GCN Circular 18396

S. D. Barthelmy (GSFC),  S. B. Cenko (GSFC), J. R. Cummings (GSFC/UMBC),
P.A. Evans (U. Leicester), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
A. Y. Lien (GSFC/UMBC), D. Malesani (DARK/NBI), C. B. Markwardt (GSFC),
C. Pagani (U. Leicester), K. L. Page (U. Leicester), D. M. Palmer (LANL),
T. Sakamoto (AGU), M. Stamatikos (OSU), and T. N. Ukwatta (LANL)
report on behalf of the Swift team:

Using the data set from T-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT trigger 657564 (the probable GRB 151004A;
Cenko et al., GCN Circ. 18389
Not found
 
 
).  The BAT ground-calculated position is
RA, Dec = 213.613, -64.956 deg which is
  RA(J2000)  =  14h 14m 27.1s
  Dec(J2000) = -64d 57' 23.3"
with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.

The BAT mask-weighted light curve shows a weak complex structure that starts
at ~T-10 s and ends at ~T+130 s. T90 (15-350 keV) is 128.40 +- 17.77 sec
(estimated error including systematics).

The BAT time-averaged spectrum from T-13.63 to T+134.42 sec is best fit by
a simple power-law model.  The power law index of the time-averaged spectrum
is 1.95 +- 0.23.  The fluence in the 15-150 keV band is 1.0 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+50.86 sec in the 15-150 keV band
is 0.4 +- 0.1 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

In the BAT energy band, the spectral properties and T90 are consistent with
those of a GRB. However, due to the low Galactic latitude (b = -3.47 deg),
we cannot rule out a Galactic origin.

The XRT began observing at 19:09:14.0 UT, 3610 seconds after the BAT trigger.
In 5.7 ks of Photon Counting (PC) data one faint X-ray source is detected inside
the BAT error circle. The astrometrically corrected X-ray position of the source
(using the XRT-UVOT alignment and matching UVOT field sources to the
USNO-B1 catalogue) is: RA, Dec = 213.63218, -64.93914 which is equivalent to:

RA (J2000): 14h 14m 31.72s
Dec (J2000): -64d 56' 20.9"

with an uncertainty of 7.0 arcsec (radius, 90% confidence). Position enhancement is
described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177).

The X-ray source is faint, at an average count rate of ~0.005 cnts/sec, below
the RASS 3-sigma limit at this position in the sky. If the source is indeed the
X-ray afterglow of GRB 151004A, it would be one of the faintest ever observed
by the XRT at that time after the GRB trigger. At this time, there is only marginal
evidence (1.5 sigma) of fading in the X-ray lightcurve and additional observations
are required to establish the nature of the source.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/657564/BA/