TITLE:   GCN CIRCULAR
NUMBER:  8454
SUBJECT: GRB 081024B: new Swift-XRT observations
DATE:    08/10/31 19:06:29 GMT
FROM:    Cristiano Guidorzi at INAF-OAB  <cristiano.guidorzi@brera.inaf.it>

C. Guidorzi, J. Mao, R. Margutti (INAF-OAB) on behalf
of the Swift-XRT team report:

Swift-XRT re-observed the field of the Fermi-LAT and Fermi-GBM
GRB 081024B (Omodei et al. GCN Circ. 8407; Connaughton et al GCN Circ. 8408)
from 2.75 to 6.1 days after the burst with a net exposure of 13 ks.
We combined this with our previous observation (Guidorzi et al. GCN Circ.
8416) for a resulting net exposure of 27 ks.
Source 2 identified in our earliest report (Guidorzi et al. GCN Circ. 8410)
as the only possibly fading source within the LAT error circle (GCN 8416)
and possibly associated with the variable optical object O2
(Cenko & Kasliwal GCN Circ. 8417; Kann et al. GCN Circ. 8423),
was finally detected with an average rate of (7.4 +- 2.2)e-4 counts/s
from 1.5 to 6.1 days. From the comparison with the first observation
(GCN 8410), in which the count rate was estimated to be (1.58 +- 0.54)e-3
counts/s, we estimate the probability of a constant rate source of 12%.
We also derived an upper limit to the possible power-law decay
index of alpha<0.8 from ~1 to ~5e5 s (90% confidence).
Therefore, at the moment we cannot confirm source 2 as the X-ray
afterglow candidate.

Further Swift-XRT observations are planned in order to clarify this issue.

This circular is an official product of the Swift-XRT team.