<figure><img src="image_1.png" /></figure>

# Reverse Shock Emission Revealed in Early Photometry in the Candidate Short GRB 180418A

R. L. Becerra<fnref n="1" />, S. Dichiara<fnref n="2" />,<fnref n="3" />, A. M. Watson<fnref n="1" />, E. Troja<fnref n="2" />,<fnref n="3" />, N. Fraija<fnref n="1" />, A. Klotz<fnref n="4" />, N. R. Butler<fnref n="5" />, W. H. Lee<fnref n="1" />, P. Veres<fnref n="6" />, D. Turpin<fnref n="7" />, J. S. Bloom<fnref n="2" />, M. Boer<fnref n="8" />, J. J. González<fnref n="1" />, A. S. Kutyrev<fnref n="3" />, J. X. Prochaska<fnref n="9" />, E. Ramirez-Ruiz<fnref n="9" />, and M. G. Richer<fnref n="10" />

Received 2019 April 11; revised 2019 May 27; accepted 2019 June 4; published 2019 August 7

## Abstract

We present observations of the possible short GRB 180418A in γ-rays, X-rays, and in the optical. Early optical photometry with the TAROT and RATIR instruments shows a bright peak (≈14.2 AB mag) between $T + 28$ and $T + 90$ s that we interpret as the signature of a reverse shock. Later observations can be modeled by a standard forward shock model and show no evidence of a jet break, allowing us to constrain the jet collimation to $\theta_j > 7°$. Using deep late-time optical observations, we place an upper limit of $r > 24$ AB mag on any underlying host galaxy. The detection of the afterglow in the *Swift* UV filters constrains the GRB redshift to $z < 1.3$ and places an upper bound on the γ-ray isotropic equivalent energy $E_{\gamma,iso} < 3 \times 10^{51}$ erg. The properties of this GRB (e.g., duration, hardness ratio, energy, and environment) lie at the intersection between short and long bursts, and we cannot conclusively identify its type. We estimate that the probability that it is drawn from the population of short GRBs is 10%–30%.

**Key words:** gamma-ray burst: individual (GRB 180418A)

## 1. Introduction

Gamma-ray bursts (GRBs) are the brightest events in the universe. There are two main populations of GRBs: short GRBs (SGRBs) and long GRBs (LGRBs). The populations are distinguished by their duration $T_{90}$, the interval in the observer's frame over which 90% of the total background-subtracted counts are observed (Kouveliotou et al. 1993), and other secondary parameters such as hardness and spectral lag (Kumar & Zhang 2015). The population of SGRBs typically has $T_{90} < 2$ s and harder spectra whereas the population of LGRBs typically has $T_{90} > 2$ s and softer spectra (Gehrels & Razzaque 2013). Nevertheless, there is some overlap between the distributions of $T_{90}$ and hardness, and in some cases it is not clear whether a burst with intermediate properties belongs to the population of SGRBs or LGRBs.

SGRBs are thought to be the consequence of mergers between compact objects driven by angular momentum and energy losses to gravitational radiation (e.g., Eichler et al. 1989; Narayan et al. 1992; Ruffert & Janka 1998; Rosswog & Ramirez-Ruiz 2002; Lee & Ramirez-Ruiz 2007; Giacomazzo et al. 2011). The resulting system can result in a stable neutron star, a black hole, or a supramassive, rotationally supported neutron star that collapses to a black hole due to the loss of angular momentum (Piro et al. 2017). The gravitational waves emitted during the merger provide stringent constraints on the individual masses of the coalescing objects (Abbott et al. 2017b). The first discovery of a binary neutron star merger (GW170817) in gravitational waves revealed the importance of

the multimessenger approach. Combined detection of electromagnetic counterparts (the kilonova, the GRB prompt, and the afterglow) is crucial for the understanding of these peculiar phenomena (Abbott et al. 2017a).

On the other hand, most LGRBs arise from the core collapse of massive stars (Hjorth et al. 2003) and are associated with hydrogen-poor, high-velocity SNe Ic (Cano 2013).

According to the standard fireball model (Paczynski & Rhoads 1993; Piran 1999; Zhang & Mészáros 2004; Kumar & Zhang 2015) in both SGRBs and LRGBs, the electromagnetic emission is produced by collimated relativistic ejecta, which initially interacts with itself and then later with the circumburst medium.

Two distinct emission phases occur in the GRB fireball scenario. First, there is a prompt phase producing the short-lived gamma-ray radiation through internal shocks within the relativistic jet that dissipate its internal kinetic energy. Then, there is an afterglow phase during which a longer fading multiwavelength emission is radiated from the external shocks between the jet and the circumstellar medium (Piran 1999; Kumar & Zhang 2015). Two kinds of external shocks are important: long-duration forward shocks, which propagate outward sweeping up the circumstellar medium, and short-lived reverse shocks, which propagate backward into the jet (Meszaros & Rees 1993).

Emission from forward shocks explains the afterglow phase of many GRBs. The dynamics of the forward shocks have been amply explored (Meszaros & Rees 1997; Sari & Piran 1999; Granot & Sari 2002). Detailed studies of the afterglow

<footnote n="1">¹Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 70-264, 04510 México, CDMX, Mexico</footnote>

<footnote n="2">²Department of Astronomy, University of Maryland, College Park, MD 20742-4111, USA</footnote>

<footnote n="3">³Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA</footnote>

<footnote n="4">⁴Institut de Recherche en Astronomique et Planétologie, 14 Avenue Edouard Belin, F-31400 Toulouse, France</footnote>

<footnote n="5">⁵School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA</footnote>

<footnote n="6">⁶Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, Huntsville, AL 35899, USA</footnote>

<footnote n="7">⁷National Astronomical Observatories/Chinese Academy of Science 20A Datun Road, Beijing, 100012, People's Republic of China</footnote>

<footnote n="8">⁸ARTEMIS, UMR 7250 (CNRS/OCA/UNS), boulevard de l'Observatoire, BP 4229, F-06304 Nice Cedex, France</footnote>

<footnote n="9">⁹Department of Astronomy and Astrophysics, UCO/Lick Observatory, University of California, 1156 High Street, Santa Cruz, CA 95064, USA</footnote>

<footnote n="10">¹⁰Instituto de Astronomía, Universidad Nacional Autónoma de México, Unidad Académica en Ensenada, 22860 Ensenada, BC, Mexico</footnote>
emission and especially the forward shock component provide valuable information about the total energy, the geometry, and the structure of the circumburst medium (e.g., Burrows et al. 2006; Stratta et al. 2007; De Pasquale et al. 2010; Troja et al. 2016; Fraija et al. 2019b, 2019c, 2019d, 2019e). On the other hand, the reverse shock emission is useful for understanding the initial bulk Lorentz factor, the ejecta composition, and magnetization (e.g., Steele et al. 2009; Mundell et al. 2013; Fraija et al. 2016, 2019f; Fraija 2014).

Under certain conditions, reverse shocks are discussed by Meszaros & Rees (1997), Sari & Piran (1999), Kobayashi (2000), and Gao & Mészáros (2015) and are predicted to generate a strong optical flash observable in the very early stages of the afterglow due to synchrotron emission, which is shown as a single-peak (Fraija & Veres 2018) for a moderately magnetized ejecta (Zhang et al. 2015). After the flare no new electrons are injected and the shell material cools adiabatically.

Rapid (within minutes) and sensitive optical observations are crucial to studying the reverse shock emission. Bright optical flashes have been observed in many LGRBs (e.g., Akerlof et al. 1999; Bloom et al. 2009; Vestrand et al. 2014; Troja et al. 2017; Fraija & Veres 2018) and are inconsistent with a simple forward shock scenario. They are normally explained as emissions emerging from a reverse shock (Japelj et al. 2014; Fraija et al. 2017). Reverse shock components in the afterglows of SGRBs have been previously suggested on the basis of gamma and radio observations (e.g., Burrows et al. 2006; Fraija et al. 2016, 2019a; Lloyd-Ronning 2018; Troja et al. 2019), but they have not previously been identified in the optical. Early radio flares have been attributed to the emission from a short-lived external reverse shock (Sari & Piran 1999; Kobayashi & Sari 2000). The particles that produce the optical flash cool adiabatically, and therefore their emission shifts to lower frequencies over time (Kobayashi & Sari 2000; Troja et al. 2019).

In this work, we present the photometric data and analysis of GRB 180418A, detected with TAROT 28 s after the gamma-ray trigger. We show that it is the first possible SGRB showing reverse shock emission in the optical. We are unable to rule out the possibility that it is an LGRB.

The paper is organized as follows: in Section 2, we present the observations with Swift, Fermi, TAROT, RATIR, and other telescopes; in Section 3 we present a temporal and spectral analysis and discuss the nature of this GRB; and in Section 4 we summarize and discuss our results. In the Appendix, we present our search for the GRB host galaxy.

## 2. Observations

### 2.1. Neil Gehrels Swift Observatory

The Swift/BAT instrument triggered on GRB 180418A at $T = 2018$ March 25 06:44:06.012 UTC (trigger 826428) (D'Elia et al. 2018). The BAT light curve showed a single FRED-like pulse that started at $T + 0$ s, peaked at about $T + 0.4$ s, and ended at about $T + 3.5$ s. The BAT light curve is shown in Figure 1.

The Swift/BAT data of GRB 180418A were processed using HEASOFT package (v6.25). The energy calibration was applied with BATECONVERT and the mask weighting was included with BATMASKWTEVT. We used BATTBLOCKS to run the Bayesian Block algorithm over the 16 ms, background-subtracted, 15–350 keV light curve and determined

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Swift/BAT light curve using a binning of 16 ms and observed in the range 15–350 keV. The red lines mark the beginning and end of the $T_{90}$ interval.</figcaption></figure>

$T_{90} = 1.504 \pm 0.380$. BATTBLOCKS was run with the default configuration options except the background-subtraction parameter `bkgsub` was set to “YES.”

The 15–150 keV spectrum integrated over the $T_{90}$ interval (from 0.272 to 1.776 s) is well fitted by a simple power law with a photon index of $1.43 \pm 0.11$ ($\chi^2$/d.o.f. = 1.02) and a fluence of $(2.72 \pm 0.11) \times 10^{-7}$ erg cm$^{-2}$. From this spectrum we derive a hardness ratio of $S(100-50)/S(25-50) = 1.48$.

The 15–150 keV peak flux was derived from the spectrum integrated in the interval from 0.144 to 1.144 s. The peak spectrum is well fitted by a power law with a photon index of $1.41 \pm 0.11$, a peak flux of $(2.36 \pm 0.17) \times 10^{-7}$ erg cm$^{-2}$ s$^{-1}$, and a peak photon flux of $(2.98 \pm 0.17) \text{ cm}^{-2} \text{ s}^{-1}$.

Due to an observing constraint, Swift did not slew to the source until $T + 49.6$ minutes (D'Elia et al. 2018), and therefore, the Swift/XRT instrument only started observing the field at $T + 3081.4$ s. It detected a fading source at R.A. decl. 11:20:29.17+24:55:59.1 J2000 with a 90% uncertainty radius of $1.8''$ (Goad et al. 2018; Liu et al. 2018; Osborne et al. 2018). The X-ray spectrum could be fitted with an absorbed power law with a photon spectral index of $2.02_{-0.26}^{+0.28}$ and an absorption column of $8.0_{-5.8}^{+7.1} \times 10^{20} \text{ cm}^{-2}$, in excess of the Galactic value of $1.1 \times 10^{20} \text{ cm}^{-2}$ (Liu et al. 2018). For our analysis in this paper, we used the X-ray light curve and the spectrum obtained from the pipeline of Butler (2007) and Butler et al. (2007).

The Swift/UVOT instrument started observing the field at $T + 3086$ s and detected a fading source at R.A. decl. 11:20:29.21+24:55:59.2 J2000 with a 90% uncertainty radius of $0.49''$ (Siegel & D'Elia 2018). We downloaded the UVOT data from the online archive$^{11}$ and derived magnitudes and signal-to-noise ratios. Table 1 shows the filter, initial time $t_i$, and the final time $t_f$ (relative to $T$), the AB magnitude, and the signal-to-noise ratio. For UVOT, the exposure time is simply $t_f - t_i$.

### 2.2. Fermi Gamma-Ray Observatory

The Fermi/GBM instrument triggered on GRB 180418A at 2018 April 18 06:44:06.28 UTC (trigger 545726651/180418281) and observed a single FRED-like peak (Figure 2), in agreement with the Swift/BAT light curve, with a $T_{90}$

<footnote n="11">11 https://swift.gsfc.nasa.gov/archive/</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** GBM light curve since the GBM trigger. The energy range is 8–1000 keV and the binning of 16 ms. The red lines mark the beginning and end of the $T_{90}$ interval.</figcaption></figure>

<table><caption>**Table 1**
UVOT Observations of GRB 180418A</caption><thead><tr><th>Filter</th><th>t<sub>i</sub> (s)</th><th>t<sub>f</sub> (s)</th><th>AB</th><th>S/N</th></tr></thead><tbody><tr><td>v</td><td>3242.6</td><td>4834.19</td><td>>20.25</td><td>...</td></tr><tr><td>b</td><td>4063.89</td><td>4263.66</td><td>20.43 ± 0.30</td><td>3.687</td></tr><tr><td>u</td><td>3858.83</td><td>4058.61</td><td>20.52 ± 0.20</td><td>5.675</td></tr><tr><td>uvw1</td><td>3654.22</td><td>3854.01</td><td>20.77 ± 0.25</td><td>4.438</td></tr><tr><td>uvm2</td><td>3449.18</td><td>3648.95</td><td>20.85 ± 0.30</td><td>3.734</td></tr><tr><td>uvw2</td><td>4475.76</td><td>4675.53</td><td>>20.90</td><td>...</td></tr><tr><td>White</td><td>3086.17</td><td>3235.93</td><td>19.99 ± 0.13</td><td>9.123</td></tr><tr><td>White</td><td>4270.14</td><td>4469.9</td><td>20.40 ± 0.15</td><td>8.060</td></tr></tbody></table>

duration of $2.56 \pm 0.20$ s and a 10–1000 keV fluence of $(5.9 \pm 0.1) \times 10^{-7}$ erg cm$^{-2}$ (Narayana Bhat et al. 2016; Bissaldi & Veres 2018). The GBM light curve is shown in Figure 2. The burst was not detected by the Fermi/LAT instrument.

## 2.3. TAROT Observations

TAROT<fnref n="12" /> La Silla is a 25 cm robotic telescope located at the European Southern Observatory, La Silla Observatory, in Chile. TAROT is equipped with a CCD camera and a filter wheel with BVRIC filters (Klotz et al. 2008).

TAROT is connected to the GCN/TAN alert system and received a BAT quick look alert packet for GRB 180418A at 06:44:21 UTC ($T + 15$ s). It immediately slewed to the burst and began observing in the clear C filter, with the first exposure starting at 06:44:34 UTC ($T + 28$ s). The first exposure is trailed with a duration of 60 s to allow continuous monitoring of the light curve (Klotz et al. 2006). Subsequent exposures were taken in sidereal tracking mode with exposure times of 30–90 s and read-out times of about 10 s. We use TAROT data from $T + 28$ s to $T + 392$ s (Klotz et al. 2018).

Table 2 gives TAROT photometry. For each exposure, it gives the initial time $t_i$ and the final time $t_f$ (relative to $T$) and the AB r magnitude (obtained from the C magnitude and not corrected for Galactic extinction) with the $1\sigma$ total uncertainties (including both statistical and systematic contributions). For TAROT, the exposure time is simply $t_f - t_i$.

<table><caption>**Table 2**
TAROT Observations of GRB 180418A</caption><thead><tr><th>t<sub>i</sub> (s)</th><th>t<sub>f</sub> (s)</th><th>r</th></tr></thead><tbody><tr><td>28.0</td><td>36.0</td><td>14.30 ± 0.04</td></tr><tr><td>36.0</td><td>43.0</td><td>14.24 ± 0.03</td></tr><tr><td>43.0</td><td>51.0</td><td>14.40 ± 0.03</td></tr><tr><td>51.0</td><td>58.0</td><td>14.61 ± 0.04</td></tr><tr><td>58.0</td><td>66.0</td><td>14.98 ± 0.05</td></tr><tr><td>66.0</td><td>73.0</td><td>15.42 ± 0.06</td></tr><tr><td>73.0</td><td>81.0</td><td>15.52 ± 0.09</td></tr><tr><td>81.0</td><td>88.0</td><td>15.83 ± 0.15</td></tr><tr><td>100.0</td><td>130.0</td><td>15.96 ± 0.06</td></tr><tr><td>141.0</td><td>171.0</td><td>16.50 ± 0.11</td></tr><tr><td>181.0</td><td>211.0</td><td>16.58 ± 0.13</td></tr><tr><td>221.0</td><td>251.0</td><td>17.10 ± 0.31</td></tr><tr><td>262.0</td><td>292.0</td><td>16.90 ± 0.25</td></tr><tr><td>302.0</td><td>392.0</td><td>17.00 ± 0.29</td></tr></tbody></table>

## 2.4. RATIR Observations

RATIR<fnref n="13" /> is a four-channel simultaneous optical and near-infrared imager mounted on the 1.5 m Harold L. Johnson Telescope at the Observatorio Astronómico Nacional in Sierra San Pedro Mártir in Baja California, Mexico (Butler et al. 2012; Watson et al. 2012; Littlejohns et al. 2015). RATIR usually obtains simultaneous photometry in riZJ or riYH, but at the time of these observations the ZY and JH channels were not operational. Therefore, RATIR only obtained photometry in ri.

RATIR is connected to the GCN/TAN alert system and received a BAT quick look alert packet for GRB 180418A at 06:44:20.3 UTC ($T + 14.3$ s). It immediately slewed to the burst and began observing, with the first exposure starting at 06:46:06.3 UTC ($T + 120.6$ s). It took simultaneous exposures in r and i with an exposure time of 80 s and a cadence of about 100 s. On the nights of 2018 April 18, 19, 20, and 21 UTC, RATIR observed from $T + 120.6$ s to $T + 3.64$ hr (Troja et al. 2018a), from $T + 20.47$ to $T + 27.09$ hr (Troja et al. 2018b), from $T + 44.62$ to $T + 51.37$ hr, and from $T + 68.55$ to $T + 75.27$ hr.

The RATIR reduction pipeline performs bias subtraction and flat-field correction, followed by astrometric calibration using the astrometry.net software (Lang et al. 2010), iterative sky-subtraction, coaddition using SWARP, and source detection using SExTRACTOR (Littlejohns et al. 2015). Images were calibrated against SDSS (Littlejohns et al. 2015).

Table 3 gives our RATIR photometry. For each exposure or coadded exposure it gives the initial time $t_i$ and final time $t_f$ (relative to $T$), the total exposure time $t_{exp}$, and the r and i magnitudes (not corrected for Galactic extinction) with their $1\sigma$ total uncertainties (including both statistical and systematic contributions).

## 2.5. Other Terrestrial Observations

Zheng & Filippenko (2018) began to observe the field of GRB 180418A with KAIT at 06:46:41 UTC ($T + 125$ s). They reported an optical transient that faded from about magnitude 15.8 at $T + 135$ s. Due to the delay in the Swift/XRT observations, this was the first precise localization of the counterpart of GRB 180418A. Additional photometry was reported by Guidorzi et al. (2018), Sota et al. (2018),

<footnote n="12">¹² http://tarotobs-hp.fr/</footnote>

<footnote n="13">¹³ http://ratir.astroscu.unam.mx/</footnote>
<table><caption>**Table 3**
RATIR Observations of GRB 180418A</caption><thead><tr><th>t<sub>i</sub> (s)</th><th>t<sub>f</sub> (s)</th><th>t<sub>exp</sub> (s)</th><th>r</th><th>i</th></tr></thead><tbody><tr><td>120.6</td><td>200.6</td><td>80</td><td>16.39 ± 0.01</td><td>16.29 ± 0.01</td></tr><tr><td>230.6</td><td>310.6</td><td>80</td><td>17.21 ± 0.01</td><td>16.96 ± 0.02</td></tr><tr><td>322.4</td><td>402.4</td><td>80</td><td>17.56 ± 0.01</td><td>17.40 ± 0.01</td></tr><tr><td>414.3</td><td>494.3</td><td>80</td><td>17.85 ± 0.02</td><td>17.93 ± 0.03</td></tr><tr><td>521.6</td><td>601.6</td><td>80</td><td>18.13 ± 0.04</td><td>18.53 ± 0.06</td></tr><tr><td>616.2</td><td>696.2</td><td>80</td><td>18.17 ± 0.02</td><td>...</td></tr><tr><td>617.1</td><td>697.1</td><td>80</td><td>...</td><td>18.04 ± 0.03</td></tr><tr><td>714.1</td><td>794.1</td><td>80</td><td>18.25 ± 0.04</td><td>18.27 ± 0.04</td></tr><tr><td>812.1</td><td>892.1</td><td>80</td><td>18.43 ± 0.02</td><td>19.33 ± 0.05</td></tr><tr><td>900.7</td><td>980.7</td><td>80</td><td>18.71 ± 0.05</td><td>18.89 ± 0.07</td></tr><tr><td>999.6</td><td>1079.6</td><td>80</td><td>18.65 ± 0.06</td><td>18.50 ± 0.07</td></tr><tr><td>1087.7</td><td>1167.7</td><td>80</td><td>18.66 ± 0.03</td><td>18.51 ± 0.04</td></tr><tr><td>1199.0</td><td>1279.0</td><td>80</td><td>18.86 ± 0.03</td><td>...</td></tr><tr><td>1199.1</td><td>1279.0</td><td>80</td><td>...</td><td>18.79 ± 0.04</td></tr><tr><td>1287.2</td><td>1367.2</td><td>80</td><td>...</td><td>18.75 ± 0.04</td></tr><tr><td>1287.6</td><td>1367.6</td><td>80</td><td>18.84 ± 0.03</td><td>...</td></tr><tr><td>1380.3</td><td>1460.3</td><td>80</td><td>18.81 ± 0.03</td><td>18.83 ± 0.05</td></tr><tr><td>1472.1</td><td>1552.1</td><td>80</td><td>...</td><td>18.77 ± 0.05</td></tr><tr><td>1570.0</td><td>1650.0</td><td>80</td><td>19.16 ± 0.06</td><td>18.94 ± 0.06</td></tr><tr><td>1683.4</td><td>1763.3</td><td>80</td><td>19.17 ± 0.04</td><td>...</td></tr><tr><td>1684.3</td><td>1764.3</td><td>80</td><td>...</td><td>19.17 ± 0.05</td></tr><tr><td>1777.3</td><td>1857.3</td><td>80</td><td>19.22 ± 0.06</td><td>19.01 ± 0.06</td></tr><tr><td>1883.6</td><td>1963.6</td><td>80</td><td>19.08 ± 0.04</td><td>18.93 ± 0.04</td></tr><tr><td>1975.8</td><td>2055.8</td><td>80</td><td>19.36 ± 0.05</td><td>19.26 ± 0.06</td></tr><tr><td>2064.0</td><td>2144.0</td><td>80</td><td>19.11 ± 0.08</td><td>19.06 ± 0.08</td></tr><tr><td>2164.6</td><td>2244.6</td><td>80</td><td>...</td><td>19.21 ± 0.07</td></tr><tr><td>2164.9</td><td>2244.9</td><td>80</td><td>19.18 ± 0.05</td><td>...</td></tr><tr><td>2284.4</td><td>2364.4</td><td>80</td><td>19.46 ± 0.07</td><td>19.16 ± 0.07</td></tr><tr><td>2377.5</td><td>2457.5</td><td>80</td><td>19.34 ± 0.05</td><td>19.53 ± 0.07</td></tr><tr><td>2466.0</td><td>2546.0</td><td>80</td><td>19.74 ± 0.14</td><td>19.51 ± 0.15</td></tr><tr><td>2558.5</td><td>2638.5</td><td>80</td><td>19.53 ± 0.06</td><td>...</td></tr><tr><td>2559.5</td><td>2639.5</td><td>80</td><td>...</td><td>19.35 ± 0.06</td></tr><tr><td>2655.9</td><td>2735.9</td><td>80</td><td>19.53 ± 0.08</td><td>19.20 ± 0.07</td></tr><tr><td>2755.1</td><td>2835.1</td><td>80</td><td>19.97 ± 0.07</td><td>19.45 ± 0.06</td></tr><tr><td>2842.3</td><td>2922.3</td><td>80</td><td>19.73 ± 0.08</td><td>19.49 ± 0.08</td></tr><tr><td>2934.2</td><td>3014.2</td><td>80</td><td>19.63 ± 0.06</td><td>19.50 ± 0.08</td></tr><tr><td>3023.6</td><td>3103.6</td><td>80</td><td>19.84 ± 0.10</td><td>19.64 ± 0.10</td></tr><tr><td>3134.3</td><td>3214.3</td><td>80</td><td>19.92 ± 0.09</td><td>...</td></tr><tr><td>3135.3</td><td>3215.3</td><td>80</td><td>...</td><td>19.64 ± 0.08</td></tr><tr><td>3247.5</td><td>3327.5</td><td>80</td><td>19.90 ± 0.08</td><td>19.88 ± 0.09</td></tr><tr><td>3341.3</td><td>3421.3</td><td>80</td><td>19.91 ± 0.11</td><td>19.73 ± 0.13</td></tr><tr><td>3428.6</td><td>3508.6</td><td>80</td><td>19.67 ± 0.07</td><td>19.67 ± 0.08</td></tr><tr><td>3521.2</td><td>3601.2</td><td>80</td><td>19.96 ± 0.12</td><td>19.77 ± 0.12</td></tr><tr><td>3608.5</td><td>3688.5</td><td>80</td><td>19.80 ± 0.10</td><td>20.13 ± 0.14</td></tr><tr><td>3714.2</td><td>3794.2</td><td>80</td><td>19.84 ± 0.09</td><td>19.80 ± 0.10</td></tr><tr><td>3812.2</td><td>3892.2</td><td>80</td><td>19.87 ± 0.12</td><td>19.80 ± 0.13</td></tr><tr><td>3910.9</td><td>3990.9</td><td>80</td><td>20.01 ± 0.12</td><td>19.81 ± 0.10</td></tr><tr><td>4090.6</td><td>4170.6</td><td>80</td><td>20.05 ± 0.13</td><td>19.69 ± 0.10</td></tr><tr><td>4207.0</td><td>4287.0</td><td>80</td><td>20.34 ± 0.15</td><td>19.70 ± 0.09</td></tr><tr><td>4387.7</td><td>4467.7</td><td>80</td><td>20.30 ± 0.13</td><td>19.99 ± 0.10</td></tr><tr><td>4479.2</td><td>4559.2</td><td>80</td><td>20.25 ± 0.12</td><td>20.06 ± 0.12</td></tr><tr><td>4567.9</td><td>4647.9</td><td>80</td><td>20.34 ± 0.13</td><td>19.97 ± 0.11</td></tr><tr><td>4666.5</td><td>4746.5</td><td>80</td><td>20.12 ± 0.11</td><td>20.08 ± 0.11</td></tr><tr><td>4778.2</td><td>4858.2</td><td>80</td><td>20.16 ± 0.11</td><td>20.01 ± 0.10</td></tr><tr><td>4778.2</td><td>4858.2</td><td>80</td><td>20.16 ± 0.11</td><td>20.20 ± 0.04</td></tr><tr><td>4778.2</td><td>6292.3</td><td>1280</td><td>20.33 ± 0.04</td><td>20.01 ± 0.10</td></tr><tr><td>4778.2</td><td>6292.3</td><td>1280</td><td>20.33 ± 0.04</td><td>20.20 ± 0.04</td></tr><tr><td>4888.0</td><td>4968.1</td><td>80</td><td>19.96 ± 0.10</td><td>...</td></tr><tr><td>4888.1</td><td>4968.1</td><td>80</td><td>...</td><td>19.89 ± 0.11</td></tr><tr><td>4976.1</td><td>5056.1</td><td>80</td><td>20.28 ± 0.15</td><td>...</td></tr><tr><td>5068.0</td><td>5148.0</td><td>80</td><td>20.30 ± 0.13</td><td>20.29 ± 0.13</td></tr><tr><td>5156.0</td><td>5236.0</td><td>80</td><td>20.36 ± 0.13</td><td>20.34 ± 0.14</td></tr><tr><td>5247.9</td><td>5327.9</td><td>80</td><td>20.32 ± 0.13</td><td>20.04 ± 0.12</td></tr><tr><td>5348.4</td><td>5428.4</td><td>80</td><td>19.92 ± 0.09</td><td>20.16 ± 0.13</td></tr></tbody></table>

<table><caption>**Table 3**
(Continued)</caption><thead><tr><th>t<sub>i</sub> (s)</th><th>t<sub>f</sub> (s)</th><th>t<sub>exp</sub> (s)</th><th>r</th><th>i</th></tr></thead><tbody><tr><td>5447.4</td><td>5527.4</td><td>80</td><td>20.48 ± 0.16</td><td>20.35 ± 0.14</td></tr><tr><td>5630.6</td><td>5710.6</td><td>80</td><td>20.47 ± 0.15</td><td>20.21 ± 0.14</td></tr><tr><td>5728.5</td><td>5808.5</td><td>80</td><td>20.23 ± 0.13</td><td>20.29 ± 0.15</td></tr><tr><td>5826.9</td><td>5906.9</td><td>80</td><td>20.34 ± 0.14</td><td>20.24 ± 0.13</td></tr><tr><td>6113.6</td><td>6193.6</td><td>80</td><td>20.40 ± 0.15</td><td>20.34 ± 0.17</td></tr><tr><td>6212.3</td><td>6292.3</td><td>80</td><td>20.43 ± 0.16</td><td>...</td></tr><tr><td>6324.4</td><td>6404.4</td><td>80</td><td>20.53 ± 0.15</td><td>20.30 ± 0.12</td></tr><tr><td>6324.4</td><td>6404.4</td><td>80</td><td>20.53 ± 0.15</td><td>20.51 ± 0.05</td></tr><tr><td>6324.4</td><td>7883.8</td><td>1280</td><td>20.72 ± 0.06</td><td>20.30 ± 0.12</td></tr><tr><td>6324.4</td><td>7883.8</td><td>1280</td><td>20.72 ± 0.06</td><td>20.51 ± 0.05</td></tr><tr><td>6435.7</td><td>6515.7</td><td>80</td><td>...</td><td>20.24 ± 0.13</td></tr><tr><td>6523.2</td><td>6603.2</td><td>80</td><td>20.44 ± 0.14</td><td>20.20 ± 0.12</td></tr><tr><td>6623.3</td><td>6703.4</td><td>80</td><td>20.42 ± 0.14</td><td>20.67 ± 0.17</td></tr><tr><td>6732.1</td><td>6812.1</td><td>80</td><td>20.61 ± 0.16</td><td>20.99 ± 0.24</td></tr><tr><td>6825.8</td><td>6905.8</td><td>80</td><td>20.43 ± 0.14</td><td>20.25 ± 0.14</td></tr><tr><td>6923.6</td><td>7003.6</td><td>80</td><td>...</td><td>20.46 ± 0.17</td></tr><tr><td>7111.3</td><td>7191.3</td><td>80</td><td>...</td><td>20.48 ± 0.17</td></tr><tr><td>7318.6</td><td>7398.6</td><td>80</td><td>...</td><td>20.53 ± 0.18</td></tr><tr><td>7418.1</td><td>7498.1</td><td>80</td><td>...</td><td>20.52 ± 0.15</td></tr><tr><td>8362.3</td><td>9887.7</td><td>1280</td><td>21.56 ± 0.15</td><td>...</td></tr><tr><td>8362.4</td><td>9887.7</td><td>1280</td><td>...</td><td>20.85 ± 0.08</td></tr><tr><td>8542.3</td><td>8622.3</td><td>80</td><td>...</td><td>20.64 ± 0.25</td></tr><tr><td>9940.3</td><td>11481.0</td><td>1280</td><td>...</td><td>20.96 ± 0.10</td></tr><tr><td>11513.1</td><td>13094.9</td><td>1280</td><td>...</td><td>21.22 ± 0.13</td></tr></tbody></table>

Fong et al. (2018), Malesani et al. (2018), Schady (2018), Misra et al. (2018), Schady & Chen (2018), Choi et al. (2018), and Horiuchi et al. (2018).

Schady (2018) mentioned that the source was not clearly point-like in their images and suggested this might be due to the presence of a host galaxy close to the afterglow. However, our RATIR observations from 2018 April 19 (T + 68.5 to T + 75.3 hr) revealed no detection to 3σ limiting magnitudes of r > 24.0 and i > 23.9, which places a limit on the magnitude of any close host galaxy. We discuss our analysis of the host galaxy search and upper limits in detail in the Appendix.

Our TAROT and RATIR observations combined with other public data are shown in Figure 3.

## 3. Analysis

### 3.1. Temporal Analysis

The prompt emission from the GRB detected by Fermi/GBM and Swift/BAT lasted until about T + 2.5 s. The earliest data from Swift/XRT start at T + 52 minutes. Our optical observations from TAROT and RATIR begin at T + 28 and T + 121 s, respectively, significantly after the end of prompt emission. Therefore, we focus our analysis on the afterglow.

#### 3.1.1. Optical Temporal Analysis

We use TAROT/RATIR r-filter data for the analysis, as we have more data in this band and the i-filter of RATIR shows the same qualitative behavior. We complement our data set with information published in GCNs Zheng & Filippenko (2018), Guidorzi et al. (2018), Fong et al. (2018), and Schady (2018). Figure 4 shows the optical and X-ray light curves for GRB 180418A.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** GRB 180418A photometry from this work (TAROT, RATIR, and UVOT) and GCNs (Fong et al. 2018; Guidorzi et al. 2018; Schady 2018; Zheng & Filippenko 2018).</figcaption></figure>

The optical light curve appears to be a smooth power-law decline but with an excess of emission at early times up to about 100 s. The later standard forward shock decay is present in most afterglows (Zhang et al. 2006).

We fit the optical light curve for $t > 1000$ s using a simple forward external shock component into a constant density ISM under the supposition of a thin-shell evolving in the slow-cooling regime below the cooling break (Kobayashi 2000), and find a temporal index of $\alpha_{forward} = 1.01 \pm 0.01$. The temporal index $\alpha_{forward}$ is expected to be related to the electron energy index $p$ by $\alpha_{forward} = 3(p-1)/4$, so we predict $p = 2.35 \pm 0.01$. A jet being driven into a decreasing-density stellar-wind environment (Fraija et al. 2017) would give a $p < 2$, which is not a typical value observed for GRBs and then, we discard this possibility for this work although we cannot reject that unlikely scenario totally.

The light curve for the forward-shock component is shown in the left panel of Figure 4. However, extrapolating this fit to earlier times, it is clear that there is a significant excess of emission, and this is shown in the right panel of Figure 4. This excess is present from our earliest observation at $t = 28$ s until $t \approx 100–300$ s and has a peak at $t \approx 40$ s. A priori, there are several different scenarios that might explain this excess.

First, we consider the possibility of late central engine activity. This scenario has been studied widely for X-rays. Figures 4 and 5 suggest an achromatic behavior for the X-ray and optical emission and, therefore, the same region of origin. In this case, we would expect a counterpart in X-ray for our detected excess. Unfortunately, we do not have data from XRT for these times.

Yi et al. (2017) suggest that the potential difference between the durations of X-ray and UV/optical flares is caused by the fact we only see the peak of the flare in the UV/optical and therefore our measured duration for the flare will be biased

relative to that of the X-ray for which we see more of the flare rise and decay but not its temporary indices (Swenson & Roming 2014). For the optical, the timing of the excess corresponds to a range of the relative duration $\delta t/t$ (duration of about 100 s over the peak time of about 40 s) of more than 2. Therefore, we would expect a $\delta t/t \ge 2$ for the predicted X-ray counterpart, which goes against the typical yields for X-rays calculated for late central activity of $\delta t/t \ll 1$ (Zhang et al. 2006). In conclusion, the late central activity scenario is unlikely to power the rebrightening observed in GRB 180418A

Second, we consider a two-component jet scenario consisting of a narrow and initially faster component and a wide and initially slower component. This model was proposed to explain the rebrightening emission in afterglow light curves (Granot 2005; Racusin et al. 2008). Granot (2005) derived the two-component jet light curves and found that this model cannot produce very sharp features in the light curve. Also, they concluded that this scenario typically shows a pronounced brightening detected in light curves of several afterglows 0.1–1 day after the GRB (depending on the environment) according to analytic solutions for typical microphysical parameters found in most GRBs ($p = 2.25$, $\epsilon_e = 0.01$, $\epsilon_B = 0.001$, $n = 0.2 \text{ cm}^{-3}$) (Peng et al. 2005). Nevertheless, in that case, the excess would appear at least a couple of hours after the trigger, and so this cannot explain a very early excess like the one seen here in GRB 180418A.

Finally, we consider emission from a reverse external shock (Kobayashi 2000) in addition to a standard forward shock. The optical light curve of GRB 180418A is similar to the *Peak + Fast Decay* curves studied by Kann et al. (2010), which show a fast rise to a peak, followed by a fast decay (with a temporal index $\alpha$ of about 1.5–2), and are typically interpreted as a reverse shock component in addition to the forward component (Japelj et al. 2014) or in some cases as central engine late activity. For this model, the temporal indices of the rise and the decay are related to the peak time of the emission of the reverse shock $t_\gamma$ and the electron index $p$ (assumed to be the same for both the forward and reverse shocks) by $\alpha_{reverse,R} = 3/2 - 3p$ and $\alpha_{reverse,D} = (27p+7)/35$, respectively. Thus, the free parameters of the model for the total emission from both shocks are $p$, $t_\gamma$, and the normalization of the two components. The final fit for both components is shown in the right panel of Figure 4 and has a $\chi^2$/d.o.f. = 0.98 with an electron index $p = 2.35 \pm 0.03$. The parameters for this model are shown in Table 4. (The temporal indices $\alpha$ are labeled with suffixes for reverse and forward and with $R$ or $D$ to refer to the rise and decay phases of the reverse component.)

### 3.1.2. X-Ray Temporal Analysis

Swift/XRT data were obtained for $t > 51$ minutes. This region can be fitted as a power law with a temporal index of $\alpha_{X,forward} = 1.04 \pm 0.05$. The similarity of the X-ray and optical temporal indices suggests that the emission in both wavelength regions arises from the same spectral regime, which we assume to be a thin-shell evolving in the slow-cooling regime below the cooling break into a constant density ISM (Kobayashi 2000).
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Left: fitting of the GRB 180418A light curve with a single power law. Red and blue points are observations in *r*-band and X-rays respectively. The X-ray and *r*-band show a non break up to $10^5$ s. Right: zoom of GRB 180418A light curve at early times with a fit using reverse shock (RS) and forward shock (FS) components. The final fit for optical is the black line (which includes the sum of the forward and reverse shocks).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Table 4**
Fitting Parameters of GRB 180418A</figcaption></figure>

<table><thead><tr><th></th><th>Parameter</th><th>Value</th></tr></thead><tbody><tr><td colspan="3"><strong>Reverse shock</strong></td></tr><tr><td>Electron index</td><td><i>p</i></td><td>2.35 ± 0.03</td></tr><tr><td></td><td><i>t</i><sub>γ</sub></td><td>35 ± 1 s</td></tr><tr><td><i>t</i> &lt; <i>t</i><sub>γ</sub></td><td>α<sub>reverse,R</sub></td><td>-5.56 ± 0.01</td></tr><tr><td><i>t</i> &gt; <i>t</i><sub>γ</sub></td><td>α<sub>reverse,D</sub></td><td>2.02 ± 0.01</td></tr><tr><td colspan="3"><strong>Forward shock</strong></td></tr><tr><td>Electron index</td><td><i>p</i></td><td>2.35 ± 0.03</td></tr><tr><td>Optical</td><td>α<sub>forward, optical</sub></td><td>1.01 ± 0.01</td></tr><tr><td>X-rays</td><td>α<sub>forward, X</sub></td><td>1.04 ± 0.05</td></tr><tr><td></td><td>β</td><td>0.80 ± 0.04</td></tr></tbody></table>

**Note.** The variable $t_γ$ is the time when a maximum is reached for the reverse shock component. The temporal indices $α$ are labeled with suffixes for reverse and forward and with *R* or *D* to refer to the rise and decay phase of the reverse component.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** The SED of GRB 180418A from X-rays to the optical at *t* = 4000 s. The data are from *Swift/XRT*, *Swift/UVOT*, and *RATIR*. The dotted line is a simple power law with index $β = 0.73$. The solid line is the best fit from XSPEC.</figcaption></figure>

## 3.2. Spectral Analysis

We retrieved the *Swift/XRT* X-ray spectrum in the time interval 3091–4831 s from the online repository.<fnref n="14" /> We interpolated our UVOT and RATIR photometry to *t* = 4000 s assuming a power-law decay with an index of $α_{forward} = 1.01$ (see Section 3.1.1). Figure 5 shows the resulting combined spectral energy distribution (SED).

From the X-ray to the optical, the SED can be fitted with a simple power law with index $β = 0.73 ± 0.03$. Under our assumption of a thin-shell evolving in the slow-cooling regime with the cooling break above the X-rays (Kobayashi 2000), we would expect the spectral index to be $(p-1)/2$ or $0.68 ± 0.03$.

for $p = 2.35$. Thus, our observed value is in good agreement with this prediction.

## 3.3. Photometric Redshift

No spectroscopic redshift for the burst has been reported; therefore we are forced to place limits on the redshift from the combined X-ray, UV, and optical broadband SED. The SED fitting was carried out using XSPECv12.10.1 (Arnaud 1996) taking into account Galactic extinction for IR, optical, and UV energy bands due to dust (Cardelli et al. 1989), the IR/optical/ UV extinction of the host galaxy, and the photoelectric absorption of soft X-rays (in the Galaxy and in the host galaxy). We set the redshift as a free parameter during the fit, and found an upper limit of $z < 1.31$ at a 90% confidence level. This limit basically comes from the fit requiring that the Lyman

<footnote n="14">¹⁴ http://www.swift.ac.uk/xrt_spectra/</footnote>
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Relation between $\tau$ and $L_{peak}$. The red data show the values for GRB 180418A assuming redshifts of $z = 0.1$, $z = 0.5$, and $z = 1.0$. The black dotted line is the relation from Norris et al. (2000) and the blue dotted lines show the $2\sigma$ scatter. The figure also shows SGRBs from Gehrels et al. (2006) including the energy band correction.</figcaption></figure>

continuum absorption fall below our detection in the UVOT uvm2 filter.

### 3.4. Classification of the Burst

The values of $T_{90}$ do not unambiguously identify GRB 180418A as an SGRB or LGRB because it is neither a hard nor definitive criteria to distinguish between the possible progenitors. The value of $T_{90} = 1.50 \pm 0.38$ from the Swift/BAT light curve would suggest an SGRB, although it is only below 2 s by $1.3\sigma$. However, the value of $T_{90} = 2.56 \pm 0.20$ s from the Fermi/GBM light curve (Bissaldi & Veres 2018) would suggest an LGRB.

Additional information on the nature of a GRB can sometimes be obtained from the spectral hardness and lag. The spectral hardness found from the Swift/BAT instrument is $S(100-50)/S(25-50) = 1.48$, which corresponds to intermediate hardness in the classification of Bromberg et al. (2013) and does not help to resolve the ambiguity. Considering both $T_{90}$ and the spectral hardness with the distributions presented by Bromberg et al. (2013), we estimate the probability that the burst is an SGRB to be 10%–30%.

We also investigate the lag between the arrival times between high- and low-energy photons $t_{lag}$ and peak luminosity $L_{peak}$ correlation for LGRBs reported by Norris et al. (2000). We used RMfit<fnref n="15" />, version 432, to analyze the spectrum around the peak of the GBM light curve in the time interval from -0.128 to 0.896 s. The spectrum fits with a single power law with index $-1.46 \pm 0.07$ and flux = $(6.0 \pm 1.2) \times 10^{-7}$ between 10 and 1000 keV. Using these parameters we derived an upper limits for the peak luminosity $L_{peak}$ as a function of redshift, obtaining $L_{peak} = 6.57 \times 10^{49}$ erg s$^{-1}$, $L_{peak} = 2.14 \times 10^{51}$ erg s$^{-1}$, and $L_{peak} = 1.01 \times 10^{52}$ erg s$^{-1}$ for $z = 0.1$, $z = 0.5$ and $z = 1.0$, respectively. The time-lag between the energy bands 50–100 keV and 15–25 keV ($\tau = 0.088 \pm 0.026$) was retrieved from the GCN notice 22658 (Palmer 2018). We applied the time dilation correction and energy correction due to the redshift considering the relation between the pulse width and energy (Norris 2002) and approximating the correction factor with $(1+z)^{0.67}$ as

<table><caption>**Table 5**
Parameters Found of the Early and Late Afterglows of GRB 180418A</caption><thead><tr><th>Parameter</th><th>Symbol</th><th>Value</th></tr></thead><tbody><tr><td>ISM density</td><td>n</td><td>0.15 cm<sup>-3</sup></td></tr><tr><td>Lorentz factor</td><td>Γ</td><td>160</td></tr><tr><td rowspan="3">Magnetization rate</td><td>R<sub>B</sub> = (B<sub>r</sub>/B<sub>t</sub>)</td><td>14</td></tr><tr><td>ε<sub>B,f</sub></td><td>10<sup>-3</sup></td></tr><tr><td>ε<sub>B,r</sub></td><td>0.2</td></tr><tr><td>ε<sub>e</sub></td><td>0.1</td></tr><tr><td>Isotropic energy</td><td>E<sub>iso</sub></td><td>0.77 &times; 10<sup>51</sup> erg</td></tr><tr><td>Redshift</td><td>z</td><td>0.5</td></tr><tr><td>Optical luminosity</td><td>L<sub>opt,7</sub></td><td>1.51 &times; 10<sup>42</sup> erg s<sup>-1</sup></td></tr><tr><td>Opening angle</td><td>θ<sub>j</sub></td><td>&gt;7°</td></tr></tbody></table>

**Note.** Most of these parameters are calculated using the work of Fraija et al. (2017) and using the model of Kobayashi (2000). The value of the redshift $z = 0.5$ is an estimation according to the average value in SGRBs and, from it, we calculate the isotropic kinetic energy $E_{iso}$, $L_{opt,7}$ and the limit value of $\theta_j$.

<table><caption>**Table 6**
GRBs with Signature of a Reverse Shock</caption><thead><tr><th>GRB</th><th>z</th><th>T<sub>90</sub></th><th>Evidence</th><th>References</th></tr></thead><tbody><tr><td>GRB 990123</td><td>1.60</td><td>63.3</td><td>B</td><td>1, 2, 3</td></tr><tr><td>GRB 021004</td><td>2.33</td><td>100.0</td><td>A</td><td>4, 5</td></tr><tr><td>GRB 021211</td><td>0.80</td><td>2.67</td><td>B</td><td>6</td></tr><tr><td>GRB 050525A</td><td>0.61</td><td>8.8</td><td>C</td><td>7, 8, 9, 10</td></tr><tr><td>GRB 050904</td><td>6.9</td><td>225.0</td><td>C</td><td>11,12</td></tr><tr><td>GRB 060111B</td><td>1-2</td><td>59</td><td>C</td><td>13, 14</td></tr><tr><td>GRB 060117</td><td>0.45</td><td>16.0</td><td>C</td><td>15, 16, 17</td></tr><tr><td>GRB 060908</td><td>1.88</td><td>19.30</td><td>A</td><td>18</td></tr><tr><td>GRB 061126</td><td>1.56</td><td>191</td><td>B</td><td>19, 20</td></tr><tr><td>GRB 080319B</td><td>0.94</td><td>&gt;50</td><td>A</td><td>21, 22</td></tr><tr><td>GRB 081007</td><td>0.53</td><td>8.0</td><td>B</td><td>23, 24</td></tr><tr><td>GRB 090102</td><td>1.55</td><td>27.0</td><td>A</td><td>18, 25, 26</td></tr><tr><td>GRB 090424</td><td>0.54</td><td>48.0</td><td>A</td><td>23,27</td></tr><tr><td>GRB 090902B</td><td>1.82</td><td>21.0</td><td>C</td><td>28, 29, 30</td></tr><tr><td>GRB 091024</td><td>1.1</td><td>1020</td><td>C</td><td>31, 32</td></tr><tr><td>GRB 110205A</td><td>2.11</td><td>257</td><td>C</td><td>33, 34, 35</td></tr><tr><td>GRB 130427A</td><td>0.34</td><td>163</td><td>B</td><td>36, 37, 38</td></tr><tr><td>GRB 160625B</td><td>1.41</td><td>35</td><td>B</td><td>39, 40</td></tr><tr><td>GRB 180418A</td><td>&lt;1.31</td><td>1.5</td><td>B</td><td>This work</td></tr></tbody></table>

**Note.** A: Confirmed. B: Strongly confirmed. C: There are other possibilities due to the lack of good early-time photometric coverage.

**References.** (1) Mészáros & Rees (1999), (2) Bloom et al. (1999), (3) Kippen (1999), (4) Castro-Tirado et al. (2010), (5) Kobayashi & Zhang (2003), (6) Wei (2003), (7) Shao & Dai (2005), (8) Cummings et al. (2005), (9) Foley et al. (2005), (10) Markwardt et al. (2005), (11) Sakamoto et al. (2005), (12) Wei et al. (2006), (13) Klotz et al. (2006), (14) Stratta et al. 2009, (15) Jelínek et al. (2006), (16) Pelangeon & Atteia (2006), (17) Cummings et al. (2006), (18) Japelj et al. (2014), (19) Gomboc et al. (2008), (20) Perley et al. 2008, (21) Yu et al. (2009), (22) Vreeswijk et al. (2008), (23) Jin et al. (2013), (24) de Ugarte Postigo et al. (2011), (25) de Ugarte Postigo et al. 2009, (26), Gendre et al. (2010), (27) Sakamoto et al. (2009), (28) Pandey et al. 2010, (29) Cucchiara et al. (2009), (30) de Palma et al. (2009), (31) Gruber et al. (2011), (32) Cucchiara et al. (2011), (33) da Silva et al. (2011), (34) Markwardt et al. (2011), (35) Gendre et al. (2012), (36) Perley et al. (2014), (37) Barthelmy et al. (2013), (38) Levan et al. (2013), (39) Troja et al. (2017), (40) Zhang et al. (2018).

proposed by Gehrels et al. (2006). The results of this analysis are shown in Figure 6 and compared with data of SGRBs presented by Gehrels et al. (2006) and the LGRB correlation of Norris et al. (2000). We see that GRB 180418A is consistent with an LGRB provided the redshift is larger than about 1 and is also consistent with an SGRB at lower redshifts.

<footnote n="15">¹⁵ https://fermi.gsfc.nasa.gov/ssc/data/analysis/rmfit/</footnote>
<table><caption>**Table 7**
Candidate Galaxies To Be the Host Galaxy of GRB 180418A</caption><thead><tr><th>Label</th><th>SDSS ID</th><th>R.A.</th><th>Decl.</th><th>r</th><th>i</th><th>δR (")</th><th>P(&lt;δR)</th></tr></thead><tbody><tr><td>G1</td><td>1237667551421006080</td><td>170.1150</td><td>24.9451</td><td>21.1390 ± 0.0274</td><td>20.8640 ± 0.0240</td><td>48.18</td><td>0.997</td></tr><tr><td>G2</td><td>1237667551421006592</td><td>170.1097</td><td>24.9475</td><td>20.0264 ± 0.0115</td><td>19.5144 ± 0.0084</td><td>64.84</td><td>0.986</td></tr><tr><td>G3</td><td>1237667551421006592</td><td>170.1216</td><td>24.9242</td><td>22.2155 ± 0.0714</td><td>21.9499 ± 0.0613</td><td>32.11</td><td>0.996</td></tr><tr><td>G4</td><td>1237667551421006848</td><td>170.1256</td><td>24.9331</td><td>22.7085 ± 0.1149</td><td>22.5791 ± 0.1089</td><td>13.39</td><td>0.674</td></tr><tr><td>G5</td><td>1237667551421006592</td><td>170.1400</td><td>24.9250</td><td>20.2362 ± 0.0141</td><td>19.7060 ± 0.0099</td><td>66.93</td><td>1.000</td></tr><tr><td>G6</td><td>1237667551421006592</td><td>170.1185</td><td>24.9224</td><td>22.1486 ± 0.0662</td><td>21.7405 ± 0.0509</td><td>39.83</td><td>0.999</td></tr><tr><td>S1</td><td>...</td><td>170.1241</td><td>24.9319</td><td>23.9651 ± 0.0000</td><td>23.7360 ± 0.3158</td><td>9.45</td><td>0.690</td></tr></tbody></table>

Unfortunately, our photometric analysis in Section 3.3 only requires $z < 1.3$, and so we are unable to rule out the possibility and again cannot conclusively decide on the nature of the burst.

In conclusion, neither $T_{90}$ nor the spectral hardness nor the spectral lag allow us to conclusively determine if GRB 180418A is drawn from the populations of SGRBs or LGRBs. Nevertheless, we constrain the redshift as $z > 0.3$ (see the Appendix) and $z < 1.3$ from the SED. For this specific case, the nondetection of a supernova associated with this event, at distances where telescopes do not have the sensibility to detect it, when there is no identification of a host galaxy cannot support nor deny the idea to eliminate the possibility of a merger of two compact objects or of the death of a massive star as the progenitor of GRB 180418A.

### 3.5. Physical Parameters

We can estimate some intrinsic parameters of the burst assuming a $\Lambda$CDM cosmology with a $H_0 = 67.8$ km Mpc$^{-1}$ s$^{-1}$ (Planck Collaboration et al. 2014). For a redshift value of $z < 1.3$ we obtained a luminosity distance of $\lesssim 9$ Gpc and $E_{\gamma,iso} \lesssim 3 \times 10^{51}$ erg, which is in the typical range for short GRBs (Berger 2014). The relation of optical luminosity for SGRBs at a rest-frame time of 7 hr ($L_{\text{opt,7}}$) as a function of $E_{\gamma,iso}$ according to Berger (2014) is given by:

$$L_{\text{opt,7}} \approx 6.9 \times 10^{42} E_{\gamma,\text{iso},51}^{0.74} \text{ erg s}^{-1}, \quad (1)$$

then $L_{\text{opt,7}} \lesssim 10^{43}$ erg s$^{-1}$, in agreement with the value reported in Table 4.

The multiwavelength afterglow is described by a simple power law decay for $t > 100$ s, and shows no evidence of a jet break. The relation between the jet break time and half opening angle $\theta_j$ is related with density, energy, time of break, and redshift as described in Sari et al. (1999):

$$\theta_j = 0.13 \left( \frac{t_j}{1+z} \right)^{3/8} \left( \frac{n}{E_{52}} \right)^{1/8}, \quad (2)$$

which, in the case of GRB 180418A, leads to a constraint of $\theta_j > 7^\circ$. Taking a $z = 0.5$ (the typical redshift of SGRBs; Berger 2014), we calculate also $\theta_j > 7^\circ$.

Within the framework discussed by Fraija et al. (2017), the afterglow parameters were calculated using the $\chi^2$ minimization within the ROOT software package (Brun & Rade-makers 1997) and following the model from Sari & Piran (1999), Fenimore et al. (1999), Soderberg & Ramirez-Ruiz (2002), Kobayashi & Zhang (2003), and Kobayashi et al. (2007).

All these values are reported in Table 5.

## 4. Discussion and Summary

We have presented a multiwavelength study of GRB 180418A. Thanks to the rapid response of TAROT, we were able to detect the optical counterpart as early as 28 s after the burst. We model the early and late light curves as emission from forward and reverse shocks in the standard fireball scenario. The reverse shock component peaks at $T + 35$ s at an observed magnitude of $r = 14.2$.

We calculated the afterglow parameters and constrained the jet collimation to $\theta_j > 7$ deg.

Optical emission from reverse shocks has been seen in many LGRBs (see Table 6), but never in SGRBs. Unfortunately, although GRB 180418A is consistent with an SGRB, it is also consistent with an LGRB with $1.0 < z < 1.3$. The absence of a spectroscopic redshift makes it impossible to exclude this possibility. Thus, GRB 180418A is simply the first candidate SGRB showing reverse-shock emission in the optical.

We analyzed all of the possible host galaxies for SGRB 180418A (Table 7). Nevertheless, we have not been able to identify the host galaxy of SGRB 180418A (see the Appendix). If deep imaging were to conclusively identify the host and if a spectroscopic redshift were obtained, this might resolve the ambiguity about the nature of the burst. This absence emphasizes the continuing need to obtain spectroscopy of bright GRBs.

We thank the referee for useful comments that helped to improve this article. We thank the staff of the Observatorio Astronómico Nacional on Sierra San Pedro Mártir. RATIR is a collaboration between the University of California, the Universidad Nacional Autónoma de México, NASA Goddard Space Flight Center, and Arizona State University, benefiting from the loan of an H2RG detector and hardware and software support from Teledyne Scientific and Imaging. RATIR, the automation of the Harold L. Johnson Telescope of the Observatorio Astronómico Nacional on Sierra San Pedro Mártir, and the operation of both are funded through NASA grants NNX09AH71G, NNX09AT02G, NNX10AI27G, and NNX12AE66G, CONACyT grants INFR-2009-01-122785 and CB-2008-101958, UNAM PAPIIT grants IG100414, IA102917 and IA102019, UC MEXUS-CONACyT grant CN 09-283, and the Instituto de Astronomía of the Universidad Nacional Autónoma de México. We acknowledge the vital contributions of Neil Gehrels and Leonid Georgiev to the early development of RATIR. TAROT has been built with the support of the Institut National des Sciences de l'Univers, CNRS, France. TAROT is funded by the CNES and is thankful for the help of the technical staff of the Observatoire de Haute Provence, OSU-Pytheas.
<figure><img src="image_9.png" /><figcaption>**Figure 7.** A deep RATIR *r* image of the neighborhood of GRB 180418A. Information on the detected galaxies is given in Table 7.</figcaption></figure>

## Appendix
Search for the GRB Host Galaxy

We obtained deep photometry of the field with RATIR on the fourth night, at about $T + 72$ hr. Our *r*-band image is shown in Figure 7 and photometry of sources is given in Table 1. The first column is the object label as shown in Figure 7, the second column is the SDSS DR12 ID, the third and fourth columns are the J2000 coordinates, the fifth and sixth columns are *r* and *i* AB magnitude, respectively, the seventh column is the angular separation $\theta$ in arcseconds, and the last column is the probability of a chance alignment $P(<\delta R)$ (Bloom et al. 2002).

We detect no source at the position of the GRB afterglow, thus any coincident host galaxy must be fainter than our $3\sigma$ limits of $r > 24.0$ and $i > 23.9$. Such a faint galaxy would be consistent with our limit $z < 1.3$ on the GRB redshift and, if compared to field galaxies (Berger et al. 2007), might suggest $z > 0.3$.

Fong et al. (2018) noted that there were no extended sources within $30''$ of the afterglow position down to a limit of $g > 23.4$ and $r > 22.5$. Thanks to our deeper photometry, we detect fainter sources, such as one at $r = 24.0$ and $i = 23.7$ at a separation of $9''$ from the afterglow. Our images have an FWHM of about $2''$, so we can say little about whether these faint sources are extended or not, although at these magnitudes we expect the majority to be galaxies (Yasuda et al. 2001).

The two galaxies with the smallest chance probabilities are G4 with $P(<\delta R) = 0.67$ and S1 with $P(<\delta R) = 0.69$. G4 appears in the SDSS DR12 catalog with a photometric redshift of $z = 0.80 \pm 0.12$ and an absolute magnitude of $M_r = -21.23$. If it were the host galaxy, the projected distance to the GRB would be 325 kpc. S1 is not in the SDSS DR12 catalog, but if it were at a typical SGRB redshift of 0.5 (Berger 2014), the projected offset to the GRB would be 137 kpc. Both of these projected distances are large compared to the samples of Troja et al. (2008) and Berger (2014), whose offsets range up to 75 kpc. Thus, based both on the large

chance probabilities $P(<\delta R)$ and the large offset distances, neither of these galaxies are likely to be the host of the GRB.

## ORCID iDs

R. L. Becerra https://orcid.org/0000-0002-0216-3415

E. Troja https://orcid.org/0000-0002-1869-7817

N. Fraija https://orcid.org/0000-0002-0173-6453

N. R. Butler https://orcid.org/0000-0002-9110-6673

P. Veres https://orcid.org/0000-0002-2149-9846

D. Turpin https://orcid.org/0000-0003-1835-1522

M. Boer https://orcid.org/0000-0001-9157-4349

J. J. González https://orcid.org/0000-0002-3724-1583

J. X. Prochaska https://orcid.org/0000-0002-7738-6875

E. Ramirez-Ruiz https://orcid.org/0000-0003-2558-3102

M. G. Richer https://orcid.org/0000-0003-4757-1153

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