GRB 230915A

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

Swift/BAT did not localize GRB 230915A onboard (T0: 2023-09-15T05:39:29.46 UTC, Fermi GBM Trig 716449174). 

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 with a sqrt(TS) of 11.3 in a 8.192 s analysis time bin, starting at T0 + 2.0482 s.

NITRATES results, independently, are ambiguous with respect to whether this burst originates from in or outside the BAT coded FOV, with a DeltaLLHOut of 10.49.

See Section 9.1 and Figures 10 and 17 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/

R. Hamburg (CNRS/IJCLab) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 05:39:29.46 UT on 15 September 2023, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 230915A (trigger 716449174 / 230915236),
which was also detected by Swift/BAT-GUANO (Tohuvavohu et al. 2023, GCN 34714
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The angle from the Fermi LAT boresight at the GBM trigger time is 56 degrees.

The GBM light curve shows a multi-peaked lightcurve
with a duration (T90) of about 42 s (50-300 keV).
The time-averaged spectrum from T0-9.73 s to T0+18.94 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -1.13 +/- 0.06 and
the cutoff energy, parameterized as Epeak, is 177 +/- 19 keV.
A Band function fits the spectrum equally well with
Epeak = 141 +/- 23 keV, alpha = -1.03 +/- 0.10 and beta = -2.18 +/- 0.20.

The event fluence (10-1000 keV) in this time interval is
(5.60 +/- 0.29)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.54 s in the 10-1000 keV band
is 5.3 +/- 0.2 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/"