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

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.

The Zoo of Isolated Neutron Stars

“…Again, the spin period (18.6 ms) does not fit the M7 family [81]. Finally, the latest discovery is the source 4XMM J022141.5-735632 [82], which for this object, the spin period has not yet been reported.…”

X-ray observations of isolated neutron stars

“…This could be caused by an unfavorable orientation of the rotation and magnetic axes, or by the intrinsic lack, or faintness, of non-thermal magnetospheric emission. The most promising INS candidates are 2XMM J104608.7−594306 [94,95], 4XMM J022141.5−735632 [96,97], eRASSU J065715.3+260428 and eRASSU J131716.9−402647 [98], but with the data release of the first eROSITA catalog we expect a significant enlargement of the sample [99,100]. [105,106].…”

A Markedly Expanded Sample of Candidate X-Ray-emitting Isolated Neutron Stars

“…The RASS Faint Source Catalogue (RASS-FSC) extension includes even more sources with detection likelihoods as low as 7 and a source count minimum of 6 (Voges et al 2000). Since the mid-2000s, there have been five additional XINS discoveries: one from within the 1RXS catalog (Rutledge et al 2008), one from within the 2XMMp catalog (Pires & Motch 2008;Pires et al 2009aPires et al , 2009bPires et al , 2015, and three that were found even more recently using data from either eROSITA in conjunction with the 4XMM-DR9 catalog (Pires et al 2022) or from eROSITA alone (Kurpas et al 2023). A list of the currently known radioquiet isolated neutron stars that emit strongly in X-rays is provided in Table 1, the final column of which will be discussed below in Section 2.…”