Astronomers have observed dramatic changes in the neutron star P13 over a decade, as it transitioned from a faint state to high luminosity. Located in the galaxy NGC 7793, about 10 million light-years away, P13's X-ray output and rotation rate shifted significantly. These observations provide new insights into supercritical accretion processes.
Neutron stars, the dense remnants of massive stars, can exhibit extreme behaviors when accreting gas. In the case of P13 in NGC 7793, researchers monitored its activity from 2011 to 2024 using telescopes including XMM-Newton, Chandra, NuSTAR, and NICER.
Initially identified as a neutron star in supercritical accretion, P13 rotates every 0.4 seconds with a steady acceleration. Over the decade, its X-ray luminosity varied by more than two orders of magnitude. A notable faint phase occurred in 2021, during which the star's brightness dropped significantly. By 2022, P13 began rebrightening, and by 2024, its luminosity had surged to over 100 times the 2021 level.
Accompanying this was a change in rotation dynamics. During the 2022 rebrightening, the acceleration rate of the rotation velocity doubled and remained elevated through 2024. This synchronization between luminosity increases and spin-up suggests evolving accretion structures, possibly involving shifts in the height of the accretion column on the neutron star's magnetic poles.
Such pulsations, detectable due to the star's rotation, offer clues to the mechanisms driving ultraluminous X-ray sources. The findings indicate that the accretion system underwent alterations during the faint period, linking gas inflow rates to both brightness and spin changes. This long-term monitoring highlights how supercritical accretion can produce extreme luminosities, advancing understanding of compact object physics.