Understanding Supercritical Accretion

When gas spirals towards a compact object due to its intense gravity-a process called accretion-it emits ⁣electromagnetic waves, often observed as X-rays. Some objects exhibit exceptionally high X-ray luminosities, potentially caused by ‍”supercritical accretion,” where an unusually large amount of gas is drawn in. Though, the⁤ exact mechanisms driving this process remain a mystery.

Focus⁢ on NGC 7793 P13

Researchers focused on NGC 7793 P13 (referred to‌ as P13), a neutron star undergoing supercritical accretion located in the NGC 7793 galaxy, approximately 10 million light-years from Earth. As gas descends onto the neutron star, it’s believed to form a column-like structure,⁣ known as an accretion column, on the star’s magnetic‍ poles, which emits intense X-rays.‍ This ​process causes⁤ the neutron star to exhibit ⁢coherent X-ray pulsations synchronized with its ‍rotation.

Previous studies established that P13 rotates with a period ⁢of 0.4 seconds and experiences a consistent acceleration in its rotation. Notably, its luminosity has fluctuated dramatically, changing by more than two ‍orders of magnitude over a decade. Both rotation velocity and luminosity are key indicators of the amount of gas being accreted, but a clear relationship between the two had not been previously identified for P13.

Recent Observations and Findings (2011-2024)

A research team analyzed archival data from the XMM-Newton, Chandra, NuSTAR, and NICER observatories, spanning from 2011 to 2024, to track the long-term changes ⁣in P13’s X-ray​ luminosity and rotation period. The data revealed that P13 was‍ in a relatively ⁣faint state in 2021 but began to brighten considerably in 2022. By 2024, its luminosity had increased to more than two orders of magnitude ⁤higher than it was in 2021.

Crucially, during the rebrightening phase in 2022, the acceleration rate of P13’s ⁣rotation increased by a factor of two and remained elevated through 2024. This ‌observation suggests a direct‍ correlation between X-ray luminosity and rotation velocity, indicating a change in the accretion system during ‌the faint phase. ⁢ Further analysis of the pulsations indicated that the height of the accretion column varied in sync with the observed 10-year flux​ modulation.

Implications for ​Supercritical Accretion Theory

These ‌findings offer⁣ valuable clues‍ for unraveling ‍the complexities of supercritical accretion. The observed relationship between luminosity, ​rotation, and accretion column height provides a new framework for understanding how gas behaves as ‍it⁢ falls onto a neutron ⁤star under extreme conditions. The research team believes these results will contribute to a more complete understanding⁤ of the​ mechanisms governing this‍ basic astrophysical process.