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X-ray observations of historical classical nova counterparts with eROSITA telescope aboard SRG orbital observatory during the all-sky survey

X-ray emission from counterparts of historical classical novae (CNe) in our Galaxy is studied. To this end, we use data from three SRG/eROSITA sky surveys in the hemisphere analyzed by the RU eROSITA consortium. Out of 309 historical CNe, X-ray emission has been detected from 52 sources with 0.3-2.3 keV luminosities in the $\rm L_X\approx 10^{30}\sim 10^{34}$ erg/s range. Among them, two sources have supersoft spectra and are associated with the post-nova supersoft X-ray emission. Hardness of X-ray spectra of some of the remaining sources tentatively suggests that magnetized white dwarfs (WDs) may account for some fraction of CN counterparts detected in X-rays. This hypothesis will be further tested in the course of the following SRG/eROSITA sky surveys. The CN counterparts represent a bona fide sample of accreting WDs with unstable hydrogen burning on their surface, while their X-ray luminosity in quiescence is a reasonable proxy for the accretion rate in the binary system. Using this fact, we have constructed the accretion rate distribution of WDs with unstable hydrogen burning and compared it with the accretion rate distribution of known steady supersoft X-ray sources in our Galaxy and nearby external galaxies. There is a pronounced dichotomy between these two distributions with the CN counterparts and the steady supersoft sources occupying different domains along the mass accretion rate axis, in accordance with the predictions of the theory of hydrogen burning on the WD surface.