TITLE: GCN CIRCULAR NUMBER: 4786 SUBJECT: GRB060218: Swift/XRT Team preliminary analysis of the possible burst DATE: 06/02/19 13:54:11 GMT FROM: Giancarlo Cusumano at INAF-IASFA Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit G. Cusumano (IASF-Pa INAF), A. Moretti (INAF-OAB), G. Tagliaferri (INAF-OAB), J. Kennea (PSU) and D. Burrows (PSU), report on behalf of the Swift/XRT Team: We analised the first 14 orbits of data (the first orbit is fully in Windowed Timing (WT) mode, the second is in WT mode for the first 84 seconds, the rest of the data are all in Photon Counting (PC) mode). The following refined position for the X-ray afterglow was determined: RA(J2000) = 03h 21m 39.7s Dec(J2000) = +16d 52' 01.8" with an estimated uncertainty of 3.6 arcsec (90% containment). This is 3.0 arcmin from the BAT position reported in GCN 4780, 3.4 arcsec from the XRT position given in GCN 4781 and 0.8 arcsec from the UVOT source given in GCN 4779. The WT light curve starts 159 s after the BAT trigger with a count rate that rises from about 40 counts/s up to about 110 counts/s. The peak occurs around 990 s after the trigger, then the count rate decays and at 2770 s after the trigger, when the first observation orbit ends, the count rate is about 50 counts/s. At the beginning of the second orbit (5944 s post-trigger) the count rate has faded to a value of about 1.1 cts/s and decays very fast. The light curve after the first orbit is modelled by a broken power law characterized by a decay index of 6.38 � 0.05 up to 9250 � 270 s post trigger, followed by an intensity decrease with a decay index of 1.15+/- 0.15. The light curve can be seen at http://www.merate.mi.astro.it/~moretti/lc_060218.gif The spectrum obtained for the first orbit of data (159 - 2770 s post-trigger) can be fitted with a power-law of photon index Gamma = 1.82 � 0.01, with an absorbing column density of (1.9 � 0.01)e21 cm^-2 in excess of the Galactic column of 1.1e21 cm^-2. Over this time span, the mean 0.2-10 keV observed (unabsorbed) flux is 3.6e-9 (5.6e-9) erg cm^-2 s^-1. The spectrum obtained in the following orbits can be fitted with a power-law of photon index Gamma = 3.3 � 0.2, with an excess of absorbing column density of (3.2 � 0.4)e21 cm^-2. The mean 0.2-10 keV observed (unabsorbed) flux is 1.5e-12 (1.4e-11) erg cm^-2 s^-1. If the burst continues to decay at the current rate we estimate an XRT count rate of 2.5e-3 counts/s at T+48hr, which corresponds to an unabsorbed 0.2-10 keV flux of 7.6e-13 erg cm^-2 s^-1. The long, slow flux increase and gradual decrease are unlike any previous GRB prompt or afterglow emission seen by the XRT. Combined with the unusual spectral evolution, this suggests that this source may be an X-ray transient rather than a GRB. This circular is an official product of the Swift XRT Team.