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    Humpy LNRF-velocity profiles in accretion discs orbiting rapidly rotating Kerr black holes: a possible relation to QPOs

    Zdeněk Stuchlík · Leden 01, 2006 · Fyzika · 0 comments
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    Publication date: Jan 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.;

    https://ui.adsabs.harvard.edu/abs/2006smqw.confE..95S

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