TITLE:   GCN CIRCULAR
NUMBER:  31006
SUBJECT: GRB 211024A: Swift/BAT-GUANO detection and candidate arcminute localization of a short burst
DATE:    21/10/26 13:53:49 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.tohu@gmail.com>

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

Swift/BAT did not trigger on GRB 211024A (T0: 2021-10-24T01:34:11.4
UTC, Fermi/GBM trigger #  656732056, GCN 30962), also detected by
GECAM-B (GCN 30974).

The Fermi/GBM 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 200 seconds of BAT event-mode data from
[-50,+150] seconds around the time of the burst. All the requested
event mode data was delivered to the ground.

The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu, in prep),
confidently detects the burst with a duration of ~0.1 seconds and
sqrt(TS) of 10.4.

A candidate localization is found with DeltaLLHOut of 13 and an
ambiguous DeltaLLHPeak of 2.5. For this reason we can confidently
claim detection, and some preference for an in-FOV origin, but the
best fit arcmin localization is not strongly preferred over other
positions.
Conventional imaging does not find a location for this burst.

The best BAT position is
RA, Dec = 28.053 -6.973 deg which is
   RA(J2000)  = 1h 52m 12.61s
   Dec(J2000) =  -6d 58′ 23″
with an estimated uncertainty of 6 arcmin.

This position is consistent with the Fermi/GBM localization.

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/