Exploring the magnetic field of the ultraluminous X-ray pulsar NGC 4631 X-8

Abstract

NGC 4631 X-8 is an ultraluminous X-ray pulsar (ULXP) having a spin period of about 9.7 s, discovered using XMM-Newton observations in 2025. The pulsar is known to show one of the largest spin-up rates ($\sim -9.6\times 10^{-8}$ s s${}^{-1}$) among the ULXP population. We explore the surface magnetic field of the neutron star in this source using different models, and find that the inferred magnetic field lies in the range of about $0.3-7\times 10^{14}$G. We study the long-term magnetic field and spin period evolution of the pulsar assuming steady accretion using prevalent theoretical mechanisms and find that the pulsar will likely evolve to become a millisecond pulsar having decayed magnetic field of about $\sim 10^9$G in about a million years. The scenario of the formation of a millisecond pulsar is also probed using an estimate of the super-Eddington duty cycle of about 14% from the literature, which suggests that the neutron star would accrete sufficient matter to likely become a recycled millisecond pulsar. Exploring the magnetic field as well as the spin period evolution properties of ULXPs may enable us to understand the poorly understood evolutionary features of ULXPs, shed light on one of the pathways of millisecond pulsar formation and also help us to understand the possibility of transient super-Eddington accretion phases in newborn magnetars, which are believed to power energetic events such as long gamma-ray bursts and Type I superluminous supernovae.

10 pages, 4 figures, accepted for publication in Journal of High Energy Astrophysics (JHEAp) Letters

References in corpus (29)

• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• Formation of Double Neutron Star Systems
• An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907
• Discovery of a 0.42-s pulsar in the ultraluminous X-ray source NGC 7793 P13
• Discovery of coherent pulsations from the Ultraluminous X-ray Source NGC 7793 P13
• A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source
• Formation of millisecond pulsars with CO white dwarf companions - II. Accretion, spin-up, true ages and comparison to MSPs with He white dwarf companions
• The critical accretion luminosity for magnetized neutron stars
• The discovery of weak coherent pulsations in the ultraluminous X-ray source NGC 1313 X-2
• Evolution of a buried magnetic field in the central compact object neutron stars
• The ultra luminous X-ray source NuSTAR J095551+6940.8: A magnetar in a high mass X-ray binary
• NuSTAR J095551+6940.8: a highly magnetised neutron star with super-Eddington mass accretion
• Insight-HXMT discovery of the highest energy CRSF from the first Galactic ultra-luminous X-ray pulsar Swift J0243.6+6124
• Study of Recent outburst in the Be/X-ray binary RX J0209.6-7427 with AstroSat: A new ultraluminous X-ray pulsar in the Magellanic Bridge?
• M51 ULX-7: super-orbital periodicity and constraints on the neutron star magnetic field
• An accreting low magnetic field magnetar for the ultraluminous X-ray source in M82
• Gravitational waves from mountains in newly born millisecond magnetars
• Super-Eddington Emission from Accreting, Highly Magnetised Neutron Stars with a Multipolar Magnetic Field
• Optical counterpart to Swift J0243.6+6124
• On the Magnetic Fields, Beaming Fractions, and Fastness Parameters of Pulsating Ultra-Luminous X-Ray Sources
• Evidence for the radiation-pressure dominated accretion disk in bursting pulsar GRO J1744-28 using timing analysis
• A unified accreting magnetar model for long-duration gamma-ray bursts and some stripped-envelope supernovae
• Constraining the dipolar magnetic field of M82 X-2 by the accretion model
• Detection of nearly periodic spin period reversals in Vela X-1 on long time-scales: inkling of solar-like cycle in the donor star?
• Constraining the ellipticity of new-born magnetar with the observational data of Long gamma-ray bursts
• Study on the magnetic field strength of NGC 300 ULX1
• Gravitational Waves from Newborn Accreting Millisecond Magnetars
• Formation of millisecond pulsar-helium star binaries
• Study on the magnetic field of the ultraluminous X-ray pulsar RX J0209.6-7427