Quasiperiodic oscillations and the global modes of relativistic, MHD accretion discs
Abstract
The highfrequency quasiperiodic oscillations that punctuate the light curves of Xray binary systems present a window on to the intrinsic properties of stellarmass black holes and hence a testbed for general relativity. One explanation for these features is that relativistic distortion of the accretion disc's differential rotation creates a trapping region in which inertial waves (rmodes) might grow to observable amplitudes. Local analyses, however, predict that largescale magnetic fields push this trapping region to the inner disc edge, where conditions may be unfavourable for rmode growth. We revisit this problem from a pseudoNewtonian but fully global perspective, deriving linearized equations describing a relativistic, magnetized accretion flow, and calculating normal modes with and without vertical density stratification. In an unstratified model we confirm that vertical magnetic fields drive rmodes towards the inner edge, though the effect depends on the choice of vertical wavenumber. In a global model we better quantify this susceptibility, and its dependence on the disc's vertical structure and thickness. Our calculations suggest that in thin discs, rmodes may remain independent of the inner disc edge for vertical magnetic fields with plasma betas as low as β ≈ 100300. We posit that the appearance of rmodes in observations may be more determined by a competition between excitation and damping mechanisms near the ISCO than by the modification of the trapping region by magnetic fields.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 May 2018
 DOI:
 10.1093/mnras/sty385
 arXiv:
 arXiv:1802.04792
 Bibcode:
 2018MNRAS.476.4085D
 Keywords:

 accretion;
 accretion discs;
 black hole physics;
 magnetic fields;
 MHD;
 waves;
 Xrays: binaries;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 20 pages, 17 figures, MNRAS accepted