TITLE: GCN CIRCULAR NUMBER: 7511 SUBJECT: GRB 080319B : Apparent spectral evolution in very early Swift/XRT WT mode data: intrinsic or pile-up effect? DATE: 08/03/24 22:35:33 GMT FROM: Binbin Zhang at UNLV Bin-Bin Zhang (University of Nevada Las Vegas), Enwei Liang (Guangxi University, China) and Bing Zhang (University of Nevada Las Vegas) report: We have processed the Swift XRT data of GRB 080319B, paying special attention to the possible spectral evolution in the WT mode data (Racusin et al. GCN 7459; cf. Butler GCN 7499). We perform a time-dependent spectral analysis using the method described in (Zhang, Liang & Zhang 2007, ApJ, 666, 1002). Since the early data are strongly affected by photon pile-up, we use a box annulus region for the WT mode data (outer radius 40*20, inner radius 8*20; see also Racusin et al. GCN 7459) and time-dependent circle annulus regions for the PC mode data to extract spectra and lightcurves. We fit the time-dependent spectra using a simple power-law model with the absorption from the MilkyWay Galaxy (NH_G=1.12e20 cm^{-2} ) and from the host galaxy (NH_host=7.3e20 cm^{-2}, obtained from fitting to the integrated 1st orbit WT mode spectrum). We confirm Butler (GCN 7499) that the apparent spectral evolution after 200 seconds is due to instrumental "pile up" effect. However, in the very early time t ~ (68-100) seconds, an apparent weak but significant hard-to-soft spectral evolution sustains even if we take into account the pile-up corrections. The photon index evolves from 1.67 ± 0.02 to 1.77 ± 0.02 during this period. Our results can be found at http://grb.physics.unlv.edu/~xrt/xrtweb/080319B/080319B.html. To make sure that this early-time spectral evolution is not due to the pile-up effect, we extract the time-dependent spectra with box annuli having different sizes. By excluding the central regions, we enlarge the outer radius up to 80 pixel * 20 pixel to make sure that there are enough photons for the spectral analysis. Our tests show that even if the inner box size is as large as 30 pixel * 20 pixel (spectra in annuli with such a large inner radius is not possible to be affected by the pile-up effect), the early time (before 200 seconds) XRT WT data still show significant spectral evolution. We therefore cautiously conclude that this early spectral evolution is likely intrinsic. Strong hard-to-soft spectral evolution has been seen in the early steep decay phase of many GRB X-ray afterglows (e.g. Zhang et al. 2007, 666, 1002), which points towards a non-forward-shock origin of the emission. We notice that the lightcurve before 200 seconds show several weak flaring/flicking features, which is more easily seen in linear scale (see http://grb.physics.unlv.edu/~xrt/xrtweb/080319B/earlylc.png). In view that some steep decay segments with overlapping flares typically show hard-to-soft spectral evolution (Group C in Zhang et al. 2007), we suspect that the weak spectral evolution in this burst is also related to the weak flaring/flicking features. It is however puzzling why this segment naturally transforms to a smooth decay after 200 seconds which show no further spectral evolution. Throughout our fit we have fixed the NH_host values. Another possibitly is that the apparent spectral evolution is caused by a varying NH_host value (Racusin et al. GCN 7459). We test such a scenario by fixing the photon index to \Gamma=1.76 (average value after 200s) and fit the time-dependent spectra before 200s using the same model (wabs*zbwas*zwabs) but allowing NH_host to be a free parameter. We obtain acceptable fits, and found that the NH_host in the early time evolves dramatically to one half of its initiall valve (from ~ 1.1e21 cm^{-2} to ~ 6.6e20 cm^{-2}). The time evolution of the NH_host value can be found at http://grb.physics.unlv.edu/~xrt/xrtweb/080319B/nh.png. This is another plausible physical scenario, although a model for the rapid depletion of NH_host is called for. The reduced chi2 in our fitting to the wabs*zwabs*powerlaw model is typically ~1. Although a possible thermal component has been suggested (cf. Racusin et al. GCN 7459), in our fitting no thermal component is required by the data. This message can be cited.