Thermally emitting isolated neutron star candidates from the SRG/eROSITA All-Sky Survey

TL;DR

This study systematically searches for X-ray dim isolated neutron stars using the eROSITA survey, identifying new candidates and analyzing their properties to enhance understanding of this elusive neutron star class.

Contribution

The paper presents a new candidate sample of XDINSs from the eROSITA survey, including 33 new candidates, and discusses their properties and the survey's completeness.

Findings

Recovered all known XDINSs and previous candidates.

Identified 33 new XDINS candidates for follow-up.

Estimated the candidate sample to be 30-50% complete.

Abstract

The SRG/eROSITA All-Sky Survey (eRASS) allows for the creation of a complete sample of X-ray dim isolated neutron stars (XDINSs), which will significantly facilitate the study of their population properties, evolution, and connection to other families of isolated neutron stars (INSs). In this work, we conduct a systematic search for XDINSs on the western Galactic hemisphere and discuss the resulting candidate sample. Consistently with the properties of the known XDINSs, we selected all eRASS sources possessing a soft X-ray spectral distribution and that are unlikely to be associated with optical or infrared sources. Our selection criteria allowed us to recover all known XDINSs and previously proposed candidates. In addition, we put forward 33 new candidate members for dedicated follow-up identification campaigns. We found the resulting candidate sample to be about 30-50% complete, mainly due to source confusion and the stringent cross-matching criteria adopted. The candidates of the sample presented here can be divided into two groups: 13 soft and 20 somewhat hard X-ray emitters. Interestingly, the thermal nature, spatial distribution, lack of known counterparts, and absence of significant flux variability of the candidates in the first group agree well with the properties of other confirmed thermally emitting INSs. For the candidates in the second group, the current observational data do not allow one to discern between rotation-powered or recycled pulsars, cataclysmic variables, or quiescent neutron stars in binary systems or even to rule out an extragalactic nature. On the basis of population synthesis and the estimated source completeness of the search, we expect that between one and three new XDINSs are among the already singled-out list of XDINS candidates - a long-sought increase in the proposed number of members of this elusive class of X-ray emitters.