X-ray emission from accreting white dwarfs and X-ray binaries

The white dwarf (WD) in a binary system accretes material from a non-degenerate donor star. Depending on the accretion rate and WD mass, the nuclear burning of the matter on the surface of the WD can proceed in stable or unstable regimes. In a stable regime, classical steady super-soft X-ray sources (SSSs) are observed, emitting extreme ultraviolet and soft X-ray emission. Unstable hydrogen burning on the surface of WDs gives rise to the explosions of classical and recurrent novae. The nova explosion is primarily observed in the optical band. However, nuclear burning of the residual hydrogen on the surface of the WD generates soft X-ray emission, and the system is observed as a post-nova SSS. In this thesis, we study X-ray emission from accreting WDs experiencing stable or unstable thermonuclear burning on their surface. Using the sample of accreting WDs, we estimate the accretion rate on these systems and probe the boundary between two regimes of thermonuclear burning. In particular, we investigate populations of soft and super-soft X-ray sources in nearby galaxies of various morphological types with particular emphasis on characterizing populations of stable nuclear burning, accreting WDs. Using the Chandra archive, we assembled a sample of nearby galaxies suitable for studying populations of SSSs. We used a combination of hardness ratio and median energy to pre-select X-ray sources with soft spectra and used the temperature--X-ray luminosity diagram to identify nuclear-burning, accreting WDs. For spiral galaxies we found, that there is a distinct and rare population of SSSs that are largely detached from the rest of the soft sources on the X-ray luminosity-temperature plane. The boundary between these sources and the much more numerous population of harder (but still soft) sources is consistent with the boundary of stable hydrogen burning on the surface of WDs. The combined spectrum of soft sources located outside this boundary shows clear emission lines of magnesium (Mg) and sulphur (S) typical for supernova remnants. The sources inside the boundary of stable nuclear burning have super-soft continua without significant line emission and are clear candidates to the accreting nuclear burning WDs. The population of SSSs in spiral galaxies is eight times larger than in lenticular and elliptical galaxies, in broad agreement with the population synthesis calculations. The low specific frequency of SSSs, especially in early-type galaxies, supports the earlier conclusion that they are not the major class of type Ia supernova progenitors. To investigate accreting WDs with unstable hydrogen burning on their surface, we studied the X-ray emission from hosts of historical Classical Novae (CNe) in our Galaxy. To this end, we used the data of the eROSITA telescope aboard the SRG orbital observatory obtained in its all-sky survey. Cross-matching the list of historical CNe with SRG/eROSITA sources catalogue, we found that the majority of them are cataclysmic variables (CVs) in a quiescent state. Their X-ray emission is associated with the accretion of material in the binary system and is produced predominantly in the boundary layer near the WD surface and the hot corona of the accretion disk. Among CVs, we discovered two new soft X-ray sources associated with the post-novae phase. A surprisingly large fraction of sources shows hard X-ray spectra, suggesting that these CVs may harbour magnetised WDs. The CN counterparts represent a bona fide sample of accreting WDs with unstable hydrogen burning on their surface. To probe the boundary between stable and unstable nuclear burning on the surface of accreting WD, we used samples of stable SSSs and CNe in quiescence. We obtained the accretion rate distribution of WDs with unstable hydrogen burning using the X-ray luminosity of CN counterparts in quiescence. We compared it to the accretion rate distribution of known stable SSSs in our and nearby galaxies. We found clear dichotomy between these two distributions, where the CN sources (Macc = 1e−12 − 1e−8 Msun/yr) and stable SSSs (Macc = 1e−7.5 − 1e−6 Msun/yr) occupy different domains in Macc, as predicted by the theory of thermonuclear hydrogen burning on the surface of the WD. In this thesis, we also study the X-ray emission of accreting black holes (BH) and neutron stars (NS) in star-forming galaxies. In high-mass X-ray binaries (HMXBs), a compact object, a NS or a BH, accretes material from a massive donor star via stellar wind or Roche-lobe overflow. The donor star with the mass of Mdonor>5 Msun has a lifetime of about 10-100 Myr, which is comparable to the time-scale of galaxies’ recent star formation activity. Therefore, the combined emission of HMXBs in a galaxy is expected to correlate with the star-formation rate (SFR). We construct the SRG/eROSITA-IRAS catalog of 1360 star-forming galaxies which is about five times bigger than samples used in previous studies. Based on the analysis of X-ray spectra of galaxies, we estimate the contribution of the nuclear activity and clean the sample galaxies containing a low luminosity AGN. Using the SRG/eROSITA - IRAS sample of star-forming galaxies, we investigate the scaling relation between X-ray luminosity and SFR for HMXBs and hot ISM. We use our measurement, along with other Lx/SFR measurements for HMXBs, to investigate the metallicity dependence of Lx/SFR for HMXBs