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, XTE J1650−500, 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 XTE J1650−500. 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 XTE J1650−500 is relatively weak compared to that of other black hole low-mass X-ray binaries, possibly because we observe as it is being “turned off” 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, XTE J1650−500 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 $a \sim 1$.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 XTE 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 250 Hz has been reported. Moreover there are indications that the system harbours a near-extreme Kerr black hole with a spin $a_{\ast}$ $\simeq$ 0.998 and mass MBH ≲ 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 $a_{\ast}$ > 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 “humpy frequency” 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 XTE J1650-500 binary system. Methods. For the Kerr black hole with spin $a_{\ast}$ $\simeq$ 0.9982 the characteristic “humpy frequency” 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 250 Hz 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 XTE J1650-500 binary system is estimated to be around 5.1 M_{\sun}