GCN Circular 3935

A. Parsons (GSFC),  C. Sarazin (U. Virginia), L. Barbier (GSFC),
S. Barthelmy (GSFC), J. Cummings (GSFC/NRC), D. Hullinger (GSFC/UMD),
E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), F. Marshall (GSFC), D. Palmer (LANL),
T. Sakamoto (GSFC/NRC), G. Sato (ISAS), T. Takahashi (ISAS),
J. Tueller (GSFC) on behalf of the Swift-BAT team:

Using the data set from T-60 to T+120 sec from the recent telemetry
downlink, we report further analysis of Swift-BAT GRB 050906
(trigger #153866)  (Krimm, et al., GCN 3926
Not found
 
 
).  The refined BAT ground position
is (RA,Dec) = 52.841,-14.652 {+03h 31m 22s,  -14d 39' 00"} [deg; J2000]
+- 2.6 arcmin, (90% containment).  This is 2.7 arcmin from the position
reported in Krimm et al. 2005, GCN 3926
Not found
 
 
. The partial coding was 77%.

The mask weighted light curve shows a small excess mostly in the
25-100 keV energy range.  This a short burst of duration T90 (15-350 keV)
128 +- 16 milliseconds. (estimated error including systematics).

The power law index of the time-averaged spectrum fit over T+0 to T+0.128 sec
is 1.91 +- 0.42.   The fluence in the 15-150 keV band is (5.9 +/- 
3.2) x 10^-8 erg/cm2.
The 1-s peak photon flux measured from T0 in the 15-150 keV
band is (0.22 +/- 0.11) ph/cm2/s.  All the quoted errors are at the 90%
confidence level.

The BAT error circle includes the relatively bright, nearby galaxy
IC 328 (Levan & Tanvir GCN 3927
Not found
 
 
).  The 2MASS K magnitude of this galaxy
is K20 = 11.51.  The density of galaxies this bright on the sky is
about 1 per square deg (Kochanek, C. S., et al. 2001, ApJ, 560, 566).
Thus, the probability of finding a galaxy this bright in the BAT 3 arcmin
error circle is about 0.008.  The redshift of IC 328 is 0.0308
(da Costa, L. N., et al. 1998, AJ, 116, 1), giving a luminosity distance
of dL = 133 Mpc for the WMAP cosmology.  At this distance, the galaxy
luminosity is ~10^11 Lsun(K), which is slightly greater than L*.
The number density of galaxies at least this luminous is ~0.0006/Mpc^3.
Thus, the probability of finding a galaxy at least this luminous at a
redshift as small as that of IC 328 within 3 arcmin of the BAT position
is about 0.001.

IC 328 appears to be a spiral galaxy, and is quite blue (U-B = -0.6;
Coziol, R. et al., 1994, AJ, 108, 405).  With these colors, it is
likely to have active star formation.  Thus, this host is quite different
than the hosts of the two short-hard bursts GRB 050509b and GRB 050724,
which appear to have occurred in or near red elliptical galaxies.

It is possible that this GRB was caused by an SGR superflare in IC 328
  (Levan & Tanvir GCN 3927
Not found
 
 
, Palmer, D.M., et al. 2005, Nature, 434, 1107;
Hurley, K. et al. 2005, Nature, 434, 1098), although an underluminous
collapsar or NS-NS merger can not be ruled out.