XMM-Newton and SRG/eROSITA observations of the isolated neutron star candidate 4XMM J022141.5-735632

TL;DR

This study investigates a candidate isolated neutron star using X-ray and optical observations, finding a very soft X-ray spectrum, no optical counterpart, and stable flux over ten years, suggesting it is a genuine INS.

Contribution

First detailed follow-up of a new INS candidate from the 4XMM-DR9 catalog using XMM-Newton and SRG/eROSITA data, confirming its properties and stability.

Findings

Soft X-ray energy distribution consistent with INS

No optical counterpart detected, high X-ray-to-optical flux ratio

Stable flux and spectral state over nearly ten years

Abstract

We report the results of follow-up investigations of a possible new thermally emitting isolated neutron star (INS), 4XMM J022141.5-735632, using observations from XMM-Newton and Spectrum Roentgen Gamma (SRG) eROSITA. The analysis is complemented by Legacy Survey imaging in the optical and near-infrared wavelengths. The X-ray source, the first to be targeted by XMM-Newton in an effort to identify new INS candidates from the fourth generation of the XMM-Newton serendipitous source catalogue Data Release 9 (4XMM-DR9), shows a remarkably soft energy distribution and a lack of catalogued counterparts; the very high X-ray-to-optical flux ratio virtually excludes any other identification than an INS. Within current observational limits, no significant flux variation nor change of spectral state is registered over nearly ten years. Future dedicated observations, particularly to search for pulsations, are crucial to shed further light on the nature of the X-ray source and relations to other Galactic neutron stars.