TITLE:   GCN CIRCULAR
NUMBER:  29877
SUBJECT: GRB 210421B: Swift/BAT-GUANO detection and arcminute localization
DATE:    21/04/21 22:47:22 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.tohu@gmail.com>

James DeLaunay (PSU), Aaron Tohuvavohu (U Toronto), Jamie A. Kennea
(PSU) report:

Swift/BAT did not trigger on GRB 210421B (T0: 2021-04-21 10:54:44 UTC,
Fermi/GBM GCN 29870, 29874; INTEGRAL/SPI-ACS trigger 9154).

The INTEGRAL/SPI-ACS notice, distributed in near real-time triggered
the Swift Mission Operations Center operated Gamma-ray Urgent Archiver
for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).

Upon trigger by this notice, GUANO sent a command to the Swift Burst
Alert Telescope (BAT) to save 90 seconds of BAT event-mode data from
[-45,+45] seconds around the time of the burst. All the requested
event mode data was delivered to the ground.

The burst is strongly detected in BAT with a duration of ~15 seconds,
confirming the long classification correction of Fermi/GBM (GCN
29874).
The burst occurred during a Swift slew.
The position of the burst was found with SNR of 9.7 using a novel slew
image mosaicing procedure (DeLaunay et al. 2021, in prep.), and so the
positional uncertainty is not yet well characterized.

The BAT position is
RA, Dec = 270.817, 56.828 deg which is
   RA(J2000)  = 18h 03m 16.08s
   Dec(J2000) =  56d 49’ 40.8”
with an estimated uncertainty of at least 5 arcmin.

This position is consistent with the Fermi GBM localization (GCN 29870).

XRT and UVOT follow-up has been requested. Results of follow-up
observations will be reported in future circulars.

GUANO is a fully autonomous, extremely low latency, spacecraft
commanding pipeline designed for targeted recovery of BAT event mode
data around the times of compelling astrophysical events to enable
more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be
found at: https://www.swift.psu.edu/guano/