Hard Solar X-Ray Bursts Observed by OSO-III.
Abstract
OSO-III has observed the solar x-ray spectrum above 7.7 keV since 9 March 1967. Solar x-ray bursts are very frequent (98 were observed in the first week of operation of the satellite) and are partially correlated with flares and radio-wave emission. The occurrence of solar x-ray bursts is well correlated with the occurrence of 2800 MHz solar radio bursts, but the detailed time structures of the events are usually different. 65% of the x-ray bursts coincide with flares; on the other hand, about 85% of flares of all importances produce detectable x-ray bursts. The peak x-ray flux for a correlated burst does not depend in a simple way on flare area or brightness. Most solar x-ray bursts exhibit a simple fast-rise, slow-decay time structure; the median rise time is about 65 sec. and the median fall time is about 340 sec. The rise and fall times are widely variable, and the peak energy flux ranges from 10-8 erg (cm2sec)-1 to more than 10-~ erg (cm2 sec)-1 above 7.7 keV. Assuming the X-ray production mechanism to be free-free transitions, the OSO data establish that the onset phase of the x-ray burst is due to a nonthermal electron distribution. The decaying phase of the burst is essentially thermal in nature. These conclusions are drawn from the strong dependence of the x-ray spectrum (or the effective temperature of the emitting region) on the magnitude of the x-ray flux. The thermal bremsstrahlung model allows a determination of neN, where N is the number of electrons radiating and ne the electron density. The distribution of values of this quantity for different bursts is strongly peaked at about 1.4X 10~~ cm-3. It is a striking fact that this value obtains for the entire observed range of sizes of solar x-ray bursts.
- Publication:
-
The Astronomical Journal Supplement
- Pub Date:
- 1968
- Bibcode:
- 1968AJS....73R..64H