GRB 240521B

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

Swift/BAT did not localize GRB 240521B onboard (T0: 2024-05-21T15:08:57.06 UTC, Fermi trigger 737996942).

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 BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), detects the burst in a 16.384 s analysis time bin starting at T0 - 12.3 s with a sqrt(TS) of 74.6.
An arcminute localization is found with DeltaLLHOut of 984.5 and a DeltaLLHPeak of 1282.7.

See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.

The BAT position is
RA, Dec = 355.6879, -32.7215 deg which is
   RA(J2000)  = 23h 42m 45.10s
   Dec(J2000) = -32° 43’ 17.4’’
with an estimated uncertainty of 3 arcmin radius.

Follow-up observations from other instruments are requested.
Swift ToO request has been submitted.

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/

S. Dalessi (UAH), C. Fletcher (USRA), and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 15:08:57.06 UT on 21 May 2024, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 240521B (trigger 737996942/240521631).
which was also detected by Swift/BAT-GUANO (Ronchini et al. 2024, GCN 36532
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 ).
The Fermi GBM on-ground location is consistent with the Swift/BAT-GUANO position.

The angle from the Fermi LAT boresight is 101 degrees.

The GBM light curve single emission episode with a duration (T90)
of about 34 s (50-300 keV). The time-averaged spectrum
from T0-16 to T0+17 s is best fit by
a simple power law function with index -1.64 +/- 0.04.

The event fluence (10-1000 keV) in this time interval is
(4 +/- 0.5)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-9.9 s in the 10-1000 keV band is 4.1 +/- 0.4 ph/s/cm^2.

A power law function with an exponential high-energy cutoff fits the spectrum equally well. The power law index is -1.48 +/- 0.11 and the cutoff energy,
parameterized as Epeak, is 222 +/- 94 keV.

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/"