A sleeping neutron star suddenly comes alive again, and the cosmos watches intently.
When gas spirals toward a compact object like a neutron star or a black hole, its intense gravity drives a process known as accretion. This infalling material heats up and emits electromagnetic radiation, with some systems shining exceedingly bright in X-rays. One leading idea is that ultraluminous X-ray sources owe their power to supercritical accretion, where an unusually large amount of gas streams onto the compact object. Yet the exact workings of supercritical accretion remain puzzling and not fully understood.
The study focused on NGC 7793 P13 (referred to here as P13), a neutron star undergoing supercritical accretion in the galaxy NGC 7793, about 10 million light-years away from Earth. As gas falls onto a neutron star, it is funneled along magnetic field lines toward the magnetic poles, creating an accretion column. This column emits strong X-rays, and the rotation of the neutron star produces coherent X-ray pulsations that we can detect. Previous observations indicated that P13 spins with a period of about 0.4 seconds and that its spin is undergoing a steady acceleration. Over roughly a decade, the system’s luminosity also varied by more than a factor of 100. Both spin behavior and luminosity are valuable clues to how much gas has been accreted, but their relationship in P13 had not yet been established.
To explore this relationship, the researchers analyzed the long-term behavior of P13’s X-ray luminosity and rotation period from 2011 through 2024, using archival data from XMM-Newton, Chandra, NuSTAR, and NICER. They found that P13 was relatively faint in 2021, then brightened again in 2022. By 2024, the luminosity exceeded its 2021 level by more than two orders of magnitude. During the 2022 rebrightening, the rate at which the neutron star’s rotation was accelerating doubled and remained elevated through 2024. This pattern points to a link between X-ray brightness and spin-up rate, suggesting that the accretion system underwent a significant change during the faint phase.
The team conducted in-depth analyses of the pulsations and proposed that the height of the accretion column—how tall it stands above the surface—modulated in step with the decade-long flux changes. These insights offer important clues about the mechanism behind supercritical accretion and bring us closer to understanding how these extraordinary systems operate.
