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/γ-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/γ-ray lightcurves for X-and M-class solar flares and, in some cases, probably also for C-class flares.

Using SPI/INTEGRAL to study the Sun

“…The result is a slight variation of these values, bringing us confidence on our calculations. The effective area refers to photons in the range 50 keV -10 MeV [8].…”

“…Goddard Space Flight Center, NASA: http://www.nasa.gov/centers/goddard/home/index.html3 The dependence of the results on the photon injection spectral index will be presented in a further paper[8] 4. http://www.isdc.unige.ch/integral/archive5 Note that the values given for the angles are the ones used for the simulations, keeping in mind that within the Monte-Carlo code, ϕ I = 180 • + ϕ Sun .…”

INTEGRAL serendipitous observations of solar and terrestrial X-rays and gamma rays

“…The Anti-Coincidence Shield (ACS) of the SPI instrument also provides valuable data for solar flare physics. The response of SPI/ACS for several selected flares detected in nonstandard observation conditions has been carefully studied through Monte Carlo simulations (Rodríguez-Gasén et al, 2014). Within the European Union's FP7 SEPServer project (Vainio et al, 2013), ACS light-curves have been extracted and studied for a set of 24 solar flares which occurred between 2002(Rodríguez-Gasén et al, 2014.…”

“…The response of SPI/ACS for several selected flares detected in nonstandard observation conditions has been carefully studied through Monte Carlo simulations (Rodríguez-Gasén et al, 2014). Within the European Union's FP7 SEPServer project (Vainio et al, 2013), ACS light-curves have been extracted and studied for a set of 24 solar flares which occurred between 2002(Rodríguez-Gasén et al, 2014. In the frame of the EU's H2020 HESPERIA project (Malandraki and Crosby, 2018), 25 more solar flare light curves measured with SPI/ACS between 2011 and 2014 were provided to the solar physicist community (Klein et al, 2018).…”

“…Thanks to its large effective area (A eff ∼ 100 -1000 cm 2 depending on the flare observation conditions; Rodríguez-Gasén et al, 2014), SPI/ACS can provide high-quality hard Xray/gamma-ray light curves for X-and M-class solar flares and, in some case, probably also for C-class flares. Detailed comparisons of ACS data with time evolutions of soft X-ray and radio emissions have shed a new light on the generation of highly energetic electrons in the solar corona and the transport of these particles to the chromosphere, where they emit hard X-ray and gamma-ray radiation via bremsstrahlung (e.g., Mann et al, 2009;Zimovets and Struminsky, 2012;Struminsky, 2013).…”