Astronomers inspect ultraluminous X-ray pulsar’s magnetic field evolution in the Whale galaxy

Indian astronomers have investigated an ultraluminous X-ray pulsar in the galaxy NGC 4631, designated X-8. The new study, described in a paper published Feb. 16 on the arXiv preprint server, provides essential information regarding the magnetic field evolution of this pulsar.

Ultraluminous and pulsating

Ultraluminous X-ray sources (ULXs) are bright point sources characterized by X-ray emissions that exceed the total multiwavelength output of a million suns. They are less luminous than active galactic nuclei, but more consistently luminous than any known stellar process. Although numerous studies of ULXs have been conducted, the basic nature of these sources still baffles scientists.

Some persistent ULXs exhibit pulsations and therefore are categorized as ultraluminous X-ray pulsars (ULXPs). Discovering and studying objects of this type could be crucial for advancing our understanding of accretion physics—for instance, mechanisms that enable the sustained X-ray luminosities of ULXs which exceed the Eddington limit.

Newfound ULXP

NGC 4631, also known as the Whale galaxy, is a late-type starburst galaxy at a distance of some 24.45 million light-years away from Earth, known to host at least eight ULXs. In one of them, designated X-8, astronomers have recently detected pulsations with a period of approximately 9.66 seconds.

This newly found ultraluminous X-ray pulsar, which accretes mass from its companion star, was found to exhibit one of the largest spin-up rates among the known ULXPs—at a level of -9.6 × 10^-8 seconds per second.

A team of astronomers led by Amar Deo Chandra of the Aryabhatta Research Institute of Observational Sciences in India decided to further investigate X-8, exploring its surface magnetic field by employing various accretion models.

Untangling magnetic field evolution

The study found that the maximum surface dipole magnetic field strength of the neutron star is about 30–200 trillion Gauss. In general, taking into account the mass of the donor star (estimated to be between 15 and 70 solar masses) and the accretion rate, the results suggest that the magnetic field will decay to a level of 1 billion Gauss at the end of the accretion phase of the binary in about 1 million years. Then, it is expected that the neutron star in the system will become a millisecond pulsar (MSP).

The astronomers explain that the evolutionary track of X-8 in the magnetic field–spin period diagram crosses the zone of magnetars and binary pulsars above the Eddington spin-up line. It then traverses through the zone of recycled binary pulsars and finally ends around the positions of recycled MSPs at the end of the accretion phase of the binary.

The researchers underline that very little is known about the properties of the companion star in order to determine the fate of X-8. Detection of its spectral type and tighter constraints on its mass will be crucial to calculate the duration of the accretion phase of the binary and to provide better estimates of the total accreted matter by the neutron star during the accretion phase.

“Future multiwavelength observations of ULXPs can be helpful to constrain the duty cycle, mass of the companion star, and hence the duration of the accretion phase in these binary systems, which can enable us to better constrain the spin and magnetic evolution of these sources,” the authors of the paper conclude.

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