The X-ray Supersoft Source State of Optical Novae

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, 2006). 21 optical novae have been detected. Optical novae show up as supersoft X-ray sources when the ejected envelope of the White Dwarf 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 the duration and 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 (Fig. 1).


Fig 1: The optical image (left, obtained at the Calar-Alto observatory near Granada in Spain) shows the outburst of a nova in the Andromeda galaxy M31 in optical light (light curve in green). About 180 days after the optical outburst the nova was detected as supersoft X-ray source. This is demonstrated in the Chandra image (right) and the blue X-ray light curve measured with XMM-Newton and Chandra.

Fig 2: Chandra HRC I and ACIS I X-ray light curves for several novae in 2004/5 which showed short X-ray bright states (Pietsch et al. 2007).



An 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 more optical nova correlations (Pietsch et al. 2007). Important findings are that the percentage of optical novae showing a X-ray SSS phase is greater than 30 % and that novae can be discovered as SSS within 30 days and may already turn off within 100 days after optical outburst (Fig. 2).

To study the SSS/optical nova connection we have initiated optical nova searches and spectroscopy as well as a 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.



Catalogs of Optical Nova in Nearby Galaxies

Supersoft Sources in M31

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