+0.12}$, $A_V = 0.12 \pm 0.03$ mag, with a blue $R_V = 2.42_{-0.10}^{+0.08}$. For this fit, we derive $dR_c = +1.161_{-0.058}^{+0.055}$ mag. The early afterglow is of very low luminosity, at a level of $R_c \approx 21$ mag. It reaches $R_c \approx 19.5$ mag at 0.43 d in the $z=1$ frame.

## Appendix B.3.21. GRB 100621A, $z = 0.5426$

Data are taken from Greiner et al. (2013), and from a GCN circular (Naito et al. 2010). Late-time host-galaxy observations are taken from Krühler et al. (2011a), see also Blanchard et al. (2016); Vergani et al. (2015). The redshift is given by Japelj et al. (2016). This was a bright but soft GRB at a moderately low redshift. The afterglow is covered very well by early GROND observations and exhibits a peculiar behavior, with a steep rise, a plateau with substructure, and a decay, which is followed by a second, extremely steep rise (Greiner et al. 2013, see also de Ugarte Postigo et al. 2018a), another plateau with substructure, and a final decay. However, there is likely another rebrightening at $\approx 1.2$ d, but data density is strongly reduced by this time. We subtract the individual host-galaxy values in each band, and fit data from $\approx 0.062$ d onward, the beginning of the second plateau phase. We find $\alpha_1 = 0.456 \pm 0.036$, $\alpha_2 = 2.14 \pm 0.06$, $t_b = 0.1065 \pm 0.0009$ d, $n = 15.3 \pm 4.4$ ($\chi^2/d.o.f. = 0.81$). This compares well with the results from Greiner et al. (2013), $\alpha_1 = 0.42 \pm 0.05$, $\alpha_2 = 2.3 \pm 0.1$, $t_b \approx 0.1$ d.

The afterglow SED ($g_G r_G i_G z_G J_G H_G K_G$) is extremely red, we find $\beta = 3.989 \pm 0.013$ ($\chi^2/d.o.f. = 340$, an extremely high result stemming from the very small errors of the SED). Free fits with MW and LMC dust find generally viable results, however, the intrinsic spectral slope is very blue ($\beta = 0.05 - 0.2$).

For SMC dust, we find $\beta = 0.778 \pm 0.085$, $A_V = 3.715 \pm 0.098$ mag ($\chi^2/d.o.f. = 24.5$, still exceedingly high because of small errors, the fit is perfectly fine visually, however). In all cases, the g' band is too bright and excluded, we caution that the very high extinction implies g' is always close to the host-galaxy level and small effects such as differing apertures for afterglow and host measurements may strongly influence the detection level. Greiner et al. (2013) find a very similar extinction value, with $\beta = 0.82 \pm 0.02$, $A_V = 3.65 \pm 0.06$ mag. The same is true for Krühler et al. (2011a), who find $\beta = \Gamma_X - 1 - 0.5 = 0.79$ and $A_V = 3.8 \pm 0.2$ mag. For the SMC fit, we derive, despite the moderately low redshift, $dR_c = -3.595_{-0.144}^{+0.140}$ mag. The afterglow peaks late, during the second peak, at a moderate $R_c \approx 15.9$ mag.

## Appendix B.3.22. GRB 100814A, $z = 1.439$

Data are taken from Nardini et al. (2014); De Pasquale et al. (2015), and from GCN circulars (Elenin et al. 2010a,b; Volnova et al. 2010c). The redshift is given by Selsing et al. (2019). The afterglow evolution is complex and reminiscent of GRB 081029 (Nardini et al. 2011). It begins with a sparsely covered flare, a decay and a small rebrightening. After this, it goes over into a smooth decay well-covered by GROND, which turns into a long rise that peaks at $\approx 1.2$ d before turning over into a steeper decay, the afterglow after host-galaxy subtraction is detectable until $\approx 26$ d. The afterglow shows color evolution across the different components (Nardini et al. 2014) and we perform two different fits, the first encompassing the first long-term decay and the beginning of the rebrightening, for which we find: $\alpha_1 = 0.5756 \pm 0.0026$, $\alpha_{rise,1} = -1.49 \pm 0.10$, $t_{b,1} = 0.340 \pm 0.014$ d, $n_1 = -1.29 \pm 0.11$ ($\chi^2/d.o.f. = 2.33$). For the second fit, our result comes to be: $\alpha_{rise,2} = -0.156 \pm 0.016$, $\alpha_2 = 2.162 \pm 0.013$, $t_{b,2} = 2.119 \pm 0.011$, $n_2 = 1.86 \pm 0.08$ ($\chi^2/d.o.f. = 5.70$). Hereby, $\alpha_{rise,1}$ and $\alpha_{rise,2}$ cover different parts of the light curve and therefore do not have to be similar. Nardini et al. (2014) even fit part of the second peak with a shallow decay. Their other results are in agreement with ours, they find the early decay is slightly chromatic and derive decay indices in the range $\alpha_1 = 0.40 - 0.58$. After correcting for the host-galaxy contribution, they find a late decay $\alpha_2 = 2.25 \pm 0.08$. Under the assumption of achromatic evolution, De Pasquale et al. (2015) derive $\alpha_1 = 0.48 \pm 0.02$, $\alpha_2 = 1.97 \pm 0.02$, $t_b = 2.52 \pm 0.028$ d, also in reasonable agreement with our results.

We also create two SEDs to take the different fits and the color evolution into account. Both SEDs are very broad (www2.uvm2.uvw1ubgGvrGRcIGzGJGHGK_G), however, we find that $g_G$ is anomalously bright, and $v$ anomalously faint so we exclude them. We also exclude www2.uvm2.uvw1 as they are clearly affected by Lyman damping. In accordance with Nardini et al. (2014), the first SED is very blue (while still exhibiting slight curvature), we find $\beta_1 = 0.348 \pm 0.020$ ($\chi^2/d.o.f. = 3.62$). Nardini et al. (2014) generally find $A_V \approx 0$ mag, and that the spectral index becomes bluer during the first decay before reaching $\beta = 0.18 \pm 0.08$. The second SED, also exhibiting curvature, is significantly redder and more typical for a GRB afterglow, we find $\beta_2 = 0.743 \pm 0.014$ ($\chi^2/d.o.f. = 13.9$), in agreement with Nardini et al. (2014), who derive $\beta = 0.82 \pm 0.15$ for a late-time SED at 0.7 Ms. For dust fits, we find that in both cases the MW fit find negative extinction. LMC dust fits are not ruled out, but the intrinsic spectral slope is very blue (negative for the first fit). We prefer the SMC fits, for which we find $\beta = 0.10 \pm 0.06$, $A_V = 0.123 \pm 0.031$ mag, ($\chi^2/d.o.f. = 1.43$), and $\beta = 0.411 \pm 0.043$, $A_V = 0.158 \pm 0.020$ mag, ($\chi^2/d.o.f. = 4.49$), respectively. The $A_V$ values agree with each other within

<footnote n="10">¹⁰ https://www.imprs-astro.mpg.de/sites/default/files/varela_karla.pdf</footnote>
errors, strongly indicating it is indeed the intrinsic spectrum that changes, without any influence on the environment, in agreement with Nardini et al. (2014). De Pasquale et al. (2015) find roughly similar results and slightly variable extinction. At 500 s, they find (assuming $\Delta\beta = 0.5$ from X-ray-to-optical fits, and LMC extinction) $\beta = 0.34 \pm 0.06$, $A_V = 0.139_{-0.044}^{+0.079}$ mag; whereas for 50 ks, they find $\beta = 0.50_{-0.04}^{+0.02}$, $A_V = 0.044 \pm 0.016$ mag. Japelj et al. (2015) also find results similar to ours, using SMC dust: $\beta = 0.52 \pm 0.07$, $A_V = 0.20 \pm 0.03$ mag. Zafar et al. (2018b) find a steeper intrinsic spectral slope, however: $\beta = 0.92_{-0.08}^{+0.12}$, $A_V < 0.07$ mag, with no $R_V$ value given. Using our second SMC fit, we derive $dR_c = -1.218 \pm 0.045$ mag. The light curve peak at a moderately luminous $R_c = 14.5$ mag when shifted to $z = 1$.

### Appendix B.3.23. GRB 100901A, $z = 1.4084$

Data are taken from Gorbovskoy et al. (2012); Hartoog et al. (2013); Laskar et al. (2015), from GCN Circulars (De Cia et al. 2010; Kuroda et al. 2010c,d,b; Hentunen et al. 2010a; Updike et al. 2010b; Sahu et al. 2010b,a; Kopac et al. 2010; Sanchez-Ramirez et al. 2010; Yoshida et al. 2010a; Andreev et al. 2010a,b; Ukwatta et al. 2010; Sposetti & Immler 2010), as well as our own observations with the Tautenberg (first reported in Kann et al. (2010a,c,d), the UKIRT/WFCAM (first reported in Im et al. 2010c), as well as an unpublished GROND data set. The redshift is given by Hartoog et al. (2013). The early afterglow of this GRB shows a remarkable evolution, with multiple rebrightenings, likely stemming from energy injections (Laskar et al. 2015). After the final peak at $\approx 0.38$ d, we find the afterglow can be fitted with a broken power-law, it is $\alpha_1 = 1.392 \pm 0.016$, $\alpha_2 = 1.930 \pm 0.042$, $t_b = 3.26 \pm 0.22$ d, $n = 10$ fixed, and no host galaxy was detected to deep limits in late GROND observations. This is likely a jet break.

The SED is very broad and rich (uww2uvm2uww1uBg$_{G}$v$_{R_G}$ $R_{CiG}$I$_{CzG}$z'$YJJ$_G$H$_G$HK$_G$K). It is well-fit by a simple power-law with $\beta = 0.75 \pm 0.06$ ($\chi^2$/d.o.f. = 0.81). All dust models yield viable solutions with low amounts of extinction. For MW dust, we find $\beta = 0.57 \pm 0.24$, $A_V = 0.13 \pm 0.17$ mag ($\chi^2$/d.o.f. = 0.83), for LMC dust, we find $\beta = 0.40 \pm 0.26$, $A_V = 0.26 \pm 0.17$ mag ($\chi^2$/d.o.f. = 0.87), and for SMC dust, we find $\beta = 0.54 \pm 0.18$, $A_V = 0.14 \pm 0.09$ mag ($\chi^2$/d.o.f. = 0.86). While mathematically indistinguishable, the lack of a clear 2175 Å bump makes us prefer the SMC solution. Japelj et al. (2015) also prefer SMC dust, finding a similar spectral slope but higher extinction: $\beta = 0.50 \pm 0.04$, $A_V = 0.29 \pm 0.03$ mag. Zafar et al. (2018b) find a slightly redder slope and also somewhat higher extinction, $\beta = 0.70_{-0.16}^{+0.13}$, $A_V = 0.25 \pm 0.08$ mag, for $R_V = 3.01 \pm 0.11$. Generally, the values are in agreement with our result. For the SMC fit, we derive $dR_c = -1.137_{-0.198}^{+0.197}$ mag. The afterglow is only moderately luminous, peaking at $R_c \approx 15.8$ mag, however, this peak is until $\approx 0.24$ d in the $z = 1$ system. At early times, the magnitude is almost always $R_c > 16$ mag.

### Appendix B.3.24. GRB 100906A, $z = 1.727$

Data are taken from Gorbovskoy et al. (2012); Page et al. (2019), from GCN Circulars (Kuroda et al. 2010e,a; Hentunen et al. 2010b; Tkachenko et al. 2010; Strobl et al. 2010; Volnova et al. 2010b), as well as our own observations with Swift UVOT (first reported in Siegel & Markwardt 2010), Tautenberg (first reported in Kann et al. 2010f,e), FTN (first reported in Melandri et al. 2010), UKIRT/WFCAM (first reported in Im et al. 2010b), and Ondřejov D50 data (reported in Strobl et al. 2010) which are not published in this paper. The redshift is given by Tanvir

et al. (2010). The GRB afterglow is very bright and a “classic” example of a forward-shock rise, captured very early by MASTER (Gorbovskoy et al. 2012). We fit data up to $\approx 0.15$ d after the trigger with a smoothly broken power-law which is best fit by an extremely smooth rollover. We find $\alpha_1 = -25.27 \pm 0.47$, $\alpha_2 = 1.0544 \pm 0.0026$, $t_b = 0.00038 \pm 0.0000004$ d (32.83 $\pm$ 0.32 s), $n = 0.1$ fixed, and no host-galaxy contribution. This extremely steep rise is not realistic, but the extremely smooth rollover implies that the afterglow takes a long logarithmic timespan to asymptotically reach this value. The short baseline of the measured rise leads to an imprecise determination of the rising slope. The post-peak decay index is very precisely measured. After $\approx 0.15$ d, the afterglow transitions into a plateau phase with additional variability as well as a following sharp flare overlaid over a break to a steep decay. We find $\alpha_1 = 0.274 \pm 0.037$, $\alpha_2 = 1.988 \pm 0.020$, $t_b = 0.370 \pm 0.0052$ d, $n = 10$ fixed. This is very likely a jet break. Gorbovskoy et al. (2012), using a significantly smaller data set, find generally similar results with $\alpha_1 = 0.14 \pm 0.02$, $\alpha_2 = 2.17_{-0.04}^{+0.03}$, $t_b = 0.404_{-0.002}^{+0.006}$ d.

The SED is very broad (uww2uvm2uww1ubg'vR_cI_Cz'YJHK). Without dust, we find a moderately red but curved afterglow SED, it is $\beta = 0.840 \pm 0.011$ ($\chi^2$/d.o.f. = 43.1). A fit with MW dust yields negative extinction. LMC dust, on the other hand, yields a negative spectral slope and high extinction, we also find no compelling evidence for a 2175 Å bump. SMC dust, finally, yields a viable solutions: $\beta = 0.151 \pm 0.039$, $A_V = 0.292 \pm 0.016$ mag, with $\chi^2$/d.o.f. = 2.20. For this fit, we derive $dR_c = -1.953 \pm 0.044$ mag. The early afterglow is luminous, peaking at $R_c \approx 10.7$ mag.

### Appendix B.3.25. GRB 101219B, $z = 0.55185 \pm 0.00005$

Data are taken from Olivares E. et al. (2015); Sparre et al. (2011). The redshift is given by Sparre et al. (2011). This was a soft GRB at moderate redshift which was followed-up in detail by Swift UVOT and GROND, the latter which discovered an SN bump (Olivares E. et al. 2015), which was confirmed spectroscopically (Sparre et al. 2011). We find the SED is best fit by a small amount of SMC dust, with $\beta = 0.578 \pm 0.067$, $A_V = 0.077 \pm 0.037$ mag ($\chi^2$/d.o.f. = 3.05 as there is some scatter). For this fit, we find $dR_c = 1.375 \pm 0.054$ mag. Our results compare well with Olivares E. et al. (2015), who find $\beta = 0.62 \pm 0.01$, $A_V = 0.12 \pm 0.01$ mag. Zafar et al. (2018b), using the X-shooter spectrum and a full Fitzpatrick-Massa parametrization, find a steeper spectral slope $\beta = 0.93_{-0.10}^{+0.14}$, but a similar extinction $A_V < 0.11$ mag. The steeper spectral slope may be the result of a different $R_V$, but no value is given.

### Appendix B.3.26. GRB 110205A, $z = 2.21442 \pm 0.00044$

Data are taken from Cucchiara et al. (2011b); Zheng et al. (2012); Gendre et al. (2012); Steele et al. (2017), and from GCN circulars (Vreeswijk et al. 2011a; Kugel 2011; Kuroda et al. 2011d,c). The redshift is given by Cucchiara et al. (2011b). This GRB afterglow has one of the densest broadband follow-ups obtained to date. The afterglow is characterized by an initial plateau phase with a superposed optical flare linked to prompt-emission activity (Guiriec et al. 2016), followed by a steep, long rise to a very bright peak. After this, the afterglow decays smoothly, experiences a short plateau phase at $\approx 0.5$ d before finally breaking to an even steeper decay. For the rising and decaying phase, before 0.5 d, we find: $\alpha_{rise} = -4.278 \pm 0.077$, $\alpha_1 = 1.4873 \pm 0.0018$, $t_b = 0.01074 \pm 0.00005$ d ($t_b = 927.9 \pm 4.3$ s), $n = 2.34 \pm 0.11$, no
host galaxy ($r' \ge 27$ mag, Cucchiara et al. 2011b) ($\chi^2/d.o.f.$ = 6.04 resulting from some scatter and small errors). Data after 0.53 d is fit with a steeper decay, we find $\alpha_2 = 1.989 \pm 0.036$ ($\chi^2/d.o.f.$ = 1.76). We note late-time detections presented in Cucchiara et al. (2011b) are significantly brighter than the data presented in Gendre et al. (2012), the latter being in good agreement with the extrapolation of earlier data after the short plateau phase. Cucchiara et al. (2011b) find $\alpha_{rise} = -6.13 \pm 0.75$, $\alpha_1 = 1.71 \pm 0.28$, $t_b = 837^{+51}_{-40}$ s, $\alpha_2 = 1.74 \pm 0.28$, roughly in agreement with our results. Zheng et al. (2012) use a multicomponent model, making comparison difficult, e.g., they fix $\alpha_{rise} = 5.5$ in a scenario where the early peak is a pure reverse shock. For a different model, the late-time decay is model-independent and they report $\alpha_1 = 1.50 \pm 0.04$, in excellent agreement with our result. Their $t_b = 1064 \pm 42$ s is somewhat later than ours. Steele et al. (2017) report $\alpha_{rise} = -4.63 \pm 0.29$, $\alpha_1 = 1.52 \pm 0.02$, $t_b = 1027 \pm 8$ s, $n = 2.18 \pm 0.45$, in good agreement with our results.

The SED is very broad (www2uvm2uww1ubg'vr'R_c'i'I_cz'JHK), shows little scatter but also has very small errors, and is clearly curved. Without dust, we find $\beta = 1.302 \pm 0.011$ ($\chi^2/d.o.f.$ = 32.1). For MW dust, we find negative extinction, but LMC and SMC dust yield viable solution, with $\beta_{LMC} = 0.561 \pm 0.062$, $A_{V,LMC} = 0.396 \pm 0.033$ mag ($\chi^2/d.o.f.$ = 16.2), and $\beta_{SMC} = 0.738 \pm 0.042$, $A_{V,SMC} = 0.191 \pm 0.013$ mag ($\chi^2/d.o.f.$ = 6.95). While the goodness-of-fit is still high due to the small errors, the SMC fit is clearly preferred and represents one of the “best-behaved” SEDs in the entire sample.

Zheng et al. (2012) obtain multiple SEDs at several epochs (their table 6). For SMC dust, they generally find somewhat bluer intrinsic power-law slopes ($\beta \approx 0.55$) and therefore somewhat higher extinction ($A_V \approx 0.27$ mag), but in general agreement with our result. Gendre et al. (2012) study the SED at two epochs, initially finding, from a joint X-ray-to-optical fit, $\beta_{OX,1} = 0.84 \pm 0.04$ and no evidence for dust. From later data, they find $\beta_{OX,2} = 1.03 \pm 0.10$, $A_{V,MW} = 0.27 \pm 0.10$ mag, $A_{V,SMC} = 0.14 \pm 0.10$ mag. The SMC result is in reasonable agreement with ours. For our SMC result, we derive $dR_c = -2.606^{+0.046}_{-0.047}$ mag, and the early afterglow peak at $z=1$, is luminous, peaking at $R_c \approx 11.3$ mag.

## Appendix B.3.27. GRB 110213A, $z = 1.4607 \pm 0.0001$

Data are taken from Cucchiara et al. (2011b); Jelínek et al. (2016); Wang et al. (2022), and from GCN circulars (Rujopakarn et al. 2011; Kuroda et al. 2011e; Nakajima et al. 2011; Wren et al. 2011; Zhao et al. 2011), and from our own Swift UVOT analysis (which completes the partial analysis already presented in Cucchiara et al. 2011b and presents late-time limits beyond the data given in Wang et al. 2022; first reported in Kuin & D'Elia 2011). The redshift is given by Cucchiara et al. (2011b). This GRB had a highly peculiar afterglow. After an initial decay, the afterglow rises slowly to a peak and then decays. We fit data from 0.00086 d to 0.024 d with a smoothly broken power-law and find: $\alpha_{rise,1} = -2.702 \pm 0.019$, $\alpha_{1,1} = 0.7455 \pm 0.0053$, $t_{b,1} = 0.00223 \pm 0.00001$ d ($t_{b,1} = 192.67 \pm 0.86$ s), $n = 1.283 \pm 0.034$. This is an extremely precise fit, but the very small errors of the dense photometric coverage leads to a very high $\chi^2/d.o.f.$ = 21.2. This decay then switches to a second rise to a peak with a following decay, in this case, the afterglow evolution is additionally characterized by rapidly variable substructure. In this case, we find: $\alpha_{rise,2} = -0.5253 \pm 0.0052$, $\alpha_{1,2} = 1.4011 \pm 0.0087$, $t_{b,2} = 0.0642 \pm 0.0001$ d, $n = 9.45 \pm 0.49$. The additional variability leads to an ever worse goodness-of-

fit, $\chi^2/d.o.f.$ = 43.2. The data indicates a plateau phase from $\approx 1.2$ d to 2.3 d, after which the afterglow decays sharply, we find $\alpha_2 = 3.52 \pm 0.15$. Such a decay is similar to what we find for GRB 120326A (see §B.3.35) and GRB 060526 (Thöne et al. 2010). This steep decay likely represents a post-jet break decay, as already remarked upon by Cucchiara et al. (2011b). These authors find $\alpha_{rise,1} = -2.08 \pm 0.23$, $\alpha_{1,1} = 1.10 \pm 0.24$, $t_{b,1} = 263^{+13}_{-19}$ s, $\alpha_{rise,2} = -2.02 \pm 0.34$, $\alpha_{1,2} = 1.80 \pm 0.15$, $t_{b,2} = 0.056$ d. These values are somewhat discrepant to our results, this is because, for one, Cucchiara et al. (2011b) use a superposition of two Beuermann equations, and secondly, likely, because we let the break smoothness be a free parameter, which influence the temporal slopes.

The SEDs of this GRB afterglow are puzzling. Our temporal fits yield results with very small errors, and the creation of the joint light curve using the normalizations as offsets reveals an extremely densely covered light curve for which even small details, especially during the variability of the second bump, can be traced. This makes it highly unlikely that there are systematic offsets involved which would lead to erroneous scatter in the SEDs. They contain no NIR data but are otherwise broad (www2uvm2uww1ubg'vr'R_c'i'I_cz', with the first fit containing no g' data, www2uvm2uww1 data are affected by Lyman damping and are not included). The SEDs show deviations from a power-law behavior that cannot be explained by the usual dust models, e.g., they are essentially flat between g' and $R_c$, only to the show a large $r' - i'$ color. This behavior negates the possibility to obtain any useful free fit with dust models. Without dust, we find $\beta = 1.322 \pm 0.006$ with an extreme $\chi^2/d.o.f.$ = 742. Using the “late” data fit from the Swift XRT repository, we fix $\beta_{opt} = 0.90$ and find that an SMC fit yields the “best” result, it is $A_V = 0.132 \pm 0.0025$ mag, with $\chi^2/d.o.f.$ = 851.

However, we note that a dust model exists which shows a similar behavior to the SED derived here, that found in quasars in the high-redshift Universe, and also discovered in a single high-redshift event, GRB 071025 (Perley et al. 2010; Jang et al. 2011), which, we note, actually shows a temporal evolution similar to GRB 110213A, with a double bump. A deeper study is beyond the scope of this paper. For the SMC fit, we derive $dR_c = -1.306^{+0.013}_{-0.016}$ mag. At $z=1$, the afterglow peaks at $R_c \approx 14$ mag.

## Appendix B.3.28. GRB 110422A, $z = 1.770 \pm 0.001$

Data are taken from Pruzhinskaya et al. (2014), from GCN Circulars (Xu et al. 2011c,a,b; Moskvitin 2011; Melandri et al. 2011; Hentunen et al. 2011a; Kuroda et al. 2011a; Rumyantsev et al. 2011a,b,c), as well as our own observations with Swift UVOT (first reported in Breeveld & Mangano 2011), the Otto Struve/CQUEAN (first reported in Jeon et al. 2011; including late host galaxy detections in $i'$z'), the LOAO (upper limits only), the AZT-22/SNUCAM, and the Bohyunsan/KASINICS (NIR upper limit only). The redshift is given by de Ugarte Postigo et al. (2011). The afterglow consists of two parts. Data up to 0.027 d are fit by a single power-law decay, we find $\alpha_1 = 1.082 \pm 0.052$ ($\chi^2/d.o.f.$ = 1.84). Between this time and 0.1 d, a rebrightening must take place which is not covered by observations. Data after 0.1 d are again fit with a simple power-law and a less steep slope ($\alpha_2 = 0.893 \pm 0.042$). Hereby, we include the host galaxy as a free parameter in $R_c'i'z'$, the latter two bands have late detections, while $R_c$ shows a flattening at $\approx 10$ d, the fit finds
$R_C = 23.26 \pm 0.16$ mag (Vega, corrected for Galactic extinction) for the host.

The SED is broad (uvw2uvm2uvw1ubg'vr$R_Ci'z'$), and red but straight (the UV filters are Lyman-damped and are not included). It is well-fit by a simple power-law ($\beta = 1.79 \pm 0.13$, $\chi^2/d.o.f. = 0.37$). However, this value is steeper than typically found in the fireball model. Free fits with all dust models yield results with little extinction and too-red intrinsic slopes. The Swift XRT repository gives $\beta_X = 0.84 \pm 0.09$ for the “late” spectrum, and we fix the intrinsic slope to this value. MW dust would require a strong 2175 Å bump which is not observed, and SMC dust shows a too-steep far-UV slope. However, we find an excellent fit with LMC dust, it is $A_V = 0.49 \pm 0.07$ mag ($\chi^2/d.o.f. = 0.49$). Using these values, we find we derive $dR_c = -2.766_{-0.181}^{+0.178}$ mag. The early afterglow is quite luminous, peaking at $R_C \approx 12.1$ mag.

### Appendix B.3.29. GRB 110715A, $z = 0.8224 \pm 0.0002$

Data are taken from Sánchez-Ramírez et al. (2017), and from GCN circulars (Piranomonte et al. 2011; Nelson 2011). The redshift is given by Sánchez-Ramírez et al. (2017). This is the first GRB for which a submillimeter afterglow was detected with ALMA (Sánchez-Ramírez et al. 2017). The afterglow shows a complex evolution with multiple rebrightening episodes. The very first detections show evidence for a steep decay which flattens quickly, we find $\alpha_{steep} = 1.85 \pm 0.70$, $\alpha_{1,a} = 1.05 \pm 0.16$, $t_b = 0.00133 \pm 0.00025$ d ($t_b = 114.9 \pm 21.6$ s), $n = -100$ fixed ($\chi^2/d.o.f. = 0.39$). After multiple rebrightening episodes which are only sparsely sampled, the afterglow resumes a normal decay at $\approx 1.8$ d, from here on, it is covered by multicolor GROND observations. We find it can be fit by a smoothly broken power-law, with $\alpha_{1,b} = 1.15 \pm 0.23$ (in full agreement with $\alpha_{1,b}$ as would be expected for energy injections), $\alpha_2 = 1.88/pm0.12$, $t_b = 3.95 \pm 0.98$ d, $n = 10$ fixed ($\chi^2/d.o.f. = 0.53$).

The early (Swift UVOT with a few $R_C$ points) and late observations (GROND with a few UVOT points) are almost disconnected in terms of filter coverage, so great care was taken in constructing the SED, which is broad despite the high Galactic foreground extinction ($ubg_GvrGi_Gz_GJ_GH_GK_G$). We find it to be moderately red and clearly curved, $\beta = 1.200 \pm 0.054$ ($\chi^2/d.o.f. = 0.99$). All dust models yield good fits, we find for MW dust: $\beta = 0.67 \pm 0.26$, $A_V = 0.46 \pm 0.22$ mag ($\chi^2/d.o.f. = $), for LMC dust: $\beta = 0.57 \pm 0.31$, $A_V = 0.54 \pm 0.26$ mag ($\chi^2/d.o.f. = 0.49$), and for SMC dust: $\beta = 0.63 \pm 0.28$, $A_V = 0.46 \pm 0.22$ mag ($\chi^2/d.o.f. = 0.50$). These results are statistically indistinguishable, however, the 2175 Å would lie between the *u* and *b* bands, both which have large errors, but we see no conclusive evidence for this spectral feature. Therefore, we prefer the SMC fit. This result is different from that of Sánchez-Ramírez et al. (2017), who find $\beta = 0.90 \pm 0.22$, $A_V = 0.09 \pm 0.18$ mag for SMC dust. This leads to us finding a significantly smaller $dR_c$ than Sánchez-Ramírez et al. (2017) do, $dR_c = -0.276_{-0.370}^{+0.367}$ mag compared to $dR_c = 0.38_{-0.32}^{+0.17}$ mag. While the multiple rebrightenings make the afterglow observationally very bright (Sánchez-Ramírez et al. 2017), the early afterglow is luminous but not exceedingly so, with $R_C \approx 13.5$ mag.

### Appendix B.3.30. GRB 110726A, $z = 1.036$

Data are taken from Steele et al. (2017), from GCN circulars (Schaefer et al. 2011; Kuroda et al. 2011b; Zheng et al. 2011; Gorosabel et al. 2011; Moskvitin & Sokolov 2011), and from our own Swift UVOT analysis (first reported in Siegel & Wolf

2011; Porterfield et al. 2011). We also present late-time $g'r'i'z'$ observations of a bright, resolved host galaxy obtained by the GTC HiPERCAM. The redshift is given by Cucchiara et al. (2011a). The initial light curve shows a plateau phase which goes over into a steep decay (Zheng et al. 2011), which then transitions into a plateau phase Steele et al. (2017). We find $\alpha_{1,1} = 1.032 \pm 0.033$, $\alpha_{plateau} = 0.16 \pm 0.09$, $t_b = 0.0189 \pm 0.0014$ d ($t_b = 1629 \pm 124$ s), $n = -10$ fixed ($\chi^2/d.o.f. = 0.73$). Steele et al. (2017) model the light curve with two overlapping components, finding $\alpha_{1,1} = 1.03 \pm 0.05$, in perfect agreement with our value. Data after the plateau are scarce, using only $R_C$ data and our HiPERCAM host magnitude, we find that the late light curve is best fit with a smoothly broken power-law, however, the break time needs to be fixed, as the post-break decay is defined by only one data point. We find, setting the break conservatively at 1 d, right after the next-to-last detection: $\alpha_{1,2} = 0.81 \pm 0.17$, $\alpha_2 = 1.75 \pm 0.34$, $t_b = 1$ d fixed, $n = 10$ fixed ($\chi^2/d.o.f. = 0.02$). Using different data, Steele et al. (2017) find $\alpha_{1,2} = 1.13 \pm 0.33$, in agreement with our value within errors.

The SED is relatively narrow $ubg'vr'R_Ci'$, mildly red and straight (additional UVOT $uvw2uvm2uvw1$ upper limits from Porterfield et al. 2011 are in agreement with $z > 1$), we find without dust: $\beta = 1.21 \pm 0.12$ ($\chi^2/d.o.f. = 1.11$). This value is slightly steeper than the X-ray slope ($\Gamma_x = 2.00_{-0.16}^{+0.23}$), and the *u* band shows there is some curvature. Free fits with dust find negative extinction (MW, LMC dust) or a negative intrinsic spectral slope (SMC dust). Fixing $\beta_{opt} = \Gamma_x - 1.0 - 0.5 = 0.5$ and using SMC dust, we find $A_V = 0.297 \pm 0.052$ mag ($\chi^2/d.o.f. = 0.55$). Using this fit, we find $dR_c = -0.638 \pm 0.096$ mag, and the early afterglow is moderately luminous, having $R_c = 15.3$ mag.

### Appendix B.3.31. GRB 110731A, $z = 2.83$

Data are taken from Ackermann et al. (2013), from GCN Circulars (Malesani et al. 2011; Tanvir et al. 2011), as well as our 2.0 m FTN and FTS data (first reported in Bersier 2011). The host galaxy is well-detected in deep HST observations (Blanchard et al. 2016), but not in ground-based follow-up (Lü et al. 2017). The redshift is given by Tanvir et al. (2019). This GRB represented, after several years of Fermi operations, the first Type II GRB to be simultaneously detected by Swift and Fermi/LAT. It was a temporally quite short, very bright event at a moderately high redshift, and has been extensively discussed in Ackermann et al. (2013). The initial light curve is found to be decaying rather steeply ($\alpha = 1.440 \pm 0.011$, $\chi^2/d.o.f. = 0.70$), in good agreement with Ackermann et al. (2013), who find $\alpha = 1.37 \pm 0.03$ from UVOT data alone. After the end of dense data coverage at 0.052 d, the light curve exhibits a flattening, before going over into another steep decay, a rise, and then a further decay. Data are too sparse to categorize the light curve with fits, but Ackermann et al. (2013) also show the final epoch (from GROND observations) to be significantly brighter than an extrapolation of the early decay.

The SED is broad ($ubg_GvrGR_Ci_Gz_GJ_GH_G$), with $ubg_G$ being affected by Lyman damping, showing a clear dropout. The GROND SED values were derived directly versus $r_G$, and for simplicity, we assume $r_G = R_C$. The SED is moderately red ($\beta = 1.20 \pm 0.14$, $\chi^2/d.o.f. = 0.48$ for no dust). The XRT spectral slope is actually bluer ($\beta_X = 0.78_{-0.13}^{+0.15}$), indicating reddening by dust. Fixing the intrinsic slope to the X-ray value, we find the best fit for LMC dust, with $A_V = 0.23 \pm 0.08$ mag ($\chi^2/d.o.f. = 0.47$). For these values, we derive $dR_c = -3.358_{-0.265}^{+0.259}$ mag. At $z = 1$, the early afterglow is very luminous at $R_C \approx 9.3$ mag.
## Appendix B.3.32. GRB 111209A, $z = 0.67702 \pm 0.00005$

Data are taken from Stratta et al. (2013); Levan et al. (2014); Kann et al. (2018b), and from GCN Circulars (Nysewander et al. 2011b). The redshift is given by Kann et al. (2018b). The afterglow of this famous ultralong GRB (Gendre et al. 2013; Levan et al. 2014, see Janiuk et al. 2013 for a potential model) has been analyzed in detail in Kann et al. (2018b). GRB 111209A was followed by the highly luminous, spectrally peculiar SN 2011kl (Greiner et al. 2015b; Mazzali et al. 2016; Kann et al. 2019a). Concerning the SED, we note Kann et al. (2018b) found a strong color change in the afterglow during a rebrightening episode, with $\beta = 0.63 \pm 0.25$, $A_V = 0.25 \pm 0.11$ mag measured before 0.09 d, and $\beta = 1.05 \pm 0.06$, $A_V = 0.12 \pm 0.04$ mag after the rebrightening. Recently, Zafar et al. (2018b) derived $\beta = 0.68_{-0.09}^{+0.12}$, $A_V = 0.18 \pm 0.08$ mag, and $R_V = 2.53_{-0.15}^{+0.13}$ (the ratio of total to selective extinction, for SMC dust it is $R_V = 2.93$.) from the X-shooter spectrum, which was taken at the beginning of the color change. These values agree well with the first SED of Kann et al. (2018b).

## Appendix B.3.33. GRB 111228A, $z = 0.71627 \pm 0.00002$

Data are taken from Xin et al. (2016); Klose et al. (2019), and from GCN circulars (Kuroda et al. 2011f; Nysewander et al. 2011a; Hentunen et al. 2011b; Klotz et al. 2011; Morgan 2011; Usui et al. 2011; Guver et al. 2011; Cenko 2011; Krushinski et al. 2011; Pandey et al. 2011). The redshift is given by Selsing et al. (2019). This was a well-observed afterglow with a clear late-time SN component (Klose et al. 2019). The afterglow shows what is likely a very early flare followed by a rise and a very smooth rollover (see also Klose et al. (2019)). Correcting for a small amount of SMC dust, we find $dR_c = 0.603_{-0.063}^{+0.062}$ mag. The early afterglow is not overly luminous, peaking at $R_c = 16.7$ mag.

## Appendix B.3.34. GRB 120119A, $z = 1.72883$

Data are taken from Morgan et al. (2014); Steele et al. (2017); Blanchard et al. (2016), from GCN Circulars (Elenin et al. 2012; Klotz et al. 2012b; Sakamoto et al. 2012), as well as our own Swift UVOT, REM, and 1.0m LOAO data, first reported in Chester & Beardmore (2012), Fugazza et al. (2012), and Jang et al. (2012a), respectively. The redshift is given by Heintz et al. (2018b). This GRB was followed-up rapidly by multiple telescopes, yielding early multicolor observations which show a clear color change, the colors becoming bluer. This has been interpreted as a sign of dust destruction, the clearest known case for GRB afterglows (Morgan et al. 2014). The GRB itself was bright, and also detected by Fermi GBM (Gruber 2012).

The light curve shows a very early decay (and possibly a peak at earliest times) which we do not fit as the afterglow evolution here is not achromatic resulting from the rapidly variable line-of-sight extinction. At $\approx 150$ s, the decay flattens and turns into a shallow rise, which peaks and goes over into a decay again. With the exception of a few NIR data points at the beginning of the rise, this evolution is achromatic. We fit it with a smoothly broken power-law (Morgan et al. 2014 also present host-galaxy magnitudes in many bands) and find $\alpha_{rise} = -0.391 \pm 0.019$, $\alpha_1 = 1.467 \pm 0.009$, $t_b = 0.0120 \pm 0.0001$ d (1038 $\pm$ 11 s), and $n = 1.644 \pm 0.075$, implying a smooth rollover. This is one of the most precise values of $n$ measured so far. Morgan et al. (2014) find a decaying slope $\alpha = 1.30 \pm 0.01$, somewhat shallower than our value, possibly stemming from them fitting only the later

data with an SPL and not with a smooth break like we do. Japelj et al. (2015) also report $\alpha \approx 1.3$.

The high data density leads to a very precise determination of the fit normalizations and to an SED with small error bars and a wide coverage ($uBg'Vr'R_Ci'I_Cz'YJHK$). The SED is very red, we find $\beta = 2.60 \pm 0.01$ as well as clear curvature and an additional deviation (leading to $\chi^2/d.o.f. = 47$) indicative of a dust bump. Indeed, we find the SED is best fit (however, it is still $\chi^2/d.o.f. = 12.5$ stemming from the small errors and some minor scatter) by LMC dust with $\beta = 0.908 \pm 0.086$, $A_V = 1.114 \pm 0.056$ mag. This represents one of the most secure detections of line-of-sight extinction toward a GRB afterglow. For MW dust, the bump is too shallow compared to the curvature, which yields a very bad fit ($\beta = 2.235 \pm 0.046$, $A_V = 0.258 \pm 0.030$ mag, $\chi^2/d.o.f. = 44$), whereas the SMC fit underestimates the data blueward of the dust bump, we find $\beta = 1.255 \pm 0.078$, $A_V = 0.799 \pm 0.046$ mag, $\chi^2/d.o.f. = 17.4$. Morgan et al. (2014) present an extensive discussion of the SED, we refer to their paper for further details, but note they state that SMC dust fits best of the standard models. Adding X-ray data, they find $\beta = 0.92$, $A_V \approx 1.15$ mag for two different LMC extinction curves, in full agreement with our value. Japelj et al. (2015) also find results fully agreeing with ours, namely $\beta = 0.89 \pm 0.01$, $A_V = 1.07 \pm 0.03$ mag, and they strongly favor LMC dust. However, they also state LMC dust over-predicts the strength of the 2175 Å bump and that none of the standard models gives a satisfactory fit. Zafar et al. (2018b) also present results in general agreement with ours, it is $\beta = 0.84_{-0.09}^{+0.10}$, $A_V = 1.02 \pm 0.11$ mag, and $R_V = 2.99_{-0.18}^{+0.24}$, very similar to the SMC extinction curve. This agrees with the best fit Morgan et al. (2014) find with a full Fitzpatrick-Massa parametrization (Fitzpatrick & Massa 1986, 2007), they deduce $R_V = 4.11 \pm 1.03$ combined with $\beta = 0.92 \pm 0.02$, $A_V = 1.09 \pm 0.16$ mag. The large extinction leads to a large correction to $z = 1$, it is $dR_c = -4.355_{-0.154}^{+0.147}$ mag. We find the early afterglow luminosity, despite the evinced capacity to burn dust, is rather unremarkable, reaching $R_C = 12.3$ mag; however, the slow rise to the second peak leads to the late light curve lying in the brighter half of the luminosity distribution. The late luminosity and decay rate are similar to the afterglow of GRB 180325A (see also Cappellazzo et al. (2022)).

## Appendix B.3.35. GRB 120326A, $z = 1.798$

Data are taken from Urata et al. (2014); Melandri et al. (2014c); Laskar et al. (2015); Jelínek et al. (2016, 2019), from GCN circulars (Klotz et al. 2012c; LaCluyze et al. 2012; Walker et al. 2012; Zhao et al. 2012a; Sahu et al. 2012), and from our own 1.0m LOAO observations (first reported in Jang et al. 2012b and partially published in Urata et al. 2014, we present a reduction of the $r'$ band using stacked frames that reduces the scatter and brings the data into alignment with that from other sources), as well as late-time GTC HiPERCAM observations that detect a bright, slightly extended host galaxy, in agreement with the final detections from Laskar et al. (2015). The redshift is given by Tello et al. (2012). The light curve shows several peculiar properties. As has already been remarked upon by Jelínek et al. (2019), at earliest times, the light curve decays in $R_C/r'$, while it rises in $i'$, causing a spectral reversal ($i' > r'$) which is not typically seen in GRB afterglows. We do not discuss this part further. Frm 0.0109 to 0.202 d, the afterglow shows a decay before smoothly turning over into a moderately steep rise to a large, second peak (Urata et al. 2014; Melandri et al. 2014c). For his segment, we find $\alpha_{1,1} = 0.80 \pm 0.16$, $\alpha_{rise} = -0.96 \pm 0.19$,
$t_b = 0.041 \pm 0.011$ d, $n = -1$ fixed ($\chi^2/d.o.f. = 0.95$). While unable to let $n$ vary freely, we find a smooth rollover fits the light curve better than a sharp break. From 0.285 d onward, the afterglow goes back over into a standard decay, breaking to an extremely steep one. Data in $r'$ from Laskar et al. (2015) already point to a transition to a host galaxy, which we fully confirm with our late-time GTC HiPERCAM imaging. We find: $\alpha_{1,2} = 0.891 \pm 0.001$, $\alpha_2 = 3.686 \pm 0.037$, $t_b = 3.417 \pm 0.014$ d, $n = 2$ fixed ($\chi^2/d.o.f. = 17.8$). The very bad quality of fit stems from a combination of small errors and scatter, mostly from the data set of Urata et al. (2014), even after replacing their $r'$ data with our own rereduction. We note that $\alpha_{1,1} = \alpha_{1,2}$ within errors, typical behavior for an energy injection. The steepness of $\alpha_2$ is dependent on the break sharpness value $n$, we found that fixing it to 2 improves $\chi^2/d.o.f.$ by 5. Allowing it to vary freely leads to a degenerate solution where $n$ continues to become softer while $\alpha_2$ increases even further.

The SED, based on the second fit, is very broad (uvw2uvm2uvw1ubg'vr'R<sub>C</sub>i'z'YJHK<sub>S</sub>), has small errors and shows some scatter. The UVOT UV and *u* filters are affected by Lyman damping and are not included. The *b* band is also anomalously bright and is excluded. Finally, JHK<sub>S</sub> data is brighter than a redward extrapolation of the optical data, however, the errors are significantly larger so they are not that strong outliers. Fits with all dust models return negative extinction, and we prefer the fit with no dust: $\beta = 0.950 \pm 0.016$ ($\chi^2/d.o.f. = 17.2$ resulting from small errors and the above-mentioned offset of NIR data).

Results from the literature for this GRB vary widely, in part because these publications only cover partial data sets. Urata et al. (2014) study the late rebrightening. They find $\alpha_{1,2} = 0.88...1.01$ in different bands, in full agreement with our value, and $\alpha_2 = 2.48...2.84$, which is less steep than our result, likely as they do not take the bright host galaxy into account. They derive three different SEDs and find a range of $\beta = 1.11...1.44$, somewhat steeper than our value. They estimate $A_V = 0.3...0.5$ mag via a joint X-ray-to-optical fit. Melandri et al. (2014c) cover the early light curve and the first days of the rebrightening, but have little data showing the late, very steep decay. For the first part of the light curve, they find $\alpha_{1,1} = 0.50 \pm 0.05$, $\alpha_{rise} = 1.53 \pm 0.18$, $\alpha_{1,2} = 1.77 \pm 0.11$. This is shallower, steeper, and also steeper than our result. The steeper late-time decay likely results from them fitting additional data beyond the break Urata et al. (2014) and we find. Spectrally, they perform a X-ray-to-optical fit and find a spectral slope very similar to ours, $\beta = 0.88 \pm 0.03$, but with a very high SMC dust column, $A_V = 1.1 \pm 0.3$ mag. Finally, Laskar et al. (2015) take the data sets of the other authors into account. For the rebrightening, they find $\alpha_{rise} = -0.52 \pm 0.06$, $\alpha_{1,2} = 1.10 \pm 0.10$, the first value being shallower than our result and the second being slightly steeper. The post-break decay slope they find is also shallower than our result, with $\alpha_2 = 2.05 \pm 0.13$, it is not made clear if the host-galaxy contribution was taken into account (it clearly is in their figure 4, top right). They derive an SED (their figure 3) that looks similar to our own, with a flatter optical part and overly bright NIR emission. Using a joint X-ray-to-optical fit, they find an optical-only slope of $\beta_{opt} = 1.80 \pm 0.16$ and in combination with the X-ray slope $\beta_X = 0.85 \pm 0.04$ an extinction of $A_V = 0.40 \pm 0.01$ mag. For the no-dust fit, we find $dR_c = -1.571 \pm 0.006$ mag. At $z=1$, the light curve is only moderately luminous, peaking at $R_c \approx 16.3$ mag, however, this is at $\approx 0.2$ d. The early afterglow remains at $R_c > 17$ mag.

## Appendix B.3.36. GRB 120327A, $z = 2.81482$

Data are taken from D'Elia et al. (2014); Steele et al. (2017); Melandri et al. (2017), and from GCN circulars (Sudilovsky et al. 2012; Klotz et al. 2012a). The redshift is given by Heintz et al. (2018b). The afterglow shows a classic rise and smooth decay, which transitions into a shallower decay and then a plateau phase and a subsequent decay. Fitting data up to $\approx 0.064$ d, we find: $\alpha_{rise} = -1.70 \pm 0.17$, $\alpha_1 = 1.3660 \pm 0.0096$, $t_b = 0.00480 \pm 0.00013$ d ($t_b = 414 \pm 11$ s), $n = 1$ fixed ($\chi^2/d.o.f. = 1.75$). Fitting data between the peak and 0.18 d, we find $\alpha_1 = 1.2959 \pm 0.0041$, $\alpha_{shallow} = 0.907 \pm 0.049$, $t_b = 0.0876 \pm 0.0061$ d, $n = -17.1 \pm 7.71$ ($\chi^2/d.o.f. = 2.43$). Fitting the shallow(er) and plateau phase, we find: $\alpha_{shallow} = 1.087 \pm 0.015$, $\alpha_{plateau} = 0.089 \pm 0.068$, $t_b = 0.174 \pm 0.005$ d, $n = -10$ fixed ($\chi^2/d.o.f. = 2.42$). Finally, in $r'i'$ we can fit the transition from plateau to subsequent decay: $\alpha_{plateau} = -0.11 \pm 0.14$, $\alpha_2 = 1.607 \pm 0.034$, $t_b = 0.248 \pm 0.006$ d, $n = 10$ fixed ($\chi^2/d.o.f. = 2.61$). Steele et al. (2017) find $\alpha_1 = 1.22 \pm 0.02$. Melandri et al. (2017) find a similar value $\alpha \approx 1.2$. Both are close to our result, likely slightly shallower as the other authors fit the initial decay together with the shallower following one.

We derive three SEDs from our first three fits (the final one uses only two bands), with the first one being the broadest (uvw1ubg'vR<sub>C</sub>I<sub>C</sub>z<sub>G</sub>J<sub>G</sub>H<sub>G</sub>K<sub>G</sub>). uwv1ubg' data are affected by Lyman damping and are excluded. The SEDs show slight curvature. We find no significant evidence for color change and fit all three SEDs simultaneously. Without dust, we find: $\beta = 1.031 \pm 0.025$ ($\chi^2/d.o.f. = 2.28$). MW and LMC dust yield negative extinction, but we find an excellent fit for SMC dust: $\beta = 0.656 \pm 0.090$, $A_V = 0.140 \pm 0.032$ mag ($\chi^2/d.o.f. = 1.07$). Melandri et al. (2017) find evidence for slight color change, modelling several SEDs either with constant slope and variable extinction, or vice versa. In the first case, they derive $\beta_{opt} = 0.55^{+0.05}_{-0.04}$ and $A_V < 0.05$ to $A_V = 0.06^{+0.04}_{-0.03}$ mag. In the other case, $A_V = 0.05 \pm 0.02$ mag, and the slope spanning from $\beta = 0.72^{+0.11}_{-0.09}$ to $\beta = 0.81^{+0.08}_{-0.02}$. These values are in decent agreement with our result. Using our SMC fit, we derive $dR_c = -3.078^{+0.139}_{-0.136}$ mag. The early afterglow at $z=1$ is luminous, peaking at $R_c \approx 11.2$ mag.

## Appendix B.3.37. GRB 120404A, $z = 2.8767$

Data are taken from Guidorzi et al. (2014b); Ershova et al. (2020), from GCN circulars (Xin et al. 2012; Tristram et al. 2012; Volnova et al. 2012b,a), as well as from our own Swift UVOT analysis (first reported in Breeveld & Stratta 2012). The redshift is given by Guidorzi et al. (2014b). The afterglow shows an initial shallow decay and a plateau phase which goes over into a long rise, smooth turnover and decay. Fitting data starting at 0.016 d, we find: $\alpha_{rise} = -2.89 \pm 0.11$, $\alpha_1 = 1.688 \pm 0.023$, $t_b = 0.02257 \pm 0.00049$ d, $n = 0.5$ fixed and no host galaxy ($\chi^2/d.o.f. = 0.78$). The final detection in (Guidorzi et al. 2014b, from GROND) lies $7\sigma$ under the extrapolation of the earlier decay, indicating another break, likely a jet break, has taken place. Guidorzi et al. (2014b) state the rising slope has a large uncertainty but do not give a value, they measure $\alpha_1 = 1.9 \pm 0.1$, and find a peak time $t_p = 0.0278 \pm 0.0069$ d. Laskar et al. (2015) find $\alpha_{rise} = -1.74 \pm 0.58$, $\alpha_1 = 1.71 \pm 0.17$ (B band, for the $R_c$ band they find $\alpha_1 = 1.90 \pm 0.02$), $t_b = 0.028 \pm 0.004$ d, $n = 0.78 \pm 0.43$. These values are generally in agreement with our own.

The SED is very broad (uvw2uvm2uvw1uB<sub>G</sub>VR<sub>C</sub>i'u<sub>C</sub>z<sub>G</sub>J<sub>G</sub>H<sub>G</sub>), with uwv2uvm2 being (deep) upper limits only, uwv1u are also strongly affected by Lyman damping and is not included
in the fitting. The rest of the SED is moderately red and nearly straight, $J_G H_G$ lie a bit above the redward extrapolation of the optical data. Without dust, we find $\beta = 1.112 \pm 0.041$ ($\chi^2/d.o.f.$ = 3.73). An SMC dust fit finds negative extinction, and an LMC dust fit yields a fit result that is worse than the fit without dust. However, we find a slight improvement using MW dust, finding: $\beta = 1.040 \pm 0.053$, $A_V = 0.064 \pm 0.029$ mag ($\chi^2/d.o.f.$ = 3.55). Using an optical-to-X-ray fit and MW, Guidorzi et al. (2014b) find their best fit results in: $\beta = 1.01 \pm 0.03$, $A_V = 0.24 \pm 0.07$ mag. That is, a very similar spectral slope and a preference for MW dust, but a higher extinction value. Laskar et al. (2015) find an unextinguished spectral slope $\beta = 1.3 \pm 0.2$ (identical to our result within errors), and derive $A_V = 0.13 \pm 0.01$ mag from a broadband analysis; no dust model is given (close to our result). Using our MW dust result, we derive $dR_c = -3.036^{+0.057}_{-0.056}$ mag. At $z=1$, the afterglow is luminous, peaking at $R_c = 13.7$ mag. However, the peak is not until $\approx 0.015$ d. The early-time afterglow is less luminous, at $R_c \approx 15$ mag.

### Appendix B.3.38. GRB 120711A, $z=1.405$

Data are taken from Martin-Carrillo et al. (2014), from GCN circulars (Levato et al. 2012; Breeveld & Page 2012), from unpublished late-time GROND observations, and from our own rereduction of the 0.6m REM/REMIR data, as well as an upper limit from FRAM/PAO. The redshift is given by Tanvir et al. (2012b). The REM reanalysis reveals a peculiarity we have not been able to successfully solve. Aside from adding as-yet-unpublished “late-time” (0.06 – 0.1 d) JHK observations, the early observations are not in agreement with those published in Martin-Carrillo et al. (2014). The times are earlier and the afterglow exhibits an extreme flare reaching $H \approx 8.2$ mag in the second image, exactly at the time the dense optical observations also peak, though they do not exhibit such a strong flaring behavior. The subsequent decay is flatter than that of the data Martin-Carrillo et al. (2014) present. A constant shift in time is able to align most of the data but would then move the flare away from the optical peak. The early data in our rereduction is sampled more finely in time which may explain why the flare is not seen in the reduction Martin-Carrillo et al. (2014) present.

### Appendix B.3.39. GRB 120714B, $z=0.3984$

Data are taken from Klose et al. (2019) as well as from the automatic UVOT analysis page<fnref n="11" />. The redshift is given by Selsing et al. (2019). This was a faint GRB at moderately low redshift, with relatively sparse early follow-up, however, the SN phase is well-covered by GROND (Klose et al. 2019). The full analysis is given in Klose et al. (2019). We follow Klose et al. (2019) and adopt $\beta = 0.7 \pm 0.4$ and no dust extinction, leading to $dR_c = 2.331^{+0.155}_{-0.158}$ mag. The g'-band light curve of this afterglow is unaffected by the accompanying SN 2012eb, and allows follow-up up to $\approx 140$ d in the $z=1$ frame. The afterglow is one of the faintest in the entire sample, being $R_c = 20.7$ mag (at $z=1$) already at very early times.

### Appendix B.3.40. GRB 120729A, $z=0.80$

Data are taken from Cano et al. (2014a); Huang et al. (2018), and from GCN circulars (Wren et al. 2012; Wang et al. 2015).

The redshift is given by Tanvir & Ball (2012). This was a bright (initial detection $R_c = 12.8$ mag correcting for Galactic extinction) afterglow which decayed steeply and showed late-time evidence for a SN bump Cano et al. (2014a) (see also Cano et al. (2014a)). Correcting for a small amount of SMC extinction, we find $dR_c = 0.269^{+0.083}_{-0.084}$ mag and an early afterglow peak magnitude $R_c = 13.1$ mag.

## Appendix B.3.41. GRB 120815A, $z=2.35820$

Data are taken from Krühler et al. (2013), as well as our own analysis of Swift UVOT data (first reported in Holland & Pagani 2012a), and VLT data (X-shooter finding charts, first reported in Malesani et al. 2012). The redshift is given by Heintz et al. (2018b). We note that we added the recommended systematic errors from Krühler et al. (2013) to the data. The light curve shows an initial, shallow rise, which goes over into a shallow decay which steepens mildly in the final data points. We find $\alpha_{rise} = -0.204 \pm 0.033$, $\alpha_1 = 0.544 \pm 0.009$, $t_{b,1} = 0.00595 \pm 0.00023$ d ($t_{b,1} = 514.1 \pm 19.9$ s) ($\chi^2/d.o.f.$ = 0.33); and $\alpha_1 = 0.515 \pm 0.017$, $\alpha_2 = 0.871 \pm 0.035$, $t_{b,2} = 0.0528 \pm 0.0066$ d ($\chi^2/d.o.f.$ = 0.43). This compares well with the results from Krühler et al. (2013), who find $\alpha_{rise} = -0.18 \pm 0.02$, $\alpha_1 = 0.52 \pm 0.01$, $t_{b,1} = 440 \pm 30$ s, $\alpha_2 = 0.86 \pm 0.03$, $t_{b,2} = 4300^{+900}_{-600}$ s ($t_{b,2} = 0.050^{+0.010}_{-0.007}$ d).

The SED is broad ($ubg_G v r_G R_c i_G z_G J_G H_G K_G$), with $u$ being affected by Lyman damping and not included. The SED is moderately red and clearly curved, without dust we find $\beta = 1.220 \pm 0.053$ ($\chi^2/d.o.f.$ = 3.86). A fit with MW dust yields negative extinction and is ruled out. A fit with LMC dust yields an improved result ($\chi^2/d.o.f.$ = 1.60) but with a negative intrinsic spectral slope, and the $i_G$ detection lies within the 2175 Å bump region and shows there is no bump, so we rule this dust model out as well. The best fit is found with SMC dust, we derive $\beta = 0.196 \pm 0.209$, $A_V = 0.423 \pm 0.085$ mag. This is in agreement with the result from Japelj et al. (2015), who derive $\beta = 0.38^{+0.07}_{-0.05}$, $A_V = 0.32 \pm 0.02$ mag, also using SMC dust. Krühler et al. (2013) also prefer SMC dust and state they find no evidence for a dust bump, especially not from their X-shooter spectrum. Krühler et al. (2013) note they do not fit the $g_G$ data as the filter extends into the Lyman-$\alpha$ region, but we note the fit results are identical whether $bg_G$ are included or not (except for the errors which become larger if the two filters are left out of the fit). Krühler et al. (2013) perform an XRT to optical fit and find no significant evidence for a cooling break between the two regimes, and $\beta = 0.78 \pm 0.01$, $A_V = 0.15 \pm 0.02$ mag. Using our SED, leaving out $bg_G$ and fixing $\beta = 0.78$, we derive $A_V = 0.13 \pm 0.02$ mag, confirming the result from Krühler et al. (2013). Finally, Zafar et al. (2018b), using a Fitzpatrick-Massa parametrization, derive and even redder intrinsic afterglow but with somewhat higher extinction compared to Krühler et al. (2013): $\beta = 0.92 \pm 0.10$, $A_V = 0.19 \pm 0.04$ mag, and $R_V = 2.38 \pm 0.09$. Using our SMC result, we find $dR_c = -3.260^{+0.312}_{-0.301}$ mag. The afterglow is moderately luminous, peaking at $R_c \approx 14.2$ mag.

## Appendix B.3.42. GRB 121024A, $z=2.30244$

Data are taken from Wiersema et al. (2014); Friis et al. (2015); Varela et al. (2016); Jelínek et al. (2016), from GCN circulars (Klotz et al. 2012d; Zhao et al. 2012b; Pandey & Kumar 2012; Cobb 2012), and from our own Swift UVOT data analysis (first reported in Holland & Pagani 2012b). The redshift is given by Heintz et al. (2018b). This GRB afterglow showed evidence for circular polarization (Wiersema et al. 2014). The

<footnote n="11">¹¹ https://swift.gsfc.nasa.gov/uvot_tdrss/526642/index.html, however, this page is not available at time of writing.</footnote>
afterglow potentially shows a bright, rapidly decaying flash at very early times (Klotz et al. 2012d). Thereafter, two rebrightenings/plateau phases are seen in the data presented by Friis et al. (2015). We begin our fit after the second plateau, at 0.0148 d. We find the afterglow can be modeled with a smoothly broken power-law, we measure: $\alpha_1 = 0.939 \pm 0.014$, $\alpha_2 = 1.72 \pm 0.12$, $t_b = 0.880 \pm 0.077$ d, $n = 10$ fixed, and the host-galaxy magnitudes given by individual late-time measurements especially from Friis et al. (2015) ($\chi^2$/d.o.f. = 1.41). (Wiersema et al. 2014) find $\alpha_1 = 0.93 \pm 0.02$, $\alpha_2 = 1.25 \pm 0.04$, $t_b = 0.4306 \pm 0.0081$ d, $n = 5.01 \pm 0.01$. Varela et al. (2016) find $\alpha_1 = 0.71 \pm 0.03$, $\alpha_2 = 1.46 \pm 0.04$, $t_b = 0.363 \pm 0.109$ d, $n = 2.7 \pm 1.1$. Our results are roughly in agreement, however, we find a steeper late-time decay and later break time.

The SED is broad ($ubG_{GVR}R_C i_G I_{CZ}G_J H_G K_G$). The *u* band is definitely Lyman-damped and is excluded. Friis et al. (2015); Varela et al. (2016) comment that $g_G$ is affected by the Lyman$\alpha$ DLA as well and exclude it from their fits. However, we find that an SMC dust fit is essentially identical whether $B_{g_G}$ are included or not, except that exclusion leads to a significantly larger error. We therefore choose to include these two filters in the SED fit. The SED is moderately red and clearly curved, without dust we find: $\beta = 1.446 \pm 0.060$ ($\chi^2$/d.o.f. = 2.36). We find negative extinction for MW dust, LMC dust yields a viable fit ($\chi^2$/d.o.f. = 1.30), but with high extinction, a very blue intrinsic SED, and no significant evidence for a 2175 Å bump. We therefore prefer the SMC dust fit, for which we find $\beta = 0.52 \pm 0.46$, $A_V = 0.39 \pm 0.24$ mag ($\chi^2$/d.o.f. = 0.58). (Wiersema et al. 2014) derive $\beta = 0.88 \pm 0.01$, $A_V = 0.22 \pm 0.02$ mag from a joint X-ray-to-optical fit. Friis et al. (2015) find $\beta = 0.90 \pm 0.02$, $A_V = 0.088 \pm 0.059$ mag from a joint X-ray-to-optical fit, using SMC dust. Also using a multi-epoch joint X-ray-to-optical fit (no evidence for color evolution is found), Varela et al. (2016) find $\beta = 0.86 \pm 0.02$, $A_V = 0.18 \pm 0.04$ mag. Zafar et al. (2018b) study the X-shooter spectrum and derive: $\beta = 0.85^{+0.09}_{-0.13}$, $A_V = 0.26 \pm 0.07$ mag, $R_V = 2.81^{+0.20}_{-0.16}$. We therefore find a bluer intrinsic SED slope, in agreement with a potential cooling break between optical and X-rays. Our extinction value is consequently higher, but only in disagreement with the value Friis et al. (2015) derive. Using our SMC fit, we derive $dR_c = -3.192^{+0.308}_{-0.312}$ mag, and find a very luminous early afterglow, $R_c \approx 9.9$ mag.

### Appendix B.3.43. GRB 130427A, $z = 0.3399 \pm 0.0002$

Data are taken from Laskar et al. (2013); Xu et al. (2013a); Vestrand et al. (2014); Maselli et al. (2014); Perley et al. (2014b); Melandri et al. (2014b); Coward et al. (2017); Becerra et al. (2017a,b), and from GCN circulars (Morgan 2013a; Yatsu et al. 2013; Im 2013; Kuroda et al. 2013b; Wiggins 2013; Takahashi et al. 2013; Keel et al. 2013; Norris & Macomb 2013; Kuroda et al. 2013a; van de Stadt et al. 2013; Flewelling et al. 2013; Hermansson et al. 2013). The redshift is given by Selsing et al. (2019). GRB 130427A was a once-per-several-decades event, the brightest GRB since 1984, however, placing it in contest, it is just a close-by example of the kind of GRBs which are usually detected at $z > 1$, earning it the moniker “a nearby, ordinary monster” (Maselli et al. 2014). It yielded very detailed high-energy observations including the longest-lasting LAT detection to date (Ackermann et al. 2014; Preece et al. 2014). The GRB was associated with SN 2013cq (de Ugarte Postigo et al. 2013; Xu et al. 2013a; Melandri et al. 2014b). Its X-ray afterglow was detected for years and shown to not break, implying a wide jet-opening angle and extreme energetics (De Pasquale et al. 2016b).

For this work, it is relevant that we derive $dR_c = +2.73$ mag. The extremely bright early prompt flash (Vestrand et al. 2014) peaks at $R_c \approx 9.6$ mag.

## Appendix B.3.44. GRB 130831A, $z = 0.4791$

Data are taken from Cano et al. (2014a); De Pasquale et al. (2016a); Klose et al. (2019), and from GCN Circulars (Xu et al. 2013c; Xin et al. 2013; Leonini et al. 2013; Sonbas et al. 2013; Khorunzhev et al. 2013; Gorbovskoy et al. 2015). The redshift is given by Cucchiara & Perley (2013). This was a low-redshift GRB with extensive follow-up and a late-time SN with spectroscopic confirmation, SN 2013fu (Cano et al. 2014a; Klose et al. 2019). The observed afterglow is bright, peaking at $R_c = 13.3$ mag. No dust is found, and we find $dR_c = 1.968^{+0.016}_{-0.017}$ mag, leading to a peak magnitude at $z = 1$ of $R_c = 15.3$ mag.

## Appendix B.3.45. GRB 131030A, $z = 1.296$

Data are taken from King et al. (2014); Huang et al. (2017), from GCN circulars (Xu et al. 2013b; Terron et al. 2013; Moskvitin 2013; Hentunen et al. 2013; Perley & Cenko 2013b; Tanigawa et al. 2013; Pandey et al. 2013), as well as our own extensive data set, consisting of Swift UVOT data (first reported in Breeveld & Troja 2013), very early Ondřejov D50 and BART (NF+WF) data (first reported in Strobl et al. 2013), Watcher data (first reported in Kubanek et al. 2013), LT data (first reported in Virgili et al. 2013a), LOAO telescope data (first reported by Im & Choi 2013 and presented but not tabulated in Huang et al. 2017), and late-time GTC HiPERCAM observations which detect a very faint host galaxy (in full agreement with the Subaru detection reported by Huang et al. 2017). The redshift is given by Selsing et al. (2019). This was a very extensively observed afterglow, over 1400 data points have been published so far, and it is the largest data set we present here, with 260 data points, including very early observations from multiple telescopes all the way to very deep host-galaxy observations. It was the first GRB afterglow to be observed by ALMA in open-use (Huang et al. 2017), and the early afterglow was polarised (King et al. 2014).

The afterglow shows classic behavior, with an initial rise which rolls over into a smooth decay, likely the forward-shock rise (initially noted by Strobl et al. 2013). We find $\alpha_{rise} = -0.54 \pm 0.11$, $\alpha_1 = 0.9745 \pm 0.0036$, $t_{b,peak} = 0.00131 \pm 0.00005$ d ($t_{b,peak} = 113.18 \pm 4.32$ s). The high data density allowed us to leave the break smoothness as a free parameter, $n = 3.29 \pm 0.56$ ($\chi^2$/d.o.f. = 4.46 stemming from some scatter and small error bars). At $\approx 0.04$ d, the light curve flattens, before completely flattening at $\approx 0.095$ d. This is followed by a data gap, the afterglow is decaying again normally when observations resume at $\approx 0.2$ d. We fit the afterglow from this point on with a broken power-law and find $\alpha_{1,2} = 0.9315 \pm 0.0067$, $\alpha_2 = 1.474 \pm 0.08$, $t_b = 1.15 \pm 0.04$ d, $n = 10$ fixed, and using our HiPERCAM host-galaxy magnitudes (we neglected the host galaxy in the NIR as it is very blue). The similarity of $\alpha_1$ and $\alpha_{1,2}$ supports the rebrightening being an energy injection. King et al. (2014) find an early decay slope $\alpha_1 = 0.78 \pm 0.02$, flatter than what we find. Huang et al. (2017), using later data, find $\alpha_1 \approx 0.84 \pm 0.04$, $\alpha_2 \approx 2.05 \pm 0.16$, $t_b = 2.91 \pm 0.56$ d. This is a later break and steeper post-break decay than our result. Nonetheless, all evidence points to this being a jet break.
We find no conclusive evidence for color change and fit the SED derived from the early fit (uvm2uww1ubvR<sub>C</sub>) and the late fit (uww2uvm2uww1ubg'vr'R<sub>C</sub>i'IC<sub>Z</sub>'YJH) together. UV data was affected by Lyman damping and was not included. A fit without dust finds β = 0.838 ± 0.035, indicating dust extinction is likely low. MW dust find negative extinction and is ruled out. A viable solution is found for LMC dust (β = 0.24 ± 0.16, A<sub>V</sub> = 0.38 ± 0.10 mag, χ<sup>2</sup>/d.o.f. = 3.54), there is no clear evidence for a 2175 Å bump, however. The best solution is found for SMC dust: β = 0.36 ± 0.11, A<sub>V</sub> = 0.26 ± 0.05 mag, χ<sup>2</sup>/d.o.f. = 2.83. Huang et al. (2017) find no evidence for host-galaxy extinction and 2σ evidence for a slope change, an early SED showing β = 0.57 ± 0.09, while a later one results in β = 1.08 ± 0.23. Using our SMC solution, we find dR<sub>c</sub> = -1.147<sup>+0.113</sup><sub>-0.115</sub> mag. The early afterglow is bright, peaking at R<sub>c</sub> ≈ 12.9 mag.

### Appendix B.3.46. GRB 140311A, z = 4.954

Data are taken from Littlejohns et al. (2015); Laskar et al. (2018b), and from GCN Circulars (Klotz et al. 2014; D'Avanzo et al. 2014c; Xu et al. 2014; Yoshida et al. 2014; Malesani et al. 2014a). The redshift is given by Selsing et al. (2019). Observations of this event at moderately high redshift are sparse. The afterglows shows an early rise ($α_{rise} = -1.05 ± 0.32$, χ<sup>2</sup>/d.o.f. = 0.34, in full agreement with Laskar et al. 2018b) later followed by a decay with no clear evidence for a further break ($α_1 = 0.771 ± 0.023$, χ<sup>2</sup>/d.o.f. = 1.35). Laskar et al. (2018b) find a flatter decay and a break, however, they start their fit from the earliest detection, excluding the following points as a superposed flare.

The SED ($R_{C,i}'z'$Y) is extremely red, with R<sub>C</sub> showing additional evidence for being Lyman-damped. We find β = 4.40 ± 0.21 (χ<sup>2</sup>/d.o.f. = 0.02), fully in agreement with Laskar et al. (2018b). It is, however, in strong contrast to Littlejohns et al. (2015), who report $β_{opt} ≈ 0.68$ – only to later report $A_{V,SMC} = 0.45^{+0.08}_{-0.06}$ mag. We fix the spectral slope to an intrinsic value derived from X-rays ($β_X = 0.584^{+0.200}_{-0.099}$ given on the Swift XRT repository) and find that MW dust strongly underestimates the R<sub>C</sub> detection. LMC dust ($A_V = 1.01 ± 0.06$ mag, χ<sup>2</sup>/d.o.f. = 1.40) and SMC dust ($A_V = 0.680 ± 0.038$ mag, χ<sup>2</sup>/d.o.f. = 0.30) yield viable solutions, we prefer the latter, as it needs lower extinction and SMC dust is more typical for GRB host galaxies (the 2175 Å bump would need redder NIR data to be confirmed or rejected). This result is the same order of magnitude as what Littlejohns et al. (2015) report. This represents one of the strongest cases for extinction along the line of sight to a GRB at high redshift, and it is therefore an outlier to the findings of Bolmer et al. (2018). The large extinction combined with the high redshift leads to a very large correction $dR_c = -8.237^{+0.250}_{-0.251}$ mag. The early flare, after this correction, is very luminous, peaking at R<sub>c</sub> ≈ 10 mag.

### Appendix B.3.47. GRB 140419A, z = 3.956

Data are taken from Littlejohns et al. (2015), from GCN Circulars (Guver et al. 2014; Cenko & Perley 2014a; Kuroda et al. 2014c,a; Pandey & Kumar 2014a; Zheng et al. 2014; Xu 2014; Volnova et al. 2014c,a,b), and from our own Swift UVOT observations (first reported in Kuin & Marshall 2014), as well as our LOAO observations (first reported in Choi et al. 2014). The redshift is given by Cucchiara et al. (2015). This was an intense GRB at moderately high redshift with a very bright early afterglow peaking at R<sub>c</sub> = 12.5 mag. The initial afterglow decay is densely

covered. It decays with a straight power-law, $α_1 = 1.185 ± 0.001$ (χ<sup>2</sup>/d.o.f. = 5.19 stemming from some scatter and very small errors). At ≈ 0.07 d, the decay flattens. Following a data gap, from 0.36 d, it is seen to have resumed its former decay slope, indicating an energy injection. There is no significant evidence for a beak until the last observation at ≈ 6.5 d.

The SED (ubg'VR<sub>C,i'</sub>IC<sub>Z</sub>'YJH) is clearly affected by the redshift of the GRB, showing Lyman-damping all the way into the R<sub>C</sub> band. The redder bands show no evidence for curvature, a fit without dust yields β = 0.76 ± 0.08. MW and LMC dust lead to negative extinction. SMC dust yields an intrinsically very blue slope whereas the small amount of extinction derived is still 0 within errors, while χ<sup>2</sup>/d.o.f. is higher than for the fit without dust. Littlejohns et al. (2015) find only a small amount of dust for the SMC model as well, $A_{V,SMC} = 0.11^{+0.06}_{-0.05}$ mag. For the dustless model, we find a correction $dR_c = -3.436 ± 0.079$ mag. At z = 1, this is among the most luminous early afterglows every detected, peaking at R<sub>c</sub> ≈ 9 mag.

### Appendix B.3.48. GRB 140423A, z = 3.26

Data are taken from Littlejohns et al. (2015); Li et al. (2020), from GCN Circulars (Ferrante et al. 2014; Cenko & Perley 2014b; Kuroda et al. 2014d; Akitaya et al. 2014; Pandey et al. 2014; Harbeck et al. 2014a,b; D'Avanzo et al. 2014a; Takahashi & Arai 2014; Cano et al. 2014b; Fujiwara et al. 2014; Bikmaev et al. 2014b; Sahu 2014; Sonbas et al. 2014), and from our own Swift UVOT observations (first reported in Chester et al. 2014). The redshift is given by Cucchiara et al. (2015). This was another bright GRB afterglow at moderately high redshift. The earliest detection (Ferrante et al. 2014) indicates a steeply decaying flare, which then goes over into a shallower rise and rollover into a typical decay. The rollover shows some substructure, including a short plateau phase from 0.0063 d to 0.0079 d. The beginning of the rise is quite steep ($α_{rise,spl} = -1.71 ± 0.10$, χ<sup>2</sup>/d.o.f. = 1.00), steeper than what is found by fitting the rollover with a smoothly broken power-law: $α_{rise,bpl} = -1.074 ± 0.027$, $α_{steep} = 1.720 ± 0.010$, $t_{b,peak} = 0.00352 ± 0.00004$ d, n = 1 fixed ($t_{b,peak} = 304.13 ± 3.46$ s). This fit, using data until ≈ 0.06 d, has a high χ<sup>2</sup>/d.o.f. = 11.9 as it does not take the short plateau mentioned above into account. The afterglow decay flattens after this point in time, fitting from after the short plateau to ≈ 2 d yields $α_{steep} = 1.849 ± 0.017$, $α_1 = 1.017 ± 0.008$, $t_{b,flattening} = 0.0385 ± 0.0012$ d, n = -10 fixed (χ<sup>2</sup>/d.o.f. = 1.53). This indicates the true decay post-peak is even steeper than the rollover fit indicated. The results indicate this is potentially a reverse- to forward-shock transition – Li et al. (2020) interpret it as a transition from a wind to a constant density circumburst medium. The final points indicate a renewed steepening, we find $α_2 = 2.09 ± 0.87$, n = 10 fixed at $t_b = 2.70 ± 0.76$ d (χ<sup>2</sup>/d.o.f. = 1.75). This is probably a jet break, but the lack of further observations prevents us from drawing firmer conclusions. Li et al. (2020) find a flatter initial rise ($α_{O,I} = -0.59 ± 0.04$) but similar values for the steep and shallow decays ($α_{O,II} = 1.78 ± 0.03$, $α_{O,III} = 1.13 ± 0.03$), respectively).

The SED shows strong influence from Lyman damping. The afterglow is not detected to deep limits in the UVOT UV and *u* filters (Sonbas et al. 2014), and the *b* band is also suppressed. The rest of the SED (g'Vr'R<sub>C,i'</sub>IC<sub>Z</sub>'YJHK<sub>S</sub>) shows some scatter and is generally flat, without dust we find β = 0.75 ± 0.06. Li et al. (2020), using only two optical filters, find an X-ray-to-optical spectral slope Γ − 1 = 0.95 ± 0.05, and quite large extinction (using SMC dust) of $A_V = 0.54 ± 0.16$ mag. Our broader SED indicates the optical SED is flatter than the X-ray
SED, indicating the existence of a cooling break between the bands, we therefore fix the intrinsic slope to $\beta = 0.55 \pm 0.08$. We find the best solution for a small amount of LMC dust, with $A_V = 0.13 \pm 0.03$ mag ($\chi^2$/d.o.f. = 2.56). A similar low amount of SMC dust cannot be ruled out either ($A_V = 0.08 \pm 0.02$ mag, $\chi^2$/d.o.f. = 2.66). However, a small depression in the z' flux is in agreement with a 2175 Å bump at this redshift, making us prefer the LMC fit. Using it, we derive $dR_c = -3.249^{+0.139}_{-0.135}$ mag. The very early flash is found to have $R_c = 10.1$ mag, with the later peak lying at $R_c = 10.9$ mag, making this a very luminous afterglow.

### Appendix B.3.49. GRB 140430A, $z = 1.601$

Data are taken from Kopač et al. (2015), from GCN Circulars (Malesani et al. 2014b; Kennedy 2014; Kennedy & Garnavich 2014), and from our own Swift UVOT observations (first reported in Breeveld & Siegel 2014), as well as a single datum from the Ondřejov D50. The redshift is given by Selsing et al. (2019). The initial optical light curve (two points) shows a shallow decay, but then the afterglow must fade very rapidly, as it is steeply rising when denser monitoring sets in shortly afterwards. From the rise, the afterglow breaks sharply to a standard decay, we find $\alpha_{rise} = -5.05 \pm 1.01$, $\alpha_1 = 0.812 \pm 0.015$, $t_b = 0.00182 \pm 0.00003$ d ($t_b = 157.25 \pm 2.59$ s), $n = 10$ fixed ($\chi^2$/d.o.f. = 1.22). The afterglow then goes over into a plateau phase before resuming decaying again, we find $\alpha_{plateau} = 0.237 \pm 0.035$, $\alpha_2 = 1.03 \pm 0.06$, $t_b = 0.094 \pm 0.0015$ d, $n = 3$ fixed ($\chi^2$/d.o.f. = 1.74). There is no evidence for a further break, however, observations extend only to 1.3 d. Kopač et al. (2015) differentiate two optical flares in the early data, finding an initial rise in perfect agreement with our results. They find later decay slopes of $\alpha_1 \approx 1$ and $\alpha_1 \approx 0.8$, respectively, a reverse of our result, however, no error bars are given so their result generally agrees with ours.

The SED (uvw2uvm2uvw1ubVR<sub>C</sub>I<sub>C</sub>) is strongly affected by Lyman damping, with the UV filters clearly suppressed. The slope is straight and moderately red, $\beta = 1.05 \pm 0.08$ ($\chi^2$/d.o.f. = 0.69). All dust models yield viable results; however, the measured extinction is zero within errors for each, and therefore we use the fit without dust. Kopač et al. (2015) find a similar slope without extinction, $\beta = 0.97 \pm 0.08$, and a joint X-ray-to-optical fit with SMC dust also only yields an upper limit $A_V < 0.14$ mag. For the fit without extinction, we derive $dR_c = -1.288 \pm 0.022$ mag. At $z = 1$, the early afterglow is moderately luminous with $R_c = 14.5$ mag.

### Appendix B.3.50. GRB 140506A, $z = 0.88911$

Data are taken from Fynbo et al. (2014); Heintz et al. (2017); Kann et al. (2024). The redshift is given by Fynbo et al. (2014). The afterglow of this otherwise ordinary GRB distinguished itself by showing strong extinction and a peculiar spectrum (Fynbo et al. 2014; Heintz et al. 2017), as well as a late blue bump that can be interpreted as a highly luminous and slow SN component (Kann et al. 2024). The full analysis is presented in Kann et al. (2024). Correcting for the very large extinction $A_V \approx 1.1$ mag leads to $dR_c = -1.537^{+0.285}_{-0.286}$ mag. The afterglow is moderately luminous, peaking at $R_c \approx 14.3$ mag at early times.

## Appendix B.3.51. GRB 140629A, $z = 2.276 \pm 0.001$

Data are taken from Xin et al. (2018); Hu et al. (2019), from GCN Circulars (Bikmaev et al. 2014a; Maehara 2014; Takaki et al. 2014; Kuroda et al. 2014b; Perley & Cenko 2014b,a; Garnavich & Rose 2014; D'Avanzo et al. 2014b; Honda et al. 2014; Moskvitin et al. 2014a,b; Yano et al. 2014; Pandey & Kumar 2014b), as well as our own Swift UVOT data set, which expands that presented in Hu et al. (2019) and also fixes errors in the upper limits presented in that work. The redshift is given by Hu et al. (2019). The light curve evolution is complex. The very first detections show a decay, before the light curve rises again to a peak, and breaks sharply into a typical decay. Using data up to 0.16 d, we find: $\alpha_{rise} = -0.848 \pm 0.097$, $\alpha_1 = 1.138 \pm 0.005$, $t_b = 0.00238 \pm 0.00007$ d ($t_b = 205.6 \pm 6.0$ s), $n = 4.34 \pm 0.86$ ($\chi^2$/d.o.f. = 1.45), with host magnitudes fixed to the values from Hu et al. (2019) or estimated (they have little influence as the afterglow is far brighter than the host). After 0.16 d, the afterglow shows a bumpy structure superposed on a steepening decay, likely an achromatic jet break (Hu et al. 2019), making the light curve similar to that of GRB 021004 (e.g., de Ugarte Postigo et al. 2005) and GRB 060526 (Thöne et al. 2010). Xin et al. (2018) find $\alpha_{rise} = -0.92 \pm 0.24$, $\alpha_1 = 1.12 \pm 0.02$, $t_b = 179 \pm 16$ s, in excellent agreement with our values. Hu et al. (2019) find a fit with an additional break in the decaying part of the light curve: $\alpha_{rise} = -0.72^{+0.15}_{-0.33}$, $\alpha_{1a} = 0.91^{+0.03}_{-0.04}$, $t_b = 176.85^{+3.48}_{-3.22}$ s, $\alpha_{1b} = 1.17 \pm 0.01$, $t_b = 638.69^{+126.31}_{-105.89}$ s. As expected their $\alpha_{1a,b}$ results straddle the single values from Xin et al. (2018) and our work.

The SED spans the full UV-to-NIR width (uvw2 uvm2 uvw1 ubg'vR<sub>C</sub>I<sub>C</sub>z'JHK<sub>S</sub>), with uvw2 uvm2 uvw1 u being affected by Lyman dropout and excluded. We note this is one of the highest redshifts at which an afterglow is still (albeit marginally) detected in uvw2 uvm2. It is moderately red and visibly curved, a fit without dust finds $\beta = 1.026 \pm 0.035$ ($\chi^2$/d.o.f. = 8.22). A fit with MW dust yields negative extinction and is ruled out. The LMC dust fit shows a marked improvement but also a negative intrinsic spectral slope. The best fit is found with SMC dust: $\beta = 0.197 \pm 0.136$, $A_V = 0.309 \pm 0.048$ mag ($\chi^2$/d.o.f. = 2.24). Xin et al. (2018) fit the X-ray-to-optical SED with a single power-law with $\beta = 0.90 \pm 0.05$, and negligible extinction. This is based on a BVR<sub>C</sub>I<sub>C</sub> SED only. Hu et al. (2019) tested two SEDs, at 775 s and at 9350 s. In the first there is a significant difference neither in the presence or absence of a cooling break between X-rays and optical, nor in which dust model is preferred. In the second SED, SMC dust is favored, but again the presence of a cooling break cannot be significantly discerned. For SMC dust, they derive $A_V = 0.29 \pm 0.01$ mag and $A_V = 0.25 \pm 0.03$ mag, respectively, in good agreement with our result. The spectral slopes are $\beta = 0.489 \pm 0.01$ and $\beta = 0.540 \pm 0.02$, respectively<fnref n="12" />, steeper than what we find. Using our SMC dust fit, we derive $dR_c = -2.772^{+0.167}_{-0.170}$ mag. The early afterglow peak is luminous, at $R_c = 10.5$ mag.

## Appendix B.3.52. GRB 140801A, $z = 1.3202 \pm 0.0001$

Data are taken from Lipunov et al. (2016); Pozanenko et al. (2017) as well as our own Swift UVOT analysis (first reported in Hagen & Troja 2014). Data from Pozanenko et al. (2017) were made brighter by 0.15 mag to bring them into accordance with

<footnote n="12">¹² Table 5 of Hu et al. (2019) gives $\Gamma = \beta + 1 = 2.040$, but this is likely missing $\Delta\Gamma = 0.5$ as in the first SED results of the table for the “BKP” models.</footnote>
contemporaneous values from Lipunov et al. (2016). The redshift is given by Lipunov et al. (2016). This GRB represents a rare case of an afterglow which was discovered very early on by wide-field observations of a Fermi GBM error box, *without* the benefit of a LAT localization (Lipunov et al. 2016). The afterglow shows a classic steep-shallow-steep decay. The initial decay slope is $\alpha_{steep} = 1.425 \pm 0.076$, in full agreement with Lipunov et al. (2016), who find $\alpha_{steep} = 1.42 \pm 0.12$. Following a data gap, the subsequent data decay following a broken power law, with the late steep decay partially based on a deep upper limit, and therefore the late-time decay slope is formally an upper limit, and also the break time may be somewhat different in reality. We find $\alpha_1 = 0.828 \pm 0.021$, $\alpha_2 = 2.37 \pm 0.16$, $t_b = 1.44 \pm 0.10$ d, $n = 5$ fixed. The shallow decay slope is also fully in agreement with Lipunov et al. (2016). The last detection by Pozanenko et al. (2017) shows a rebrightening must have taken place, as those authors already point out.

The SED is relatively broad (uvw1 ug'Vr'R_C i'I_Cz'JH), with uvw1 being affected by Lyman damping and not included. The SED shows some scatter but is generally straight, we find a perfectly acceptable fit with no dust, $\beta = 0.67 \pm 0.16$ ($\chi^2$/d.o.f. = 0.90). SMC dust yields negative extinction. For MW and LMC dust, we also find acceptable fits with blue intrinsic spectral slopes and some extinction: $\beta = 0.17 \pm 0.46$, $A_V = 0.24 \pm 0.21$ mag ($\chi^2$/d.o.f. = 0.86) for MW dust and $\beta = 0.19 \pm 0.71$, $A_V = 0.25 \pm 0.36$ mag ($\chi^2$/d.o.f. = 0.97). These fits do not improve the extinction-less fit significantly and have large errors. Furthermore, Lipunov et al. (2016) show the X-ray-to-optical SED is fit perfectly well with no dust and a simple power-law spanning the entire frequency range, with $\beta = 0.81^{+0.04}_{-0.03}$, in agreement within errors with our result. We therefore continue with the fit without extinction, for which we derive $dR_c = -7.011^{+0.027}_{-0.026}$ mag. The early afterglow is moderately luminous, at $R_c \approx 13.3$ mag.

### Appendix B.3.53. GRB 141221A, $z = 1.452$

Data are taken from Bardho et al. (2016), from GCN circulars (Guidorzi et al. 2014a; Schweyer et al. 2014), and from our own Swift UVOT analysis (first reported in Marshall & Sonbas 2014) and REM seven-band observations (first reported in Covino 2014). The redshift is given by Perley et al. (2014a). The afterglow shows a steep rise, which goes over into a shallow decay/plateau phase (Bardho et al. 2016) before breaking to a typical decay. We fit the data in two epochs. For the first epoch, we use data until the end of the plateau phase (0.0031 d), we find: $\alpha_{rise} = -4.69 \pm 0.86$, $\alpha_{plateau} = 0.50 \pm 0.11$, $t_b = 0.00117 \pm 0.0005$ d ($t_b = 101.1 \pm 4.3$ s), $n = 1$ fixed, and no host galaxy has been detected ($\chi^2$/d.o.f. = 2.10). The second begins at 0.0014 d and yields: $\alpha_{plateau} = 0.160 \pm 0.032$, $\alpha_1 = 1.221 \pm 0.015$, $t_b = 0.00383 \pm 0.0012$ d ($t_b = 331 \pm 10$ s), $n = 5$ fixed, no host galaxy ($\chi^2$/d.o.f. = 3.51, resulting from some scatter in the data). The difference in decay slopes for the plateau is likely a result from the break to the regular decay not being sharp, making it hard to evaluate when to end the first fit. Bardho et al. (2016) obtain fits in several single bands, finding e.g., $\alpha_{rise} = -1.6 \pm 0.9$, $\alpha_{plateau} = 0.5 \pm 0.2$, $t_b = 110 \pm 13$ s. They also find the late decay is better fit by another broken power-law, with data starting at 337 s (almost exactly where we find the second break) they find $\alpha_1 = 1.0 \pm 0.2$, $\alpha_2 = 1.6 \pm 0.4$. Their rise index is shallower than what we find, they have probably used a sharper break. Their plateau decay is identical to our result, and our late-time decay index represents a mean value of what they find. The afterglow is only significantly detected until 0.11 d.

The SED is broad (ubg'Vr'R_C i'I_Cz'JHK), red and clearly curved. Without dust, we find: $\beta = 1.629 \pm 0.021$ ($\chi^2$/d.o.f. = 39.6). MW dust yields negative extinction. LMC dust find a very blue intrinsic slope and high extinction, there is no evidence for a 2175 Å bump and the steepness of the far-UV slope is underestimated. SMC dust yields the only plausible solution, with $\beta = 0.26 \pm 0.09$, $A_V = 0.736 \pm 0.048$ mag ($\chi^2$/d.o.f. = 2.67). This $15\sigma$ detection of extinction is one of the most secure measured so far. For a broken power-law X-ray-to-optical fit, Bardho et al. (2016) find $\beta = 0.3^{+0.3}_{-0.1}$, $A_V = 0.569^{+0.063}_{-0.190}$ mag for LMC dust and $\beta = 0.37^{+0.03}_{-0.09}$, $A_V = 0.498^{+0.088}_{-0.147}$ mag for SMC dust. Their spectral slope is slightly redder than our result, and accordingly their extinction result is slightly lower, but generally, they confirm the quite high extinction toward this GRB. For our SMC result, we derive $dR_c = -2.536^{+0.112}_{-0.115}$ mag. At $z = 1$, the afterglow is luminous, peaking at $R_c \approx 12.7$ mag.

### Appendix B.3.54. GRB 150301B, $z = 1.5169$

Data are taken from Gorbovskoy et al. (2016), from GCN circulars (Kann et al. 2015; Guidorzi et al. 2015), and from our own Swift UVOT analysis (first reported in Chester & Lien 2015), and VLT X-shooter acquisition camera analysis (first reported in de Ugarte Postigo et al. 2015a). The redshift is given by Selsing et al. (2019). The afterglow evolution is simple, we find it can be fit with a simple power-law decay from the earliest to the last detection (0.00097 to 0.82 d). We find $\alpha_1 = 0.9781 \pm 0.0094$ ($\chi^2$/d.o.f. = 1.70). Based only on their MASTER data set, Gorbovskoy et al. (2016) find $\alpha_1 = 1.16$. The SED is broad (uvw1 ubg_G vr_G R_C i_G z_G J_G H_G) and perfectly well fit by a simple power-law without dust, we find $\beta = 0.806 \pm 0.089$ ($\chi^2$/d.o.f. = 0.13). a fit with MW dust yields negative extinction. Fits with LMC and SMC dust are viable but only find small amounts of extinction, 0 within errors. We therefore prefer the fit without dust. Gorbovskoy et al. (2016) use only unfiltered data but do find that the X-ray column density toward the GRB is equal to the (low) Galactic value, generally an indication of a low-$A_V$ line of sight (at this redshift). Using this fit, we find $dR_c = -1.081 \pm 0.022$ mag. The early afterglow, which has one of the earliest detections in our sample, is luminous, peaking at $R_c \approx 14$ mag.

### Appendix B.3.55. GRB 150910A, $z = 1.3585$

Data are taken from Xie et al. (2020), from GCN circulars (Kuroda et al. 2015c,a,b; Xu et al. 2015; de Ugarte Postigo et al. 2015b; Moskvitin & Goranskij 2015; Schmidl et al. 2015; Yanagisawa et al. 2015; Cano et al. 2015; Mazaeva et al. 2015c,b,a; Perley & Cenko 2015; Butler et al. 2015a,b; Andreev et al. 2015; Sonbas et al. 2015; Volnova et al. 2015a,b; Rumyantsev et al. 2015), and from our own Swift UVOT analysis (first reported in McCauley & Pagani 2015), as well as our FTN observations (first reported in Dichiara et al. 2015a). The redshift is given by Xie et al. (2020). The afterglow shows a short decay at very early times which goes over into a very steep rise to a peak, covered only by our UVOT data. After a short data gap, the afterglow is found to be decaying in our FTN data as well as the observations presented by Xie et al. (2020), the dense coverage shows some substructure in this decay. The late evolution is anomalous, the decay seems to steepen after 1.5 d to 2 d, but then slowly rises until at least 4 d, our final low-significance UVOT white detections at 5.9 d shows the decay must have commenced again. We fit the steep rise and following decay, and find: $\alpha_{rise} = -5.60 \pm 0.40$, $\alpha_1 = 1.240 \pm 0.005$, $t_b = 0.01061 \pm 0.0017$ d
(t<sub>b</sub> = 917 ± 15 s), n = 1 fixed, no host galaxy (χ<sup>2</sup>/d.o.f. = 2.38). Xie et al. (2020), using a significantly smaller data set, especially at early times, find α<sub>rise</sub> = −2.24 ± 0.16, α<sub>1</sub> = 1.36 ± 0.03, t<sub>peak</sub> = 1451 ± 51 s.

The SED is very broad (www2uwm2uvw1uu'bg'vr'R<sub>C</sub>i'I<sub>Cz</sub>' K<sub>S</sub>), the UV filters are affected by Lyman damping and are excluded, the rest of the SED is moderately red and slightly curved. We find β = 0.965 ± 0.034, which is similar to β ≈ 0.9 that Xie et al. (2020) find using an X-ray-to-optical fit but only two optical filters. With MW dust, we find negative extinction. We find viable fits for both LMC and SMC dust, for LMC dust: β = 0.50 ± 0.17, A<sub>V</sub> = 0.245 ± 0.085 mag (χ<sup>2</sup>/d.o.f. = 0.90), and for SMC dust: β = 0.53 ± 0.14, A<sub>V</sub> = 0.169 ± 0.050 mag (χ<sup>2</sup>/d.o.f. = 0.42). There is no clear evidence for a 2175 Å bump so we prefer the SMC fit. Using this fit, we derive dR<sub>C</sub> = −1.115<sup>+0.110</sup><sub>−0.112</sub> mag, at z = 1 the afterglow peak is moderately luminous, R<sub>C</sub> ≈ 14.3 mag.

Appendix B.3.56. GRB 151027B, z = 4.06463

Data are taken from Greiner et al. (2018), from GCN circulars (Watson et al. 2015; Buckley et al. 2015; Dichiara et al. 2015b), from the automatic Swift UVOT analysis page<fnref n="13" />, and from our Ondřejov D50 observations (partially already presented in Jelínek et al. 2019). The redshift is given by Heintz et al. (2018b). This is Swift's 1000th detected GRB, which showed strong radio variability (Greiner et al. 2018). The afterglow shows an initial rise (based on an upper limit and a single detection), which goes over into a shallow decay, before breaking into a steeper decay. The final detections at ≈ 9 d (which were not included in the fitting) lie 2σ under the extrapolation of the earlier decay, and even later deep upper limit adds further evidence that another break to an even steeper decay (likely a jet break) must have taken place, however, the lack of data prevents a more quantitative analysis. We find α<sub>plateau</sub> = 0.42 ± 0.10, α<sub>1</sub> = 1.088 ± 0.021, 0.23 ± 0.07 d, n = 10 fixed, no host galaxy (χ<sup>2</sup>/d.o.f. = 1.10). Greiner et al. (2018) find α<sub>plateau</sub> ≈ 0.4, α<sub>1</sub> ≈ 1.4, the steeper late decay likely stems from their inclusion of the data at ≈ 9 d.

The SED is clearly influenced by the moderately high redshift. The UVOT data yields relevant upper limits in uvw1 and u. For the detections (g<sub>GR</sub>′r<sub>G</sub>R<sub>C</sub>iG′i'I<sub>Cz</sub>G′z′J<sub>G</sub>H<sub>G</sub>K<sub>G</sub>), bands blueward of i<sub>G</sub> are also influenced by Lyman damping and are not included. We find the SED is best fit by a simple power-law with no dust extinction: β = 0.526 ± 0.087 (χ<sup>2</sup>/d.o.f. = 1.18). MW dust yields negative extinction, whereas LMC and SMC dust yield high extinction and negative slopes. From an X-ray-to-optical fit, Greiner et al. (2018) find a steeper spectral slope (β = 0.81 ± 0.01) but also no evidence for extinction. Using this fit, we find dR<sub>C</sub> = -3.265<sup>+0.086</sup><sub>-0.088</sub> mag. The early afterglow is moderately luminous, peaking at R<sub>C</sub> ≈ 14.6 mag. However, even in the z = 1 system, the first detection is only 370 s after trigger, at higher redshift, this moves out of the initial detection window of Gamow. We therefore do not include this afterglow in the statistics.

Appendix B.3.57. GRB 161023A, z = 2.71067

Data are taken from de Ugarte Postigo et al. (2018b), where a full analysis is presented. The redshift is given by Heintz et al. (2018b). The afterglow light curve shows an initial sharp flare likely linked to the prompt emission, followed by a large rise and

a rollover into a typical decay, followed by a jet break at several days, all in all a very classical afterglow. The SED is fit by a small amount of SMC dust, and the very luminous afterglow peaks at R<sub>C</sub> = 9.75 mag.

Appendix B.3.58. GRB 161219B, z = 0.1475

Data are taken from Cano et al. (2017a), Ashall et al. (2019), Laskar et al. (2018a), from GCN circulars (Martin-Carrillo et al. 2016; Fujiwara et al. 2016; Buckley et al. 2016b,a). The redshift is given by Selsing et al. (2019). This is a low-redshift low-luminosity GRB with extensive optical follow-up. The very broad SED yields no evidence for dust and a spectral slope β = 0.497 ± 0.011, leading to dR<sub>C</sub> = 4.603<sup>+0.006</sup><sub>-0.007</sub> mag. Based on mostly the Swift UV data, which is unaffected by SN 2016jca, the afterglow is detected up to 100 d in the observer frame. In the z = 1 frame, it is one of the least luminous detected afterglows, being R<sub>C</sub> ≈ 20.5 mag even at very early times.

Appendix B.3.59. GRB 180325A, z = 2.2486

Data are taken from Zafar et al. (2018a); Becerra et al. (2021), from GCN Circulars (Littlefield & Garnavich 2018; Lipunov et al. 2018b; Malesani & Fynbo 2018), as well as our own Swift UVOT analysis (first reported in Marshall & Troja 2018), REM data (first reported in D'Avanzo et al. 2018), LT data (first reported in Guidorzi et al. 2018), as well as data from the Ondřejov D50, first reported in Strobl et al. (2018), which will not be published in this work. The redshift is given by Zafar et al. (2018a). After initial rapid variability, the afterglow exhibits a bright rapidly rising and decaying flare (see also Becerra et al. 2021), for which we find α<sub>flare,rise</sub> = −8.75 ± 1.80, α<sub>flare,decay</sub> = 2.12 ± 0.14, t<sub>b,flare</sub> = 0.00112 ± 0.00005 d (t<sub>b</sub> = 96.77 ± 4.32 s), n = 1.23 ± 0.61, and no host (χ<sup>2</sup>/d.o.f. = 0.92). Beginning at 0.005 d, the afterglow can be described by a second smoothly broken power-law with α<sub>1</sub> = 0.442 ± 0.008, α<sub>2</sub> = 1.521 ± 0.030, t<sub>b</sub> = 0.086 ± 0.0015 d (t<sub>b</sub> = 7456 ± 128 s), a very sharp break n = 25.6 ± 14.3, and no host (χ<sup>2</sup>/d.o.f. = 1.59). Zafar et al. (2018a), using a significantly sparser data set, find a similar break time (t<sub>b</sub> = 6480 ± 1200 s) but more extreme slopes (α<sub>1</sub> = 0.02 ± 0.03, α<sub>2</sub> = 2.00 ± 0.13). They do not report a break sharpness, but a smaller value combined with the sparser data may explain the shallower, then steeper decay they report. Becerra et al. (2021) report α<sub>1</sub> = 0.46 ± 0.01, α<sub>2</sub> = 1.48 ± 0.18, in good agreement with our values (they also note the predicted decay slope from the α-β relations should be α = 1.52, in perfect agreement with our measurement).

The broad SED (ubgGvr'i'z'J<sub>G</sub>HK<sub>G</sub>) has been extensively discussed in Zafar et al. (2018a), being very red and showing clear evidence for a 2175 Å bump. For a fit without dust, we find β = 2.537 ± 0.017 (χ<sup>2</sup>/d.o.f. = 65.9). Generally, χ<sup>2</sup>/d.o.f. values are very high in this case as the well-defined optical light curve leads to very small errors in the normalization which form the SED, and even small deviations lead to large values. The existence of the bump also leads to SMC dust not yielding a viable result, with β = 2.598 ± 0.052, A<sub>V</sub> = 0.027 ± 0.022 mag (χ<sup>2</sup>/d.o.f. = 76.4). MW dust, while yielding a much better fit, is not able to account for the steepness of the SED, we find β = 1.969 ± 0.040, A<sub>V</sub> = 0.431 ± 0.028 mag (χ<sup>2</sup>/d.o.f. = 35.2). LMC dust, finally, yields a somewhat worse fit but values in agreement with GRB theories, namely β = 1.137 ± 0.104, A<sub>V</sub> = 0.819 ± 0.060 mag (χ<sup>2</sup>/d.o.f. = 47.1). To directly compare with the results of Zafar et al. (2018a), we fix the spectral slope to the value derived from the X-rays (Zafar et al. 2018a

<footnote n="13">13 https://swift.gsfc.nasa.gov/uvot_tdrss/661869/index.html, offline at the time of writing.</footnote>
find $\beta_X = 0.84 \pm 0.10$, in full agreement with the XRT repository value of $\beta_X = 0.79 \pm 0.09$), and find $A_V = 1.011 \pm 0.010$ mag. Using a multiparameter Fitzpatrick-Massa parameterization, Zafar et al. (2018a), using four different SEDs, derive a higher $A_V \approx 1.6$ mag, albeit also with a much higher value $R_V \approx 4.5$. For the values we derive, we calculate a very large $dR_c = -4.957 \pm 0.056$ mag. Correcting for this, we find the early flare of GRB 180325A was very luminous, peaking at $R_c \approx 11$ mag.

## Appendix B.3.60. GRB 180720B, $z = 0.654$

This GRB does not yet have any optical observations published beyond GCN Circulars. We here present our own observations from telescopes of the LCOGT (the FTS, with later observations obtained by the FTN and the Haleakala Observatory; first reported by Martone et al. 2018), the OSN T90 and T150 telescopes, first reported in Kann et al. (2018a), the JAST/T80 telescope, first reported in Izzo et al. (2018b), the REM telescope, first reported in Covino & Fugazza (2018), as well as data from the Ondřejov D50, first reported in Jelinek et al. (2018), which will not be published in this work. Swift UVOT did not observe the GRB due to a very bright star in the FoV. We also add data from GCN Circulars (Sasada et al. 2018; Crouzet & Malesani 2018; Schmalz et al. 2018; Lipunov et al. 2018a; Zheng & Filippenko 2018). We furthermore obtained a late-time host-galaxy observation in $u'g'r'i'z'$ with the GTC HiPERCAM, discovering the host and detecting it in all bands. The redshift is given by Vreeswijk et al. (2018). This was a very bright GRB at moderately low redshift which was also the first GRB for which evidence of ultrahigh-energy emission was discovered (Abdalla et al. 2019, however, the early high-GeV/TeV emission of GRB 190114C was reported first, MAGIC Collaboration et al. 2019a). This discovery resulted in extensive discussion in the literature (Duan & Wang 2019; Wang et al. 2019; Fraija et al. 2019; Ronchi et al. 2020; Sahu & Fortín 2020; Huang et al. 2020; Chen et al. 2021). The GRB was also detected prominently in the radio range (Rhodes et al. 2020).

The early afterglow is among the brightest ever discovered, at $R_c \approx 9.4$ mag (Sasada et al. 2018). We present an optical light curve of this event here for the first time based on our extensive observations spanning from early to late times. It initially shows a steep decay going over into an ill-defined plateau phase, we find $\alpha_{steep} = 1.702 \pm 0.051$, $\alpha_{plateau} = -0.12 \pm 1.69$, $t_{b,early} = 0.0126 \pm 0.0016$ d ($t_{b,early} = 1089 \pm 134$ s), $n = -10$ fixed ($\chi^2/d.o.f. = 0.04$). The initial steep decay is in accordance with an interpretation of a reverse-shock flash. While it may be assumed the plateau could be the onset of the forward-shock afterglow caught at peak, a back-extrapolation of the later data slightly underestimates this data, indicating another steeper decay must have taken place that then transits into the shallow decay we find next. The rest of our data is described by a smoothly broken power-law with $\alpha_1 = 0.856 \pm 0.021$, $\alpha_2 = 1.574 \pm 0.020$, $t_b = 0.434 \pm 0.023$ d, $n = 10.10 \pm 3.95$, and host galaxy values based on our GTC HiPERCAM detections. No SN component in the light curve has been reported so far, our data do not cover the time frame when a SN is expected. The broad SED ($Bg'Vr'R_c i'I_c z' JHK$) shows a small amount of scatter, but is otherwise straight. All three dust models yield slightly negative extinction, we prefer the fit with no dust, which yields $\beta = 0.851 \pm 0.078$ ($\chi^2/d.o.f. = 0.23$). Using this spectral slope, we derive $dR_c = 1.114 \pm 0.016$ mag.

## Appendix B.3.61. GRB 181201A, $z = 0.450$

Data are taken from Laskar et al. (2019); Belkin et al. (2020), and from GCN and ATel Circulars (Kong 2018; Heintz et al. 2018a; Watson et al. 2018; Ramsay et al. 2018; Srivastava et al. 2018; Kumar et al. 2019). The redshift is given by Izzo et al. (2018a). This was a bright INTEGRAL GRB at moderately low redshift. It shows evidence for a rising SN component at late times (Belkin et al. 2020). The SED shows no evidence for dust, we find $\beta = 0.578 \pm 0.030$ and derive $dR_c = 1.984 \pm 0.010$ mag. The early peak reaches $R_c = 14.1$ mag in the $z = 1$ frame, but early coverage is sparse and the true peak mag is likely brighter.

## Appendix B.3.62. GRB 190114C, $z = 0.4250$

Data are taken from MAGIC Collaboration et al. (2019b); Jordana-Mitjans et al. (2020); Misra et al. (2021); Melandri et al. (2022), Thöne et al., in prep., and GCN Circulars (Kim & Im 2019; Kim et al. 2019; Mazaeva et al. 2019; Watson et al. 2019; Im et al. 2019), as well as some automatically reduced UVOT data (https://swift.gsfc.nasa.gov/uvot_tdrss/883832/index.html). The SN component (SN 2019jrj) was first reported by Melandri et al. (2019) and is discussed in more detail in Melandri et al. (2022). The host galaxy was first reported by de Ugarte Postigo et al. (2019) and has been discussed in detail in de Ugarte Postigo et al. (2020b). The redshift is given by Kann et al. (2019b). GRB 190114C was an exceptionally bright GRB, similar to GRB 130427A, it was a "full-fledged cosmological" GRB that occurred at a relatively low redshift. It is especially exceptional as a rapid reaction by the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC Florian Goebel Telescopes) led to an early detection of radiation up to TeV energies (MAGIC Collaboration et al. 2019a,b). The bright prompt emission was detected by many satellites (Ajello et al. 2020; Ursi et al. 2020; Minaev & Pozanenko 2019; Xiao et al. 2019). The afterglow was detected right after the GRB at $10^{th}$ magnitude (Jordana-Mitjans et al. 2020), yielding a very rich data set, despite the high line-of-sight extinction (Kann et al. 2019b; MAGIC Collaboration et al. 2019b; Misra et al. 2021). Campana et al. (2021) detect a decrease by a factor of two in the late X-ray column density, which they explain as a compact absorbing cloud that initially covers the entire radiating surface of the jet.

## Appendix B.3.63. GRB 210312B, $z = 1.0690 \pm 0.0005$

Data are taken from Jelinek et al., in prep. The redshift is given by Kann et al. (2021b, see also Jelinek et al., in prep.). This was a relatively short ($T_{90} \approx 5$ s) and faint GRB localized by INTEGRAL/IBAS (Mereghetti et al. 2021). It was followed-up rapidly by the Ondřejov D50 telescope, which discovered the afterglow (Jelinek et al. 2021), which decayed rapidly (Kann et al. 2021a,b). The full analysis is presented in Jelinek et al., in prep.; we here focus on the late-time afterglow. We fit a small flaring episode and the subsequent decay with a smoothly broken power-law, finding $\alpha_1 = -0.16 \pm 0.75$, $\alpha_2 = 1.18 \pm 0.09$, $t_b = 0.046 \pm 0.014$ d. We fixed $n = 10$, and all data have been host-galaxy subtracted. The afterglow is very faint, being $R_c \approx 24.7$ mag at one day already. The SED ($Bg'Vr'R_c i'I_c z'$) is red but shows no significant sign of curvature. All three dust fits yield negative extinction, and therefore we proceed with a simple power-law fit with no dust which is perfectly acceptable: $\beta = 1.186 \pm 0.077$, $\chi^2/d.o.f. = 0.77$. Using this slope, we find $dR_c = -0.188 \pm 0.003$ mag. Both observationally and at $z = 1$,
the afterglow of GRB 210312B represents one of the faintest ever
followed-up in such detail.

<table><caption>**Table B.1.** GRB afterglow/supernova/host observations.</caption><thead><tr><th>GRB</th><th>Δt (days)</th><th>mag</th><th>filter</th><th>Telescope/Instrument</th></tr></thead><tbody><tr><td rowspan="5">080413A</td><td>0.061861</td><td>19.249<sup>+0.263</sup><sub>-0.212</sub></td><td>b</td><td>Swift UVOT</td></tr><tr><td>1.587929</td><td>&gt; 22.091</td><td>b UL</td><td>Swift UVOT</td></tr><tr><td>1.718968</td><td>&gt; 21.616</td><td>b UL</td><td>Swift UVOT</td></tr><tr><td>0.003005</td><td>14.605<sup>+0.099</sup><sub>-0.091</sub></td><td>v</td><td>Swift UVOT</td></tr><tr><td>0.003177</td><td>14.701<sup>+0.072</sup><sub>-0.068</sub></td><td>v</td><td>Swift UVOT</td></tr></tbody></table>

**Notes.** This table is available in its entirety at the CDS. All data are in AB magnitudes and not corrected for Galactic foreground extinction. Midtimes are derived logarithmically. $t = 10^{(\log(t_1-t_0)+\log(t_2-t_0))/2}$, hereby $t_{1.2}$ are the absolute start and stop times, and $t_0$ is the GRB trigger time.