Andrej Čadež

Line emission from accretion discs around black holes: the analytic approach

Abstract The broad X-ray iron line observed in many active galactic nuclei spectra is thought to originate from the accretion disc surrounding the putative supermassive black hole. We show here how to perform the analytical integration of the geodesic equations that describe the photon trajectories in the general case of a rotating black hole (Kerr metric), in order to write a fast and efficient numerical code for modelling emission line profiles from accretion discs.

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

Context. 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. Aims. 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. Methods. Earlier analysis of the evolution of classical Keplerian orbits was extended to relativistic orbits around a Schwarzschild black hole. Results. 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. Conclusions. 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.

Apparent horizons in the two-black-hole problem

Abstract The apparent horizon of the two-black-hole problem on the time-symmetric spacelike hypersurface is studied. Its area is computed as a function of the separtion parameter. The critical value of the separation parameter for which the two black holes merge is computed.

Effects of a Black Hole's Gravitational Field on the Luminosity of a Star during a Close Encounter

To complement hydrodynamic studies of the tidal disruption of the star by a massive black hole, we present the study of stellar luminosity and its variations produced by the strong gravitational field of the black hole during a close encounter. By simulating the relativistically moving star and its emitted light and taking into account general relativistic effects on particle and light trajectories, our results show that the black holes gravity alone induces apparent stellar luminosity variations on typical timescales of a few rg/c [=(5 s)mbh/(106 M☉)] to a few 100 rg/c [~(10 minutes)mbh/(106 M☉)], where rg = Gmbh/c2. We discern different cases with respect to the strength of tidal interaction and focus on two: (1) a star encountering a giant black hole traces spacetime almost as a point particle, so the apparent luminosity variations are dominated by clearly recognizable general relativistic effects, and (2) in a close encounter of a star with a black hole of similar size, the stellar debris is spread around the black hole by processes in which hydrodynamics plays an important role. We discuss limitations and results of our approach.

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.

Aqueye optical observations of the Crab Nebula pulsar

We observed the Crab pulsar in October 2008 at the Copernico Telescope in Asiago - Cima Ekar with the optical photon counter Aqueye (the Asiago Quantum Eye) which has the best temporal resolution and accuracy ever achieved in the optical domain (hundreds of picoseconds). Our goal was to perform a detailed analysis of the optical period and phase drift of the main peak of the Crab pulsar and compare it with the Jodrell Bank ephemerides. We determined the position of the main peak using the steepest zero of the cross-correlation function between the pulsar signal and an accurate optical template. The pulsar rotational period and period derivative have been measured with great accuracy using observations covering only a 2 day time interval. The error on the period is 1.7 ps, limited only by the statistical uncertainty. Both the rotational frequency and its first derivative are in agreement with those from the Jodrell Bank radio ephemerides archive. We also found evidence of the optical peak leading the radio one by ~230 microseconds. The distribution of phase-residuals of the whole dataset is slightly wider than that of a synthetic signal generated as a sequence of pulses distributed in time with the probability proportional to the pulse shape, such as the average count rate and background level are those of the Crab pulsar observed with Aqueye. The counting statistics and quality of the data allowed us to determine the pulsar period and period derivative with great accuracy in 2 days only. The time of arrival of the optical peak of the Crab pulsar leads the radio one in agreement with what recently reported in the literature. The distribution of the phase residuals can be approximated with a Gaussian and is consistent with being completely caused by photon noise (for the best data sets).

Optical Spectrum of Main-, Inter-, and Off-Pulse Emission from the Crab Pulsar

A dedicated stroboscopic device was used to obtain optical spectra of the Crab pulsar main pulse and interpulse as well as the spectrum of the underlying nebula when the pulsar is turned off. Since the nebular emission is very inhomogeneous, our ability to effectively subtract the nebular background signal is crucial. No spectral lines intrinsic to the pulsar are detected. The main pulse and the interpulse behave as power laws, both with the same dereddened index α = +0.2 ± 0.1. This value was obtained by subtracting the nebular spectrum at the exact position of the pulsar. The underlying nebula is redder, α = -0.4 ± 0.1. Its emission lines are split into approaching (~-1200 km s-1) and receding (~+600 km s-1) components. The strength of the emission line components and the flux in the nebular continuum vary on an arcsecond scale. The nebular line and continuum intensities along the north-south slit are given.

Crab pulsar photometry and the signature of free precession

Optical photometry for the pulsar PSR 0531+21 has been extended with new observations that strengthen evidence for a previously observed 60 s periodicity. This period is found to be increasing with time at approximately the same rate as the rotational period of the pulsar. The observed period and its time dependence t a simple free precession model.

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

The Crab pulsar seen with AquEYE at Asiago Cima Ekar observatory

\approx 1 \, \mu