GRB 230415A

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

Swift/BAT did not localize GRB 230415A onboard (T0: 2023-04-15 20:48:48.63 UTC, Fermi/GBM trig #703284533). 

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 12.6 in a 16.384 s analysis time bin.
The burst duration is ~30 seconds.

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

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/

S. Poolakkil (UAH) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 20:48:48.63 UT on 15 April 2023, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 230415A (trigger 703284533 / 230415867)
which was also detected by the Swift/BAT-GUANO (Tohuvavohu et al. 2023, GCN 33639
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 ).

The angle from the Fermi LAT boresight at the GBM trigger time is 50 degrees.

The GBM light curve consists of multiple peaks followed
by some extended emission with a duration (T90) of about 40 s (50-300 keV).
The time-averaged spectrum from T0 s to T0+34.8 s
is best fit by a power law function with an exponential
high-energy cutoff. The power law index is -0.87 +/- 0.09  and
the cutoff energy, parameterized as Epeak, is 250 +/- 31 keV.

The event fluence (10-1000 keV) in this time interval is
(6.3 +/- 0.4)E-06  erg/cm^2. The 1-sec peak photon flux measured
starting from T0+8.3 s in the 10-1000 keV band
is 2.5 +/- 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/"