GCN Circular 4108

O. Godet (U. Leicester), A. Parsons (GSFC) K. Page, J. Osborne, A.
Beardmore (U. Leicester), D. Burrows (PSU), N. Gehrels (GSFC) report on
behalf the Swift XRT team:

We have analysed the first four orbits of data from the Swift XRT
observation of GRB051016B (Parsons et al., GCN 4103
Not found
 
 
). The refined ground
processed coordinates are :
 RA(J2000)  =  08h 48m 27.5s
 Dec(J2000) = +13d 39m 20.8s
with an estimated uncertainty of 8 arcsecs radius (90% containment).
This is 4.9 arcsecs away from the XRT position reported in Parsons et
al.

The XRT light curve shows a steep initial decline with a decay slope
of 5.3+0.6/-0.7, followed by a break at ~T+ 200s to a shallower decay
slope with a flare at ~T+409s superposed. At ~T+6000s, another break can
be seen in the X-ray light-curve with a steeper decay slope of
0.9+/-0.2.

The X-ray spectra show strong evolution in the early phase.  The
Windowed Timing (WT) data from the first orbit (from T+82s to T+107s)
can be fit with an absorbed power-law with a photon index of
6.2+2.4/-1.5 and a column density excess of 4.8+/-2.0 E21 cm-2 over
the Galactic column density of 3.66E+20 cm-2 in this direction. The
Photon Counting (PC) spectrum from the first orbit (containing the
flare) can be fit with an absorbed power-law with a photon index of
1.8+0.2/-0.4 and no absorption excess over the Galactic absorption. The
PC spectrum of the second, third and fourth orbits can be fit by an
absorbed power-law with a photon index of 1.5+/-0.1 and no absorption
excess over the Galactic absorption.

The observed 0.2-10 keV flux for the PC data from the second, third
and fourth orbits (exposure time ~7600s, starting at ~T+3950s) was
3.82E-12 ergs cm**-2 s**-1, corresponding to an unabsorbed flux of
4.15E-12 ergs cm**-2 s**-1.

Assuming the X-ray emission from the GRB continues to decline at the
same rate (alpha~0.94), the predicted XRT count rate at T+24hrs is
0.0077 counts/s, which  corresponds to an observed 0.2-10 keV flux of
4.2e-13 ergs cm**-2 s**-1.