Variable line profiles due to non-axisymmetric patterns in an accretion disc around a rotating black hole

We have explored spectral line profiles due to spiral patterns in accretion discs around black holes. A parametrization was employed for the shape and emissivity of spiral waves, which can be produced by non-axisymmetric perturbations affecting the disc density and ionization structure. The effects of the light-travel time, energy shift, and gravitational focusing near to a rotating black hole were taken into account. A high-resolution ray-tracing code was used to follow the time variations of the synthetic line profile. A variety of expected spectral features were examined and the scheme applied to a broad iron line observed in MCG-6-30-15.Comment: 11 pages (LaTeX), 7 figures (EPS); PASJ accepte

Star-disc interactions in a galactic centre and oblateness of the inner stellar cluster

Structure of a quasi-stationary stellar cluster is modelled assuming that it is embedded in the gravitational field of a super-massive black hole. Gradual orbital decay of stellar trajectories is caused by the dissipative interaction with an accretion disc. Gravitational field of the disc is constructed and its effect on the cluster structure is taken into account as an axially symmetric perturbation. Attention is focused on a circumnuclear region (r<10^4 gravitational radii) where the effects of the central black hole and the disc dominate over the influence of an outer galaxy. It is shown how the stellar system becomes gradually flattened towards the disc plane. For certain combinations of the model parameters, a toroidal structure is formed by a fraction of stars. Growing anisotropy of stellar velocities as well as their segregation occur. The mass function of the inner cluster is modified and it progressively departs from the asymptotic form assumed in the outer cluster. A new stationary distribution can be characterized in terms of velocity dispersion of the stellar sample in the central region of the modified cluster.Comment: Accepted for publication in MNRAS; 12 pages, 10 figure

Intracerebral Implantation of Hydrogel-Coupled Adhesion Peptides: Tissue Reaction

Arg-Gly-Asp peptides (RGD) were synthesized and chemically coupled to the bulk of N-(2-hydroxypropyl) methacrylamide-based polymer hydrogels. Fourier Transform Infrared Spectroscopy (FFIR) and amino acid analysis confirmed the peptide coupling to the polymer. Activated and control (unmodified) polymer matrices were stereotaxically implanted in the striata of rat brains, and two months later the brains were processed for immunohistochemistry using antibodies for glial acidic fibrillary protein (GFAP), laminin and neurofilaments. RGD-containing polymer matrices promoted stronger adhesion to the host tissue than the unmodified polymer matrices. In addition, the RGD-grafted polymer implants promoted and supported the growth and spread of GFAP-positive glial tissue onto and into the hydrogels. Neurofilament-positive fibers were also seen running along the surface of the polymer and, in some instances, penetrating the matrix. These findings are discussed in the context of using bioactive polymers as a new approach for promoting tissue repair and axonal regeneration of damaged structures of the central nervous system

Enhanced activity of massive black holes by stellar capture assisted by a self-gravitating accretion disc

We study the probability of close encounters between stars from a nuclear cluster and a massive black hole. The gravitational field of the system is dominated by the black hole in its sphere of influence. It is further modified by the cluster mean field (a spherical term) and a gaseous disc/torus (an axially symmetric term) causing a secular evolution of stellar orbits via Kozai oscillations. Intermittent phases of large eccentricity increase the chance that stars become damaged inside the tidal radius of the central hole. Such events can produce debris and lead to recurring episodes of enhanced accretion activity. We introduce an effective loss cone and associate it with tidal disruptions during the high-eccentricity phases of the Kozai cycle. By numerical integration of the trajectories forming the boundary of the loss cone we determine its shape and volume. We also include the effect of relativistic advance of pericentre. The potential of the disc has the efffect of enlarging the loss cone and, therefore, the predicted number of tidally disrupted stars should grow by factor of ~10^2. On the other hand, the effect of the cluster mean potential together with the relativistic pericentre advance act against the eccentricity oscillations. In the end we expect the tidal disruption events to be approximately ten times more frequent in comparison with the model in which the three effects -- the cluster mean field, the relativistic pericentre advance, and the Kozai mechanism -- are all ignored. The competition of different influences suppresses the predicted star disruption rate as the black hole mass increases. Hence, the process under consideration is more important for intermediate-mass black holes, M_bh~10^4M_s.Comment: 10 pages, 5 figures; Astronomy & Astrophysics accepte

Numerical simulation of small perturbation on an accretion disk due to the collision of a star with the disk near the black hole

In this paper, perturbations of an accretion disk by a star orbiting around a black hole are studied. We report on a numerical experiment, which has been carried out by using a parallel-machine code originally developed by D\"{o}nmez (2004). An initially steady state accretion disk near a non-rotating (Schwarzschild) black hole interacts with a "star", modeled as an initially circular region of increased density. Part of the disk is affected by the interaction. In some cases, a gap develops and shock wave propagates through the disk. We follow the evolution for order of one dynamical period and we show how the non-axisymetric density perturbation further evolves and moves downwards where the material of the disk and the star become eventually accreted onto the central body. When the star perturbs the steady state accretion disk, the disk around the black hole is destroyed by the effect of perturbation. The perturbed accretion disk creates a shock wave during the evolution and it loses angular momentum when the gas hits on the shock waves. Colliding gas with the shock wave is the one of the basic mechanism of emitting the $X-$rays in the accretion disk. The series of supernovae occurring in the inner disk could entirely destroy the disk in that region which leaves a more massive black hole behind, at the center of galaxies.Comment: 20pages, 8 figures, accepted for publication in Astrophysics and Space Scienc

On highly eccentric stellar trajectories interacting with a self-gravitating disc in Sgr A*

We propose that Kozai's phenomenon is responsible for the long-term evolution of stellar orbits near a supermassive black hole. We pursue the idea that this process may be driven by a fossil accretion disc in the centre of our Galaxy, causing the gradual orbital decay of stellar trajectories, while setting some stars on highly elliptic orbits. We evolve model orbits that undergo repetitive transitions across the disc over the period of ~10^7 years. We assume that the disc mass is small compared to the central black hole, and its gravitational field comparatively weak, yet non-zero, and we set the present values of orbital parameters of the model star consistent with those reported for the S2 star in Sagittarius A*. We show how a model trajectory decays and circularizes, but at some point the mean eccentricity is substantially increased by Kozai's resonance. In consequence the orbital decay of highly eccentric orbits is accelerated. A combination of an axially symmetric gravitational field and dissipative environment can provide a mechanism explaining the origin of stars on highly eccentric orbits tightly bound to the central black hole. In the context of other S-stars, we can conclude that an acceptable mass of the disc (i.e., M_d<=1 percent of the black hole mass) is compatible with their surprisingly young age and small pericentre distances, provided these stars were formed at r<=10^5 gravitational radii.Comment: Accepted for publication in A&A; 9 pages, 6 figures. Revised version with minor language corrections (no change in content