K.L. Page (U Leicester), N. Gehrels (NASA/GSFC) and T. Sakamoto
(NASA/UMBC) report on behalf of the Swift-XRT team:
We have analysed three orbits of Swift-XRT data obtained for GRB 071021
(Sakamoto et al. GCN Circ. 6958), totalling 1.7 ks of Windowed Timing (WT)
data and 5.3 ks of Photon Counting (PC) data. Using the PC data we derive
a refined position of RA, Dec = 340.64311, +23.71808, which is equivalent
RA(J2000) = 22 42 34.35
Dec(J2000) = +23 43 05.1
with an estimated error radius of 3.9 arcsec (90 percent containment).
This is 4.8 arcsec from the onboard XRT position given by Sakamoto et al.
in GCN Circ. 6958). The position is also 15.1 arcsec from the Faulkes
Telescope South optical candidate (Guidorzi et al., GCN Circ. 6959) and
therefore not consistent with it.
The X-ray light-curve is initially quite flat, remaining at around 40-50
count s^-1 until 230 seconds after the trigger. There follows a very steep
decay, until about 700 seconds post-trigger, when a series of strong
flares are seen; this behaviour continues into the second orbit, with the
third orbit showing the afterglow emission has decayed further.
There is spectral evolution during the initial light-curve, although the
WT data before the steep decay (135-245 seconds) can be modelled as an
absorbed power-law, with Gamma = 2.12 +/- 0.09 and a total absorbing
column of NH = (1.8 +/- 0.2)e21 cm^-2, considerably in excess of the
Galactic value of 4.8e20 cm^-2. The 0.3-10 keV observed (unabsorbed) flux
during this time is 1.3e-10 (1.9e-10) erg cm^-2 s^-1.
Because of the flaring activity, it is not feasible accurately to predict
a count-rate at 24 hours.
This circular is an official product of the Swift-XRT team.