[vsnet-grb-info 21942] GRB 190331B: Swift observations of a ground-detected burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24041
SUBJECT: GRB 190331B: Swift observations of a ground-detected burst 
DATE:    19/04/01 20:40:32 GMT
FROM:    Kim Page at U.of Leicester  <klp5 at leicester.ac.uk>

K. L. Page (U. Leicester), D. M. Palmer (LANL) and N. P. M. Kuin
(UCL-MSSL) report on behalf of the Neil Gehrels Swift Observatory Team:

Ground processing of Swift-BAT data has discovered a possible long
duration burst, GRB 190331B, in onboard images covering 03:59:05 -
04:25:29 UT of 2019-03-31. The onboard-calculated source location is RA,
Dec = 326.258, -54.555 deg which is
 RA(J2000)  =  21h 45m 01.9s
 Dec(J2000) = -54d 33' 17.1"
As is usual for image triggers, the light curve shows no obvious
variation.  However, the source appears both in a single image covering
the time range given above, and in individual shorter images covering
subintervals, specifically 03:59:05-04:05:21; 04:05:21-04:11:21; and
04:23:37-04:25:29 The location was outside of the BAT Field of View both
before and after this time interval due to Swift's pre-planned observing
schedule, so this 26 minute span is the lower limit of the emission
interval.

Swift-XRT has performed follow-up observations of this source, collecting
2.0 ks of Photon Counting (PC) mode data between T0+73.3 ks and T0+79.0
ks.

One uncatalogued X-ray source has been detected consistent with being
within 394 arcsec of the Swift/BAT position, it is below the RASS limit
and shows no definitive signs of fading. Therefore, at the present time
we cannot confirm this as the afterglow. Details of this source are
given below:

Source 1:
  RA (J2000.0):  326.2238  =  21:44:53.71
  Dec (J2000.0): -54.4717  =  -54:28:18.2
  Error: 3.7 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: 0.0124 +/- 0.0030 ct s^-1
  Distance: 261 arcsec from Swift/BAT position.
  Flux: (5.5 +/- 1.3)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Another uncatalogued source was also detected, however this was too far
from the GRB position to be the afterglow.

The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
http://www.swift.ac.uk/ToO_GRBs/00020885.

The Swift-UVOT observations of GRB 190331B  did not find a new source,
although a source, present in the  DSS near the XRT position, is also seen
in the UVOT.

Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) are: 

Filter         T_start(s)   T_stop(s)      Exp(s)       3-sigma UL

white          73752         79024          283         >18.71
v              74187         74615          421         >17.35
u              73318         78999         1075         >18.73

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.02 in the direction of the burst
(Schlegel et al. 1998).

Although the gamma-ray emission duration is unusually long for a GRB, the
source's location is 45 degrees from the Galactic plane, reducing the
likelihood that this is a Galactic transient.

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)