The origin of the early-time optical emission of Swift GRB 080310★

Abstract

We present broad‐band multiwavelength observations of GRB 080310 at redshift z= 2.43. This burst was bright and long‐lived, and unusual in having extensive optical and near‐infrared (IR) follow‐up during the prompt phase. Using these data we attempt to simultaneously model the gamma‐ray, X‐ray, optical and IR emission using a series of prompt pulses and an afterglow component. Initial attempts to extrapolate the high‐energy model directly to lower energies for each pulse reveal that a spectral break is required between the optical regime and 0.3 keV to avoid overpredicting the optical flux. We demonstrate that afterglow emission alone is insufficient to describe all morphology seen in the optical and IR data. Allowing the prompt component to dominate the early‐time optical and IR and permitting each pulse to have an independent low‐energy spectral indices we produce an alternative scenario which better describes the optical light curve. This, however, does not describe the spectral shape of GRB 080310 at early times. The fit statistics for the prompt‐ and afterglow‐dominated models are nearly identical making it difficult to favour either. However one enduring result is that both models require a low‐energy spectral index consistent with self‐absorption for at least some of the pulses identified in the high‐energy emission model.

Time-dependent excitation and ionization modelling of absorption-line variability due to GRB 080310

“…This V-band limit is in agreement with the Swift V-band measurement of F V = 247 ± 140 µJy at the same epoch, but our R-band limit is well below (almost a factor of three) the prompt-emission fit featured in Fig. 10 of Littlejohns et al (2012). Instead, it is fully consistent with the alternative afterglow fit shown in their Fig.…”

“…The resulting slope values do not show any trend in time, and cluster around the value β = −1.0, with a standard deviation of 0.4 and an error in the mean of 0.07. The RAPTOR-T measurements are generally consistent with those reported in Littlejohns et al (2012). The RAPTOR V-and R-band magnitude limits (3σ), at a mid-exposure time of 84 s after the burst, correspond to F V < 136 µJy and F R < 78 µJy, respectively.…”

“…Following a rapid increase in brightness to a peak value at R 16.6 mag, the optical emission fluctuates by a few tenths of a magnitude and begins a slow decline after ∼30 min. While the BAT light curve still shows a detectable gamma-ray emission between 150 and 320 s after the trigger (see Littlejohns et al 2012), the optical emission over this time interval appears uncorrelated with the gamma rays. The optical light curves from RAPTOR-T show no significant colour evolution.…”

“…The R-band light curve is interpolated in log space to obtain the brightness at any given time to be used in the model calculations. To determine the flux at different frequencies, we initially adopt a value for the spectral slope of β = −0.75 (the same as that adopted by Littlejohns et al 2012). We note that β represents the intrinsic spectral slope, i.e.…”

“…X-ray photons photoionize species such as Fe , Si  and Fe  mainly via the ejection of inner-shell electrons. Since the X-ray flux for GRB 080310 is not a simple extrapolation of the optical/UV flux with the optical spectral slope (see Littlejohns et al 2012), we include the X-ray light curve as measured by the Swift XRT. We retrieved the 0.3−10 keV XRT afterglow light curve in count rate from the Swift repository (Evans et al 2009) and separated it in eight different time intervals (0-141, 141-185, 185-269, 269−393, 393−545, 545-615, 615-796, 796-7261 s after the trigger) in order to limit the possible X-ray spectral evolution in each single isolated light curve track.…”