TITLE:   GCN CIRCULAR
NUMBER:  28241
SUBJECT: GRB 200809B: Swift-XRT afterglow detection
DATE:    20/08/10 09:24:54 GMT
FROM:    Antonino D'Ai at IASF-PA  <antonino.dai@inaf.it>

M. Perri (ASDC), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), J.A. Kennea
(PSU), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), A.P. Beardmore
(U. Leicester), J.P. Osborne (U. Leicester) and P.A. Evans (U.
Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the GRB 200809B
(detected by Fermi/GBM, GCN 28236, Swift/BAT, GCN 28237, and MAXI/GSC,
GCN 28240), collecting 3.1 ks of Photon Counting (PC) mode data between
T0+14.0 ks and T0+41.4 ks. 

Two uncatalogued X-ray sources are detected, of which one ("Source 1")
is  believed to be the afterglow. Using 1323 s of PC mode data and 2
UVOT images, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 15.82953, -73.84557 which is equivalent to:

RA (J2000): 01h 03m 19.09s
Dec(J2000): -73d 50' 44.1"

with an uncertainty of 4.5 arcsec (radius, 90% confidence). This
position is 111 arcsec from the Swift/BAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.7 (+2.0, -1.2).

If the light curve continues to decay with a power-law decay index of
1.7, the count rate at T+24 hours will be 1.1 x 10^-3 count s^-1

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021026.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021026.

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