arXiv subjects

Agnieszka Janiuk

Publications and source records attributed to Agnieszka Janiuk.

At least 55 records · Page 3Linked to original sources

arXiv2020

Accretion induced black hole spin up in magnetized collapsars

We study the gravitational collapse and formation of the Kerr black hole from the rotating progenitor star. We follow the evolution of black hole spin, coupled with its increasing mass. We study the effect of different level of rotation endowed in the progenitor's envelope, and we out some constraints on the final black hole parameters. Our method is based on semi-analytical computations that involve stellar-evolution models of different progenitors. We also follow numerically the black hole evolution and spacetime metric changes during the collapse, via General Relativistic MHD modeling. (abridged)

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arXiv2020

On the maximum stellar rotation to form a black hole without an accompanying luminous transient

The collapse of a massive star with low angular momentum content is commonly argued to result in the formation of a black hole without an accompanying bright transient. Our goal in this Letter is to understand the flow in and around a newly-formed black hole, involving accretion and rotation, via general relativistic hydrodynamics simulations aimed at studying the conditions under which infalling material can accrete without forming a centrifugally supported structure and, as a result, generate no effective feedback. If the feedback from the black hole is, on the other hand, significant, the collapse would be halted and we suggest that the event is likely to be followed by a bright transient. We find that feedback is only efficient if the specific angular momentum of the infalling material at the innermost stable circular orbit exceeds that of geodesic circular flow at that radius by at least $\approx 20\%$. We use the results of our simulations to constrain the maximal stellar rotation rates of the disappearing massive progenitors PHL293B-LBV and N6946-BH1, and to provide an estimate of the overall rate of disappearing massive stars. We find that about a few percent of single O-type stars with measured rotational velocities are expected to spin below the critical value before collapse and are thus predicted to vanish without a trace.

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arXiv2020

Clumpy wind accretion in Cygnus X-1

Cygnus X-1 is one of the brightest X-ray sources observed and shows the X-ray intensity variations on time scales from milliseconds to months in both the soft and hard X-rays. The accretion onto the black hole is believed to be wind fed due to focused stellar wind from the binary companion HDE-226868. We aim to understand the physical mechanism responsible for the short timescale X-ray variability ($<$100 s) of the source in its Hard/Low state. We compute the 2D relativistic hydrodynamic simulation of the low angular momentum accretion flow with a time dependent outer boundary condition that reflects the focused, clumpy wind from the super-giant in this X-ray binary system. We follow the dynamical evolution of our model for about 100 s and present the results showing an oscillatory shock, being a potential explanation of variability observed in hard X-rays. The simulated model with shock solutions is in good agreement with the observed power density spectra of the source.

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arXiv2020

Effects of self-gravity of the accretion disk around rapidly rotating black hole in long GRBs

We prescribe a method to study the effects of self-gravity of accretion disk around a black hole associated with long Gamma Ray Bursts (GRBs) in an evolving background Kerr metric. This is an extension to our previous work where we presented possible constraints for the final masses and spins of these astrophysical black holes. Incorporating the self-force of the accreting cloud around the black hole is a very important aspect due to the transient nature of the event, in which a huge amount of mass is accreted and changes the fundamental black hole parameters i.e. its mass and spin, during the process. Understanding of the GRBs engine is important because they are possible sources of high-energy particles and gravitational waves as most of the energy released from the dynamical evolution is in the form of gravitational radiation. Here, we describe the analytical framework we developed to employ in our numerical model. The numerical studies are planned for the future work.

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arXiv2020

Multi-messenger signals from short gamma ray bursts

We present the results of simulations done with the code HARM-COOL developed in the CTP PAS Warsaw research group over the years 2017-2019. It is based in the original GR MHD scheme proposed by Gammie et al. (2003) for the simulation of Active Galactic Nucleus, but now it has been suited for the engine of a short Gamma Ray Burst event. We compute time-dependent evolution of a black hole accretion disk, in two-dimensional, axisymmetric scheme. The code includes neutrino cooling and accounts for nuclear structure of dense, degenerate matter. Free protons, neutrons, and electron-positron pairs form a neutron-rich, magnetically driven outflow that provides site for subsequent r-process nucleosynthesis. Here the heavy elements up to the Uranium and Gold are synthesized and may contribute to the chemical enrichment of the circum-burst medium. Their radio-active decay will give signal in lower energies in a timescale of weeks-months after the GRB prompt phase. In addition, the magnetic fields are responsible for the launching of ultra-relativistic jets along the rotation axis of the central black hole, according to the well-known Blandford-Znajek mechanism. These jets are sites of variable high energy emission in gamma rays. We find that the magnetic field and the black hole spin account for the observed variability timescales and jet energetics.

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arXiv2020

Simulations of black hole accretion torus in various magnetic field configurations

Using axisymetric general relativistic magnetohydrodynamics simulations we study evolution of accretion torus around black hole endowed with different initial magnetic field configurations. Due to accretion of material onto black hole, parabolic magnetic field will develop in accretion torus funnel around vertical axis, for any initial magnetic field configuration.

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arXiv2020

Time variability of low angular momentum accretion flows around black hole

We present the relativistic 2D and 3D GRMHD simulation of axisymmetric, inviscid, hydrodynamic accretion flows in a fixed Kerr black hole gravitational field. The flow is having low angular momentum with respect to Keplerian one. A relativistic fluid where its bulk velocity is comparable to the speed of light, flowing in the accretion disk very close to the horizon should be described by adiabtic index: 4=3 < g < 5=3 .The time dependent evolution of shock position and respective effect on mass accretion rate and oscillation frequency with varying adiabatic index has been studied. Here we present some of the results for adiabatic index = 1.4 in a 2D and 3D model.

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arXiv2019

Mass and spin constraint on black holes in long GRBs

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arXiv2019

GLADIS: GLobal Accretion Disk Instability Simulation

I present the publicly available code GLADIS (GLobal Accretion Disk Instability Simulation) developed in my reserach group over the years 2002-2017. It can be freely downloaded and modified by the users via the link from the Astrophysics Source Code Library. The software computes time-dependent evolution of a black hole accretion disk, in one-dimensional, axisymmetric, vertically integrated scheme. The main applications are to explain the variability of accretion disks that can be subject to radiation-pressure instability. The phenomenon is relevant for fast variable microquasars, as well as for a class of changing-look AGN.

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arXiv2019

The r-process nucleosynthesis in the outflows from short GRB accretion disks

Short gamma-ray bursts require a rotating black hole, surrounded by a magnetized relativistic accretion disk, such as the one formed by coalescing binary neutron stars or neutron star - black hole systems. The accretion onto a Kerr black hole is the mechanism of launching a baryon-free relativistic jet. An additional uncollimated outflow, consisting of sub-relativistic neutron-rich material which becomes unbound by thermal, magnetic and viscous forces, is responsible for blue and red kilonova. We explore the formation, composition and geometry of the secondary outflow by means of simulating accretion disks with relativistic magneto-hydrodynamics and employing realistic nuclear equation of state. We calculate the nucleosynthetic r-process yields by sampling the outflow with a dense set of tracer particles. Nuclear heating from the residual r-process radioactivities in the freshly synthesized nuclei is expected to power a red kilonova, contributing independently from the dynamical ejecta component, launched at the time of merger, and neutron-poor broad polar outflow, launched from the surface of the hypermassive neutron star by neutrino wind. Our simulations show that both magnetisation of the disk and high black hole spin are able to launch fast wind outflows ($v/c\sim 0.11-0.23$) with a broad range of electron fraction $Y_{\rm e}\sim 0.1-0.4$, and help explain the multiple components observed in the kilonova lightcurves. The total mass loss from the post-merger disk via unbound outflows is between 2\% and 17\% of the initial disk mass.

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arXiv2019

Effects of adiabatic index on the sonic surface and time variability of low angular momentum accretion flows

We study the role of adiabatic index in determining the critical points in the transonic low angular momentum accretion flow onto a black hole. We present the general relativistic 2D hydrodynamic simulations of axisymmetric, inviscid accretion flows in a fixed Kerr black hole gravitational field. A relativistic fluid where its bulk velocity is comparable to the speed of light, flowing in the accretion disk very close to the horizon can be described by an adiabatic index of 4/3 < γ < 5/3. The time dependent evolution of the shock position and respective effects on mass accretion rate and oscillation frequency with varying adiabatic index is discussed in the context of the observed microquasars.

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arXiv2019

Catching Element Formation In The Act

Gamma-ray astronomy explores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays and relativistic-particle acceleration, and the evolution of galaxies. MeV gamma-rays provide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nuclear de-excitation, and positron annihilation. The substantial information carried by gamma-ray photons allows us to see deeper into these objects, the bulk of the power is often emitted at gamma-ray energies, and radioactivity provides a natural physical clock that adds unique information. New science will be driven by time-domain population studies at gamma-ray energies. This science is enabled by next-generation gamma-ray instruments with one to two orders of magnitude better sensitivity, larger sky coverage, and faster cadence than all previous gamma-ray instruments. This transformative capability permits: (a) the accurate identification of the gamma-ray emitting objects and correlations with observations taken at other wavelengths and with other messengers; (b) construction of new gamma-ray maps of the Milky Way and other nearby galaxies where extended regions are distinguished from point sources; and (c) considerable serendipitous science of scarce events -- nearby neutron star mergers, for example. Advances in technology push the performance of new gamma-ray instruments to address a wide set of astrophysical questions.

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arXiv2018

Accretion in a dynamical spacetime and spinning up of the black hole in the gamma ray burst central engine

We compute the evolution of a quasi-spherical, slowly rotating accretion flow around a black hole, whose mass and spin evolve adequately to the mass-energy transfer through the horizon. Our model is relevant for the central engine driving a long gamma ray burst, that originates from the collapse of a massive star. The computations of a GRB engine in a dynamically evolving spacetime metric are important specifically due to the transient nature of the event, in which a huge amount of mass is accreted and changes the fundamental black hole parameters, its mass and spin, during the process. We discuss the results in the context of angular momentum magnitude of the collapsing star. We also study the possible formation and evolution of shocks in the envelope, which may temporarily affect accretion. Our results are important for the limitations on the mass and spin range of black holes detected independently by electromagnetic observations of GRBs and gravitational waves. We speculate on the possible constraints for the final masses and spins of these astrophysical black holes. It is shown that the most massive BHs were rahter not formed in a powerful GRB explosion, if the cores of their progenitors were only weakly rotating.

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arXiv2018

The Athena X-ray Integral Field Unit

The X-ray Integral Field Unit (X-IFU) is the high resolution X-ray spectrometer of the ESA Athena X-ray observatory. Over a field of view of 5' equivalent diameter, it will deliver X-ray spectra from 0.2 to 12 keV with a spectral resolution of 2.5 eV up to 7 keV on ~5 arcsecond pixels. The X-IFU is based on a large format array of super-conducting molybdenum-gold Transition Edge Sensors cooled at about 90 mK, each coupled with an absorber made of gold and bismuth with a pitch of 249 microns. A cryogenic anti-coincidence detector located underneath the prime TES array enables the non X-ray background to be reduced. A bath temperature of about 50 mK is obtained by a series of mechanical coolers combining 15K Pulse Tubes, 4K and 2K Joule-Thomson coolers which pre-cool a sub Kelvin cooler made of a 3He sorption cooler coupled with an Adiabatic Demagnetization Refrigerator. Frequency domain multiplexing enables to read out 40 pixels in one single channel. A photon interacting with an absorber leads to a current pulse, amplified by the readout electronics and whose shape is reconstructed on board to recover its energy with high accuracy. The defocusing capability offered by the Athena movable mirror assembly enables the X-IFU to observe the brightest X-ray sources of the sky (up to Crab-like intensities) by spreading the telescope point spread function over hundreds of pixels. Thus the X-IFU delivers low pile-up, high throughput (>50%), and typically 10 eV spectral resolution at 1 Crab intensities, i.e. a factor of 10 or more better than Silicon based X-ray detectors. In this paper, the current X-IFU baseline is presented, together with an assessment of its anticipated performance in terms of spectral resolution, background, and count rate capability. The X-IFU baseline configuration will be subject to a preliminary requirement review that is scheduled at the end of 2018.

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arXiv2018

Numerical simulations of black hole accretion flows

We model the structure and evolution of black hole accretion disks, and their neighboring regions, using numerical simulations. The numerics is governed by the equations of general relativistic magneto-hydrodynamics (GRMHD). In particular, such disks and outflows can be found at the base of very energetic ultra-relativistic jets produced by cosmic explosions, so called gamma-ray bursts (GRBs). Another, more persistent type of the jet phenomena, are blazars, emitted from the centers of galaxies. Long-lasting, detailed computations are essential to properly determine the physics of these explosions, and confront the theoretical models with any potential observables. From the point of view of numerical methods and computational techniques, three ingredients need to be considered. First, the numerical scheme must work in a conservative manner, which is achieved by solving a set of non-linear equations at each time-step, to advance the conserved quantities from one time step to the next. Second, the efficiency of computations intrinsically depends on the code parallelization methods, which may use various techniques. Third, the analysis of results is possible via the post-processing of the computed time-dependent physical quantities, and visualization of the flow properties. This is done via implementing various packages and libraries that are standardized in the field of computational astrophysics and supported by community developers. In the present paper, we discuss the physical picture of the cosmic sources which are modeled using numerical framework. We also describe several technical issues, in the particular context of our own experience with the performance of the GRMHD code which we develop. We also present a suite of performance tests, done on the High-Performance Computer cluster (HPC) in the Center for Mathematical Modeling of the Warsaw University.

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arXiv2018

Gamma Ray Bursts. Progenitors, accretion in the central engine, jet acceleration mechanisms

The collapsar model was proposed to explain the long-duration gamma-ray bursts (GRBs), while the short GRBs are associated with the mergers of compact objects. In the first case, mainly the energetics of the events is consistent with the proposed progenitor models, while the duration, time variability, as well as the afterglow emission may shed some light on the detailed properties of the collapsing massive stars. In the latter case, the recent discovery of the binary neutron star (NS-NS) merger in the gravitational wave observation made by LIGO (GW170817), and the detection of associated electromagnetic counterparts, for the first time gave a direct proof of the NS-NS merger being a progenitor of a short GRB. In general, all GRBs are believed to be powered by accretion through a rotationally supported torus, or by fast rotation of a compact object. For long ones, the rotation of the progenitor star is a key property in order to support accretion over relatively long activity periods, and also to sustain the rotation of the black hole itself. The latter is responsible for ejection of the relativistic jets, which are powered due to the extraction of the BH rotational energy, mitigated by the accretion torus and magnetic fields. The jets must break through the stellar envelope though, which poses a question on the efficiency of this process. Similar mechanisms of powering the jet ejection may act in short GRBs, which in this case may freely propagate through the interstellar medium. The power of the jets launched from the rotating black hole is at first associated mostly with the magnetic Poynting flux, and then at large distances it is transferred to the kinetic and finally radiative energy of the expanding shells.

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arXiv2017

Microphysics in GRB central engine

We study the structure and evolution of the accreting plasma in gamma ray burst central engines. The models are based on the general relativistic MHD simulations. The nuclear equation of state adequate for dense and degenerate plasma, is incorporated to the numerical scheme instead of a simple polytropic EOS. Plasma is cooled by neutrinos and energy is extracted from the rotating BH by magnetic fields. We discuss our results in the frame of the observable GRBs, and speculate about the origin of variability in the GRB jet emission. We also discuss the possible contribution from the heavy elements synthesized in winds from GRB engines to the kilonova emission.

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arXiv2017

Shocks in the relativistic transonic accretion with low angular momentum

We perform 1D/2D/3D relativistic hydrodynamical simulations of accretion flows with low angular momentum, filling the gap between spherically symmetric Bondi accretion and disc-like accretion flows. Scenarios with different directional distributions of angular momentum of falling matter and varying values of key parameters such as spin of central black hole, energy and angular momentum of matter are considered. In some of the scenarios the shock front is formed. We identify ranges of parameters for which the shock after formation moves towards or outwards the central black hole or the long lasting oscillating shock is observed. The frequencies of oscillations of shock positions which can cause flaring in mass accretion rate are extracted. The results are scalable with mass of central black hole and can be compared to the quasi-periodic oscillations of selected microquasars (such as GRS 1915+105, XTE J1550-564 or IGR J17091-3624), as well as to the supermassive black holes in the centres of weakly active galaxies, such as Sgr $A^{*}$.

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