Old Galactic novae in the eROSITA All-Sky Survey

Context. Nova explosions occur on accreting white dwarfs. A thermonuclear runaway in the H-rich accreted envelope causes its ejection without destroying the white dwarf, and an increase in the luminosity by several magnitudes. Accretion is re-established some time after the explosion. The explosion of the nova itself is expected to affect the mass-transfer rate from the secondary and the accretion rate, but these effects have been little explored observationally. Most novae are observed only in outburst, and the properties of the host systems are unknown. Aims. X-ray observations of novae happen mostly during outburst; only a few have been the target of dedicated X-ray observations years or decades after outburst. However, the X-ray emission long after the outburst provides a powerful diagnostic of the accretion rate and the possible magnetic nature of the white dwarf. Methods. We explored the first two years of the SRG/eROSITA All-Sky Survey (eRASS) for X-ray sources correlated with Galactic historical novae. We present the first population study of nova hosting systems in X-rays, focusing on the evolution of the accretion rate as a function of time since the last outburst, and look for new candidates for magnetic systems. Results. In total, 32 X-ray counterparts of novae are found in the western Galactic hemisphere. Combined with 53 nova detections published for the eastern hemisphere, the fraction of X-ray detected novae in quiescence is 18% of the Galactic novae. We have, for the first time, enough statistics to observationally determine the evolution of accretion rate as a function of time since the last nova outburst for a time span of 120 years. The results confirm that magnetic systems remain systematically at higher fluxes and that accretion is enhanced during the first years after the outburst, as theoretically predicted. We also identify new intermediate polar candidates in AT Cnc and RR Cha.