TITLE:   GCN CIRCULAR
NUMBER:  27497
SUBJECT: GRB 200405B: Swift/BAT-GUANO detection and *possible* arcminute localization of a short GRB
DATE:    20/04/05 22:52:50 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.tohu@gmail.com>

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

Swift/BAT did not trigger on GRB 200405B (T0: 2020-04-05 03:53:38 UTC).

The INTEGRAL/SPI-ACS Notice #8579, distributed at T0+35 seconds
triggered the Swift Mission Operations Center operated Gamma-ray
Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al.
2020, in prep).

Upon trigger by these notices, 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.

In a ground analysis of the data, we detect the burst in the full
detector-summed rates (no localization information) with an SNR of ~9.
The detected duration is ~0.5 seconds.

With a maximum likelihood analysis (DeLaunay et al. 2020, 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 12.4. The sqrt(TS) behaves
similarly to SNR.
Using the normal BAT imaging technique, we find an SNR of ~4 at the
same location.

The BAT ground-calculated position is
RA, Dec = 62.7894, -51.5326 deg which is
   RA(J2000)  = 4h 11m 9.5s
   Dec(J2000) =  -51d 31’ 57.4”
with an uncertainty of 4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 11.6%.

Due to the low partial coding, and the lack of any localization
information from SPI-ACS for a consistency check, we stress that while
this localization is significant, we cannot be certain of whether or
not the burst actually originated from outside the BAT coded field-of-view.

XRT and UVOT follow-up has been triggered and observations of this
region have already begun. 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/