G5 Artikkeliväitöskirja
Super-critical accretion onto black holes and neutron stars
Tekijät: Chashkina Anna
Kustantaja: University of Turku
Kustannuspaikka: Turku
Julkaisuvuosi: 2019
ISBN: 978-951-29-7788-8
eISBN: 978-951-29-7789-5
Verkko-osoite: http://urn.fi/URN:ISBN:978-951-29-7789-5
Rinnakkaistallenteen osoite: http://urn.fi/URN:ISBN:978-951-29-7789-5
Accretion is one of the most important processes in astrophysics. Accretion discs are observed in a very large wavelength range, from infrared to X-rays, and in a very large range of scales: from several kilometers (accretion onto a neutron star) to parsecs (gas tori in active galactic nuclei). Physics of accretion discs is diverse, but the basic equations, equations of magnetohydrodynamics, remain the same.
Accretion can also be a radiatively efficient process. As a consequence, radiation pressure easily becomes dynamically important, and a large number of accreting sources exceed their Eddington limit – the luminosity limit beyond which radiation pressure is sufficient to destroy the accretion flow itself.
Ultraluminous X-ray sources (ULXs) are extragalactic non-nuclear sources with huge luminosities L È few£1039 erg s¡1 exceeding the Eddington luminosity limits for a stellar mass black hole. Though studied for already about 30 years, they remain interesting both as an extreme case of accreting sources and as an outcome of violent and poorly understood stellar evolution. Besides, recent discoveries have shown many ULXs to contain neutron stars with strong magnetic fields, making them even more extreme both in the sense of fundamental physics and from the point of view of the luminosity excess over the Eddington limit. To meaningfully interpret ULX observations, detailed models of super-Eddington accretion onto compact objects, both of the accretion disc and of the disc-magnetosphere interaction, are highly desirable. However, theoretical models describing accretion at such high rates are complicated by the dominance of radiation pressure, the need to account for advection of energy towards the compact object, and the possible presence of powerful outflows.
We have started our studies by deriving the critical luminosity for a disc around a black hole taking into account disc finite thickness, heat advection, and general relativity (GR) effects. GR effectively makes vertical gravity stronger, and so does inward heat advection. At the same time, normally ignored non-linear vertical gravity dependence on height in a thick disc slightly lowers the limit, resulting in an overall correction by about a factor of two with respect to the classical approach. More accurate result surprisingly depends on the two-dimensional rotation profile of the disc.
For the case of a neutron star with a strong magnetic field, the structure of the disc is not that important by itself as it is essentially invisible and unimportant for the energy budget. However, the position of the disc-magnetosphere boundary is the most important link between the fundamental parameters of the system (magnetic moment of the star, mass accretion rate, viscosity parameter etc.) and the observables (such as spin period and its derivative, power density spectrum of the variability of the source). We developed a model of an accretion disc around a neutron star taking into account the effects of advection and wind. This model may be applied to a large variety of magnetized neutron stars accreting close to or above their Eddington limits: ULX pulsars, Be/X-ray binaries in outbursts, and other systems.