GCN Circular 13641

B.-B. Zhang (PSU), S. R. Oates (UCL-MSSL),  K. L. Page (U Leicester), P.A.
Evans (U. Leicester), D. N. Burrows (PSU), J. R. Cummings (GSFC/UMBC),  J. A.
Kennea (PSU),H. A. Krimm (GSFC/USRA), E. Helder (PSU) and S. D. Barthelmy
(GSFC) report on behalf of the Swift-XRT and Swift-UVOT team: 

We report further Swift XRT-UVOT observations of GRB 120811A (Zhang et al,  GCN
13618
Not found
 
 
).

We performed 9.1 ks follow-up XRT observations of the field at T0+1.6 hr,
T0+0.27 day, T0+0.94 days and T0+3.1 days (where T0 = 2012 Aug 11 at 02:35:18
UT ), in order to determine whether or not the X-ray source had faded. We find
a fading source with count rates in the 0.3-10 keV band as follows :

T-T0                     Count Rate
(5.8+/-0.2)x10^3 s   0.05+/-0.01 count/s
(2.2+/-0.01)x10^4 s   0.02+/-0.01 count/s
(8.1+/-0.03)x10^4 s  0.02+/-0.001 count/s
(2.7+/-0.2)x10^5 s   < 0.005 count/s

The data are entirely in Photon Counting (PC) mode. Using 1227 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 =
257.16542, -22.71054 , which is equivalent to:

RA (J2000): 17h 08m 39.70s
Dec(J2000): -22d 42' 38.0"

with an uncertainty of 2.8 arcsec (radius, 90% confidence).

The light curve can be modelled with  a power-law decay with a decay index of
alpha=0.31 (+0.13, -0.12).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+/-0.5). The best-fitting
absorption column is  7.4 (+4.2, -3.0) x 10^21 cm^-2, in excess of the Galactic
value of 1.5 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is
5.5 x 10^-11 (9.7 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.4 (+4.2, -3.0) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^21 cm^-2
Excess significance: 3.2 sigma
Photon index:	     1.9 (+/-0.5)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00530581.

The fact that the XRT source is fading suggests that Swift trigger 530581
(Zhang et al., GCN Circ. 13618
Not found
 
 
) is most likely a GRB.

The Swift/UVOT began settled observations of the field of GRB 120811A 5627 s
after the BAT trigger (Zhang  et al., GCN Circ. 13618
Not found
 
 
). No optical afterglow
consistent with the XRT position is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system(Breeveld et
al. 2011,AIP Conf. Proc. 1358, 373) for the initial exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)    Mag

white           6243        6443         197        > 20.7    
v               6654        6853         197        > 19.1    
b               6038        7593         313        > 20.2    
u               5832        7468         393        > 20.0     
uvw1            5627        7263         393        > 19.9
uvw2            6449        6649         197        > 19.8    

The magnitudes in the table are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 0.77 in the direction of the burst (Schlegel et
al. 1998).

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