Novae as a Class of Transient X-Ray Sources

Motivated by the recently discovered class of faint (10³⁴-10³⁵ ergs s^-1) X-ray transients in the Galactic center region, we investigate the 2-10 keV properties of classical and recurrent novae. Existing data are consistent with the idea that all classical novae are transient X-ray sources with durations of months to years and peak luminosities in the 10³⁴-10³⁵ ergs s^-1 range. This makes classical novae a viable candidate class for the faint Galactic center transients. We estimate the rate of classical novae within a 15^' radius region centered on the Galactic center (roughly the field of view of XMM-Newton observations centered on Sgr A*) to be ~0.1 yr^-1. Therefore, it is plausible that some of the Galactic center transients that have been announced to date are unrecognized classical novae. The continuing monitoring of the Galactic center region carried out by Chandra and XMM-Newton may therefore provide a new method to detect classical novae in this crowded and obscured region, where optical surveys are not, and can never hope to be, effective. Therefore, X-ray monitoring may provide the best means of testing the completeness of the current understanding of the nova populations.