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Discovery of a 9.67-s pulsar in an ultraluminous X-ray source in NGC 4631 with XMM-Newton

L. Ducci, S. Mereghetti, F. Pintore, S. Allak, A. Santangelo, M. Sasaki, P. Kavanagh

TL;DR

This study reports the discovery of a new pulsating ultraluminous X-ray source, X-8, in NGC 4631 with XMM-Newton. The NS exhibits a spin period of about 9.67 s and an exceptionally rapid spin-up (Ṗ ≈ −9.6 × 10^−8 s s^−1), with a peak luminosity near 3.4 × 10^39 erg s^−1. Phase-resolved spectroscopy reveals a pulsed component distinguishable from a steady, disk-like emission, consistent with other pulsating ULXs. The work discusses both orbital Doppler effects and accretion torque as drivers of the spin evolution and uses accretion-torque models to estimate the magnetic field (B ∼ 10^13–10^14 G) and disk structure, providing a key dataset to constrain the physics of super-Eddington accretion in NS ULXs and guiding future timing observations to determine orbital parameters and beaming geometry.

Abstract

Thanks to a recent observation with XMM-Newton, we discovered periodic pulsations at P= 9.6652 +/- 0.0002 s in a new ultraluminous X-ray source (ULX) in the galaxy NGC 4631. This source, dubbed as X-8, shows one of the largest spin-up rates ever observed, dP/dt = (-9.6 +/- 0.5)*1E-8 s/s. These findings indicate that the compact object is a neutron star, and X-8 is a new member of the pulsating ULX class. The 0.3-10 keV luminosity of X-8 is ~3.4E39 erg/s, and its X-ray spectrum can be described by an absorbed disk blackbody or a cut-off power law, similar to what is observed in other pulsating ULXs. We discuss two possible causes for the large spin-up rate: Doppler shift from orbital motion of the neutron star and intrinsic spin-up due to accretion torque. This new ULX pulsar adds a key source to the small known population, and will enable future studies to better constrain the physical mechanisms responsible for their super-Eddington luminosities.

Discovery of a 9.67-s pulsar in an ultraluminous X-ray source in NGC 4631 with XMM-Newton

TL;DR

This study reports the discovery of a new pulsating ultraluminous X-ray source, X-8, in NGC 4631 with XMM-Newton. The NS exhibits a spin period of about 9.67 s and an exceptionally rapid spin-up (Ṗ ≈ −9.6 × 10^−8 s s^−1), with a peak luminosity near 3.4 × 10^39 erg s^−1. Phase-resolved spectroscopy reveals a pulsed component distinguishable from a steady, disk-like emission, consistent with other pulsating ULXs. The work discusses both orbital Doppler effects and accretion torque as drivers of the spin evolution and uses accretion-torque models to estimate the magnetic field (B ∼ 10^13–10^14 G) and disk structure, providing a key dataset to constrain the physics of super-Eddington accretion in NS ULXs and guiding future timing observations to determine orbital parameters and beaming geometry.

Abstract

Thanks to a recent observation with XMM-Newton, we discovered periodic pulsations at P= 9.6652 +/- 0.0002 s in a new ultraluminous X-ray source (ULX) in the galaxy NGC 4631. This source, dubbed as X-8, shows one of the largest spin-up rates ever observed, dP/dt = (-9.6 +/- 0.5)*1E-8 s/s. These findings indicate that the compact object is a neutron star, and X-8 is a new member of the pulsating ULX class. The 0.3-10 keV luminosity of X-8 is ~3.4E39 erg/s, and its X-ray spectrum can be described by an absorbed disk blackbody or a cut-off power law, similar to what is observed in other pulsating ULXs. We discuss two possible causes for the large spin-up rate: Doppler shift from orbital motion of the neutron star and intrinsic spin-up due to accretion torque. This new ULX pulsar adds a key source to the small known population, and will enable future studies to better constrain the physical mechanisms responsible for their super-Eddington luminosities.

Paper Structure

This paper contains 9 sections, 8 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Data analysis
  3. Results
  4. Discovery of a pulsation and a new ULX
  5. Timing analysis
  6. Spectral analysis
  7. Discussion
  8. Conclusions
  9. Log of the observations

Figures (8)

  • Figure 1: Combined XMM-Newton EPIC image of NGC 4631 in the 0.3--10 keV band from the July 2025 observation. The locations of known ULXs and the new ULX X$-$8 are marked.
  • Figure 2: $Z^2_1$ test computed for the pn data of X$-$8, in the energy range 0.2--12 keV.
  • Figure 3: EPIC images of the field around ULXs X$-$2 and X$-$8 in three energy bands. The images are a combination of pn, MOS1, and MOS2 data using the SAS emosaic task. The circles indicate the 95% positional uncertainty for each source (X$-$2: $0.7\arcsec$; X$-$8: $0.97\arcsec$).
  • Figure 4: pn images (0.2$–$12 keV) of the On-Pulse (left) and Off-Pulse (right) phase intervals for the region containing X$-$8 and X$-$2. The circles indicate the 95% positional uncertainty for each source.
  • Figure 5: Pulse profile of X$-$8 obtained from pn data in the energy band 0.2$-$12 keV. The profile is background subtracted and corrected for the spin-up. The inset shows the pulsed fractions in two sub-bands.
  • ...and 3 more figures