Calibration of the INTEGRAL SPI Anti Coincidence Shield with Gamma Ray Bursts observations

TL;DR

This paper calibrates the INTEGRAL SPI Anti Coincidence Shield's response to Gamma Ray Bursts using known localizations, deriving conversion factors and assessing directional and spectral dependencies to improve flux measurements.

Contribution

It provides the first detailed calibration of the ACS response to GRBs, including flux conversion factors and directional dependence, based on observed GRBs with known spectra.

Findings

1 ACS count ~ 1E-10 erg/cm^2 in 75keV-1MeV range for orthogonal directions.

ACS effective area broadly agrees with Monte Carlo simulations but may overestimate sensitivity near the instrument axis.

Calibration improves flux estimation from ACS data for GRB observations.

Abstract

The Anti Coincidence Shield (ACS) of the INTEGRAL SPI instrument provides an excellent sensitivity for the detection of Gamma Ray Bursts (GRBs) above ~ 75keV, but no directional and energy information is available. We studied the ACS response by using GRBs with known localizations and good spectral information derived by other satellites. We derived a count rate to flux conversion factor for different energy ranges and studied its dependence on the GRB direction and spectral hardness. For a typical GRB spectrum, we found that 1 ACS count corresponds on average to ~ 1E-10 erg/cm^2 in the 75keV-1MeV range, for directions orthogonal to the satellite pointing axis. This is broadly consistent with the ACS effective area derived from the Monte Carlo simulations, but there is some indication that the latter slightly overestimates the ACS sensitivity, especially for directions close to the instrument axis.