X-ray, optical, and radio follow-up of five thermally emitting isolated neutron star candidates^★

¹ Leibniz-Institut für Astrophysik Potsdam (AIP) An der Sternwarte 16 14482 Potsdam, Germany
² Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences 99 West Lincheng Rd. 550051 Guiyang, China
³ College of Physics, Guizhou University Guiyang 550025, China
⁴ Max-Planck-Institut für extraterrestrische Physik Gießenbachstraße 1 85748 Garching, Germany
⁵ Potsdam University, Institute for Physics and Astronomy Karl-Liebknecht-Straße 24/25 14476 Potsdam, Germany
⁶ Shanghai Astronomical Observatory, Chinese Academy of Sciences Shanghai 200030, China
⁷ Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences Beijing, PR China

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 August 2025
Accepted: 3 November 2025

Abstract

We report on follow-up observations with XMM-Newton, the FORS2 instrument at the ESO-VLT, and FAST, aiming to characterise the nature of five thermally emitting isolated neutron star (INS) candidates recently discovered from searches in the footprint of the Spectrum Roentgen Gamma (SRG)/eROSITA All-sky Survey. We find that the X-ray spectra are predominantly thermal and can be described by low-absorbed blackbody models with effective temperatures ranging from 50 to 210 eV. In two sources, the spectra also show narrow absorption features at 300–400 eV. Additional non-thermal emission components are not detected in any of the five candidates. The soft X-ray emission, the absence of optical counterparts in four sources, and the consequent large X-ray-to-optical flux ratios > 3000 − 5400 confirm their INS nature. For the remaining source, eRASSU J144516.0–374428, the available data do not allow a confident exclusion of an active galactic nucleus nature. However, if the source is Galactic, the small inferred X-ray emitting region is reminiscent of a heated pulsar polar cap, possibly pointing to a binary pulsar nature. X-ray timing searches do not detect significant modulations in all candidates, implying pulsed fraction upper limits of 13–19% (0.001–13.5 Hz). The absence of pulsations in the FAST observations targeting eRASSU J081952.1–131930 and eRASSU J084046.2–115222 excludes periodic magnetospheric emission at 1–1.5 GHz with an 8σ significance down to 4.08 μJy and 2.72 μJy, respectively. The long-term X-ray emission of all sources does not imply significant variability. Additional observations are warranted to establish exact neutron star types. At the same time, the confirmation of the predominantly thermal neutron star nature in four additional sources highlights the power of SRG/eROSITA to complement the Galactic INS population.