The critical accretion luminosity for magnetized neutron stars

Abstract

The accretion flow around X-ray pulsars with a strong magnetic field is funnelled by the field to relatively small regions close to the magnetic poles of the neutron star (NS), the hotspots. During strong outbursts regularly observed from some X-ray pulsars, the X-ray luminosity can be so high, that the emerging radiation is able to stop the accreting matter above the surface via radiation-dominated shock, and the accretion column begins to rise. This border luminosity is usually called the "critical luminosity". Here we calculate the critical luminosity as a function of the NS magnetic field strength B using exact Compton scattering cross section in strong magnetic field. Influence of the resonant scattering and photon polarization is taken into account for the first time. We show that the critical luminosity is not a monotonic function of the B-field. It reaches a minimum of a few 10 36 erg s −1 when the cyclotron energy is about 10 keV and a considerable amount of photons from a hotspot have energy close to the cyclotron resonance. For small B, this luminosity is about 10 37 erg s −1 , nearly independent of the parameters. It grows for the B-field in excess of 10 12 G because of the drop in the effective cross-section of interaction below the cyclotron energy. We investigate how different types of the accretion flow and geometries of the accretion channel affect the results and demonstrate that the general behaviour of the critical luminosity on B-field is very robust. The obtained results are shown to be in a good agreement with the available observational data and provide a necessary ground for the interpretation of upcoming high quality data from the currently operating and planned X-ray telescopes.

High-mass X-ray binaries in the Milky Way

“…This behaviour was interpreted with a change of the geometry of the accretion column, which is rising above the neutron star surface at luminosities higher than the critical one (Basko and Sunyaev, 1976;Mushtukov et al, 2015). Nishimura (2008Nishimura ( , 2014 modelled the cyclotron line by the sum of the contributions emerging from individual line-forming regions along the accretion column with different magnetic field strength, temperature, and density.…”

“…The plasma falls in the accretion column at almost the speed of light and heats to 10 8 K close to the neutron star surface (see, e.g. Basko and Sunyaev, 1976;Nagel, 1981;Meszaros and Nagel, 1985;Araya-Góchez and Harding, 2000;Nishimura, 2008;Mushtukov et al, 2015). Bulk and thermal Comptonization plays a key role in the formation of the non thermal X-ray emission (Becker and Wolff, 2007).…”

Suzakuview of the Be/X-ray binary pulsar GX 304-1 during Type I X-ray outbursts

“…The magnetospheric radius of the pulsar can be used to constrain the location of additional absorbing matter in such narrow streams. The size of magnetosphere of the pulsar depends on the mass accretion rate and the strength of the magnetic field (Mushtukov et al 2015a and references therein) through the relation Rm = 2.6 × 10 8 M 1/7 R 10/7 6 B 4/7 12 L −2/7 37 cm…”

On the radiation beaming of bright X-ray pulsars and constraints on neutron star mass–radius relation

“…At mass accretion rates slightly above the critical value (Mushtukov et al 2015b) most of the X-ray flux is intercepted by the NS surface and beam pattern is defined by reflected component. The higher the mass accretion rate, the higher the accretion column, the bigger the illuminated part of NS surface, the smaller the variations of detected X-ray flux and the smaller the corresponding PF.…”

“…Mukherjee et al 2013a,b) on the NS surface at relatively low mass accretion rates ( 10 17 g s −1 ). However, at high mass accretion rates the radiation pressure becomes strong enough to stop accretion flow above NS surface (Mushtukov et al 2015b), which leads to appearance of accretion columns (this critical luminosity has been recently detected by Doroshenko et al 2017). The material is stopped at the top of accretion column at a radiation dominated shock and then settles down to the NS surface.…”