Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21537101m&link_type=abstract
American Astronomical Society, AAS Meeting #215, #371.01; Bulletin of the American Astronomical Society, Vol. 42, p.579
Astronomy and Astrophysics
Astronomy
Scientific paper
Accretion discs around black holes fed with angular momentum that is misaligned with the spin of the black hole can become warped. The inner parts align with the black hole spin by the Lense-Thirring effect. We solve analytically the accretion disc equations in steady state, neglecting the non-linear term, in the case where the viscosity is a power law in radius from the black hole. With this we can make disc models more quickly than with a numerical solution. We show that such models fit the shape of the maser observations in the active galactic nucleus NGC 4258 well. We conclude that the Lense-Thirring effect could be responsible for the warping but the timescale on which it happens is long. A companion star to a black hole exerts a tidal torque on the disc that aligns the outer parts with the binary orbital plane. We solve steady state accretion disc equations for the shape of a disc warped by this torque. We then analytically match our two solutions with the different torques to find a solution for the whole disc shape. We find it is a good approximation to the numerical solution up to a misalignment of about 60 degrees.
We find the torque on a misaligned black hole from the disc and the timescale on which the warped disc causes the black hole to precess and align with the outer parts of the disc. We calculate the alignment timescale and lifetime of steady mass-transfer in the microquasar GRO J1655-40 and find it is not unexpected that the system is still misaligned.
Martin Rebecca G.
Pringle James E.
Tout Christopher A.
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