arXiv subjects

Agnieszka Janiuk

Publications and source records attributed to Agnieszka Janiuk.

At least 37 records · Page 2Linked to original sources

arXiv2024

Alive but Barely Kicking: News from 3+ years of Swift and XMM-Newton X-ray Monitoring of Quasi-Periodic Eruptions from eRO-QPE1

Quasi-periodic Eruptions (QPEs) represent a novel class of extragalactic X-ray transients that are known to repeat at roughly regular intervals of a few hours to days. Their underlying physical mechanism is a topic of heated debate, with most models proposing that they originate either from instabilities within the inner accretion flow or from orbiting objects. At present, our knowledge of how QPEs evolve over an extended timescale of multiple years is limited, except for the unique QPE source GSN 069. In this study, we present results from strategically designed Swift observing programs spanning the past three years, aimed at tracking eruptions from eRO-QPE1. Our main results are: 1) the recurrence time of eruptions can vary between 0.6 and 1.2 days, 2) there is no detectable secular trend in evolution of the recurrence times, 3) consistent with prior studies, their eruption profiles can have complex shapes, and 4) the peak flux of the eruptions has been declining over the past 3 years with the eruptions barely detected in the most recent Swift dataset taken in June of 2023. This trend of weakening eruptions has been reported recently in GSN 069. However, because the background luminosity of eRO-QPE1 is below our detection limit, we cannot verify if the weakening is correlated with the background luminosity (as is claimed to be the case for GSN 069). We discuss these findings within the context of various proposed QPE models.

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arXiv2024

Collapsing massive stars and their EM transients

We investigate the fate of a collapsing stellar core, which is the final state of evolution of a massive, rotating star of a Wolf-Rayet type. Such stars explode as type I b/c supernovae, which have been observed in association with long gamma ray bursts (GRBs). The core of the star is potentially forming a black hole, which is embedded in a dense, rotating, and possibly highly magnetized envelope. We study the process of collapse using General Relativistic MHD simulations, and we account for the growth of the black hole mass and its spin, as well as related evolution of the spacetime metric. We find that some particular configurations of the initial black hole spin, the content of angular momentum in the stellar core, and the magnetic field configuration and its strength, are favored for producing a bright electromagnetic transient (i.e., a gamma ray burst). On the other hand, most of the typical configurations studied in our models do not lead to a transient electromagnetic explosion and will end up in a direct collapse, accompanied by some residual variability induced by changing accretion rate. We also study the role of self-gravity in the stellar core and quantify the relative strength of the interfacial instabilities, such as Self-Gravity Interfacial (SGI) instability and Rayleigh-Taylor (RT), which may account for the production of an inhomogeneous structure, including spikes and bubbles, through the inner radii of the collapsing core (inside $\sim 200~r_{g}$). %Furthermore, we investigate the axisymmetric modes of gravitational instability based on the generalized Toomre parameter. We find that in self-gravitating collapsars the RT modes cannot grow efficiently. We also conclude that transonic shocks are formed in the collapsing envelope, but they are weaker in magnetized stars.

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arXiv2024

Following the jet interaction with a post-merger disk outflow

Short GRBs are produced by relativistic jets arising from binary NS-NS or NS-BH mergers. Since the detection of the first unambiguous off-axis GRB 170817A, we learned that energy distribution in the jet plays an important role in explaining the GRB emission. The structure and dynamics are modified during the first seconds of the jet interaction with a post-merger environment. Conventional studies often assume this environment as a simple homologous and symmetrically expanding wind. However, post-merger outflows exhibit complex dynamics influenced by the accretion disc evolution. Moreover, the r-process nucleosynthesis influences the thermodynamics and properties of the post-merger neutron-rich environment. In this work, we study the impact of realistic post-merger disc outflow over the jet dynamics at large scales. We find the results are substantially different from the typical model with symmetric homologous wind.

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arXiv2024

Numerical GR MHD simulations of the post-merger system with a composition-dependent equation of state

By means of HARM\_COOL\_EOS, which is our code for conservative relativistic magnetohydrodynamics, we developed a new scheme for the simulation of a system formed after compact binary merger. Our code works with a tabulated equation of state of dense matter, accounts for the neutrino leakage, and follows the mass outflows via the tracer particle method. We discuss the numerical scheme, and present the recovery method included in our code. We also show results of a numerical simulation, addressed to the post-merger system after the coalescence of binary neutron stars, or a neutron star with a stellar mass black hole. The plasma is very neutron-rich, so the r-process nucleosynthesis in the ejected material may lead to unstable heavy isotopes creation. They are responsible for an electromagnetic signal, observed as a kilonova. In addition, the magnetized, neutrino-driven wind can act as a collimating mechanism for the relativistic jet.

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arXiv2023

Accretion processes onto black holes: theoretical problems, observational constraints

We shortly summarize the standard current knowledge on the structure of the accretion flow onto black holes in galactic binary systems and in active galactic nuclei. We stress the similarities and differences between the two types of systems, and we highlight the complementarity of the data caused by these differences. We highlight some new developments and list the unsolved problems.

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arXiv2023

Neutrino cooled disk in post-merger system studied via numerical GR MHD simulation with a composition-dependent equation of state

The code HARM\_COOL, a conservative scheme for relativistic magnetohydrodynamics, is being developed in our group and works with a tabulated equation of state of dense matter. This EOS can be chosen and used during dynamical simulation, instead of the simple ideal gas one. In this case, the inversion scheme between the conserved and primitive variables is not a trivial task. In principle, the code needs to solve numerically five coupled non-linear equations at every time-step. The 5-D recovery schemes were originally implemented in HARM and worked accurately for a simple polytropic EOS which has an analytic form. Our current simulations support the composition-dependent EOS, formulated in terms of rest-mass density, temperature and electron fraction. In this proceeding, I discuss and compare several recovery schemes that have been included in our code. I also present and discuss their convergence tests. Finally, I show set of preliminary results of a numerical simulation, addressed to the post-merger system formed after the binary neutron stars (BNS) coalescence.

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arXiv2023

Self-gravitating collapsing star and black hole spin-up in long gamma ray bursts

Long Gamma Ray Bursts (GRBs) originate from the collapse of massive, rotating stars. We aim to model the process of stellar collapse in the scenario of a self-gravitating collapsing star. We account for the changes in Kerr metric induced by the growth of the black hole, accretion of angular momentum, as well as the self-gravity effect due to a large mass of the collapsing stellar core falling onto black hole in a very short time. We also investigate the existence of accretion shocks in the collapsar, and role of magnetic field in their propagation. We compute the time-dependent axially-symmetric General Relativistic magnetohydrodynamic model of a collapsing stellar core in the dynamical Kerr metric. We explore the influence of self-gravity in such star, where the newly formed black hole is increasing the mass, and changing its spin. The Kerr metric evolves according to the mass and angular momentum changes during the collapse. We parameterize the rotation inside the star, and account for the presence of large-scale poloidal magnetic field. For the set of the global parameters, such as the initial black hole spin, and initial content of specific angular momentum in the stellar envelope, we determine the evolution of black hole parameters (mass and spin) and we quantify the strength of the gravitational instability, variability timescales and amplitudes. We find that the role of the gravitational instability measured by the value of the Toomre parameter is relatively important in the innermost regions of the collapsing star. The character of accretion rate variability strongly depends on the assumption of self-gravity in the model, and is also affected by the magnetic field. Additional factors are initial spin and rotation of the stellar core.

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arXiv2022

Studying Postmerger Outflows from Magnetized Neutrino-cooled Accretion Disks

Neutrino-cooled accretion flow around a spinning black hole, produced by a compact binary merger is a promising scenario for jet formation and launching magnetically-driven outflows. Based on GW170817 gravitational wave detection by LIGO and Virgo observatories followed by electromagnetic counterparts, this model can explain the central engine of the short duration gamma ray bursts (GRB) and kilonova radiations. Using the open-source GRMHD HARM-COOL code, we evolved several 2D magnetized accretion disk-black hole models with realistic equation of state in the fixed curved space-time background. We applied particle tracer technique to measure the properties of the outflows. The disk and black hole's initial parameters are chosen in a way to represent different possible post-merger scenarios of the merging compact objects. Our simulations show a strong correlation between black hole's spin and ejected mass. Generally, mergers producing massive disks and rapidly spinning black holes launch stronger outflows. We observed our models generate winds with moderate velocity ($v/c \sim 0.1-0.2$), and broad range of electron fraction. We use these results to estimate the luminosity and light curves of possible radioactively powered transients emitted by such systems. We found the luminosity peaks within the range of $10^{40}-10^{42}$ erg/s which agrees with previous studies for disk wind outflows.

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arXiv2022

Modeling the GRB jet properties with 3D general relativistic simulations of magnetically arrested accretion flows

We investigate the dependence of the GRB jet structure and its evolution on the properties of the accreting torus in the central engine. Our models numerically evolve the accretion disk around a Kerr black hole using 3D general relativistic magnetohydrodynamic simulations. We use two different analytical hydrodynamical models of the accretion disk, based on the Fishbone-Moncrief and Chakrabarti solutions, as our initial states for the structure of the collapsar disk and the remnant after a binary neutron star merger, respectively. We impose poloidal magnetic fields of two different geometries upon the initial stable solutions. We study the formation and evolution of the magnetically arrested disk state and its effect on the properties of the emitted jet. The jets produced in our models are structured and have a relatively hollow core and reach higher Lorentz factors at an angle $\gtrsim 9{^\circ}$ from the axis. The jet in our short GRB model has an opening angle of up to $\sim 25^{\circ}$ while our long GRB engine produces a narrower jet, of up to $\sim 11^{\circ}$. We also study the time variability of the jets and provide an estimate of the minimum variability timescale in our models. The application of our models to the GRB jets in the binary neutron star post-merger system and to the ultra-relativistic jets launched from collapsing stars are briefly discussed.

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arXiv2022

Modified models of radiation pressure instability in application to 10, 10$^5$, and 10$^7$ $M_{\odot}$ accreting black holes

Some of the accreting black holes exhibit much stronger variability patterns than the usual stochastic variations. Radiation pressure instability is one of the proposed mechanisms which could account for this effect. We aim to model luminosity changes for objects with black hole mass of 10, 10$^5$, and 10$^7$ solar masses, using the time-dependent evolution of an accretion disk unstable due to the dominant radiation pressure. We use a 1-dimensional, vertically integrated time-dependent numerical scheme which models simultaneous evolution of the disk and corona, coupled by the vertical mass exchange. We also discuss the possibility of presence of an inner optically thin flow, namely the Advection-Dominated Accretion Flow (ADAF). We found that the outburst character strongly depends on the magnetic field and the outer radius of the disk if this radius is smaller (due to TDE phenomenon) than the size of the instability zone in a stationary disk with infinite radius. For microquasars, the dependence on the magnetic field is monotonic, and the period decreases with the field strength. For larger black hole masses, the dependence is non-monotonic, and initial rise of the period is later replaced with the relatively rapid decrease as the magnetic field continues to rise. Still stronger magnetic field stabilizes the disk. Our computations confirm that the radiation pressure instability model can account for heartbeat states in microquasars. Rapid variability detected in IMBH in the form of Quasi-Periodic Ejection can be consistent with the model but only if combined with TDE phenomenon. Yearly repeating variability in Changing Look AGN also requires, in our model, small outer radius either due to the recent TDE or due to the presence of the gap in the disk related to the presence of a secondary black hole.

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arXiv2022

Accretion of the magnetized neutrino-cooled torus

Neutrino-cooled accretion flow around a black hole, produced by a compact binary merger, is a promising scenario for jet formation and magnetic-driven winds to explain short duration gamma ray bursts (GRBs) central engine and kilonovae based on GW170817 gravitational wave observation. Magnetorotational instability (MRI) turbulence and Blandford-Znajek (BZ) mechanism are expected to play key roles in the thermal equilibrium of the disk (balancing neutrino cooling) and in driving accretion and creating jets. Using the open-source GRMHD HARM-COOL code, we study the magnetically-driven evolution of an accretion disk with realistic equation of state in the fixed curved space-time background. We identify the effects of the neutrino cooling and the magnetic field, paying particular attention to the dynamical, thermal and composition evolution of the disk and outflows.

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arXiv2021

Many faces of accretion in gamma ray bursts

Accretion powers relativistic jets in GRBs, similarly to other jet sources. Black holes that are at heart of long GRBs, are formed as the end product of stellar evolution. At birth, some of the black holes must be very rapidly spinning, to be able to power the GRBS. In some cases, the black holes may be born without formation of a disk/jet engine, and then the star will collapse without an electromagnetic transient. In this proceeding, we discuss the conditions for launching variable jets from the magnetized disk in an arrested state. We also discuss properties of collapsing massive stars as progenitors of GRBs, and the conditions which must be satisfied for the star in order for the collapsar to produce a bright gamma-ray transient. We find that the black hole rotation is further affected by self-gravity of the collapsing matter. Finally, we comment on the properties of the accretion disk under extreme conditions of nuclear densities and temperatures, while it can contribute to the kilonova accompanying short GRBs.

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arXiv2021

Modeling changing-look (CL) AGN phenomenon in 1D using accretion disk instabilities

Apart from regular, low-level stochastic variability, some AGN occasionally show exceptionally large changes in the luminosity, spectral shape, and/or X-ray absorption. The most notable are the changes of the spectral type when the source classified as a Seyfert 1 becomes a Seyfert 2 galaxy or vice versa. Thus a name was coined of 'Changing-Look AGN' (CL AGN). The origin of this phenomenon is still unknown, but for most of the sources, there are strong arguments in favor of the intrinsic changes. Understanding the nature of such rapid changes is a challenge to the models of black hole accretion flows since the timescales of the changes are much shorter than the standard disk viscous timescales. We aim to model the CL AGN phenomenon assuming that the underlying mechanism is the time-dependent evolution of a black hole accretion disk unstable due to the dominant radiation pressure. We use a 1-dimensional, vertically integrated disk model, but we allow for the presence of the hot coronal layer above the disk and the presence of the inner purely hot flow. We focus on the variability timescales and amplitudes, which can be regulated by the action of large-scale magnetic fields, the description of the disk-corona coupling, and the presence of an inner optically thin flow, like Advection-Dominated Accretion Flow (ADAF). We compare model predictions for the accretion disk around black hole mass 10$^7$M$_{\odot}$.

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arXiv2021

Unraveling circular polarimetric images of magnetically arrested accretion flows near event horizon of a black hole

Magnetically arrested accretion flows are thought to fuel some of the supermassive black holes and to power their relativistic jets. We calculate and study a time sequence of linear and circular polarimetric images of numerical, high resolution and long duration simulations of magnetically dominated flows to investigate observational signatures of strong magnetic fields near the event horizon of a non-rotating black hole. We find that the magnitude of resolved linear and circular polarizations is rather sensitive to the assumption of the coupling of electron and ions in the accretion flow. Models with cooler electrons have higher Faraday rotation and conversion depths which results in scrambled linear polarization and enhanced circular polarization. In those high Faraday thickness cases the circular polarization is particularly sensitive to dynamics of toroidal-radial magnetic fields in the accretion flows. The models with high Faraday thickness are characterized by nearly constant handedness of circular polarization, consistent with observations of some accreting black holes. We also find that the emission region produced by light which is lensed around the black hole shows inversion of circular polarization handedness with respect to the handedness of the circular polarization of the entire emission region. Such polarity inversions are unique to near horizon emission.

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arXiv2021

Variability of magnetically-dominated jets from accreting black holes

Structured jets are recently invoked to explain the complex emission of gamma ray bursts, such as GW 170817. Based on the accretion simulations, the jets are expected to have a structure that is more complex than a simple top-hat. Also, the structure of launching regions of blazar jets should influence their large scale evolution. This is recently revealed by the interactions of jet components in TXS 0506+056, where the jet is observed at a viewing angle close to zero. Observational studies have also shown an anti-correlation between the jet variability, measured e.g. by its minimum variability time scale, and the Lorentz factor, that spans several orders of magnitude and covers both blazars and GRBs samples. Motivated by those observational properties of black hole sources, we investigate the accretion inflow and outflow properties, by means of numerical GR MHD simulations. We perform axisymmetric calculations of the structure and evolution of central engine, composed of magnetized torus around Kerr black hole that is launching a non-uniform jet. We probe the jet energetics at different points along the line of sight, and we measure the jet time variability as localized in these specific regions. We quantify our results by computing the minimum variability timescales and power density spectra. We reproduce the MTS-$Γ$ correlation and we attribute it to the black hole spin as the main driving parameter of the engine. We also find that the PDS slope is not strongly affected by the black hole spin, while it differs for various viewing angles.

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arXiv2021

Accretion induced black hole spin up revised by numerical GR MHD simulations

We investigate the accretion induced spin up of the black hole via numerical simulations. Our method is based on general-relativistic magneto-hydrodynamics of the slowly-rotating flows in the Kerr metric, where possibly transonic shock fronts may form. We account for the changing black hole mass and spin during accretion which enforces dynamical evolution of the space-time metric. We first study non-magnetized flows with shocks, and we also include magnetic field endowed in the gas. The aim of this study is to verify whether the high mass black holes may be produced with large spins, even though at birth the collapsars might have contained slowly, or moderately spinning cores. In this way, we put constraints on the content of angular momentum in the collapsing massive stars. Our studies are also showing that shock fronts and magnetic fields may halt accretion and limit the black hole spin-up in the exploding supernovae.

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arXiv2020

Magnetically Ejected Disks: Equatorial Outflows Near Vertically Magnetized Black Hole

Black holes attract gaseous material from the surrounding environment. Cosmic plasma is largely ionized and magnetized because of electric currents flowing in the highly conductive environment near black holes; the process of accretion then carries the magnetic flux onto the event horizon, $r\simeq R_+$. On the other hand, magnetic pressure acts against accretion. It can not only arrest the inflow but it can even push the plasma away from the black hole if the magnetic repulsion prevails. The black hole does not hold the magnetic field by itself. In this contribution we show an example of an equatorial outflow driven by a large scale magnetic field. We initiate our computations with a spherically symmetric distribution of gas, which flows onto the domain from a large distance, $r\gg R_+$. After the flow settles in a steady (Bondi) solution, we impose an axially symmetric configuration of a uniform (Wald) magnetic field aligned with the rotation axis of the black hole. Then we evolve the initial configuration numerically by employing the MHD code that approaches the force-free limit of a perfectly conducting fluid. We observe how the magnetic lines of force start accreting with the plasma while an equatorial intermittent outflow develops and goes on ejecting some material away from the black hole.

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arXiv2020

The Fate of the Merger Remnant in GW170817 and its Imprint on the Jet Structure

The first neutron star binary merger detected in gravitational waves, GW170817 and the subsequent detection of its emission across the electromagnetic spectrum showed that these systems are viable progenitors of short $γ$-ray bursts (sGRB). The afterglow signal of GW170817 has been found to be consistent with a structured GRB jet seen off-axis, requiring significant amounts of relativistic material at large angles. This trait can be attributed to the interaction of the relativistic jet with the external wind medium. Here we perform numerical simulations of relativistic jets interacting with realistic wind environments in order to explore how the properties of the wind and central engine affect the structure of successful jets. We find that the angular energy distribution of the jet depends primarily on the ratio between the lifetime of the jet and the time it takes the merger remnant to collapse. We make use of these simulations to constrain the time it took for the merger remnant in GW170817 to collapse into a black hole based on the angular structure of the jet as inferred from afterglow observations. We conclude that the lifetime of the merger remnant in GW170817 was $\approx 1-1.7$s, which, after collapse, triggered the formation of the jet.

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