////////////////////////////////////////////////////////////////////////
TITLE:   GCN CIRCULAR
NUMBER:  31934
SUBJECT: GRB 220421A: Swift/BAT-GUANO detection outside the coded FOV
DATE:    22/04/21 21:25:41 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.tohu@gmail.com>

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

Swift/BAT did not localize GRB 220421A onboard (T0:
2022-04-21T08:17:54.5 UTC, CALET trig #1334564169, Fermi/GBM trig
#672221879).

The CALET and Fermi notices, 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 with a sqrt(TS) of 34.7 in a
4.096 s analysis time bin.
The burst episode as seen by BAT is ~10s long.

NITRATES results indicate a burst coming from outside the coded FoV,
with DeltaLLHOut of -25. An out of FOV origin is consistent with the
Fermi/GBM localization.

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

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/

////////////////////////////////////////////////////////////////////////
TITLE:   GCN CIRCULAR
NUMBER:  31936
SUBJECT: GRB 220421A: Fermi GBM observation
DATE:    22/04/23 15:07:33 GMT
FROM:    Andreas von Kienlin at MPE  <azk@mpe.mpg.de>

A. von Kienlin (MPE) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 08:17:54.53 UT on 21 April 2022, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 220421A (trigger 672221879/220421346), which was
also 
detected by the Swift/BAT-GUANO (Raman et al. 2022, GCN 31934)

The on-ground calculated location, using the GBM trigger
data, is RA = 74.4, DEC = 23.1 (J2000 degrees,
equivalent to 04 h 57 m, 23 d 06 '), with a statistical uncertainty
of 2.1 degrees (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which we have characterized as a core-plus-tail model, with 90% of
GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg
systematic error. [Connaughton et al. 2015, ApJS, 216, 32] ).

The angle from the Fermi LAT boresight at the GBM trigger time is 59
degrees.

The GBM light curve shows a single pulse with a duration (T90) of about 6.4
s (50-300 keV).
The time-averaged spectrum from T0+0.002 s to T0+7.008 s is best fit by a
Band function 
with Epeak = 133.20 +/- 5.40 keV, alpha = -0.41 +/- 0.05, and beta = -2.48
+/- 0.11

The event fluence (10-1000 keV) in this time interval is
(7.2 +/- 0.2)E-06. The 1.024-sec peak photon flux measured
starting from T0+1.056 s in the 10-1000 keV band
is 13.2 +/- 0.3 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support
Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"