arXiv subjects

Agnieszka Janiuk

Publications and source records attributed to Agnieszka Janiuk.

At least 73 records · Page 4Linked to original sources

arXiv2017

Local stability and global instability in iron-opaque disks

The thermal stability of accretion disk and the possibility to see a limit-cycle behaviour strongly depends on the ability of the disk plasma to cool down. Various processes connected with radiation-matter interaction appearing in hot accretion disk plasma contribute to opacity. For the case of geometrically thin and optically thick accretion disk, we can estimate the influence of several different components of function κ, given by the Roseland mean. In the case of high temperatures, the electron Thomson scattering is dominant. At lower temperatures atomic processes become important. The slope d log κ/d log T can have locally stabilizing or destabilizing effect on the disk. Although the local MHD simulation postulate the stabilizing influence of the atomic processes, only the global time-dependent model can reveal the global disk stability range estimation. This is due to global diffusive nature of that processes. In this paper, using previously tested GLADIS code with modified prescription of the viscous dissipation, we examine the stabilizing effect of the Iron Opacity Bump.

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arXiv2017

Modified viscosity in accretion disks. Application to Galactic black hole binaries, intermediate mass black holes and AGN

Black holes (BHs) surrounded by accretion disks are present in the Universe at different scales of masses, from microquasars up to the active galactic nuclei (AGNs). The current picture remains ad hoc due to the complexity of the magnetic field action. In addition, accretion disks at high Eddington rates can be radiation-pressure dominated and, according to some of the heating prescriptions, thermally unstable. The observational verification of their resulting variability patterns may shed light on both the role of radiation pressure and magnetic fields. We compute the structure and time evolution of an accretion disk, using the code GLADIS (which models the global accretion disk instability). We supplement this model with a modified viscosity prescription, which can to some extent describe the magnetisation of the disk. We study the results for a large grid of models, to cover the whole parameter space, and we derive conclusions separately for different scales of black hole masses, which are characteristic for various types of cosmic sources. We show the dependencies between the flare or outburst duration, its amplitude, and period, on the accretion rate and viscosity scaling. We show that if the heating rate in the accretion disk grows more rapidly with the total pressure and temperature, the instability results in longer and sharper flares. In general, we confirm that the disks around the supermassive black holes are more radiation-pressure dominated and present relatively brighter bursts. Our method can also be used as an independent tool for the black hole mass determination, which we confront now for the intermediate black hole in the source HLX-1.

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arXiv2017

Microphysics in the gamma ray burst central engine

We calculate the structure and evolution of a gamma ray burst central engine where an accreting torus has formed around the newly born black hole. We study the general relativistic,MHD models and we self-consistently incorporate the nuclear equation of state. The latter accounts for the degeneracy of relativistic electrons, protons, and neutrons, and is used in the dynamical simulation, instead of a standard polytropic -law. The EOS provides the conditions for the nuclear pressure in the function of density and temperature, which evolve with time according to the conservative MHD scheme. We analyze the structure of the torus and outflowing winds, and compute the neutrino flux emitted through the nuclear reactions balance in the dense and hot matter. We also estimate the rate of transfer of the black hole rotational energy to the bipolar jets. Finally, we elaborate on the nucleosynthesis of heavy elements in the accretion flow and the wind, through computations of the thermonuclear reaction network. We discuss the possible signatures of the radioactive elements decay in the accretion flow. We suggest that further detailed modeling of the accretion flow in GRB engine, together with its microphysics, may be a valuable tool to constrain the black hole mass and spin. It can be complementary to the gravitational wave analysis, if the waves are detected with an electromagnetic counterpart.

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arXiv2017

Black hole accretion in gamma ray bursts

We study the structure and evolution of the hyperaccreting disks and outflows in the gamma ray bursts central engines. The torus around a stellar mass black hole is composed of free nucleons, Helium, electron-positron pairs, and is cooled by neutrino emission. Accretion of matter powers the relativistic jets, responsible for the gamma ray prompt emission. The significant number density of neutrons in the disk and outflowing material will cause subsequent formation of heavier nuclei. We study the process of nucleosynthesis and its possible observational consequences. We also apply our scenario to the recent observation of the gravitational wave signal, detected on September 14th, 2015 by the two Advanced LIGO detectors, and related to an inspiral and merger of a binary black hole system. A gamma ray burst that could possibly be related with the GW150914 event was observed by the Fermi satellite. It had a duration of about 1 second and appeared about 0.4 seconds after the gravitational-wave signal. We propose that a collapsing massive star and a black hole in a close binary could lead to the event. The gamma ray burst was powered by a weak neutrino flux produced in the star remnant's matter. Low spin and kick velocity of the merged black hole are reproduced in our simulations. Coincident gravitational-wave emission originates from the merger of the collapsed core and the companion black hole.

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arXiv2016

The universal "heartbeat" oscillations in black hole systems accross the mass-scale

The hyperluminous X-ray source (HLX-1, the peak X-ray luminosity $\sim 10^{42}\rm erg\ s^{-1}$) near the spiral galaxy ESO 243-49 is possibly the best candidate for intermediate mass black hole (IMBH), which underwent recurrent outbursts with a period of $\sim 400$ days. The physical reason for this quasi-periodic variability is still unclear. We explore the possibility of radiation-pressure instability in accretion disk by modeling the light curve of HLX-1, and find that it can roughly reproduce the duration, period and amplitude of the recurrent outbursts HLX-1 with an IMBH of ~10^5Msun. Our result provides a possible mechanism to explain the recurrent outbursts in HLX-1. We further find a universal correlation between the outburst duration and the bolometric luminosity for the BH sources with a very broad mass range (e.g., X-ray binaries, XRBs, HLX-1 and active galactic nuclei, AGNs), which is roughly consistent with the prediction of radiation-pressure instability of the accretion disk. These results imply that "heartbeat" oscillations triggered by radiation-pressure instability may appears in different-scale BH systems.

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arXiv2016

Non-linear variability in microquasars in relation with the winds from their accretion disks

The microquasar IGR J17091-3624, which is the recently discovered analogue of the well known source GRS 1915+105, exhibits quasi-periodic outbursts, with a period of 5-70 seconds, and regular amplitudes, referred to as "heartbeat state". We argue that these states are plausibly explained by accretion disk instability, driven by the dominant radiation pressure. Using our GLobal Accretion DIsk Simulation hydrodynamical code, we model these outbursts quantitatively. We also find a correlation between the presence of massive outflows launched from the accretion disk and the stabilization of its oscillations. We verify the theoretical predictions with the available timing and spectral observations. Furthermore, we postulate that the underlying non-linear differential equations that govern the evolution of an accretion disk are responsible for the variability pattern of several other microquasars, including XTE J1550-564, GX 339-4, and GRO J1655-40. This is based on the signatures of deterministic chaos in the observed lightcurves of these sources, which we found using the recurrence analysis method. We discuss these results in the frame of the accretion disk instability model.

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arXiv2016

Nucleosynthesis of elements in gamma ray bursts engines

We consider the gamma ray burst (GRB) central engine that is powered by the collapse of a massive rotating star or compact binary merger. The engine is a hot and dense accretion disk, which is composed of free nucleons, electron-positron pairs, and helium, and cooled by neutrino emission. A significant number density of neutrons in the inner disk body provide conditions for neutron rich plasma in the GRB outflows or jets. Helium is synthesized in the inner disk if the accretion rate is large, and heavy nuclei are also formed in the outer disk at distances above 150-250 $r_{g}$ from the black hole. We study the process of nucleosynthesis in the GRB engine, depending on its physical properties. The GRB central engine is hydrodynamically modeled in the frame of a dense and hot disk which accretes with a high rate (up to 1 Solar mass per second) onto a maximally spinning, stellar mass black hole. The synthesis of heavy nuclei up to germanium and gallium is then followed by the nuclear reaction network. The accretion at high rate onto a Kerr black hole feeds the engine activity and establishes conditions for efficient synthesis of heavy nuclei in the disk. These processes may have important observational implications for the jet deceleration process and heavy elements observed in the spectra of GRB afterglows.

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arXiv2016

Interplay between heartbeat oscillations and wind outflow in microquasar IGR J17091-3624

During the bright outburst in 2011, the black hole candidate IGR J17091-3624 exhibited strong quasi-periodic flare-like events (on timescales of tens of seconds) in some characteristic states, the so-called heartbeat state. From the theoretical point of view, these oscillations may be modeled by the process of accretion disk instability, driven by the dominant radiation pressure and enhanced heating of the plasma. Although the mean accretion rate in this source is probably below the Eddington limit, the oscillations will still have large amplitudes. As the observations show, the source can exhibit strong wind outflow during the soft state. This wind may help to partially or even completely stabilize the heartbeat. Using our hydrodynamical code GLADIS, we modeled the evolution of an accretion disk responsible for X-ray emission of the source. We accounted for a variable wind outflow from the disk surface. We examined the data archive from the Chandra and XMM-Newton satellites to find the observed limitations on the wind physical properties, such as its velocity and ionization state. We also investigated the long-term evolution of this source, which lasted over about 600 days of observations, using the data collected by the Swift and RXTE satellites. During this long period, the oscillations pattern and the observable wind properties changed systematically. We found that this source probably exhibits observable outbursts of appropriate timescales and amplitudes as a result of the disk instability. Our model requires a substantial wind component to explain the proper variability pattern, and even complete suppression of flares in some states. The wind mass-loss rate extracted from the data agrees quantitatively well with our scenario.

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arXiv2016

The Athena X-ray Integral Field Unit (X-IFU)

The X-ray Integral Field Unit (X-IFU) on board the Advanced Telescope for High-ENergy Astrophysics (Athena) will provide spatially resolved high-resolution X-ray spectroscopy from 0.2 to 12 keV, with 5 arc second pixels over a field of view of 5 arc minute equivalent diameter and a spectral resolution of 2.5 eV up to 7 keV. In this paper, we first review the core scientific objectives of Athena, driving the main performance parameters of the X-IFU, namely the spectral resolution, the field of view, the effective area, the count rate capabilities, the instrumental background. We also illustrate the breakthrough potential of the X-IFU for some observatory science goals. Then we briefly describe the X-IFU design as defined at the time of the mission consolidation review concluded in May 2016, and report on its predicted performance. Finally, we discuss some options to improve the instrument performance while not increasing its complexity and resource demands (e.g. count rate capability, spectral resolution). The X-IFU will be provided by an international consortium led by France, The Netherlands and Italy, with further ESA member state contributions from Belgium, Finland, Germany, Poland, Spain, Switzerland and two international partners from the United States and Japan.

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arXiv2016

On the possible gamma-ray burst -- gravitational wave association in GW150914

Data from the Fermi Gamma-ray Burst Monitor satellite observatory suggested that the recently discovered gravitational wave source, a pair of two coalescing black holes, was related to a gamma-ray burst. The observed high-energy electromagnetic radiation (above 50 keV) originated from a weak transient source and lasted for about 1 second. Its localization is consistent with the direction to GW150914. We speculate about the possible scenario for the formation of a gamma-ray burst accompanied by the gravitational-wave signal. Our model invokes a tight binary system consisting of a massive star and a black hole which leads to the triggering of a collapse of the star's nucleus, the formation of a second black hole, and finally to the binary black hole merger. For the most-likely configuration of the binary spin vectors with respect to the orbital angular momentum in the GW150914 event, the recoil speed (kick velocity) acquired by the final black hole through gravitational wave emission is of the order of a few hundred km/s and this might be sufficient to get it closer to the envelope of surrounding material and capture a small fraction of matter from the remnant of the host star. The gamma-ray burst is produced by the accretion of this remnant matter onto the final black hole. The moderate spin of the final black hole suggests that the gamma-ray burst jet is powered by weak neutrino emission rather than the Blandford-Znajek mechanism, and hence explains the low power available for the observed GRB signal.

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arXiv2016

Simulations of coalescing black holes

We describe the methods and results of numerical simulations of coalescing black holes. The simulation in dynamical spacetime covers the inspiral, merger, and ringdown phases. We analyze the emission of gravitational waves and properties of a black hole being the merger product. We discuss the results in the context of astrophysical environment of black holes that exist in the Universe.

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arXiv2016

Non-linear behaviour of XTE J1550-564 during its 1998-1999 outburst, revealed by recurrence analysis

We study the X-ray emission of the microquasar XTE J1550-564 and analyze the properties of its light curves using the recurrence analysis method. The indicators for non-linear dynamics of the accretion flow are found in the very high state and soft state of this source. The significance of deterministic variability depends on the energy band. We discuss the non-linear dynamics of the accretion flow in the context of the disc-corona geometry and propagating oscillations in the accretion flow.

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arXiv2016

Numerical test of the method for revealing traces of deterministic chaos in the accreting black holes

The high energy radiation emitted by black hole X-ray binaries originates in an accretion disk, hence the variability of the lightcurves mirrors the dynamics of the disc. We study the time evolution of the emitted flux in order to find evidences, that low dimensional non-linear equations govern the accretion flow. Here we test the capabilities of our novel method to find chaotic behaviour on the two numerical time series describing the motion of a test particle around a black hole surrounded by a thin massive disc, one being regular and the other one chaotic.

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arXiv2016

Relativistic low angular momentum accretion in 3D

We study the low angular momentum accretion of matter onto Schwarzschild or Kerr black hole using fully relativistic numerical simulations. Our aim is to include the rotation of both the gas and the black hole and the presence of the magnetic field into the picture. We compare resulting structure of transonic flow with results of 1D pseudo-Newtonian semi-analytical computations of non-magnetized flow.

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arXiv2015

Chaotic and stochastic processes in the accretion flows of the black hole X-ray binaries revealed by recurrence analysis

The black hole candidates exhibit fast variability of their X-ray emission on a wide range of timescales. The short, coherent variations, with frequencies above 1 Hz, are referred to as quasi-periodic oscillations, and are generally explained by resonant effects in the black hole accretion flow. The purely stochastic variability that occurs due to turbulent conditions in the plasma, is quantified by the power density spectra and appears practically in all types of sources and their spectral states. The specific kind of quasi-periodic flares is expected, when the global structure of the accretion flow, governed by the nonlinear hydrodynamics, induces fluctuations around a fixed point solution. These conditions, which occur at high accretion rates, lead to the variability in the sense of deterministic chaos. We study the nonlinear behaviour of X-ray sources using the recurrence analysis method. We estimate quantitatively the indications for deterministic chaos, such as the Renyi's entropy, for the observed time series, and we compare them with the surrogate data. Using the data collected by RXTE satellite, we reveal the oscillations pattern and the observable properties of six black hole systems. We confirm that both the well known microquasar GRS 1915+105, as well as its recently discovered analogue, IGR J17091-3624, exhibit variability characteristic to deterministic chaotic system. We conclude therefore that the underlying nature of the process must be intrinsically connected in these sources with the accretion flow instability, that leads to the limit cycle oscillations around a fixed point. Furthermore, we studied four other black hole candidates with high accretion rates. For GX 339-4, GRO J1655-40 and XTE J1550-564 we obtained significant results for some observations, whereas for XTE J1650-500 no such data set was found.

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arXiv2015

Long-term quasi-periodicity of 4U 1636-536 resulting from accretion disc instability

We present the results of a study of the low-mass X-ray binary 4U 1636-536. We have performed temporal analysis of all available RXTE/ASM, Swift/BAT and MAXI data. We have confirmed the previously discovered quasi-periodicity of ~45 d present during ~2004, however we found it continued to 2006. At other epochs, the quasi-periodicity is only transient, and the quasi-period, if present, drifts. We have then applied a time-dependent accretion disc model to the interval with the significant X-ray quasi-periodicity. For our best model, the period and the amplitude of the theoretical light curve agree well with that observed. The modelled quasi-periodicity is due to the hydrogen thermal-ionization instability occurring in outer regions of the accretion disc. The model parameters are the average mass accretion rate (estimated from the light curves), and the accretion disc viscosity parameters, for the hot and cold phases. Our best model gives relatively low values of viscosity parameter for cold phase 0.01 and for hot phase 0.03.

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arXiv2015

Nucleosynthesis of heavy elements in gamma ray bursts

The ultrarelativistic jets responsible for prompt and afterglow emission in gamma ray bursts are presumably driven by a central engine that consists of a dense accretion disk around a spinning black hole. We consider such engine, composed of free nucleons, electron-positron pairs, Helium nuclei, and cooled by neutrino emission. A significant number density of neutrons in the disk provide conditions for neutron rich plasma in the outflows and jets. Heavy nuclei are also formed in the accretion flow, at the distances 150-250 gravitational radii from the black hole. We study the process of nucleosynthesis in the GRB engine, depending on its physical properties. Our results may have important observational implications for the jet deceleration process and heavy elements observed in the spectra of GRB afterglows.

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arXiv2014

Oscillating shocks in the low angular momentum flows as a source of variability of accreting black holes

We derive the conditions for shock formation in a quasi-spherical, slightly rotating flows. We verify the results of semi-analytical, stationary calculations with the time evolution studied by numerical hydro-simulations, and we study the oscillations of the shock position. We also study the behaviour of flows with varying specific angular momentum, where the 'hysteresis' type of loop is found when passing through the multiple sonic points region. Our results are in agreement with the timescales and shapes of the luminosity flares observed in Sgr A*. These models may also be applicable for the Galactic stellar mass black holes, like GX 339-4 or GRS 1915+105, where periodic oscillations of X-ray luminosity are detected.

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