Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

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

J. Kurpas, A. M. Pires, A. D. Schwope, B. Li, D. Yin, F. Haberl, M. Krumpe, S. Sheth, I. Traulsen, Z. L. Zhang

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 $μ$Jy and 2.72 $μ$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.

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

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 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 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% ( 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 significance down to 4.08 Jy and 2.72 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.

Paper Structure

This paper contains 13 sections, 2 equations, 5 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. XMM-Newton
  4. ESO-VLT
  5. FAST
  6. Results
  7. X-ray spectral analysis
  8. Limits on high-energy excess
  9. X-ray short- and long-term variability
  10. Radio results for J0819 and J0840
  11. Limits on the optical emission
  12. Discussion
  13. Summary and conclusions

Figures (5)

  • Figure 1: EPIC-pn spectra and best-fit spectral models of all five candidates. The spectral parameters of the presented fits are listed in Table \ref{['tab_fitres']}.
  • Figure 2: Power spectra from a $Z^2_1$-search in the $10^{-3}-13.5$ Hz band using the event arrival times from EPIC-pn. The dashed green lines indicate the $1-3\sigma$ significance levels (from lowest to highest).
  • Figure 3: Long-term X-ray light curves in the 0.2 -- 12 keV band, including flux values and $3\sigma$ upper limits from archival X-ray observations and measurements obtained from spectral fitting (green; Table \ref{['tab_fitres']}).
  • Figure 4: FORS2 $R\_SPECIAL$ band finding charts of the fields containing the five INS candidates. We indicate the X-ray sky localisation from XMM-Newton (green; 90% confidence region; Table \ref{['tab_pos']}), the X-ray sky position from eROSITA 2024AA...687A.251K, and the nearby field sources identified from a SExtractor run 1996AAS..117..393B. (Bottom right:) X-ray-to-optical flux ratio vs hardness ratio diagram, indicating the five candidates (black markers), the known XDINSs on the western Galactic hemisphere, including the recently discovered candidate eRASSU J131716.9--402647 2024AA...683A.164K, as well as other soft X-ray emitting sources, namely AGNs 2025arXiv250902842S, coronal emitters 2024AA...684A.121F, CVs 2003AA...404..301R2017ApJ...835...64G, low-mass XRBs 2023AA...675A.199A, and high-mass XRBs 2023AA...677A.134N.
  • Figure 5: Hardness-ratio diagram presenting the candidates (black), XDINSs including eRASSU J131716.9--402647 (magenta), and prevalent soft X-ray emitting source types such as AGNs (grey), coronal emitters (green), CVs (blue), low-mass XRBs (red), and high-mass XRBs (orange).