GRB 070923

M. C. Stroh (PSU), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
N. Gehrels (NASA/GSFC), D. Grupe (PSU), H. A. Krimm (GSFC/USRA),
D. M. Palmer (LANL), A. M. Parsons (GSFC), J. L. Racusin (PSU),
R. L. C. Starling (U Leicester) and T. N. Ukwatta (GSFC/GWU) report
on behalf of the Swift Team:

At 19:15:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070923 (trigger=292004). 
The BAT on-board calculated location is 
RA, Dec 184.626, -38.268 which is 
   RA(J2000)  =  12h 18m 30s
   Dec(J2000) = -38d 16' 03"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single spike
with a duration of about 100 msec.  The peak count rate
was ~8000 counts/sec (15-350 keV), at ~0.0 sec after the trigger. 

Because Swift is in the process of returning to normal operations, 
automatic slewing to GRBs is currently disabled.  Therefore, there 
are no prompt XRT or UVOT observations of this burst.  This burst is
38 deg from the Sun and so is too close for later XRT observations. 

Burst Advocate for this burst is M. C. Stroh (stroh AT astroh.org). 
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.)

D.B. Fox (Penn State) and E. Ofek (Caltech) report:

"We note the presence of two 2MASS-cataloged galaxies within the BAT
localization region for the short-hard burst GRB 070923 (Stroh et al.,
GCN 6818
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 ).  The galaxy coordinates, distances (D) from the center of
the BAT error circle in arcmin, and total broadband magnitudes (J, H,
Ks) as given by the NASA/IPAC Extragalactic Database* are:

            RA (J2000)   Dec (J2000)   D      J      H     Ks
   =================================================================
   2MASS-A  12:18:29.35  -38:16:44.1  0.7   15.01  14.77  13.73
   2MASS-B  12:18:21.87  -38:18:24.0  2.8   15.35  14.44  13.88
   =================================================================

Although neither galaxy has a measured redshift, the somewhat more
distant galaxy 2MASX J12180474-3818108 (D=5.4 arcmin) and 2MASX
J12175556-3818208 (D=7.2 arcmin) both have z~0.076 according to the
6dF redshift survey (Jones et al. 2004)."


*This research has made use of the NASA/IPAC Extragalactic Database
(NED) which is operated by the Jet Propulsion Laboratory, California
Institute of Technology, under contract with the National Aeronautics
and Space Administration.

J. Cummings (GSFC/UMBC), L. Barbier (GSFC), S. D. Barthelmy (GSFC),
E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC),
T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU),
M. C. Stroh (PSU), J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. the Swift-BAT team):

Using the data set from T-240 to T+230 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 070923 (trigger #292004)
(Stroh, et al., GCN Circ. 6818
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 ).  The BAT ground-calculated position is
RA, Dec = 184.623, -38.294 deg which is
    RA(J2000)  = 12h 18m 29.5s
    Dec(J2000) = -38d 17' 38"
with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 75%.

The mask weighted light curve shows a single peak of duration ~50 msec.
T90 (15-350 keV) is 0.05 +- 0.01 sec (estimated error including systematics).

The time-averaged spectrum from T+0.16 to T+0.37 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.02 +- 0.29.  The fluence in the 15-150 keV band is 5.0 +- 0.9 x 
10^-8 erg/cm2.
The 1-sec peak photon flux measured from T-0.40 sec in the 15-150 keV band
is 2.4 +- 0.4 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

P. Chandra (UVA/NRAO) and  D. A. Frail (NRAO) report on behalf of the
Caltech-NRAO-Carnegie GRB
Collaboration:

"We observed the field centered on the BAT position of the Swift short
hard burst GRB 070923 (GCN#6818
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 ) using the VLA at a frequency of 8.46 GHz
and starting at 18.70 UT on Sept 28, 2007, about 5 days after the burst. 
There is no detection of the afterglow at the GRB Swift-BAT position
(GCN#6821
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  )with 2-sigma upper limit of 90 microJy on its flux density.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc."