The Hard Solar X-Ray Spectrum Observed from the Third Orbiting Solar Observatory

The hard solar X-ray scintillation-counter telescope on the OSO-Ill satellite covers the energy range 7.7-210 keV with 15-sec time resolution, and six logarithmically spaced energy channels. Approximately 55 per cent time coverage has been obtained for the period following the date of launching, March 8, 1967, until failure of the on-board tape recorder June 28, 1968 This paper is based mainly on solar X-ray events observed during the first two weeks of data accumulation. Approximately ten bursts per day were de- tected during the interval March 9-March 23, 1967, above the threshold sensitivity of 10-8 erg (cm2 sec)-1 for 7.7 «= liv «= 12.5 keY. About once per day a burst of peak energy flux greater than 1.6 X 1O~ erg (cm2 sec)' was observed. Although many variations were observed, the typical event had an e-folding rise time of 86 sec and a decay time of 458 sec. The bursts occurred in correlation with almost all listed flares and subflares (88 per cent), microwave bursts (92 per cent), and SID's (100 per cent). Numerous bursts were also detected without these accompanying phenomena. The correlation with type III radio bursts, although still positive, is not as good (31 per cent), a fact which suggests that coronal disturbances are not an inevitable consequence of the process which produces X-ray emission. The X-ray spectrum is appreciably non-thermal in the initial phase of the burst and thermal in the decay phase, with an effective temperature often exceeding 50 X 106 °K. The average peak temperature of subfiares exceeds 10 X 106° K, while that of importance 1 or greater exceeds 14 X 1060 K. The emission measure flefliV has a constant value of about 1.4 X 1O~ cm3 both during an X-ray burst and from burst to burst. The solar X-ray bursts therefore differ mainly in the maximum temperature attained. A less detailed examination of later data shows that the phenomena we describe are not peculiar characteristics of a single active re- gion