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Astron. Astrophys. 334, 299-313 (1998)

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Deka-keV X-ray observations of solar bursts with WATCH/GRANAT: frequency distributions of burst parameters

N. Crosby 1, 2, N. Vilmer 1, N. Lund 3 and R. Sunyaev 4

1 LPSH, URA 2080 CNRS, DASOP Observatoire de Paris, Section d'Astrophysique de Meudon, 5, Place J. Janssen, F-92195 Meudon Cedex, France
2 LPCE, UPR 4010 du CNRS, conventionee avec l'Universite d'Orleans, 3A, Avenue de la Recherche Scientifique, F- 45071 Orléans la Source Cedex 2, France
3 Danish Space Research Institute, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark
4 Space Research Institute, Profsoyuznaja 84/32, 117296 Moscow, Russia

Received 22 July 1997 / Accepted 18 November 1997


Solar flare observations in the deka-keV range are performed by the WATCH experiment on board the GRANAT satellite. The WATCH experiment is presented, including the energy calibration as applied in the present work. The creation of the solar burst catalogue covering two years of observation is described and some examples of solar observations are given. The estimated energy releases in the flares presented here are found to extend below the range of hard X-ray flares which were previously studied by ISEE-3 and HXRBS/SMM detectors. The X-ray emitting component cannot be exclusively explained by contributions from a thermal plasma around a few keV. Either a hotter component or a non-thermal population of particles must also be present to produce the observed deka-keV emission. The WATCH data furthermore shows that the relative contributions of these components may change during an event or from event to event and that the injection of energy contained in suprathermal electrons may occur throughout an event and not only during the rise phase. For the most energetic WATCH flares simultaneous observations performed by other experiments at higher energies further indicate that non-thermal emission can be observed as low as 10 keV. A statistical study is performed on the total WATCH solar database and frequency distributions are built on measured X-ray flare parameters. It is also investigated how the properties of these frequency distributions behave when subgroups of events defined by different ranges of parameters are considered. No correlation is found between the elapsed time interval between successive flares arising from the same active region and the peak intensity of the flare.

Key words: Sun: activity – Sun: flares – Sun: X-rays, gamma rays

Send offprint requests to: N. Crosby

© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998