X-ray detection of Nova Herculis 1991 five days after optical outburst

CLASSICAL nova outbursts are thought to occur in binary systems in which a white dwarf accretes material from a main-sequence dwarf. The outburst is due to thermonuclear runaway in the accreted material, and results in the ejection of about 10 −5 –10 −4 solar masses of material at velocities of up to several thousand kilometres per second (ref. 1). Previous X-ray observations of classical novae in the early stages of outburst have resulted only in upper limits to the X-ray flux 2–4 . Here we report a positive detection by the Rosat satellite of X-ray emission from Nova Herculis 1991 five days after its optical discovery. Standard nova models predict X-ray emission to arise directly from nuclear burning on the surface of the white dwarf, and suggest that X-rays should not be seen until later in the outburst 5 . We argue that the emission from Nova Her 1991 comes from hot, shocked circuin-stellar material, which may be the ejected material itself or preexisting circumstellar matter. In either case, however, the required density of material is higher than models of nova binary systems would suggest.