GCN Circular 4786

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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
Not found
 
 
, 3
Not found
 
 
.4 arcsec from 
the XRT position given in GCN 4781
Not found
 
 
 and 0
Not found
 
 
.8 arcsec from the UVOT source 
given in GCN 4779
Not found
 
 
.

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.