Solar Hard X-Ray Flares with GRBM

Several of the GRB candidates (174) found with our systematic GRB search have been classified as solar flares. In fact, these 174 solar flares automatically detected do not correspond to distinct flares: for the longest flares, more than one trigger occurred in the GRB quest; after visual inspection, it has come out that these 174 off-line triggers refer to 146 distinct flares occurred during the scanned interval (July 1996 - October 2001). In order to determine whether a transient event could have come from the Sun, cross-checks with the NASA mission GOES have been performed: the 146 distinct flares have been identified as solar flares thanks to the simultaneous GOES detections, performed in the 1-8 Å energy range.

On the other hand, since they triggered the HR condition (eq. ), it means that this set of solar flares collects the hardest ones, among those that could be detected with the GRBM. Due to this, all the X-ray solar flares that have been observed in the only 40-700 keV and not in the $>$100 keV GRBM bands, have been missed by the off-line GRB quest, that was optimzed for GRB search. This strong selection prevented from obtaining a complete catalog of solar flares seen by the GRBM. In addition, since the brightest flares detected with the GRBM lasted several hundreds seconds, it often happened that many of them have been partially Earth-blocked (fig. ), and/or for some of them the time profiles are only partially available, owing to SAGA gaps in the ratemeters.

Figure: The huge April 02, 2001, solar flare as seen by the GRBM (UT 21:36:40): no dead time correction. The 1 s ratemeters recycled in several 1 s bins. (GRBM unit 2, 40-700 keV).

Figure: May 4, 1998, solar flare (UT 09:29:17), as seen by the GRBM (40-700 keV and $>$ 100 keV time profiles are shown in the top and bottom panels, resp.): this flare lasted $\sim$ 24 min (GRBM units 1+3).

Furthermore, there are two other factors contributing to making the solar flares' analysis delicate: first, due to the long durations (fig. ), the background subtraction for these events is not trivial; second, in several cases, the peak count rates ($\sim 10^5$ c/s) have determined a recycling of the 1 s ratemeters (16 bit counters) (fig. ). A complete catalog of solar flares has not been accomplished in this work, because it goes beyond our purposes. Two examples of outstanding solar flares are shown in figg.  and  .