[vsnet-grb-info 2269] GRB 051016B: refined analysis of the Swift/XRT data

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4108
SUBJECT: GRB 051016B: refined analysis of the Swift/XRT data
DATE:    05/10/17 16:26:18 GMT
FROM:    Olivier Godet at U.of Leicester  <og19 at star.le.ac.uk>

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). 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.