Relativistic tidal compressions of a star by a massive black hole

Aims: We investigate the stellar pancake mechanism during which a solar-type star is tidally flattened within its orbital plane passing close to a 10^6 solar masses black hole. We simulate the relativistic orthogonal compression process and follow the associated shock waves formation. Methods: We consider a one-dimensional hydrodynamical stellar model moving in the relativistic gravitational field of a non-rotating black hole. The model is numerically solved using a Godunov-type shock-capturing source-splitting method in order to correctly reproduce the shock waves profiles. Results: Simulations confirm that the space-time curvature can induce several successive orthogonal compressions of the star which give rise to several strong shock waves. The shock waves finally escape from the star and repeatedly heat up the stellar surface to high energy values. Such a shock-heating could interestingly provide a direct observational signature of strongly disruptive star - black hole encounters through the emission of hard X or soft gamma-ray bursts. Timescales and energies of such a process are consistent with some observed events such as GRB 970815.Comment: 8 pages, 11 figures, submitted to Astron. Astrophy

Shock waves in tidally compressed stars by massive black holes

We interest in the case of a main-sequence star deeply penetrating within the tidal radius of a massive black hole. We focus on the compression phase leading to a so-called pancake configuration of the star at the instant of maximal compression. The aim is to study the tidal compression process paying particular attention to the development of shock waves;to deduce reliable estimates of the thermodynamical quantities involved in the pancake star; and to solve a controversy about whether or not thermonuclear reactions can be triggered in the core of a tidally compressed star. We have set up a one-dimensional hydrodynamical model well-adapted to the geometry of the problem. Based on the high-resolution shock-capturing Godunov-type approach, it allows to study the compression phase undergone by the star in the direction orthogonal to its orbital plane. We show the existence of two regimes depending on whether shock waves develop before or after the instant of maximal core compression. In both cases we confirm high compression and heating factors in the stellar core able to trigger a thermonuclear explosion. Moreover, we show that the shock waves carry outwards a brief but very high peak of temperature from the centre to the surface of the star. We tentatively conclude that the phenomenon could give rise to hard electromagnetic radiation, to be compared to some X-ray flares already observed in some galactic nuclei harbouring massive black holes. Finally, we estimate that the rate of pancake stars should be about $10^{-5}$ per galaxy per year. If generated in hard X- or $\gamma$-ray band, several events of this kind per year should be detectable within the full observable universe.Comment: 19 pages, 38 figures, 7 tables; v2 : corrected to match version accepted in Astron. Astrophys. Tables and references added, new simulations also performed for adiabatic index 4/

Relativistic Effects on the Appearance of a Clothed Black Hole

For an accretion disk around a black hole, the strong relativistic effects affect every aspect of the radiation from the disk, including its spectrum, light-curve, and image. This work investigates in detail how the images of a thin disk around a black hole will be distorted, and what the observer will see from different viewing angles and in different energy bands.Comment: 4 pages, 5 figures. Based on the poster presented at the Sixth Pacific Rim Conference on Stellar Astrophysics (Xi'an, China, July 11-17, 2002). Color versions of figures are given separatel

Entropic issues in contemporary cosmology

Penrose [1] has emphasized how the initial big bang singularity requires a special low entropy state. We address how recent brane cosmological schemes address this problem and whether they offer any apparent resolution. Pushing the start time back to $t=-\infty$ or utilizing maximally symmetric AdS spaces simply exacerbates or transfers the problem. Because the entropy of de Sitter space is $S\leq 1/\Lambda$, using the present acceleration of the universe as a low energy $(\Lambda\sim 10^{-120}$) inflationary stage, as in cyclic ekpyrotic models, produces a gravitational heat death after one cycle. Only higher energy driven inflation, together with a suitable, quantum gravity holography style, restriction on {\em ab initio} degrees of freedom, gives a suitable low entropy initial state. We question the suggestion that a high energy inflationary stage could be naturally reentered by Poincare recurrence within a finite causal region of an accelerating universe. We further give a heuristic argument that so-called eternal inflation is not consistent with the 2nd law of thermodynamics within a causal patch.Comment: brief discussion on Poincare recurrence include

Shadow of a rotating braneworld black hole

We investigate the shadow cast by a rotating braneworld black hole, in the Randall-Sundrum scenario. In addition to the angular momentum, the tidal charge term deforms the shape of the shadow. For a given value of the rotation parameter, the presence of a negative tidal charge enlarges the shadow and reduces its deformation with respect to Kerr spacetime, while for a positive charge, the opposite effect is obtained. We also analyze the case in which the combination of the rotation parameter and the tidal charge results in a naked singularity. We discuss the observational prospects corresponding to the supermassive black hole at the Galactic center.Comment: 11 pages, 6 figures. v2: improved version (includes 2 new figures

General Relativistic Flux Modulations from Disk Instabilities in Sagittarius A*

Near-IR and X-ray flares have been detected from the supermassive black hole Sgr A* at the center of our Galaxy with a (quasi)-period of ~17-20 minutes, suggesting an emission region only a few Schwarzschild radii above the event horizon. The latest X-ray flare, detected with XMM-Newton, is notable for its detailed lightcurve, yielding not only the highest quality period thus far, but also important structure reflecting the geometry of the emitting region. Recent MHD simulations of Sgr A*'s disk have demonstrated the growth of a Rossby wave instability, that enhances the accretion rate for several hours, possibly accounting for the observed flares. In this Letter, we carry out ray-tracing calculations in a Schwarzschild metric to determine as accurately as possible the lightcurve produced by general relativistic effects during such a disruption. We find that the Rossby wave induced spiral pattern in the disk is an excellent fit to the data, implying a disk inclination angle of ~77 deg. Note, however, that if this association is correct, the observed period is not due to the underlying Keplerian motion but, rather, to the pattern speed. The favorable comparison between the observed and simulated lightcurves provides important additional evidence that the flares are produced in Sgr A*'s inner disk.Comment: 5 Pages, 3 Figures, accepted for publication in ApJ Lette