GCN Circular 13012

T. Sakamoto (GSFC/UMBC), V. Mangano (INAF-IASF/Palermo),
A. Maselli (INAF-IASF/Palermo), M. H. Siegel (PSU), 
M. J. R. Cummings (GSFC/UMBC), P.A. Evans (U. Leicester), 
F. E. Marshall (NASA/GSFC), K.L. Page (U. Leicester) and 
B. Sbarufatti (INAF-OAB/IASFPA) on behalf of the Swift team:

We report further analysis of BAT ground detected GRB 120302A
(Sakamoto, et al., GCN Circ. 13000
Not found
 
 
).  By carefully examining the BAT image,
we found a position with a higher significance (9.2 sigma in the 14-200 keV image)
than that originally reported (Sakamoto, et al., GCN Circ. 13000
Not found
 
 
).  The best
BAT position is RA, Dec = 122.426, +29.642 deg which is
   RA(J2000)  =  08h 09m 42.2s
   Dec(J2000) = +29d 38' 31.2"
with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment).
The center of this position differs by 1.6 arcmin from the position reported
on the initial circular (Sakamoto, et al., GCN Circ. 13000
Not found
 
 
).

A 5 ksec Swift ToO observation was performed on 18:17 UT on March 2
(~16.4 hr after the burst).  The XRT observation was performed in PC mode.
Using 666 s of XRT Photon Counting mode data and 1 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; Evans et al. 2009):
RA, Dec = 122.39761, 29.62781 which is equivalent to:
   RA(J2000)  =  08h 09m 35.43s
   Dec(J2000) = +29d 37' 40.1"
with an uncertainty of 3.2 arcsec (radius, 90% confidence).  This XRT
refined position is consistent with an afterglow candidate reported by
GROND (Elliott et al., GCN Circ. 13003
Not found
 
 
).

The X-ray light curve shows a roughly constant count rate at the average
level of 1.2e-2 counts s^-1.  A fit with a power-law model cannot constrain
the slope.  A spectrum formed from the PC mode data can be fitted with an
absorbed power-law with a photon spectral index of 1.29 (+0.68, -0.35).  The best-fitting
absorption column is less than 1.9 x 10^21 cm^-2, in excess of the Galactic value
of 3.77 x 10^20 cm^-2 (Kalberla et al. 2005).  The observed (unabsorbed) 0.3-10 keV
flux is 6.1 (-2.1,+2.2) x 10^-13 (6.1 (-1.7, +2.3) x 10^-13) erg cm^-2 s^-1.
All the quoted erros are at the 90% confidence level.

The Swift/UVOT data were taken entirely in the u filter.  Using the first u 
data (T0+58.9 ksec, 1391 s exposure), we find a candidate optical afterglow at: 
RA, Dec = 122.39810, 29.62807 which is equivalent to:
    RA(J2000)  =  08h 09m 35.54s 
    DEC(J2000) = +29d 37' 41.1"  
with a 90%-confidence error radius of about 0.6".  The source has a u magnitude of
20.25+/-0.14 (1 sigma) on the system of Breeveld et al. 2011, AIP Conf. Proc. 1358, 373.
This position is consistent with the GROND position.  The magnitude is not corrected 
for the Galactic extinction due to the reddening of E(B-V) = 0.04 in the direction 
of the burst (Schlegel et al. 1998).

The 2nd epoch observation of Swift ToO has been approved.  This observation
will be scheduled on March 8.