SIRI-2 Detection of the Gamma-ray Burst 221009A

TL;DR

SIRI-2, a gamma-ray detector on STPSat-6, successfully observed GRB 221009A without saturation, providing consistent spectral data and contributing valuable fluence measurements that suggest such bright bursts are extremely rare.

Contribution

This paper reports the first detection of GRB 221009A by SIRI-2, demonstrating its capability to measure bright gamma-ray bursts without saturation and providing new spectral and fluence data.

Findings

SIRI-2 detected GRB 221009A without saturation.

Spectral measurements are consistent with other instruments.

The fluence of the burst is 0.140 erg cm-2, indicating such bright bursts are very rare.

Abstract

SIRI-2 is a collection of Strontium Iodide gamma-ray detectors sensitive at approximately 400 keV to 10 MeV, launched on the Department of Defense's STPSat-6 to geosynchronous orbit. SIRI-2 detected the gamma-ray burst (GRB) 221009A and, unlike most GRB detectors, was not saturated and did not require any pulse pile-up corrections. The energetics of this burst as measured by SIRI-2 are consistent with those found by other instruments, and the Band function fits to the spectra are consistent with that from the unsaturated Insight and GECAM instruments, and similar to corrected spectra from the Fermi Gamma-ray Burst Monitor, but softer than those found by Konus-Wind when that instrument was saturated. The total fluence measured with SIRI-2 was measured to be 0.140 +/- 0.002 erg cm-2, lower than other instruments, likely due to the increasing background of SIRI-2 forcing the calculation to use a smaller time interval. An extrapolation of the distributions of fluences from GRBs to the fluence of 221009A measured with SIRI-2 indicates bursts brighter than this one should occur about once every 4,000 years.