TITLE:   GCN CIRCULAR
NUMBER:  31848
SUBJECT: GRB 220408A: Swift detection of a burst with an optical counterpart, possibly associated with M51
DATE:    22/04/08 06:04:40 GMT
FROM:    Kim Page at U.of Leicester  <kimlpage1978@gmail.com>

R. Caputo (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL),
B. Sbarufatti (PSU) and A. Tohuvavohu (U Toronto) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 05:46:04 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 220408A (trigger=1101675).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 202.402, +47.061 which is 
   RA(J2000) = 13h 29m 36s
   Dec(J2000) = +47d 03' 41"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 5 sec.  The peak count rate
was ~3500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 05:47:53.1 UT, 109.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 202.41670, 47.06870 which is equivalent
to:
   RA(J2000)  = 13h 29m 40.01s
   Dec(J2000) = +47d 04' 07.3"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 45 arcseconds from the BAT onboard position, within the BAT
error circle. 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 (1.98 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.6
(+4.76/-3.79) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 112 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	13:29:39.72 = 202.41548
  DEC(J2000) = +47:04:09.5  =  47.06931
with a 90%-confidence error radius of about 0.91 arc sec. This position is 5.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
20.76 with a 1-sigma error of about  0.22. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.042. 

We note that this GRB is in a direction near to that of M51, the 
Whirlpool Galaxy, although well outside of its optical disk. 
Determining whether this is a coincidence will require further study. 

Burst Advocate for this burst is R. Caputo (regina.caputo AT nasa.gov). 
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/)