{
  "bibcode": "2010GCN.11457....1C",
  "body": "S. Campana (INAF-OAB), S. Covino (INAF-OAB), A. Cucchiara (UCB/LBNL/UCSC),\nP. A. Evans  (U Leicester), D. Grupe (PSU), D. Malesani (DARK/NBI),\nT. Sakamoto (NASA/UMBC) and R. L. C. Starling (U Leicester):\n\nSwift BAT triggered on the early activity of GRB 101213A (Sakamoto et\nal. 2010, GCN 11448). Swift XRT started observing 106 s after the\ntrigger observing the main event. We selected the time interval between\n112-154 s during which the hardness ratio (as judged from the Swift/XRT\nspectrum repository at http://www.swift.ac.uk/xrt_spectra/00440285/ )\nand the power law photon index (as judged from the Swift/XRT burst analyser\nhttp://www.swift.ac.uk/burst_analyser/00440285/ ) are consistent\nwith a constant. We bin the spectrum to 30 counts per energy bin and\nused the latest response matrices v012. The spectrum is in Windowed Timing\nmode (WT) and contains about 6,000 counts. We fit the WT spectrum with \nXSPEC\nusing the model tbabs*ztbabs*(cutoff), (the cutoff power-law model provides\nmuch better results in terms of column density evaluation with respect to a\nsimple power-law model when small spectral variations are present).\nWe assume a Galactic column density of 4.6x10^20 cm^-2 (Kalberla et al.\n2005, A&A 440 775). In the intrinsic column density vs.\nredshift plane there is just one deep minimum hinting for a redshift\nz=2.9^+0.2_-0.8 (90% confidence level) and an intrinsic column density\nN_H(z)=(1.5^+0.7_-0.3)x10^23 cm^-2. The X-ray spectrum is soft with\nGamma~2.5 at the end of the prompt phase and no signs of spectral curvature\n(cut-off energy > 30 keV).\n\nThe absorption pattern towards GRB 101213A is complicated by the galaxy\nreported by Malesani & de Ugarte Postigo (2010, GCN 11449). We tried to fit\nan extra absorption component fixed either to z=0.1 or z=0.5 (i.e. the two\npossible photometric redshifts of the putative host galaxy observed in the\nSDSS, z=0.10+/-0.07 and z=0.47+/-0.22, respectively), with basically no\neffects on the overall results. This low-redshift component is not able\nalone to account for the observed absorption pattern.\n\nLooking at the burst analyser page (see above), we also note that there is\na mismatch in the BAT and XRT power law photon indices during the short\ninterval during which there are contemporanous data. Motivated by this we\nfit the XRT spectrum with a broken power law.\nThe fit is as good as the power law fit. In this case we obtain a very soft\nlow-energy photon index of Gamma1=3.1+/-0.2 and a high-energy photon index\nGamma2=2.0+/-0.2 (consistent with the BAT extrapolation). The transition\nenergy is around 3 keV. In this case, the redshift is z=0.3^+0.4_-0.1 \nand the\nintrinsic column density is (1.1^+0.3_-0.2)x10^22 cm^-2. In this case the\nredshift determination is consistent with the putative host galaxy but the\nresulting X-ray spectrum is peculiar, with a spectral variation Delta \nGamma~1\nand a spectrum that remains (almost) constant for ~150 s, becoming even\nsofter later on.\nAlternatively, the X-ray spectrum could be fit (equally well) with a black\nbody plus a power law component. In this case the redshift is \nz=0.3^+0.1_-0.2\nand the intrinsic column density NH=(1.1^+0.3_-0.2)x10^22 cm^-2. The power\nlaw is very soft with Gamma~3.3 and the black body (rest-frame) temperature\nis high kT=2.3^+1.3_-0.5 keV. The black body radius is ~2x10^11 cm.\nWe note that the black body temperature is much higher than what is observed\nin, e.g., GRB 060218 (kT~0.2 keV, Campana et al. 2006, Nat 442 1008) or\nGRB 100316D (kT~0.1 keV, Starling et al. 2010, MNRAS in press).\n\nWe can conclude that in all cases, the X-ray afterglow flux is heavily\nabsorbed and this is in line with the non-detection with UVOT.\nGiven the presence of a candidate host galaxy within the XRT refined error\ncircle (Malesani & de Ugarte Postigo 2010), we cannot rule out a low\nredshift solution, but in this case a peculiar X-ray spectrum is observed.",
  "circularId": 11457,
  "createdOn": 1292421347000,
  "email": "sergio.campana@brera.inaf.it",
  "subject": "Redshift solution(s) for GRB101213A from Swift-XRT data",
  "submitter": "Sergio Campana at INAF-OAB  <sergio.campana@brera.inaf.it>",
  "eventId": "GRB 101213A"
}