Solar hard X-ray and γ-ray bursts observed by the sigma anti-coincidence shield aboard granat

The anti-coincidence shield of the SIGMA telescope aboard GRANAT provides observations of solar X-ray and -ray bursts in the energy range 200 keV–15 MeV. The characteristics of the experiment are presented here as well as the observations of the solar bursts detected during the first year of operation. Among these events, two of these bursts associated respectively with a limb and a disk flare were observed with significant flux above 10 MeV. Advantage is taken of the experiment large detector area to examine fine time structures even at high energies ( 10 MeV).     

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

The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is a European Space Agency hard X-ray/γ-ray observatory for astrophysics, covering photon energies from 15 keV to 10 MeV. It was launched in 2002, and since then the Bismuth Germanate (BGO) detectors of the Anti-Coincidence Shield (ACS) of the Spectrometer on INTEGRAL (SPI) have detected many hard X-ray (HXR) bursts from the Sun, producing light curves at photon energies above ≈?100 keV. The spacecraft has a highly elliptical orbit, providing long uninterrupted observing (about 90 % of the orbital period) with nearly constant background due to the shorter time needed to cross Earth’s radiation belts. However, because of technical constraints, INTEGRAL cannot be pointed at the Sun, and high-energy solar photons are always detected in nonstandard observation conditions. To make the data useable 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 checked the performance of the model employed for the Monte Carlo simulations using the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations for the same sample of solar flares. We conclude that although INTEGRAL was not designed to perform solar observations, ACS is a useful instrument for solar-flare research. In particular, its relatively large effective area allows determining good-quality HXR/γ-ray light curves for X- and M-class solar flares and, in some cases, probably also for C-class flares.     

Hard x-ray and Metric/Decimetric Radio Observations of the 20 February 2002 Solar Flare

The GOES C7.5 flare on 20 February 2002 at 11:07 UT is one of the first solar flares observed by RHESSI at X-ray wavelengths. It was simultaneously observed at metric/decimetric wavelengths by the Nançay radioheliograph (NRH) which provided images of the flare between 450 and 150 MHz. We present a first comparison of the hard X-ray images observed with RHESSI and of the radio emission sites observed by the NRH. This first analysis shows that: (1) there is a close occurrence between the production of the HXR-radiating most energetic electrons and the injection of radio-emitting non-thermal electrons at all heights in the corona, (2) modifications with time in the pattern of the HXR sources above 25 keV and of the decimetric radio sources at 410 MHz are observed occurring on similar time periods, (3) in the late phase of the most energetic HXR peak, a weak radio source is observed at high frequencies, overlying the EUV magnetic loops seen in the vicinity of the X-ray flaring sites above 12 keV. These preliminary results illustrate the potential of combining RHESSI and NRH images for the study of electron acceleration and transport in flares.     

Deka-keV X-ray emission associated with the onset of radio noise storms

Radio noise storms show that suprathermal electrons (a few tens of keV) are present in the vicinity of active regions during several hours or even a few days. Where and how these electrons are energized is not yet well known. A flare-like sudden energy release in the active region is in general observed at the onset of noise storms, either as a fully developed flare or, more often, as a soft X-ray brightening without conspicuous H signature. In order to investigate to what extent electrons energized in the active region contribute to the noise-storm emission in the overlying coronal structures, we combine radio imaging (Nançay radioheliograph) with X-ray spectral observations at photon energies of a few keV (GOES) and - for the first time - around 10 keV (WATCH/GRANAT). In two of four studied events the WATCH data show a significant excess of the deka-keV count rate above the expectation from an isothermal fit to the GOES fluxes. Although the electron population producing the deka-keV X-ray emission would be energetic enough to power the simultaneous radio noise storm, the much longer duration of the radio emission requires time-extended particle acceleration. The acceleration probably occurs in the corona overlying the X-ray emitting region, triggered by the processes which give rise to the X-ray brightenings.