[vsnet-grb-info 25409] GRB 200409A: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  27509
SUBJECT: GRB 200409A: Swift detection of a burst with an optical counterpart
DATE:    20/04/09 03:30:47 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

J.D. Gropp (PSU), S. D. Barthelmy (GSFC), J. A. Kennea (PSU),
N. J. Klingler (PSU), S. Laha (GSFC/UMBC/CRESST),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), M. J. Moss (GWU),
D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 03:19:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 200409A (trigger=965484).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 60.553, -50.247, which is 
   RA(J2000)  =  04h 02m 13s
   Dec(J2000) = -50d 14' 49"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple peaks
with a total duration of about 20 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 03:21:13.1 UT, 73.4 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 60.53820, -50.22509 which is equivalent
to:
   RA(J2000)  = 04h 02m 09.17s
   Dec(J2000) = -50d 13' 30.3"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 85 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.27 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 5
(+4.06/-3.42) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 77 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	04:02:08.75 =  60.53646
  DEC(J2000) = -50:13:28.4  = -50.22455
with a 90%-confidence error radius of about 0.77 arc sec. This position is 4.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.14 with a 1-sigma error of about  0.16. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is J.D. Gropp (jdg44 AT psu.edu). 
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/)