GRB 151004A

M. De Pasquale (UCL-MSSL) and S. B. Cenko (GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of
the trigger 657564 (possible GRB 151004A) 3613 s after the
BAT trigger (Cenko et al., GCN Circ. 18389
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 ).
No optical afterglow consistent with the BAT position
is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC          3613         3763          147         >20
white             3613         4994          344         >20.3
v                 3770         5404          393         >18.7
b                 4590         4789          197         >19.2
u                 4384         4584          197         >18.9
w1                4179         4379          197         >19.0
m2                3974         5565          350         >19.3
w2                5000         5200          197         >19.1

The magnitudes in the table are not corrected for the heavy Galactic
extinction due to the reddening of E(B-V) = 1.03 in the direction of
the burst (Schlegel et al. 1998).

-- 

Dr. Massimiliano De Pasquale
Research associate - Swift UVOT scientist
Mullard Space Science Laboratory, University College London

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
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 ).  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/

C. Pagani, K. L. Page, P. A. Evans (U. Leicester) and S. B. Cenko (GSFC) 
report on behalf of the Swift-XRT team:

A second epoch of XRT data was collected on the field of the Swift 
Trigger 657564 / GRB 151004A (Cenko et al., GCN Circ. 18389
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 ) on 2015 
October 15. The observations consist of 7.7 ks of PC mode data, from 
T0+885 ks to T0+932 ks.

The candidate X-ray source reported in Barthelmy et al., GCN Circ. 
18396
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 , originally detected at a count rate of 4.8 (+/- 1.14) x 10^-3 
cnts/sec (0.3-10 keV) is not detected in this second epoch of 
observations, with a 3 sigma upper limit of 1.5 x 10^-3 cnts/sec (0.3-10 
keV). The fading confirms that this is the X-ray afterglow of the Swift 
Trigger 657564 / GRB 151004A.

This circular is an official product of the Swift-XRT team.