Researchers study the pulsations of the ultra-luminous X-ray pulsar M82 X-2

Using NASA’s Chandra spacecraft and ESA’s XMM-Newton satellite, Chinese astronomers inspected the pulsations of an ultra-luminous X-ray pulsar known as M82 X-2. The study results indicate that the pulsar exhibits a long-term downward trend. The study was detailed in an article published December 28 on the preprint server. arXiv.

Ultraluminous X-ray (ULX) sources are point sources in the sky that are so bright in X-rays that each emits more radiation than a million suns emit at all wavelengths. Although they are less luminous than active galactic nuclei, they are more constantly luminous than any known stellar process.

Astronomers generally think that because of their brightness, most ULXs are black holes. However, recent observations have shown that some ULXs exhibit coherent pulsations. These sources, known as ultraluminous X-ray pulsars (ULXPs), are neutron stars that are generally less massive than black holes. The list of known ULPs is still relatively short; thus, the study of objects in this class is essential for researchers exploring the universe with X-rays.

M82 X-2 is the first ULXP discovered. It is located in the Cigar Galaxy (also known as Messier 82), at a distance of approximately 12 million light years. The system consists of a neutron star, with a rotation period of about 1.37 seconds, and a much more massive companion.

Since many properties of M82 X-2 still remain unclear, a team of astronomers led by Jiren Liu analyzed archival data from Chandra and XMM-Newton to better understand its nature, including its pulsation.

Liu’s team successfully measured the pulsation behavior of the M82 period of 20 years, with occasional rotation events. Such spin behavior is typically observed in Be-type X-ray binaries (BeXB).

“BeXBs typically exhibit a rotational tendency during a giant blast or explosion, and display a continued downward trend when there is much less mass to accumulate,” the researchers explained.

According to the study, the changing spin-down/up behavior of the M82 X-2 seems to suggest that its accretion torque, and therefore its accretion rate, must vary. Based on these results, astronomers assume that M82 X-2 may have a variable accretion rate and that for long periods it remains in a relatively quiet state and slows down.

In trying to explain the cause of the downward trend observed in M82 X-2, the authors of the paper consider a few possibilities. They assume that the most plausible hypothesis is that such a trend is due to magnetic threading. If this scenario is true, they estimate that the dipole magnetic field of this pulsar is at 12,000 billion Gauss.

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