A distant neutron star unexpectedly reawakened, unleashing a powerful X-ray surge that may reveal how the universe’s most extreme stars feed.
A research team has studied the long-term changes in X-ray emissions from the neutron star NGC 7793 P13, which is believed to be powered by supercritical accretion. This process involves a massive amount of gas falling onto the neutron star, generating intense X-rays. The researchers discovered a connection between the star’s X-ray brightness and its rotation speed, offering valuable insights that may help explain the underlying mechanism of supercritical accretion.
When gas is pulled in by the intense gravity of a compact object such as a neutron star or a black hole, a process known as accretion, it releases energy in the form of electromagnetic radiation. Advances in high-sensitivity telescopes have revealed sources that shine with exceptionally powerful X-ray output.
One leading idea is that these extreme emissions occur when unusually large amounts of gas stream onto a compact object through a phenomenon known as supercritical accretion. Despite this progress, the physical processes that enable supercritical accretion are still not well understood.
To explore this mystery, the research team examined NGC 7793 P13 (hereafter, P13), a neutron star undergoing supercritical accretion in the galaxy NGC 7793 (about 10 million light-years from the Earth). As matter spirals toward a neutron star, its strong magnetic field guides the inflowing gas toward the magnetic poles, where it forms tall, narrow structures known as accretion columns. These regions are believed to be the source of the system’s intense X-ray emission.
As the neutron star spins, its X-ray emission can appear as regular, repeating pulses that follow the star’s rotation. Earlier research showed that P13 has a rotation period of about 0.4 s and that its spin rate increases at a nearly steady pace.
Over a timescale of roughly 10 years, the system’s brightness also varied dramatically, changing by more than a hundredfold. Both the spin behavior and the X-ray brightness are useful indicators of how much material is falling onto the neutron star, yet for P13 no clear connection between these two measurements had been identified.
Long-Term Variability and Clues to Supercritical Accretion
The research team investigated the long-term evolution of the X-ray luminosity and rotation period of P13 from 2011 to 2024, using the archival data of XMM-Newton, Chandra, NuSTAR, and NICER. It was found that P13 was in a faint phase in 2021 and started to be bright again in 2022. By 2024, it reached a high luminosity, more than two orders of magnitude higher than in 2021. Moreover, in the rebrightening phase in 2022, the acceleration rate of the rotation velocity was increased by a factor of 2, and it was maintained until 2024.
This result suggests a relationship between X-ray luminosity and rotation velocity, and that the accretion system changed during the faint phase. The research team then focused on the pulsation and performed detailed analyses. It was suggested that the height of the accretion column was changed with the 10-year flux modulation. Those results are expected to be clues to reveal the mechanism of supercritical accretion.
Reference: “Monitoring of the Spectral and Timing Properties of the Ultraluminous X-Ray Pulsar NGC 7793 P13” by Marina Yoshimoto, Tomokage Yoneyama, Shogo B. Kobayashi, Hirokazu Odaka, Taiki Kawamuro and Hironori Matsumoto, 29 October 2025, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ae018f
Funding: Japan Science and Technology Agency, Japan Society for the Promotion of Science
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