Optical novae (i.e. thermonuclear outbursts of White Dwarfs in binary systems) are among the brightest objects in galaxies. Optical novae have been identified as a main contributor to supersoft X-ray sources (SSS) in nearby galaxies in an analysis of XMM-Newton, Chandra and ROSAT archival data of M31 and M33 (Pietsch et al. 2005). Optical novae show up as supersoft X-ray sources when the envelope of the White Dwarf ejected during the outburst gets transparent to the X-ray emission emitted from its Hydrogen burning surface. From the delay of the onset of the X-ray emission and its duration and the X-ray spectrum, one can determine the ejected and burned mass during the outburst as well as other binary parameters. The big advantage of observations of the center area of M31 is that many novae can be monitored in the field of view of the instruments at the same time.
The analysis of archival Chandra HRC-I and ACIS-I and XMM-Newton data of the center area of M31 from July 2004 to February 2005 revealed many correlations of supersoft X-ray sources with optical novae (Pietsch et al. 2007). Important findings were that the percentage of optical novae showing an X-ray SSS phase is greater than expected from theoretical work and that novae can be discovered as SSS even within 30 days and may already turn off within 100 days after optical outburst. Based on these initial results we performed several dedicated monitoring campaigns of the central region of M 31 over the years from 2006 to 2009, involving both XMM-Newton and Chandra X-ray observatories.
To study the SSS/optical nova connection we also have initiated optical nova searches and spectroscopy as well as an optical monitoring program of the center area of M31. In this area more than 25 optical novae are expected to explode every year, which can be monitored in one XMM-Newton EPIC and Chandra HRC-I field.