[vsnet-grb-info 8166] Swift XRT and UVOT refned analysis of GRB 090720

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9697
SUBJECT: Swift XRT and UVOT refned analysis of GRB 090720
DATE:    09/07/22 13:34:27 GMT
FROM:    Dirk Grupe at PSU/Swift-XRT  <grupe at astro.psu.edu>

D. Grupe (PSU), S. R Oates (MSSL-UCL), E. A. Hoversten (PSU), P.A.  
Evans and M.R. Goad (U. Leicester) report on behalf of the Swift team:


Swift XRT and UVOT started observing the field of GRB 090720 11 hours  
after the Fermi GBM  and Swift/BAT triggers (Rau et al. GCN circ 9688,  
Cummings et al. GCN circ 9694).

Using 1233 s of XRT Photon Counting mode data and one UVOT image, we  
find an astrometrically corrected X-ray position (using the XRT-UVOT  
alignment and matching UVOT field sources to the USNO-B1 catalogue):  
RA, Dec = 203.68020, -10.33480 which is equivalent to:

RA (J2000):  13 34 43.25
Dec (J2000): -10 20 05.3

with an uncertainty of 3.0 arcsec (radius, 90% confidence).

This position may be improved as more data are received.  The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, arXiv:0812.3662).

Swift has continued observing the field of GRB 090720 and the source  
is clearly fading. Currently it is at a level of less than 1.6e-3  
counts/s (or 5.0 e-14 ergs/s/cm2). Due to the low number of counts, no  
spectral analysis can be performed.

With the Swift/UVOT,  we do not detect any source at the Swift XRT  
position. The 3-sigma upper limits for the finding chart exposure is:

Filter   T_start(s)   T_stop(s)   Exp(s)  Mag (3-sigma upper limit)
-------------------------------------------------------------
white      41,776      63,952      579          > 21.28
u          39,573      63,855      3513         > 21.10
-------------------------------------------------------------

The quoted upper limits have not been corrected for the expected
Galactic extinction along the line of sight of E_(B-V) = 0.05 mag.
All photometry is on the UVOT photometric system described in Poole et
al. (2008, MNRAS, 383, 627).

This circular is an official product of the Swift team.