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

TL;DR

This paper estimates the detection rate of neutron star-white dwarf coalescence transients by eROSITA and ART-XC, predicting thousands of events over four years but noting detection challenges due to short transient durations and limited photon counts.

Contribution

It provides the first estimates of detection rates for neutron star-white dwarf mergers with eROSITA and ART-XC, highlighting observational challenges.

Findings

Thousands of transients could be detected over four years with eROSITA.

Detection is hindered by short durations and low photon counts.

Collimation of X-ray emission may reduce observable events.

Abstract

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 [1] can be an example of this type of events as suggested by [2]. 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 - $\lesssim 10^4$ s, - mostly such transients might be visible just in one scan of telescopes ($\sim 40$ s), and so only a few photons are expected to be detected which makes definite identification without additional information very problematic.