[vsnet-grb-info 9851] GRB 101008A: Swift/XRT Detection of an Afterglow with no UVOT Counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  11319
SUBJECT: GRB 101008A: Swift/XRT Detection of an Afterglow with no UVOT Counterpart
DATE:    10/10/08 18:34:31 GMT
FROM:    Stephen Holland at USRA/NASA/GSFC/SSC  <Stephen.T.Holland at nasa.gov>

W. H. Baumgartner (GSFC/UMBC),
P. A. Evans (U Leicester),
S. T. Holland (CRESST/GSFC/USRA),
N. P. M. Kuin (UCL-MSSL),
C. Pagani (U Leicester),
K. L. Page (U Leicester), and
M. H. Siegel (PSU)
report on the behalf of the Swift UVOT team:

    The XRT began observing the field of GRB 101008A (Baumgartner et al. 2010,
GCNC 11318) at 17:35:56.9 UT, 3161.7 seconds after the BAT trigger.  Using promptly
downlinked data we find an uncatalogued X-ray source located at RA, Dec 328.87463,
37.06580 which is equivalent to:

    RA(J2000) =  21h 55m 29.91s
   Dec(J2000) = +37d 03' 56.9"

with an uncertainty of 3.9 arcseconds (radius, 90% containment).  This
location is 48 arcseconds from the BAT onboard position, within the BAT
error circle.  This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper.  We
cannot determine whether the source is fading at the present time.

    A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.44 x
10^21 cm^-2, Kalberla et al. 2005), with an excess column of 3
(+4.38/-2.94) x 10^21 cm^-2 (90% confidence).

    UVOT took a finding chart exposure of 150 seconds with the white filter
starting 3167 seconds after the BAT trigger.  No credible afterglow candidate
has been found in the initial data products.  The 2.7' x 2.7' sub-image covers
100% of the XRT error circle.  The preliminary 3-sigma upper limit in the white
filter is 20.3 mag.  No correction has been made for the expected Galactic
extinction corresponding to a reddening of E_{B-V} =  0.19 mag.  Photometry is
on the UVOT photometry system of Poole et al. (2008).