TITLE:   GCN CIRCULAR
NUMBER:  31919
SUBJECT: GRB 220418B: Swift/BAT-GUANO candidate arcminute localization of a short burst
DATE:    22/04/19 05:52:20 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 localize GRB 220418B onboard (T0:
2022-04-18T17:16:21 UTC, Fermi/GBM trig #671994986).

The Fermi 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,
arXiv:2111.01769), detects the burst in a 2.048 s analysis time bin
with a sqrt(TS) of 10.4.

A candidate arcminute localization is found with DeltaLLHPeak of 11.8.

See Section 9.1 and Figure 20 in the NITRATES paper for brief
descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and
DeltaLLHOut.

The BAT position is
RA, Dec = 224.329, -17.514 deg which is
   RA(J2000)  = 14h 57m 18.96s
   Dec(J2000) = -17d 30′ 15.4″
with an estimated uncertainty of 5 arcmin.

This position is consistent with the Fermi/GBM localization (GCN 31915).

XRT and UVOT follow-up has been requested, but will be delayed due to
the position's proximity to the Moon.
We encourage prompt followup from other facilities.

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/