GRB 100413B

J. L. Racusin (NASA/GSFC), S. D. Barthelmy (GSFC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (NASA/GSFC), and 
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 08:42:41 UT on 2010-04-13, BAT detected a rate increase 
GRB 100413B (trigger=419380).  On-board analysis of two 
separate 1.024 second time intervals found sub-threshold peaks (~5 sigma)
at a consistent location in both images, but further on-board analysis was 
truncated by a preplanned slew. Subsequent ground analysis of the data 
improved our confidence in the detection and refined the location to 
RA, Dec = 356.818, +51.321 which is
 RA(J2000)  = 23h 47m 16s
 Dec(J2000) = +51d 19m 18s
with an uncertainty of of 3 arcmin of error (radius, 90% containment, 
including systematic uncertainty).  The BAT light curve shows a single peak 
rising to a peak count rate of ~1800 counts/sec (15-350 keV), at ~12 sec 
after the trigger, with the data truncated by the slew at ~18 sec after the trigger.

A Target of Opportunity observation has been made with the XRT and 
UVOT starting at 31.4 hours after the trigger.  Due to a telemetry backlog, 
data from these observations will not be available until tomorrow.

If this source is confirmed it will mark Swift's 500th GRB.

Burst Advocate for this burst is J. L. Racusin (judith.racusin AT nasa.gov). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

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
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 ), 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).