Publication date: n/a 2006
Abstract:
Change of sign of the velocity gradient (mesured with respect to locally
non-rotating frames) has been found for accretion discs orbiting rapidly
rotating Kerr black holes with spin a > 0.9953 for Keplerian discs
[1] and a > 0.99979 for marginally stable thick discs [2]. Such
"humpy" orbital velocity profiles occur close to but above the
marginally stable circular geodesic of the black hole spacetimes. The
maximal positive rate of change of the orbital velocity in terms of the
proper radial distance introduces a locally defined critical frequency
characterizing any processes in the disc capable to excite possible
oscillations connected with the velocity hump. Comparing the "humpy
frequency" related to distant observers with the epicyclic frequencies
we show that in Keplerian discs orbiting extremely rapid Kerr holes (1 –
a < 10-4 ) the ratio of the epicyclic frequencies and the humpy
frequency is nearly constant, i.e., almost independent of a, being
˜ 3 : 2 for the radial epicyclic frequency and ˜ 11 : 2 for
the vertical epicyclic frequency. For black holes with a ˜ 0.996,
i.e., when the resonant phenomena with ratio 3 : 1 between the vertical
and radial epicyclic oscillations occur near the radius of the critical
humpy frequency, the ratio of the radial epicyclic and the humpy
frequency is ˜ 12 : 1, which is close to the ratio between high-
and low- frequency QPO in X-ray systems. For a > 0.996 the resonant
orbit r4:1 (with the ratio 4 : 1 between the vertical and radial
epicyclic oscillations) occurs in the region of the hump. Applying the
model on the nearly extreme black hole candidate GRS 1915+105, we
conclude that for black hole parameters M = 14.8M⊙ and a = 0.9998
the observed high-frequency QPOs could be related to the hump-induced
oscillations in thin accretion disc, as the first two QPOs, 41 Hz and 67
Hz, can be identified with the "humpy frequency" and the radial
epicyclic frequency (at the same orbit). The other observed
QPO-frequencies, 113 Hz and 166 Hz, can be explained as the
combinational ones of the "humpy" and epicyclic frequencies.
Authors:
Stuchlik, Zdenek; Slany, P.; Török, G.;