Detectability of neutron star — White dwarf coalescences by eROSITA and ART-XC

Coalescences of neutron stars and white dwarfs are relatively frequent phenomena, outnumbering other types of compact object mergers (neutron stars and black holes without involving white dwarfs) altogether. Such event potentially can produce not only optical, but also an X-ray burst. Transient source CDF-S XT2 ( Xue et al., 2019 ) can be an example of this type of events as suggested by Fernández et al. (2019). In this note we estimate the rate of these transients in the field of view of X-ray instruments on-board Spectrum-RG satellite. We demonstrate that during four years of the survey program several thousand of events related to neutron star — white dwarf mergers might appear in the field of view of eROSITA. Collimation of X-ray emission can reduce this number. Smaller, but comparable number of transients is expected in the case of ART-XC telescope. However, due to relatively short duration — ≲104 s, — mostly such transients might be visible just in one scan of telescopes (∼40 s), and so only a few photons are expected to be detected which makes definite identification without additional information very problematic.