Black hole mergers: can gas discs solve the 'final parsec' problem?

We compute the effect of an orbiting gas disc in promoting the coalescence of a central supermassive black hole binary. Unlike earlier studies, we consider a finite mass of gas with explicit time dependence: we do not assume that the gas necessarily adopts a steady state or a spatially constant accretion rate, i.e. that the merging black hole was somehow inserted into a pre-existing accretion disc. We consider the tidal torque of the binary on the disc, and the binary's gravitational radiation. We study the effects of star formation in the gas disc in a simple energy feedback framework.
The disc spectrum differs in detail from that found before. In particular, tidal torques from the secondary black hole heat the edges of the gap, creating bright rims around the secondary. These rims do not in practice have uniform brightness either in azimuth or time, but can on average account for as much as 50 per cent of the integrated light from the disc. This may lead to detectable high-photon-energy variability on the relatively long orbital time-scale of the secondary black hole, and thus offer a prospective signature of a coalescing black hole binary.
We also find that the disc can drive the binary to merger on a reasonable time-scale only if its mass is at least comparable with that of the secondary black hole, and if the initial binary separation is relatively small, i.e.   a ₀≲ 0.05  pc. Star formation complicates the merger further by removing mass from the disc. In the feedback model we consider, this sets an effective limit to the disc mass. As a result, binary merging is unlikely unless the black hole mass ratio is ≲0.001. Gas discs thus appear not to be an effective solution to the 'last parsec' problem for a significant class of mergers.     

The gravitational wave background from star–massive black hole fly-bys

Stars on eccentric orbits around a massive black hole (MBH) emit bursts of gravitational waves (GWs) at periapse. Such events may be directly resolvable in the Galactic Centre. However, if the star does not spiral in, the emitted GWs are not resolvable for extragalactic MBHs, but constitute a source of background noise. We estimate the power spectrum of this extreme mass ratio burst background (EMBB) and compare it to the anticipated instrumental noise of the Laser Interferometer Space Antenna (LISA). To this end, we model the regions close to an MBH, accounting for mass segregation, and for processes that limit the presence of stars close to the MBH, such as GW inspiral and hydrodynamical collisions between stars. We find that the EMBB is dominated by GW bursts from stellar mass black holes, and the magnitude of the noise spectrum  ( fS _GW)^1/2  is at least a factor of ∼10 smaller than the instrumental noise. As an additional result of our analysis, we show that LISA is unlikely to detect relativistic bursts in the Galactic Centre.     

Constraining jet/disc geometry and radiative processes in stellar black holes XTE J1118+480 and GX 339−4

We present results from modelling of quasi-simultaneous broad-band (radio through X-ray) observations of the Galactic stellar black hole (BH) transient X-ray binary (XRB) systems XTE J1118+480 and GX 339−4 using an irradiated disc + compact jet model. In addition to quantifying the physical properties of the jet, we have developed a new irradiated disc model which also constrains the geometry and temperature of the outer accretion disc by assuming a disc heated by viscous energy release and X-ray irradiation from the inner regions. For the source XTE J1118+480, which has better spectral coverage of the two in optical and near-infrared (OIR) wavelengths, we show that the entire broad-band continuum can be well described by an outflow-dominated model + an irradiated disc. The best-fitting radius of the outer edge of the disc is consistent with the Roche lobe geometry of the system, and the temperature of the outer edge of the accretion disc is similar to those found for other XRBs. Irradiation of the disc by the jet is found to be negligible for this source. For GX 339−4, the entire continuum is well described by the jet-dominated model only, with no disc component required. For the two XRBs, which have very different physical and orbital parameters and were in different accretion states during the observations, the sizes of the jet base are similar and both seem to prefer a high fraction of non-thermal electrons in the acceleration/shock region and a magnetically dominated plasma in the jet. These results, along with recent similar results from modelling other galactic XRBs and AGNs, may suggest an inherent unity in diversity in the geometric and radiative properties of compact jets from accreting black holes.     

A steady-state solution for warped accretion discs

We consider a thin accretion disc warped due to the Bardeen–Petterson effect, presenting both analytical and numerical solutions for the situation in which the two viscosity coefficients vary with radius as a power law, with the two power-law indices not necessarily equal. The analytical solutions are compared with numerical ones, showing that our new analytical solution is more accurate than the previous one, which overestimated the inclination change in the outer disc. Our new analytical solution is appropriate for moderately warped discs, while for extremely misaligned discs only a numerical solution is appropriate.     

Magnetic connection and current distribution in black hole accretion discs

We discuss one of the possible origins of large-scale magnetic fields based on a continuous distribution of toroidal electric current flowing in the inner region of the disc around a Kerr black hole (BH) in the framework of general relativity. It turns out that four types of configuration of the magnetic connection (MC) are generated, i.e. MC of the BH with the remote astrophysical load (MCHL), MC of the BH with the disc (MCHD), MC of the plunging region with the disc (MCPD) and MC of the inner and outer disc regions (MCDD). It turns out that the Blandford–Znajek process can be regarded as one type of MC, i.e. MCHL. In addition, we propose a scenario for fitting the quasi-periodic oscillations in BH binaries based on MCDD associated with the magnetic reconnection.     

A Study on the Long-term Spectral Variability of the Black-hole Candidate X-ray Binary Cyg X-1

Using the All-Sky Monitor (ASM, 1.5–12–kev) data of Rossi X-ray Timing Explorer (RXTE) from January 1996 to May 2005, we have made a detailed analysis of the correlation between photon-count rate and spectral hardness ratio HR2 (5–12 keV/3–5 keV) of the black-hole candidate X-ray binary Cyg X-1 in 3 energy bands, namely the A-band (1.5–3 keV), B-band (3–5 keV) and Cband (5–12 keV). By the study on the ASM data of 1-day time scale, we find: (1) When Cyg X-1 is in the soft state, the A-band photon-count rate and hardness ratio HR2 exhibit an anticorrelation, but in B-band and C-band there appears the positive correlation. When Cyg X-1 is in hard state, the photon-count rates in the A,B,C bands are all inversely correlated with the hardness ratio HR2; (2) No matter whether Cyg X-1 is in the soft state or the hard state, the hardness ratios HR2 and HR1 are always positively correlated. In addition, we have analyzed the “dwell by dwell” data of the ASM, and obtained the following interesting results: (1) In the period of MJD = 52600–52760 (while Cyg X-1 is in the hard state), the photon-count rates in the A-band and B-band are inversely correlated with HR2, but in the C-band there appears a relatively strong positive correlation; (2) During the hard state, a clear anticorrelation exists between the hardness ratios HR2 and HR1.     

Dynamical modelling of the elliptical galaxy NGC 2974

In this paper we analyse the relations between a previously described oblate Jaffe model for an ellipsoidal galaxy and the observed quantities for NGC 2974, and obtain the length and velocity scales for a relevant elliptical galaxy model. We then derive the finite total mass of the model from these scales, and finally find a good fit of an isotropic oblate Jaffe model by using the Gauss-Hermite fit parameters and the observed ellipticity of the galaxy NGC 2974. The model is also used to predict the total luminous mass of NGC 2974, assuming that the influence of dark matter in this galaxy on the image, ellipticity and Gauss-Hermite fit parameters of this galaxy is negligible within the central region, of radius 0.5R _e.     

Can the known millisecond pulsars help in the detection of intermediate-mass black holes at the centers of globular clusters?

We consider the possibility of detecting intermediate-mass (10³–10⁴ M _⊙) black holes, whose existence at the centers of globular clusters is expected from optical and infrared observations, using precise pulse arrival timing for the millisecond pulsars in globular clusters known to date. For some of these pulsars closest to the cluster centers, we have calculated the expected delay times of pulses as they pass in the gravitational field of the central black hole. The detection of such a time delay by currently available instruments for the known pulsars is shown to be impossible at a black hole mass of 10³ M _⊙ and very problematic at a black hole mass of 10⁴ M _⊙. In addition, the signal delay will have a negligible effect on the pulsar periods and their first derivatives compared to the current accuracy of their measurements.