Modelling the light-curves of objects tidally disrupted by a black hole

Tidal disruption by massive black holes is a phenomenon, during which a large part of gravitational energy can be released on a very short time-scale. The time-scales and energies involved during X-ray and IR flares observed in Galactic centre suggest that they may be related to tidal disruption events. Furthermore, aftermath of a tidal disruption of a star by super-massive black hole has been observed in some galaxies, e.g. RX J1242.6-1119A. All these discoveries increased the demand for tools for tidal disruption study in curved space-time. Here we summarise our study of general relativistic effects on tidal deformation of stars and compact objects.Comment: 2 pages, to appear in the proceedings of the JENAM 2008, Symposium 7: "Grand Challenges in Computational Astrophysics

What brakes the Crab pulsar?

Optical observations provide convincing evidence that the optical phase of the Crab pulsar follows the radio one closely. Since optical data do not depend on dispersion measure variations, they provide a robust and independent confirmation of the radio timing solution. The aim of this paper is to find a global mathematical description of Crab pulsar's phase as a function of time for the complete set of published Jodrell Bank radio ephemerides (JBE) in the period 1988-2014. We apply the mathematical techniques developed for analyzing optical observations to the analysis of JBE. We break the whole period into a series of episodes and express the phase of the pulsar in each episode as the sum of two analytical functions. The first function is the best-fitting local braking index law, and the second function represents small residuals from this law with an amplitude of only a few turns, which rapidly relaxes to the local braking index law. From our analysis, we demonstrate that the power law index undergoes "instantaneous" changes at the time of observed jumps in rotational frequency (glitches). We find that the phase evolution of the Crab pulsar is dominated by a series of constant braking law episodes, with the braking index changing abruptly after each episode in the range of values between 2.1 and 2.6. Deviations from such a regular phase description behave as oscillations triggered by glitches and amount to fewer than 40 turns during the above period, in which the pulsar has made more than 2.0e10 turns. Our analysis does not favor the explanation that glitches are connected to phenomena occurring in the interior of the pulsar. On the contrary, timing irregularities and changes in slow down rate seem to point to electromagnetic interaction of the pulsar with the surrounding environment.Comment: 11 pages, 8 figures, 3 tables; accepted for publication in Astronomy & Astrophysic

Optical phase coherent timing of the Crab nebula pulsar with Iqueye at the ESO New Technology Telescope

The Crab nebula pulsar was observed in 2009 January and December with a novel very fast optical photon counter, Iqueye, mounted at the ESO 3.5 m New Technology Telescope. Thanks to the exquisite quality of the Iqueye data, we computed accurate phase coherent timing solutions for the two observing runs and over the entire year 2009. Our statistical uncertainty on the determination of the phase of the main pulse and the rotational period of the pulsar for short (a few days) time intervals are $\approx 1 \, \mu$s and ~0.5 ps, respectively. Comparison with the Jodrell Bank radio ephemerides shows that the optical pulse leads the radio one by ~240 $\mu$s in January and ~160 $\mu$s in December, in agreement with a number of other measurements performed after 1996. A third-order polynomial fit adequately describes the spin-down for the 2009 January plus December optical observations. The phase noise is consistent with being Gaussian distributed with a dispersion $\sigma$ of $\approx 15 \, \mu$s in most observations, in agreement with theoretical expectations for photon noise-induced phase variability.Comment: 10 pages, 5 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Societ

On the tidal evolution of the orbits of low-mass satellites around black holes

Low-mass satellites, like asteroids and comets, are expected to be present around the black hole at the Galactic center. We consider small bodies orbiting a black hole, and we study the evolution of their orbits due to tidal interaction with the black hole. In this paper we investigate the consequences of the existence of plunging orbits when a black hole is present. We are interested in finding the conditions that exist when capture occurs. The main difference between the Keplerian and black hole cases is in the existence of plunging orbits. Orbital evolution, leading from bound to plunging orbits, goes through a final unstable circular orbit. On this orbit, tidal energy is released on a characteristic black hole timescale. This process may be relevant for explaining how small, compact clumps of material can be brought onto plunging orbits, where they may produce individual short duration accretion events. The available energy and the characteristic timescale are consistent with energy released and the timescale typical of Galactic flares.Comment: 7 pages, 6 figure

Tidal effects on small bodies by massive black holes

The compact radio source Sagittarius A (Sgr A) at the centre of our Galaxy harbours a supermassive black hole, whose mass has been measured from stellar orbital motions. Sgr A is therefore the nearest laboratory where super-massive black hole astrophysics can be tested, and the environment of black holes can be investigated. Since it is not an active galactic nucleus, it also offers the possibility of observing the capture of small objects that may orbit the central black hole. We study the effects of the strong gravitational field of the black hole on small objects, such as a comet or an asteroid. We also explore the idea that the flares detected in Sgr A might be produced by the final accretion of single, dense objects with mass of the order of 10^20 g, and that their timing is not a characteristic of the sources, but rather of the space-time of the central galactic black hole in which they are moving. We find that tidal effects are strong enough to melt the solid object, and present calculations of the temporal evolution of the light curve of infalling objects as a function of various parameters. Our modelling of tidal disruption suggests that during tidal squeezing, the conditions for synchrotron radiation can be met. We show that the light curve of a flare can be deduced from dynamical properties of geodesic orbits around black holes and that it depends only weakly on the physical properties of the source.Comment: 10 pages, 14 figures, A&A accepte

Super-spinning compact objects and models of high-frequency quasi-periodic oscillations observed in Galactic microquasars

We have previously applied several models of high-frequency quasi-periodic oscillations (HF QPOs) to estimate the spin of the central Kerr black hole in the three Galactic microquasars, GRS 1915+105, GRO J1655-40, and XTE J1550-564. Here we explore the alternative possibility that the central compact body is a super-spinning object (or a naked singularity) with the external space-time described by Kerr geometry with a dimensionless spin parameter a = cJ/GM2 > 1.We calculate the relevant spin intervals for a subset of HF QPO models considered in the previous study. Our analysis indicates that for all but one of the considered models there exists at least one interval of a > 1 that is compatible with constraints given by the ranges of the central compact object mass independently estimated for the three sources. For most of the models, the inferred values of a are several times higher than the extreme Kerr black hole value a = 1. These values may be too high since the spin of superspinars is often assumed to rapidly decrease due to accretion when a >> 1. In this context, we conclude that only the epicyclic and the Keplerian resonance model provides estimates that are compatible with the expectation of just a small deviation from a = 1.Comment: 6 pages, 4 figures, accepted by A&

Modelling the light-curves of objects tidally disrupted by a black hole

Tidal disruption by massive black holes is a phenomenon, during which a large part of gravitational energy can be released on a very short time-scale. The time-scales and energies involved during X-ray and IR flares observed in Galactic centre suggest that they may be related to tidal disruption events. Furthermore, aftermath of a tidal disruption of a star by super-massive black hole has been observed in some galaxies, e.g. RX J1242.6-1119A. All these discoveries increased the demand for tools for tidal disruption study in curved space-time. Here we summarise our study of general relativistic effects on tidal deformation of stars and compact objects

Tidal disruption of asteroids by supermassive black holes

The compact radio source Sgr A* at the centre of our Galaxy harbours a super-massive black hole, and is therefore the nearest laboratory for testing the super-massive black hole astrophysics and environment. Since it is not an active galactic nucleus, it also offers the possibility of observing the capture of low-mass objects, such as comets or asteroids, that may orbit the central black hole. In this paper we discuss conditions for tidal disruption of low-mass objects and predictions of the appearance and light curve of such events, as well as their relevance for the X-ray and infra-red flares detected in Sgr A*. The modelled light curves of such tidal disruption events bear marks of the strong gravitational field: tidal squeezing and elongation of the object, gravitational lensing, aberration of light, and Doppler effects. Finally, we show that this model is able to reproduce and fit X-ray flares

On X-ray emission lines from active galactic nuclei and disk models

An efficient numerical code to calculate line profiles from warped disks around nonrotating black holes is presented. Extensive numerical experiments suggest a method making it possible to distinguish between line profiles belonging to flat and warped accretion disks. The extension of our code to rotating black holes is briefly discussed