Perspectives of monitoring of cataclysmic variables in the very soft X‐ray band

Abstract

Some components and important regions of X-ray spectra of cataclysmic variables (CVs) are present only in the very soft band with energy E < 1.5 keV, which is beyond the region of sensitivity of most available X-ray monitors. We show that a population study of a large ensemble of such systems with various states of activity that will be observable by eROSITA (on board the SRG satellite) will be a suitable method of investigation even with its very sampled data. We demonstrate that the hardness ratios of such X-ray spectra show specific positions and patterns of the individual types of CVs in the hardness vs. hardness diagrams. Quiescent dwarf novae and intermediate polars constitute a long tail extending from a group of the high-state polars. Interstellar absorption plays only a marginal role in this respect. A complicated distribution of CVs in these diagrams thus mainly reflects the complexity of the emitting regions, the absorbing regions intrinsic to the source, and the emission mechanisms in various CVs and CV types. Although this is a case study using incomplete data as examples, it demonstrates serendipitous scientific potentials of the eROSITA archives.