Relativistic effects on coronal ejection in variable X-ray sources

TL;DR

This paper investigates how general relativistic effects influence the ejection of coronal plasma in neutron star systems during X-ray bursts, highlighting the role of radiation pressure and luminosity fluctuations.

Contribution

It provides a relativistic analysis of coronal ejection mechanisms, emphasizing the impact of radiation drag and luminosity changes near neutron stars.

Findings

Coronal ejection occurs at luminosities near the Eddington limit.

Outer coronae can be ejected by sub-Eddington outbursts.

Results at large distances are unaffected by Poynting-Robertson drag.

Abstract

Optically thin coronae around neutron stars suffering an X-ray burst can be ejected as a result of rapid increase in stellar luminosity. In general relativity (GR), radiation pressure from the central luminous star counteracts gravitational attraction more strongly than in Newtonian physics. However, motion near the neutron star is very effectively impeded by the radiation field. We discuss coronal ejection in a general relativistic calculation of the motion of a test particle in a spherically symmetric radiation field. At every radial distance from the star larger than that of the ISCO, and any initial luminosity of the star, there exists a luminosity change which leads to coronal ejection. The luminosity required to eject from the system the inner parts of the optically thin neutron-star corona is very high in the presence of radiation drag and always close to the Eddington luminosity. Outer parts of the corona, at a distance of ~20 $R_G$ or more, will be ejected by a sub-Eddington outburst. Mildly fluctuating luminosity will lead to dissipation in the plasma and may explain the observed X-ray temperatures of coronae in low mass X-ray binaries (LMXBs). At large radial distances from the star ($3\cdot 10^3 R_G$ or more) the results do not depend on whether or not Poynting-Robertson drag is included in the calculation.