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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, 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
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
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.