Strongly Variable $z=1.48$ FeII and MgII Absorption in the Spectra of $z=4.05$ GRB060206



Introduction
============

The optical spectrum of a gamma-ray burst (GRB) afterglow is generally a power-law with absorption lines superimposed. These absorption lines correspond to intervening material between the source and the observer. Typically, the highest-redshift lines are thought to be associated with the host galaxy [e.g., see discussion in @jha01], and thus provide a measurement of the redshift of the burst. There might be several lower-redshift absorption lines caused by intervening systems. Spectra of luminous GRB afterglows have opened a new window on absorption-line studies along lines of sight that are not associated with quasars [cf. @vreeswijk03].

Recent studies of GRB absorption spectra have produced an unexpected result. @prochter06 reported statistically significant evidence for a much higher incidence of strong absorbers along GRB sight lines than that expected from studies of quasars of comparable redshift. They considered and rejected three possible scenarios to produce such an effect, namely dust associated with the absorbers, ejection of by the GRB, and strong gravitational lensing by galaxies associated with the emission.

@frank06 proposed a simple geometric solution to the quandary, with resulting predictions that are readily testable. Namely, if the absorbers seen in both GRB and quasar spectra are truly intervening, then the Prochter et al. result could be caused by different sizes of the GRB and quasar emission regions. If the absorbers have physical sizes comparable to the emitting regions of quasars and GRBs, then a background source with a larger emission region would "see" a smaller effective absorption column density than a background source with a smaller emission region (see Frank et al. for details of their proposed scenario).

One immediate prediction of the Frank et al. scenario is that for strongly dynamic objects like GRB afterglows, the strength and structure of intervening absorption lines in GRB spectra should vary in some, if not all, cases. Motivated by this prediction, we have examined our existing, multi-epoch spectroscopic data of GRB060206 and have discovered such a behavior. In this paper, we provide the first observational evidence of this prediction. In fact, as far as we know this is the first report ever of clearly evolving intervening (i.e., non-intrinsic) ground state absorption lines seen in spectra of any cosmological object. There is evidence for variation in fine structure lines in the GRB host environment [cf. @vreeswijk06; @zavadsky06].

GRB060206 triggered <span>*Swift*</span>-BAT on 2006 Feb 6 at 04:46:53 UT [@morris2006]. A likely afterglow was identified by @fynbo06, who also determined that the afterglow was at high redshift ($z=4.048$). The evolution of a bright and complex afterglow was reported by several groups, and @wozniak06, @stanek06, and @monfardini06 have published detailed photometric data for this "anomalous" event. The burst also showed complex behavior in the X-rays as seen with <span>*Swift*</span>-XRT instrument, but the overall X-ray evolution follows that seen in the optical [@stanek06].

Spectroscopic Observations
==========================

The brightness of the optical transient (OT) associated with GRB060206 allowed us to take spectra for several hours after the burst. Spectra of the OT were obtained with the FAST spectrograph [@fabricant98] at the Cassegrain focus of the 1.5m Tillinghast telescope at the F. L. Whipple Observatory at 08:25, 09:02, 09:33, 10:05, 10:41, 11:37, 12:09 (all times UT). The OT was visible on the telescope acquisition camera and could be identified and placed on the spectrograph slit for the observations. Each spectrum was taken at or near the parallactic angle [@filippenko82]. The spectra were reduced in the standard manner with IRAF[^1] using the optimal extraction package of @horne86. Night-sky emission lines were removed by fitting a line to two sky regions on either side of the OT column-by-column; the value determined from the linear function at the position of the OT extraction was then subtracted. Wavelength calibration was provided with an HeNeAr lamp taken immediately after each OT spectrum. Minor adjustments to the wavelength solution were made based upon night-sky lines in the OT frames. There were clouds during some of the exposure, but the signal-to-noise ratio (S/N) is comparable for all the spectra. The resolution of each spectrum is $~6.2$ Å as measured from the FWHM of night-sky emission lines.

We flux-calibrated the spectra with spectrophotometric standard stars using our own software. Relative fluxes in our OT spectrum are accurate to $~4\%$ over the observed wavelength range. Following the arguments of @wade88 and @matheson00, telluric correction was accomplished by dividing the GRB spectra with a normalized spectrum of the flux standard, which was set to unity for regions unaffected by atmospheric features. For our spectra, telluric correction was only applied over the regions 6250-6360 Å and 6840-7410 Å. We scaled the strength of the absorption by the difference of the airmasses to the $0.6$ power to correct the saturated B band properly [@wade88]. Most of the correction is $~2\%$ or less, with a $~5\%$ correction in the 6250-6360 Å region and $~25\%$ in the strong region of the B band.

In the S/N weighted average spectrum, shown in Figure \[grb-all\], a damped Lyman-alpha absorption system (DLA) is present at $\lambda=6145$ Å. A number of strong and weak absorption metal line systems are superimposed on the continuum, identified as systems at $z=1.48$ and $z=4.05$ [reported by @fynbo06]. The very strong DLA system at $\lambda=6145$ Å indicates that the other absorbers at $z=4.05$ most likely represent the host galaxy of the GRB. In the intervening metal line system at $z=1.48$, our data show absorption from the transitions $\lambda$2600 and $\lambda\lambda$2796, 2803. These are fairly typical in absorption line systems [e.g., @steidel92]. Two absorption lines at $\lambda=6331.3$ Å and $6338.3$ Å represent a system at $z=3.54$, with a strong corresponding Ly$\alpha$ absorption line at $5525.9$ Å.

Spectroscopic Monitoring
========================

Motivated by the prediction of Frank et al. (2006) that the absorption lines along GRB sight lines should vary in some cases, we examined our time series of GRB 060206 spectra to look for this effect. Variation of the intervening $z=1.48$ and lines is obvious even upon casual inspection of the spectra, as shown in Figure \[compare\]. These lines are seen to vary in strength, profile, and possibly velocity. We have checked the original 2-D spectra and no cosmic ray features are apparent that would cause this variation. In addition, night-sky emission lines and telluric features were carefully removed with several variations in technique to ensure that reduction processes did not introduce the observed variability. In fact, there are no sky lines or telluric features at all near the strongly variable $z=1.48$ .

To measure the magnitude of variations in the equivalent widths (EWs) of the $z=1.48$ absorption lines, we fit each of the lines with multiple Gaussians and the line with a single Gaussian. The error of the EWs are calculated from the conservative fit uncertainties. The resulting EW measurements are listed in Table \[MgII\] and plotted in Figure \[MgIIf\]. The first three spectra show a gradual decrease in the EW of the $\lambda 2796$ line accompanied by a sharp decline in the EW of the weaker $\lambda 2803$ line. As a result, the ratio of the doublet increases from $1.2\!:\!1$ in the first spectrum to about $2\!:\!1$ in the third. This is followed in the fourth spectrum by a sharp rise in the EW of both components of the doublet and a sharp decrease in the doublet ratio to $1.3:1$. While the EW of the line also decreases throughout the first three spectra, it seems to completely disappear in the fourth spectrum. The two small features in this spectrum at the location of the feature are consistent with noise (see Figure \[compare\]). The absorption line gradually returns throughout the last three spectra, but it never regains its original strength. The only other spectral feature to show possible variability is the $\lambda 1393.76$ absorption line at the presumed redshift of the host galaxy. The variability in this line does not follow the behavior of either the or systems at $z = 1.48$. We are still investigating the nature and significance of variability in this line.

Variation in the EW of an absorption line is a direct consequence of variation in the column density of the absorber as seen by the background source. The and systems are not necessarily expected to be distributed homogeneously throughout an absorbing "cloud." As the beam size of the GRB increases due to its physical expansion, the beam could include various dense or dilute regions of the "cloud," leading to different behaviors in the measured EWs of the absorbers. Additionally, the and may not be co-spatial, so their behaviors may not be directly comparable as seen here.

To estimate the statistical significance of these observed variations, we apply $\chi^2$ statistics to the "null hypothesis" that the and lines are not variable. Fitting the multiple measurements of the EW with a constant value gives $\chi^2=38.8$ with six degrees of freedom. Such a value of $\chi^2$ results in a probability of $p=0.0000794\%$ ($\sim 5 \sigma$ Gaussian significance) that the line is not varying. A similar analysis for the $\lambda 2803$ line results in $\chi^2 = 14.0$, which gives $p = 2.93\%$ ($2.2 \sigma$ Gaussian significance) for lack of variation in the line EW. Finally, for the $\lambda 2796$ line, we find $\chi^2=7.4$, resulting in $p=28.9\%$ for the assumption that the EW does not vary. The combined data clearly show statistically significant evidence for variability. Note that while we only test here the significance of variations in the line EWs, it is also immediately apparent from the spectra that the line profiles are varying as well (see Figure \[compare\]). Each of these lines is likely composed of several components that are blended in our spectra, and it is variations in individual components that are causing the observed variations in the total EWs and line profiles.

To further investigate the variability of the lines, we use the spectral subtraction algorithm developed by Hartman & Stanek (2007, in preparation), which uses the entire spectrum in the wavelength range $6250\AA \sim 7200\AA$ to obtain the convolution kernel for matching individual observed spectra to the reference spectrum. We choose the first observed spectrum as the reference, then calculate the difference between the matched spectra and the observed spectra for the rest of the observed spectra, divided by the variance spectra we can calculate the significance of the difference. The shows a $4.9\sigma$ variation in the line region $(6439\AA\sim6450\AA)$ and the $\lambda 2796$ line shows a $2.5\sigma$ variation in the line region $(6926\AA\sim6942\AA)$.

Discussion
==========

Variable intervening and was predicted by @frank06 as a direct consequence of their geometric solution to the absorption over-abundance problem discovered by @prochter06. Our result immediately provides constraints on the physical size of the absorbers that are several orders of magnitude stronger than those determined from studies of lensed quasars — $\sim 0.01$ pc [@frank06] vs. $< 200-300$ pc [@rauch02; @petitjean00]. Observations of lensed quasars such as those proposed by @dong06 may provide additional constraints on the physical sizes of the absorbers. Significant spectral variability on the timescales of months to years induced by microlensing can be used to probe the $10^{14}-10^{16}\;$cm cloudlets along the sightlines of lensed quasars. Properties of the cloudlets (physical size, sub-structure, etc.) can be studied statistically if such spectral variations are seen.

We can also investigate the average physical density of the absorbers. Assuming the "cloud" is optically thin, we can obtain a lower limit on the column density $N_{\rm MgII}=
1.7\times10^{13}\;{\rm{cm}}^{-2}$. To evaluate this result, we have used our third spectrum taken $\sim 5\;$hours after the burst, where the line ratio is close to $2\!:\!1$, indicating the lines are not strongly saturated (see Table \[MgII\]). The beam size of the GRB during that epoch can be estimated [@waxman97; @loeb98] via $$R_s(t) = 4.1\times10^{15}\left(\frac{E_{52}}{n_1}\right)^{\frac 1
8}
         (1+z_s)^{-5/8}(t/hour)^{5/8}\; {\rm{cm}}$$ where $z_s$ is the source redshift, $E_{52}$ is the "isotropic-equivalent" of the energy release in units of $10^{52}~{\rm erg~s^{-1}}$, and $n_1$ is the ambient gas density in units of  $1~{\rm cm^{-3}}$. For GRB 060206, $E_{52}=5.8$ [@palmer06], and assuming $n_1 = 1\;{\rm{cm}}^{-3}$, then $R_s=5.1\times 10^{15}\;$cm for our third spectrum. With the physical size of the individual "clumps" constrained to be comparable to the GRB beam size (i.e., $\sim 10^{16}\;$cm), one can immediately derive the average physical density of atoms to be about $0.0015~\rm{cm}^{-3}$. This is the first time that such a strong constraint has been obtained. Given its large equivalent width, the z=1.48 absorber is likely a Lyman limit system with $\log{N(HI)} > 18$, although there is a $\sim 20-30$% probability that it is a damped Lyman-alpha system (Rao et al. 2006). Then the physical density of the hydrogen of this cloud is $>200~\rm{cm}^{-3}$. The estimate of physical density of absorbers is very important for understanding the cloud origins and properties, and more generally the metal lines and metal enrichment (see discussion in Schaye et al 2007).

Another consequence of this discovery relates to GRBs themselves. In the case of GRB060206, we see the EW initially decreasing, but then it shows a dramatic increase. This behavior is either a result of the GRB beam becoming much smaller in a short time (possibly due to an episode of energy injection) or part of the afterglow ring suddenly brightening. Our discovery points the way to a new method for studying GRB phenomena by monitoring afterglows with high-resolution optical spectroscopy.

Additional implications relate to chemical abundances for intervening systems as measured using absorption lines. In our first spectrum, the ratio of EWs for compared to is about 0.7. In our fourth spectrum, where the is only marginally present, and is very strong, that ratio is less than 0.1. Estimates of relative abundances for these two elements would be dramatically different when analyzing these two low-resolution spectra. If the system is at the lower end of this column density range$\log{N(HI)} >
18$, some of the difference in / ratio could be the result of spatially-varying ionization levels. However, it may be that there are intrinsic variations in the relative metal abundances (although such variations are not seen in damped Lyman-alpha systems: Prochaska 2003, Rodriguez et al 2006).

The observations presented in this paper open up the exciting possibility of studying time-variability of intervening absorption lines in the spectra of cosmological objects. Strongly and rapidly variable objects, such as GRBs and blazars, are obvious targets for further studies, but quasars (including both lensed and non-lensed) should also be investigated. Variable narrow absorption lines have been observed in spectra of quasars (e.g., Hamann et al. 1997), but they have been interpreted as originating in gas physically associated with the quasar. In fact, variability of narrow absorption lines is now used as one of the criteria to select intrinsic absorbers associated with quasars. Results presented in this paper suggest that using this criterion should be re-examined—quasars are optically variable sources, so size of their continuum emission region is bound to vary, just like it does for GRBs. As already discussed by Frank et al. (2006), quasars with smaller continuum emitting regions (i.e., intrinsically fainter ones), should have higher incidence of the strong absorbers.

Note that throughout this paper we considered the $z=1.48$ variable absorption system not to be physically associated with the $z=4.05$ GRB, i.e., to be truly intervening due to cosmological expansion. For these two objects to be physically associated would require and absorbing gas to have ejection velocity $v\sim 0.6\;$c, while maintaining relatively low temperature in order not to be completely ionized. We consider such scenario most unlikely. Detection of a galaxy associated with this particular $z=1.48$ variable absorption system would settle this issue beyond any reasonable doubt. As discussed in Prochter et al. (2006), such foreground galaxies were already identified for a number of strong, intervening absorbers seen in GRB spectra.

We thank J. Hartman, P. Martini, S. Gaudi, S. Mathur, R. Pogge, S. Frank, X. Dai, J. Prieto, D. An, M. Elvis, R. Kirshner, and R. Narayan for helpful comments and discussions. M. C. Bentz is supported by a National Science Foundation Graduate Fellowship.

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<span>llccc</span> 2.86384 & 4.13 & 2.53 $\pm$ 0.23 & 2.05 $\pm$ 0.25 & 1.72 $\pm$ 0.25\
2.88946 & 4.50 & 2.14 $\pm$ 0.25 & 1.50 $\pm$ 0.27 & 1.30 $\pm$ 0.15\
2.91110 & 5.02 & 2.06 $\pm$ 0.30 & 0.92 $\pm$ 0.33 & 0.60 $\pm$ 0.20\
2.93323 & 5.55 & 3.14 $\pm$ 0.44 & 2.47 $\pm$ 0.41 & 0.28 $\pm$ 0.21\
2.95857 & 6.15 & 2.24 $\pm$ 0.29 & 1.93 $\pm$ 0.34 & 0.36 $\pm$ 0.17\
2.99745 & 7.09 & 1.90 $\pm$ 0.44 & 2.34 $\pm$ 0.58 & 0.96 $\pm$ 0.29\
3.01990 & 7.63 & 1.96 $\pm$ 0.35 & 1.41 $\pm$ 0.42 & 0.67 $\pm$ 0.40

.

[^1]: IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.

{'source': 'arXiv_out/astro-ph0612409_extract_ms'}