Exploring the Capabilities of the Anti-Coincidence Shield of the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) Spectrometer to Study Solar Flares

Abstract

INTEGRAL is a hard X-ray/gamma-ray observatory for astrophysics (ESA) covering photon energies from 15 keV to 10 MeV. It was launched in 2002 and since then the BGO detectors of the Anti-Coincidence shield (ACS) of the SPI spectrometer have detected many hard X-ray (HXR) bursts from the Sun, producing lightcurves at photon energies above ~ 100 keV. The spacecraft has a highly elliptical orbit, providing a long uninterrupted observing time (about 90% of the orbital period) with nearly constant background due to the reduction of the crossing time of the Earth's radiation belts. However, due to technical constraints, INTEGRAL cannot point to the Sun and high-energy solar photons are always detected in non-standard observation conditions. To make the data useful for solar studies, we have undertaken a major effort to specify the observing conditions through Monte-Carlo simulations of the response of ACS for several selected flares. We check the performance of the model employed for the Monte-Carlo simulations using RHESSI observations for the same sample of solar flares. We conclude that, despite the fact that INTEGRAL was not designed to perform solar observations, ACS is a useful instrument in solar flare research. In particular, its relatively large effective area allows the determination of good-quality HXR/gamma-ray lightcurves for X- and M-class solar flares and, in some cases, probably also for C-class flares.