[vsnet-grb-info 20336] Swift trigger 811074 (possible GRB 180221A): Swift refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22432
SUBJECT: Swift trigger 811074 (possible GRB 180221A): Swift refined analysis
DATE:    18/02/22 04:48:13 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien at nasa.gov>

A. P. Beardmore (U. Leicester), C. B. Markwardt (GSFC), M. H. Siegel (PSU),
S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), J. R. Cummings (CPI),
P. A. Evans (U Leicester), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL), T. Sakamoto (AGU),
M. Stamatikos (OSU), T. N. Ukwatta (LANL)
report on behalf of the Swift team:

Using the data set from the recent telemetry downlink, we report further
analysis of the possible GRB 180221A (trigger #811074)
(Stamatikos et al., GCN Circ. 22427).

Using the BAT data from T-239 to T+963 sec,
the BAT ground-calculated position is
RA, Dec = 326.787, -72.891 deg which is
   RA(J2000)  =  21h 47m 08.9s
   Dec(J2000) = -72d 53' 27.3"
with an uncertainty of 2.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 31%.

The BAT mask-weighted light curve shows a hint of a weak pulse that starts
at ~ T+2 s and ends at ~T+15 s. However, the mask-weighted light curve also
suggests a negative average count rate for the background interval (~T-200 s
to ~T0), which indicates incorrect background estimation that is likely caused
by the high count rates around the SAA. When performing the analysis without
subtracting the background image, the image significance of the pulse in the
15-350 keV drops to ~ 3.5 sigma, which is consistent with noise fluctuation.

Because of the uncertainty in the background estimation, it is difficult to
confirm the nature of the source with BAT data alone.

Swift-XRT observed the field of the possible GRB 180221A (BAT trigger
811074) in Photon Counting mode for 5.73 ks from 3.1 ks to 16.4 ks
after the trigger. We detect a candidate counterpart within the BAT
error circle with an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue) of
RA, Dec (J2000) = 326.77306, -72.86947, which is equivalent to:

RA (J2000):  21h 47m 05.53s
Dec(J2000): -72d 52' 10.1"

with an uncertainty of 2.2 arcsec (radius, 90% confidence).

Although faint, with a peak rate of 0.035 c/s (over the 0.3-10.0 keV
energy band), the source appears to be fading with a power-law decay
slope alpha = 0.57 +/- 0.31.

A spectrum formed from the PC mode data over this interval can be
fitted with an absorbed power-law with a photon index 1.8 (+0.5 -0.4)
and column density of 4.4(+14.2 -4.4)E20 cm^-2, in addition to the
Galactic column density of 3.98e20 cm^-2 in the direction of the
source. The observed 0.3-10 keV flux is 7.8E-13 erg cm^-2 s^-1.

The Swift/UVOT began settled observations of the field of the possible
GRB 180221A 2950 s after the BAT trigger.  No optical afterglow consistent
with the XRT position is detected in the initial UVOT exposures nor is any
other optical transient detected down to a 3-sigma upper limit of 21.4 in
the white filter.

Further Swift observations are planned to determine if the
source fades further.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/811074/BA/