TITLE:   GCN CIRCULAR
NUMBER:  29844
SUBJECT: GRB 210420B: Swift detection of a burst
DATE:    21/04/20 19:34:55 GMT
FROM:    Kim Page at U.of Leicester  <klp5@leicester.ac.uk>

M. J. Moss (GWU), P. A. Evans (U Leicester), C. Gronwall (PSU),
J.D. Gropp (PSU), J. A. Kennea (PSU), K. L. Page (U Leicester) and
B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

BAT triggered on GRB 210420B (trigger 1044382) at some time before 18:50
UT. Due to telemetry gaps, we have only XRT data for this burst and
therefore at the present time cannot give a precise trigger time, or
information about the BAT light curve. The most likely trigger time is
between 18:20 and 18:50 UT. Once the full dataset has been downlinked we
will be able to derive a precise trigger time and analyse the BAT light
curve. 

Using promptly downlinked data we find an uncatalogued X-ray source
located at RA, Dec 254.32245, 42.56951 which is equivalent to:
   RA(J2000)  = 16h 57m 17.39s
   Dec(J2000) = +42d 34' 10.2"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
position may be improved as more data are received; the latest position
is available at https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (2.09 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.1
(+1.75/-1.59) x 10^21 cm^-2 (90% confidence). 

Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com). 
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/)