HEAO-1 soft X-ray observations of Sirius

Sirius was observed on October 4-9, 1977, with the low-energy detectors of the HEAO 1 A-2 experiment. It was detected only in the 0.15- to 0.28-keV band with a flux corresponding to a luminosity of 10 to the 28th power erg/sec, confirming the ANS result. The X-ray intensity is steady to within + or - 30% over the six days studied. The spectral shape is variable. The X-ray spectrum can be characterized by a bremsstrahlung temperature of 200,000 to 500,000 K. This latter finding is of significance because recently Cash, Bowyer, and Lampton (1978) obtained EUV upper limits which rule out the possibility that the soft X-ray flux originates in the photosphere of Sirius B, the white dwarf member of the system. Thus, the soft X-ray emission is interpreted in terms of coronae surrounding Sirius A or B, and the properties of such models are derived.