Bimodal character of black hole accretion

Details Bimodal character of black hole accretion Bimodal character of black hole accretion

Jun 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988cha%26a..12..119j&link_type=abstract

Chinese Astronomy and Astrophysics, Volume 12, Issue 2, p. 119-128.

Astronomy and Astrophysics

Astrophysics

Scientific paper

Accretion onto a black hole must be transonic. For a stationary, adiabatic flow the specific energy E, specific angular momentum L and mass accretion rate M are constant in space. The condition for regularity of the transonic solution, F(E, L, M) = 0, reduces the number of independent parameters to two. For a fixed pair of E (>0) and L satisfying Ec > E > EBarrL where Ec is a critical
value and EBarr is the potential barrier connected with the centrifugal force, the regularity condition equation gives two different formal accretion rates corresponding to different locations of the sonic point in the flow. However, the physically acceptable global solution is unique: it is always realized for the smaller of the two accretion rates.
For a non-rotating black hole the accretion occurs in one of two modes: quasi-spherical or the Bondi type in which both the rotational and relativistic effects are negligible in the transonic part, and disk type in which they are dominant. Transition between the two, which is based on a discontinuous jump in the location of the sonic point, is caused by a continuous change in the flow parameter (angular momentum, say). The Bondi type accretion defines a high state and the disk type a low state, in the sense that the former always requires a higher accretion rate.
A time-dependent accretion flow would probably oscillate between the two modes, exhibiting a periodic or quasi-periodic or chaotic behaviour. This could be used to explain the luminosity variability of active galactic nuclei and Cyg X-l, thus providing a strong observational support for the existence of black holes.

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