[vsnet-grb-info 2842] BAT GRB 060204C is probably not a GRB

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4679
SUBJECT: BAT GRB 060204C is probably not a GRB
DATE:    06/02/05 23:09:37 GMT
FROM:    Takanori Sakamoto at NASA/GSFC  <takanori at milkyway.gsfc.nasa.gov>

T. Sakamoto (GSFC/ORAU), L. Barbier (GSFC), S. Barthelmy (GSFC),
P. Boyd (GSFC-UMBC), J. Cummings (GSFC/ORAU), E. Fenimore (LANL),
N. Gehrels (GSFC), D. Hullinger (UMD), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC),
G. Sato (ISAS), J. Tueller (GSFC), W. Voges (MPE)
on behalf of the Swift-BAT team:

Using the data set from T-299.0 to T+303.1 sec from the recent telemetry
downlink, we report further analysis of BAT trigger #180274 
(Grupe, et al., GCN 4663).  The BAT ground-calculated position is
(RA,Dec) = 92.715, 70.174 deg {6h 10m 58.5s, 70d 10' 41.7"} (J2000)
+- 3.6 arcmin, (radius, sys+stat, 90% containment).  The partial coding
was 39%.  The mask-weighted light curve shows a single broad peak in 
the 15-50 keV band with no significant emission above 50 keV.  
T90 (15-350 keV) is (60 +- 5) sec (estimated error including systematics).

The time-averaged spectrum from T+0 to T+64 is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.4 +- 0.6.  The fluence in the 15-150 keV band is
(3.5 +- 1.2) x 10^-07 erg/cm2.  The average photon flux measured from
T+0 to T+64 sec in the 15-150 keV band is (0.10 +- 0.03) ph/cm2/sec.
All the quoted errors are at the 90% confidence level.  

This source was detected at the 4.8-sigma level 1.5 days earlier
at the same location.  There is 0.1% probability that this pre-trigger
detection is a chance coincidence.  This plus the very soft nature
of the spectrum forces us to conclude that this is probably not a 
real GRB, but is very likely a hard X-ray transient.  

Further analysis has shown that even though the star tracker had a
loss-of-lock during the initial trigger, the tracking error was much 
smaller than our stated location uncertainty.