{
  "bibcode": "2010GCN.11501....1C",
  "body": "S. Campana (INAF-OAB), S. Covino (INAF-OAB),\nJ.L. Racusin (NASA/GSFC) and K. L. Page (U Leicester)\nreport on behalf of the Swift Team:\n\nGRB 101225A was detected by Swift BAT as an image trigger (Racusin\nal. 2010, GCN 11493). Swift XRT started observing 1383 s after the\ntrigger observing the main event. The afterglow is very bright and\nthe XRT remained in the Windowed Timing (WT) mode for two consecutive\norbits. Within each orbit the spectrum remain constant as judged from\nthe hardness ratio and from the power law photon index.\nWe extract the XRT spectra for each of the first two orbits, separately.\nWe then fit the two spectra within XSPEC using the model\ntbabs * ztbabs * (cutoff), keeping the same absorption pattern and leaving\nfree the cutoff power-law model for the two spectra (the cutoff power-law\nmodel provides much better results in terms of column density evaluation\nwith respect to a simple power-law model when small spectral variations\nare present).\nWe assume a Galactic column density of 7.9x10^20 cm-2 (Kalberla et al.\n2005, A&A 440 775) and we allow for a 30% uncertainty in the fit. We\nbinned the spectra to 50 counts per energy bin and used the latest (v12)\nresponse matrix. The detection in the UVW2 filter by UVOT (Siegel &\nRacusin 2010, GCN 11499) constrains the redshift to be <1.4.\nThe fit with the cutoff power-law is relatively good with a reduced\nchi^2 of 1.13 (508 dof).\nMotivated by the similarities with GRB 060218 (extremely long duration,\nsoft prompt-emission spectrum, bright smoothly decaying X-ray afterglow\nlight curve, UV detection with optical non-detection), we fit the X-ray\nspectrum including a blackbody component.\nThe fit improves considerably (and the cutoff energy goes well above the\nXRT energy band, becoming unimportant for both spectra).\nThe new fit provides a reduced chi^2 of 1.07 (504 dof), based on an F-test\nthe addition of the blackbody component improves the fit by 4.2 sigma.\nIn the intrinsic column density vs. redshift plane there is just one\ndeep minimum hinting for a redshift z=0.07^+0.13_-0.04 (90% confidence\nlevel) and an intrinsic column density N_H(z)=(1.5^+0.5_-0.2)x10^21 cm^-2.\nThe power law component shows a small softening from the first to the\nsecond orbit, with a photon index around 2.2-2.4.\nThe blackbody component is hot with a temperature of ~1 keV, consistent\namong the two observations. The blackbody radius decreases from the first\nobservation to the second from ~2x10^10 cm to ~0.6x10^10 cm.\n\nGround-based optical follow-up observations are strongly encouraged in order\nto get spectroscopic confirmation for the redshift estimate and, eventually,\nto monitor the coming out of a supernova.",
  "circularId": 11501,
  "createdOn": 1293467392000,
  "email": "sergio.campana@brera.inaf.it",
  "subject": "Tentative redshift of GRB101225A (Christmas's burst) from Swift-XRT data",
  "submitter": "Sergio Campana at INAF-OAB  <sergio.campana@brera.inaf.it>",
  "eventId": "GRB 101225A"
}