Effects of magnetic coupling on radiation from accretion disc around a Kerr black hole

ABSTRACT The effects of magnetic coupling (MC) process on the inner edge of the disc are dis-cussed in detail. It is shown that the inner edge can deviate from the innermost stablecircular orbit (ISCO) due to the magnetic transfer of energy and angular momentumbetween a Kerr black hole (BH) and its surrounding accretion disc. It turns out thatthe inner edge could move inward and outward for the BH spin a ∗ being greater andless than 0.3594, respectively. The MC effects on disc radiation are discussed based onthe displaced inner edge. A very steep emissivity can be provided by the MC process,which is consistent with the observation of MCG-6-30-15. In addition, the BH spinsof GRO J1655-40 and GRS 1915+105 are detected by X-ray continuum fitting basedon this model.Key words: accretion, accretion disc— black hole physics — magnetic field 1 INTRODUCTIONAs is well shown, disc accretion is a very efficient energysource in astrophysics, which is widely used to explain thehigh-energy radiation of AGNs, X-ray binaries and so on. Inthe model of standard accretion disc (SAD), the motion ofthe accreting matter is assumed to be Keplerian with a smallradial velocity, and the inner edge of the disc lies at the in-nermost stable circular orbit (ISCO) of radius r