[vsnet-grb-info 27881] GRB 200716A: Swift/BAT-GUANO archival detection and arcminute localization

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29984
SUBJECT: GRB 200716A: Swift/BAT-GUANO archival detection and arcminute localization
DATE:    21/05/10 21:02:08 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.tohu at gmail.com>

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

Swift/BAT did not trigger on GRB 200716A (T0: 2020-07-16 01:26:37.76
UTC),  Fermi/GBM team (GCNs 28109, 28120), BALROG (GCN 28112), AGILE
(GCN 28116), AstroSat CZTI (GCN 28117), IPN (GCN 28121).

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 at least 60 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 110.96. 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 8.5.

The partial coding fraction for this burst was 1%.  The SNR in the
image is significantly lower than the sqrt(TS) as the maximum
likelihood analysis is capable of considering counts from the burst
deposited on the detector that arrive via lines of sight that do not
pass through the coded mask. For the case of bursts at low partial
coding fractions, this yields a substantial sensitivity improvement
over imaging.

The BAT position is
RA, Dec = 139.348, -16.712 deg which is
   RA(J2000)  = 09h 17m 23.5s
   Dec(J2000) =  -16d 42′ 43.2″
with an estimated uncertainty of 5 arcmin.

This position is consistent with the ~1/2 square degree IPN
localization (GCN 28121).

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/