GCN Circular 10602

J. L. Racusin (NASA/GSFC), S. D. Barthelmy (NASA/GSFC), J. Cummings  
(GSFC/UMBC), P. A. Evans (U. Leicester), N. Gehrels (NASA/GSFC), W. B.  
Landsman (NASA/GSFC), O. M. Littlejohns (U. Leicester), F. .E.  
Marshall (NASA/GSFC), K. L. Page (U. Leicester), D. Palmer (LANL)  
report on behalf of the Swift team:

After further analysis of the BAT ground detected GRB 100413B (Racusin  
et al., GCN 10593
Not found
 
 
), we can now confirm that it is a real burst.  It  
was detected at the level of 12.4 sigma in 10 seconds of data.  The  
light curve shows a peak beginning at ~T-7 sec, and lasting until at  
least T+18 sec, when the spacecraft slewed away from the burst due to  
a pre-planned target.

Our best position is the BAT ground calculated RA/Dec = 356.8264,  
+51.2704 deg, which is
    RA(J2000) = 23h 47m 18.3s
    Dec(J2000) = +51d 16' 13.4"
with an uncertainty of 2 arcmin (radius, 90% containment, including  
systematic uncertainty).

The time-averaged spectrum from T-2 to T+8 sec is best fit by a power- 
law with an exponential cutoff.  The fit gives a power law index of  
-0.5+/-0.4, and an Epeak of 28+/-6 keV.  The fluence in the 15-150 keV  
band is (4.3 +/- 0.4) x10^-7 ergs cm-2.  All the quoted errors are at  
the 90% confidence level.

GRB 100413B was also detected by Konus-Wind in the waiting mode (V.  
Pal'shin, private communication).

A Target of Opportunity observation began ~31.5 hours after the BAT  
trigger.  In 16.5 ks of XRT data, no X-ray afterglow was detected down  
to a limit of 1.6 x 10^-3 counts s-1 (3-sigma).  Assuming an average  
observed flux to counts conversion factor of 3.8 x 10^-11 (Evans et  
al., 2009, MNRAS, 397, 1177), this corresponds to a flux limit of 6 x  
10^-14 erg cm-2 s-1.  Given the late start to follow-up observations  
and exposure time, the lack of a X-ray counterpart is not surprising.

Preliminary 3-sigma upper limits using the UVOT photometric system  
(Poole et al. 2008, MNRAS, 383, 627) from a 2538 sec u band image  
taken 31.4 hours after the burst shows no new source within the BAT  
error circle with a limit of u > 21.4.  This value is not corrected  
for the Galactic extinction due to the reddening of E(B-V) = 0.19 in  
the direction of the burst (Schlegel et al. 1998).