GRB 230421A, GRB 230421

E. Bissaldi (Politecnico and INFN Bari) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 09:42:46.37 UT on 21 April 2023, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 230421A (trigger 703762971 / 230421405),
which was also detected by the Swift/BAT-GUANO (DeLaunay et al. 2023, GCN 33672
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 ).
The GBM on-ground location is consistent with the Swift position.

The angle from the Fermi LAT boresight
at the GBM trigger time is 104 degrees.

The GBM light curve consists of a single emission episode
with a duration (T90) of about 10 s (50-300 keV).
The time-averaged spectrum from T0-1 s to T0+4 s is
adequately fit by a power law function with an exponential
high-energy cutoff. The power law index is -1.1 +/- 0.2 and
the cutoff energy, parameterized as Epeak, is 360 +/- 160 keV.

The event fluence (10-1000 keV) in this time interval is
(1.3 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-0.3 s in the 10-1000 keV band
is 3.2 +/- 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/"

James DeLaunay (U Alabama), Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Samuele Ronchini (PSU), Tyler Parsotan (UMBC/GSFC) report:

Swift/BAT did not localize GRB 230431A onboard (T0: 2023-04-21 09:42:46.370000UTC, Fermi/GBM GCN 33668
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 ).

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 burst occured during a Swift slew. 
Slew mosaic imaging finds the position of the burst with SNR of 10.5.

The BAT position is
RA, Dec = 41.704, +1.119 deg which is
   RA(J2000)  = 02h 46m 48.96s
   Dec(J2000) = +01d 07′ 08.4″
with an estimated uncertainty of 5 arcmin radius.

The burst location is Sun constrained for Swift, so there will be no XRT or UVOT followup.

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/

The Fermi GBM team reports the detection of a likely LONG GRB

At 09:42:46 UT on 21 Apr 2023, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 230421A (trigger 703762971.373066 / 230421405).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 44.5, Dec = 6.0 (J2000 degrees, equivalent to J2000 02h 58m, 6d 00'), with a statistical uncertainty of 5.5 degrees.

The angle from the Fermi LAT boresight is 99.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230421405/quicklook/glg_skymap_all_bn230421405.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230421405/quicklook/glg_healpix_all_bn230421405.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230421405/quicklook/glg_lc_medres34_bn230421405.gif