TITLE: GCN CIRCULAR NUMBER: 11457 SUBJECT: Redshift solution(s) for GRB101213A from Swift-XRT data DATE: 10/12/15 13:55:47 GMT FROM: Sergio Campana at INAF-OAB S. Campana (INAF-OAB), S. Covino (INAF-OAB), A. Cucchiara (UCB/LBNL/UCSC), P. A. Evans (U Leicester), D. Grupe (PSU), D. Malesani (DARK/NBI), T. Sakamoto (NASA/UMBC) and R. L. C. Starling (U Leicester): Swift BAT triggered on the early activity of GRB 101213A (Sakamoto et al. 2010, GCN 11448). Swift XRT started observing 106 s after the trigger observing the main event. We selected the time interval between 112-154 s during which the hardness ratio (as judged from the Swift/XRT spectrum repository at http://www.swift.ac.uk/xrt_spectra/00440285/ ) and the power law photon index (as judged from the Swift/XRT burst analyser http://www.swift.ac.uk/burst_analyser/00440285/ ) are consistent with a constant. We bin the spectrum to 30 counts per energy bin and used the latest response matrices v012. The spectrum is in Windowed Timing mode (WT) and contains about 6,000 counts. We fit the WT spectrum with XSPEC using the model tbabs*ztbabs*(cutoff), (the cutoff power-law model provides much better results in terms of column density evaluation with respect to a simple power-law model when small spectral variations are present). We assume a Galactic column density of 4.6x10^20 cm^-2 (Kalberla et al. 2005, A&A 440 775). In the intrinsic column density vs. redshift plane there is just one deep minimum hinting for a redshift z=2.9^+0.2_-0.8 (90% confidence level) and an intrinsic column density N_H(z)=(1.5^+0.7_-0.3)x10^23 cm^-2. The X-ray spectrum is soft with Gamma~2.5 at the end of the prompt phase and no signs of spectral curvature (cut-off energy > 30 keV). The absorption pattern towards GRB 101213A is complicated by the galaxy reported by Malesani & de Ugarte Postigo (2010, GCN 11449). We tried to fit an extra absorption component fixed either to z=0.1 or z=0.5 (i.e. the two possible photometric redshifts of the putative host galaxy observed in the SDSS, z=0.10+/-0.07 and z=0.47+/-0.22, respectively), with basically no effects on the overall results. This low-redshift component is not able alone to account for the observed absorption pattern. Looking at the burst analyser page (see above), we also note that there is a mismatch in the BAT and XRT power law photon indices during the short interval during which there are contemporanous data. Motivated by this we fit the XRT spectrum with a broken power law. The fit is as good as the power law fit. In this case we obtain a very soft low-energy photon index of Gamma1=3.1+/-0.2 and a high-energy photon index Gamma2=2.0+/-0.2 (consistent with the BAT extrapolation). The transition energy is around 3 keV. In this case, the redshift is z=0.3^+0.4_-0.1 and the intrinsic column density is (1.1^+0.3_-0.2)x10^22 cm^-2. In this case the redshift determination is consistent with the putative host galaxy but the resulting X-ray spectrum is peculiar, with a spectral variation Delta Gamma~1 and a spectrum that remains (almost) constant for ~150 s, becoming even softer later on. Alternatively, the X-ray spectrum could be fit (equally well) with a black body plus a power law component. In this case the redshift is z=0.3^+0.1_-0.2 and the intrinsic column density NH=(1.1^+0.3_-0.2)x10^22 cm^-2. The power law is very soft with Gamma~3.3 and the black body (rest-frame) temperature is high kT=2.3^+1.3_-0.5 keV. The black body radius is ~2x10^11 cm. We note that the black body temperature is much higher than what is observed in, e.g., GRB 060218 (kT~0.2 keV, Campana et al. 2006, Nat 442 1008) or GRB 100316D (kT~0.1 keV, Starling et al. 2010, MNRAS in press). We can conclude that in all cases, the X-ray afterglow flux is heavily absorbed and this is in line with the non-detection with UVOT. Given the presence of a candidate host galaxy within the XRT refined error circle (Malesani & de Ugarte Postigo 2010), we cannot rule out a low redshift solution, but in this case a peculiar X-ray spectrum is observed.