X‐Ray Emission from Classical Novae

Classical novae are expected to be bright soft X-ray sources after out-burst, due to the steady hydrogen burning of the material left on top of the white dwarf after the outburst. But observations performed by ROSAT of 39 recent classical novae showed that only three objects where bright soft X-ray sources. A harder X-ray emission was found to be a more common feature, and it was understood to be caused by the heating of the gas in the ejecta by internal or external shocks. To elucidate the problem of the missing soft X-ray emission, we have performed XMM-Newton observations to monitor recent classical novae. Four out of the five observed sources are still emitting two to four years after outburst.