Optically thick envelopes around ULXs powered by accreating neutron stars

Abstract

Magnetized neutron stars power at least some ultra-luminous X-ray sources. The accretion flow in these cases is interrupted at the magnetospheric radius and then reaches the surface of a neutron star following magnetic field lines. Accreting matter moving along magnetic field lines forms the accretion envelope around the central object. We show that, in case of high mass accretion rates 10 19 g s −1 the envelope becomes closed and optically thick, which influences the dynamics of the accretion flow and the observational manifestation of the neutron star hidden behind the envelope. Particularly, the optically thick accretion envelope results in a multi-color black-body spectrum originating from the magnetospheric surface. The spectrum and photon energy flux vary with the viewing angle, which gives rise to pulsations characterized by high pulsed fraction and typically smooth pulse profiles. The reprocessing of radiation due to interaction with the envelope leads to the disappearance of cyclotron scattering features from the spectrum. We speculate that the super-orbital variability of ultra-luminous X-ray sources powered by accreting neutron stars can be attributed to precession of the neutron star due to interaction of magnetic dipole with the accretion disc.

Lense-Thirring precession in ULXs as a possible means to constrain the neutron star equation of state

“…Although we cannot rule out a scenario where a radiative warp is generated in the outer disc, this would seem to demand that r sph < rm which would require extremely high (magnetar) surface dipole field strengths which can explain much of the details of ULPs including their spectra and smooth pulse profile (Dall'Osso 2016; Mushtukov et al 2017) yet is disputed by a number of authors (e.g. Kluzniak & Lasota 2015;King & Lasota 2016;Christodoulou et al 2016).…”

“…As discussed in Mushtukov et al (2017), very high field strengths (>10 14 G) can lead to precession periods ∼months-year which has the attraction that it can self-consistently explain other features where very high field strengths are invoked -although see §6 for a Figure 9. Ratio of the vertical (polar) and azimuthal components of Maxwell stress from the 3D RMHD simulation of Jiang et al (2014).…”

Super-Eddington accretion on to a magnetized neutron star

“…Usually, the adopted picture of magnetospheric accretion assumes that matter is captured by magnetic field at the boundary of the disc. If the radiation pressure from the accretion column is larger than the magnetic pressure, optically thin infalling matter may be blown away by the radiation pressure force escaping the magnetosphere (Mushtukov et al 2017). Optically thick accretion flow, on the other hand, can remain bound, but its dynamics will still be affected by the radiation pressure.…”

Predicting ultraluminous X-ray source demographics from geometrical beaming

“…However, the interpretation of the brightness of these ULPs is still contentious as the nature of the accretion flow will depend heavily on the surface dipole magnetic field strength (see e.g. Mushtukov et al 2017) and whether this has been effectively diluted by the highṁ0 (which itself is not in question). For field strengths < 10 12 G it is probable that most ULPs will be geometrically beamed as the discs reach the Eddington limit before being magnetically truncated (e.g.…”