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

TL;DR

This study uses multi-wavelength follow-up observations to confirm the thermally emitting isolated neutron star nature of five candidates discovered by SRG/eROSITA, revealing their thermal spectra, high X-ray-to-optical flux ratios, and lack of pulsations.

Contribution

It provides the first detailed multi-wavelength characterization of five new thermally emitting isolated neutron star candidates from SRG/eROSITA data, confirming their nature and properties.

Findings

Four sources confirmed as thermally emitting INSs with thermal spectra.

No significant pulsations detected in timing analyses.

One source possibly an active galactic nucleus or binary pulsar.

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 $8\sigma$ significance down to 4.08 $\mu$Jy and 2.72 $\mu$Jy, 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.