TITLE:   GCN CIRCULAR
NUMBER:  27516
SUBJECT: GRB 200405B: ATCA follow-up and ASKAP limits on pre-burst radio emission
DATE:    20/04/10 01:56:01 GMT
FROM:    Dougal Dobie at VAST  <ddobie94@gmail.com>

Dougal Dobie (USYD/CSIRO), Tara Murphy (USYD), James Leung (USYD/CSIRO),
Adam Stewart (USYD), Joshua Pritchard (USYD), David Kaplan (UWM)

GRB 200405B (GCN 27497) occurred in a field that has been observed 5 times
as part of the ASKAP Variables And Slow Transients (VAST; Murphy et al.
2013)
pilot survey between 2019-08-27 and 2020-01-25. We have searched for radio
emission from the BAT ground-calculated position and the 4 uncatalogued
X-ray sources detected by Swift (GCN 27500) and find no radio counterparts
to a detection limit of ~1.5 mJy at 888 MHz.

We also performed follow-up observations of all 5 positions with the ATCA
between 2020-04-09 05:00-10:30 UTC with 2x2 GHz bands centered on
5.5 and 9 GHz. No radio emission was detected coincident with any of the
sources, we list 3-sigma upper limits below

Source      5.5 GHz (uJy)    9 GHz (uJy)
BAT pos     81               54
Source 1    90               120
Source 2    84               96
Source 3    87               90
Source 4    90               114

We do detect a radio source at coordinates of 04:10:26.8, -51:31:55
(offset 7.2 arcsec from Source 4), coincident with WISEA
J041026.82-513155.2,
with a flux density of ~4 mJy at 5.5 GHz and ~7 mJy at 9 GHz. This source is
also detected in the VAST pilot survey with a flux density of ~10 mJy. We do
not consider this a candidate afterglow for the GRB.

Further observations with ATCA and as part of the VAST program are planned.

Thank you to CSIRO staff for supporting these observations during these
especially difficult times.