[vsnet-grb-info 19320] Swift trigger 748858: Swift BAT, XRT, and UVOT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  21017
SUBJECT: Swift trigger 748858: Swift BAT, XRT, and UVOT refined analysis
DATE:    17/04/19 22:30:06 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien at nasa.gov>

T. Sakamoto (AGU), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), D. N. Burrows (PSU),
N. Gehrels (GSFC), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU),
M. H. Siegel (PSU), M. Stamatikos (OSU),
T. N. Ukwatta (LANL) on behalf of the Swift team:

Using the data set from the recent telemetry downlink, we report further analysis
of the Swift/BAT trigger #748858 (Siegel et al., GCN Circ. 21011).

The BAT data analysis is performed using data from T-239 to T+963 sec.
The BAT ground-calculated position is
RA, Dec = 199.804, 15.442 deg which is
  RA(J2000)  =  13h 19m 13.0s
  Dec(J2000) = +15d 26' 31.1"
with an uncertainty of 2.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.

The BAT mask-weighted light curve shows a very weak pulse that starts at ~T0
and ends at ~T+65 s. T90 (15-350 keV) is 61 +- 15 sec (estimated error
including systematics).

The time-averaged spectrum from T-2.00 to T+65.00 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.19 +- 0.58.  The fluence in the 15-150 keV band is 1.8 +- 0.7 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+7 sec in the 15-150 keV band
is 0.2 +- 0.1 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

Results from the BAT data analysis are consistent with those of a long GRB.
Also, the image significance of 7.4 sigma suggests that the detection is likely
to be real. However, because there is no confirmed XRT and UVOT detections
of a fading counterpart, we cannot rule out the possibility that the BAT signal is
from some other origin.

In 3.8 ks of XRT Photon Couning data covering T+158 s to T+16.8 ks we detect
a single source inside the BAT error circle:

RA (J2000): 13h 19m 24.41s = 199.8517d
Dec (J2000): +15d 30’ 00.4” = 15.5001d
Pos. error: 7.9" (radius, 90% confidence)
Peak rate: 4.45e-3 (+1.43e-3, -1.20e-3) ct/sec

The source is much fainter than expected for early time XRT observations of
a long GRB and shows no evidence of fading, so we cannot rule out the possibility
that it is an unrelated object or a non-GRB source.

The Swift/UVOT began settled observations of the field 150s after the BAT trigger.
No optical afterglow consistent with the BAT or XRT positions was detected in the
initial UVOT exposures down to a limit of 21.0 in the white filter and 20.0 in the u filter.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/748858/BA/