24 research outputs found
LNRF-velocity hump-induced oscillations of a Keplerian disc orbiting near-extreme Kerr black hole: A possible explanation of high-frequency QPOs in GRS 1915+105
At least four high-frequency quasiperiodic oscillations (QPOs) at frequencies
41Hz, 67Hz, 113Hz, and 167Hz were reported in a binary system GRS 1915+105
hosting near-extreme Kerr black hole with a dimensionless spin a>0.98. We use
the idea of oscillations induced by the hump of the orbital velocity profile
(related to locally non-rotating frames - LNRF) in discs orbiting near-extreme
Kerr black holes, which are characterized by a "humpy frequency" f_h, that
could excite the radial and vertical epicyclic oscillations with frequencies
f_r, f_v. Due to non-linear resonant phenomena the combinational frequencies
are allowed as well. Assuming mass M=14.8M_sun and spin a=0.9998 for the GRS
1915+105 Kerr black hole, the model predicts frequencies f_h=41Hz, f_r=67Hz,
(f_h+f_r)=108Hz, (f_v-f_r)=170Hz corresponding quite well to the observed ones.
For black-hole parameters being in good agreement with those given
observationally, the forced resonant phenomena in non-linear oscillations,
excited by the "hump-induced" oscillations in a Keplerian disc, can explain
high-frequency QPOs in GRS 1915+105 within the range of observational errors.Comment: 4 pages, 2 figures, accepted for publication in Astronomy and
Astrophysics, added references, corrected typo
Humpy LNRF-velocity profiles in accretion discs orbiting nearly extreme Kerr black holes. A possible relation to QPOs
Change of sign of the LNRF-velocity gradient has been found for accretion
discs orbiting rapidly rotating Kerr black holes with spin a > 0.9953 for
Keplerian discs and a > 0.99979 for marginally stable thick discs. Aschenbach
(2004) has identified the maximal rate of change of the orbital velocity within
the "humpy" profile with a locally defined critical frequency of disc
oscillations, but it has been done in a coordinate-dependent form. We define
the critical "humpy" frequency H in general relativistic, coordinate
independent form, and relate the frequency defined in the LNRF to distant
observers. At radius of its definition, so-called "humpy" radius r_h, the
"humpy" frequency H is compared to the radial (R) and vertical (V) epicyclic
frequencies and the orbital frequency of the disc. For Keplerian thin discs, we
show that the epicyclic resonance radii r_31 and r_41 (with V:R = 3:1 or 4:1)
are located in vicinity of r_h where efficient triggering of oscillations with
frequencies ~ H could be expected. Asymptotically (for 1-a < 10^(-4)) the ratio
of the epicyclic and Keplerian frequencies and the humpy frequency is nearly
constant, i.e., almost independent of spin, being for the radial epicyclic
frequency R:H ~ 3:2. For thick discs the situation is more complex due to
dependence on distribution of the specific angular momentum l determining the
disc properties. For l = const tori and 1-a < 10^(-6) the frequency ratios of
the humpy frequency and the orbital and epicyclic frequencies are again nearly
constant and independent of both a and l, being for the radial epicyclic
frequency R:H close to 4. In the limiting case of very slender tori (l ~ l_ms)
the epicyclic resonance radius r_41 ~ r_h for spin 1-a < 2x10^(-4).Comment: 11 pages,10 figures, 1 table. Accepted for publication in Astronomy
and Astrophysic
Report on workshop A1: Exact solutions and their interpretation
I report on the communications and posters presented on exact solutions and
their interpretation at the GRG18 Conference, Sydney.Comment: 9 pages, no figures. Many typos corrected. Report submitted to the
Proceedings of GR18. To appear in CQ
Disentangling the NIR/optical emission of the black hole XTE J1650-500 during outburst
Context: While the sources of X-ray and radio emission in the different states of low-mass X-ray binaries are relatively well understood, the origin of the near-infrared (NIR) and optical emission is more often debated. It is likely that the NIR/optical flux originates from an amalgam of different emission regions, because it occurs at the intersecting wavelengths of multiple processes. Aims: We aim to identify the NIR/optical emission region(s) of one such low-mass X-ray binary and black hole candidate, XTEâJ1650â500, via photometric, timing, and spectral analyses.Methods: We present unique NIR/optical images and spectra, obtained with the ESO-New Technology Telescope, during the peak of the 2001 outburst of XTEâJ1650â500. Results: The data suggest that the NIR/optical flux is due to a combination of emission mechanisms including a significant contribution from X-ray reprocessing and, at early times in the hard state, a relativistic jet that is NIR/radio dim compared to similar sources. Conclusions: The jet of XTEâJ1650â500 is relatively weak compared to that of other black hole low-mass X-ray binaries, possibly because we observe as it is being âturned offâ or quenched at the state transition. While there are several outliers to the radio-X-ray correlation of the hard state of low-mass X-ray binaries, XTEâJ1650â500 is the first example of an outlier to the NIR/optical-X-ray correlation
Appearance of Keplerian discs orbiting Kerr superspinars
We study optical phenomena related to appearance of Keplerian accretion discs
orbiting Kerr superspinars predicted by the string theory. The superspinar
exterior is described by the standard Kerr naked singularity geometry breaking
the black hole limit on the internal angular momentum (spin). We construct
local photon escape cones for a variety of orbiting sources that enable to
determine the superspinars silhouette in the case of distant observers. We show
that the superspinar silhouette depends strongly on the assumed edge where the
external Kerr spacetime is joined to the internal spacetime governed by the
string theory and significantly differs from the black hole silhouette. The
appearance of the accretion disc is strongly dependent on the value of the
superspinar spin in both their shape and frequency shift profile. Apparent
extension of the disc grows significantly with growing spin, while the
frequency shift grows with descending spin. This behavior differs substantially
from appearance of discs orbiting black holes enabling thus, at least in
principle, to distinguish clearly the Kerr superspinars and black holes. In
vicinity of a Kerr superspinar the non-escaped photons have to be separated to
those captured by the superspinar and those being trapped in its strong
gravitational field leading to self-illumination of the disc that could even
influence its structure and causes self-reflection effect of radiation of the
disc. The amount of trapped photons grows with descending of the superspinar
spin. We thus can expect significant self-illumination effects in the field of
Kerr superspinars with near-extreme spin .Comment: accepted in Classical and Quantum Gravity (September 1st, 2010
Equilibria of charged dust tori in a dipole magnetic field: hydrodynamic approach
The authors present Newtonian model of non-conductive charge perfect fluid tori orbiting in combined spherical gravitational and dipolar magnetic fields, focusing on stationary, axisymmetric toroidal structures
Mass estimate of the XTEÂ J1650-500 black hole from the extended orbital resonance model for high-frequency QPOs
Context. XTE J1650-500 is a Galactic black-hole binary system for which at least one high-frequency QPO at 250âHz has been reported. Moreover there are indications that the system harbours a near-extreme Kerr black hole with a spin 0.998 and mass MBH âČ 7.3 M_{\sun}. Recently it was discovered that the orbital 3-velocity of test-particle (geodesical) discs orbiting Kerr black holes with a spin > 0.9953, analyzed in the locally non-rotating frames, reveals a hump near the marginally stable orbit. It was suggested that the hump could excite the epicyclic motion of particles near the ISCO with frequencies typical for high-frequency QPOs. The characteristic frequency of the hump-induced oscillations was defined as the maximal positive rate of change of the LNRF-related orbital velocity with the proper radial distance. If the characteristic âhumpy frequencyâ and the radial epicyclic frequency are commensurable, strong resonant phenomena are expected.
Aims. We apply the idea of hump-induced oscillations in accretion discs around near-extreme Kerr black holes to estimate the black-hole mass in the XTEÂ J1650-500 binary system.
Methods. For the Kerr black hole with spin 0.9982 the characteristic âhumpy frequencyâ and the radial epicyclic frequency are in the ratio 1:3 at the orbit where the positive rate of change of the LNRF-related orbital velocity with the proper radial distance is maximal. Identifying the radial epicyclic frequency with the observed 250âHz QPO, we arrive at the mass of the black hole. In this method the ratio of frequencies determines the spin (and vice versa), and the values of the frequencies determine the black-hole mass.
Results. The mass of the Kerr black hole in XTEÂ J1650-500 binary system is estimated to be around 5.1 M_{\sun}