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GCN Circular 29985

GRB 201228B: Swift/BAT-GUANO archival detection and arcminute localization
2021-05-10T21:03:44Z (3 years ago)
Aaron Tohuvavohu at U Toronto <>
James DeLaunay (PSU), Aaron Tohuvavohu (U Toronto), Jamie A. Kennea
(PSU) report:

Swift/BAT did not trigger on GRB 201228B (T0: 2020-12-28 15:21:45 UTC,
Fermi/GBM trigger num 630861710, GCN 29188).

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 burst is detected in BAT with a duration of ~16 seconds.

This burst was not localized in our low-latency analysis at the time.
An updated and improved version of the targeted search pipeline was
recently re-run on archival data and this burst was recovered.

With a maximum likelihood analysis (DeLaunay et al. 2021, in prep.) on
the event-mode data we detect a location for the burst with a square
root of the test statistic, sqrt(TS), of 27.88. The sqrt(TS) behaves
similarly to SNR.
Using the normal BAT imaging technique, we find the same location for
the GRB with an SNR of 13.1.

The BAT position is
RA, Dec = 35.594, 56.015 deg which is
   RA(J2000)  = 02h 22m 22.56s
   Dec(J2000) =  +56d 00��� 54.0���
with an estimated uncertainty of 3 arcmin.

This position is consistent with the Fermi GBM team localization (GCN 29188).

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:
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