arXiv subjects

Agnieszka Janiuk

Publications and source records attributed to Agnieszka Janiuk.

At least 19 recordsLinked to original sources

arXiv2026

The Hidden Evolution of Gamma-Ray Burst Jets Revealed by Gravitational Waves

Gamma-ray burst (GRB) jets can propagate through massive stars, where the high opacity of the stellar envelope traps electromagnetic radiation and limits observations of the jet at early times. Gravitational waves (GWs), in contrast, can escape these dense environments and provide direct information of the jet dynamics. Previous studies have shown that GRB jets can emit GWs at low frequencies, while high-frequency emission may arise from jet-driven cocoons. However, the cocoon contribution typically requires energies much larger than those inferred from observations and produces stochastic fluctuations that can resemble noise. In this work, we show that high-frequency GW signals, can instead be generated by variability in the central engine. We consider jet models with different luminosity histories. We find that the GW emission depends on both the central engine history and the interaction of the jet with the stellar envelope. Rapid luminosity fluctuations are progressively suppressed as the jet propagates through the star, while the GW signal preserves information about the early jet evolution. Jets powered by strongly variable engines produce a GW signal peaking at tens of Hz, whereas smoothly varying engines are dominated by low-frequency emission. For a source at 36 Mpc, comparable to the distance of the closest GRB observed to date, the high-frequency signal from our most energetic variable models can reach amplitudes detectable by current ground-based interferometers. Our results suggest that the detection or non-detection of GWs from nearby GRBs and Type Ic broad-lined SNe could help constrain the mechanisms operating in the central engine.

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arXiv2026

Exploring Jet Structure and Dynamics in Short Gamma Ray Bursts: A Case Study on GRB 090510

Gamma-ray bursts observed in high-energies allow the investigation of the emission processes of these still puzzling events. In this study, we perform general relativistic magnetohydrodynamic (GRMHD) simulations to investigate GRB 090510, a peculiar short GRB detected by Fermi-LAT. Our primary goal is to model the energetics, jet structure, variability, and opening angle of the burst to understand its underlying physical conditions. We tested the 2D and 3D models and estimated the time scale of variability. The predicted energetics and the jet opening angle reconcile with the observed ones with 1$σ$ when considering that the jet opening angles also evolve with redshift. Furthermore, we extend our analysis by incorporating dynamical ejecta into selected models to study its impact on jet collimation at smaller distances. In addition, we investigated a suite of models exhibiting a broad range of observable GRB properties, thereby extending our understanding beyond this specific event.

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arXiv2026

Numerical simulations of jet launching and breakout from collapsars

Long Gamma-Ray Bursts (LGRBs) are often associated with the collapse of stripped-envelope massive stars. Powerful relativistic jets drill through the stellar envelope before the gamma emission. Previous hydrodynamical studies imposed jets artificially, neglecting accretion dynamics, while the central engine simulations have reproduced jet launching via the Blandford-Znajek mechanism focusing on the inner core regions. However, both the central engine and the progenitor structure are crucial to determining the jet's evolution. In this study, we present axisymmetric (2.5-D) GRMHD simulations that self-consistently follow jet formation from the black-hole horizon to breakout at the stellar surface ($R_\star \sim 10^{10}$~cm). The setup assumes a Kerr black hole with spin $a \sim 0.9$ in the centre of three progenitor models, varying the magnetic-field strength and geometry. Relativistic jets are successfully launched by a strong dipolar magnetic field ($B_0 \gtrsim 10^{12}$-$10^{14}$~G) from magnetically arrested disks. These jets, initially magnetically dominated, convert energy into thermal and kinetic during their propagation. We found breakout times within $1.8 \lesssim t_{\rm bo} \lesssim 3.5$~s and luminosities $L_j \sim 5\times10^{49}-7\times10^{52}$~erg\,s$^{-1}$. Our results highlight the role of the initial magnetic field strength and its geometry, emphasizing the progenitor's density distribution as a key factor impacting the final structure and dynamics of LGRB jets.

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arXiv2026

Chemical Evolution and Kilonova Implications of Post-Merger Accretion Disk Winds

Several gamma ray bursts have recently been associated with a kilonova emission. We study the mechanisms which could account for this effect, by means of radioactive decay of elements synthesized in accretion disk wind. We model the r-process nucleosynthesis in the accretion disk wind system, asscociated with the prompt GRB phase. We compute the time-dependent GR MHD evolution of a GRB central engine where the newly formed black hole is accreting the mass from post-merger remnant. We explore the wind properties, for a range of the initial parameters of the system, and study representative cases for compact binary merger progenitors. We compute a suite of 2D and 3D time-dependent General Relativistic numerical simulations with a tabulated 3-parameter equation of state that allows for evolution of chemical composition evolution of the accretion flow. The neutrino emission is accounted for by incorporating the leakage scheme, where neutrino optical depth is calculated along the radial rays. We parameterize the optically thick and thin tori with different values of the pressure maximum and entropy in the disk, while the strength of large-scale poloidal magnetic fields is parameterized according to the chosen gas-to-magnetic pressure ratio. To probe the winds, we follow the particle trajectories. Upon this, we derive the nucleosynthetic yields of heavy elements in the outflows, and we map the regions of Lanthanide rich and poor ejecta. We find that the outflow carries high mass of neutron rich material expanding with mildly relativistic velocities. Our accretion disks operating under the SANE mode can power the GRB jets via neutrino annihilation, if the disk to BH mass ratio is larger than about 0.01 and the black hole is spinning. Slowly spinning black holes surrounded by massive post-merger disks can power these jets, and also be the sites of efficient nucleosynthesis of Lanthanides.

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arXiv2026

Three-dimensional GRMHD simulations of jet formation and propagation in self-gravitating collapsing stars

We investigate collapsar models with and without self-gravity under identical initial conditions to directly compare the effects of self-gravity on jet properties, such as opening angle, jet power, terminal Lorentz factor, and its variability. We compute a suite of time-dependent, three-dimensional GRMHD simulations of collapsars in evolving spacetime. We update the Kerr metric components due to the growth of the black hole mass and changes its angular momentum. The self-gravity is considered via perturbative terms. We present for the first time the process of jet formation in self-gravitating collapsars. We find that self-gravity leads to temporary jet quenching, which can explain some features in the gamma-ray burst prompt emission. We find no substantial difference in jet launching times between models with and without self-gravity. We observe that in the absence of self-gravity, the jet can extract more rotational energy from the black hole, while self-gravitating models produce narrower jet opening angles. We show that under certain conditions, self-gravity can interrupt the jet formation process, resulting in a failed burst. Our computations show that self-gravity significantly modifies the process of jet propagation, resulting in notably different jet properties. We show that the timescales, variability, and opening angle of jet depend on whether self-gravity is included or not. We argue that self-gravity can potentially explain certain prompt emission properties due to the jet quenching.

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arXiv2025

The propagation of long GRB jets through and beyond its progenitor star

Long gamma-ray bursts (lGRB) are produced by relativistic jets arising from the collapse of massive stars. Such progenitor environments present complex physical conditions that are challenging to model by numerical simulations. The difficulty increases when solving the accretion process and propagation of the outflows, as it requires covering distances from the black hole horizon to beyond the progenitor star. General Relativistic Magnetohydrodynamic (GRMHD) simulations provide a convenient framework to study high-luminosity jets, where magnetic flux plays an important role in the process of jet launching from the central engine. To follow the propagation of the jet through and beyond its progenitor environment, we use multi-scale simulations (i.e., AMR-based). In this work, we report results of 2.5-dimensional GRMHD simulations of a lGRB progenitor. We present highly magnetized, weakly magnetized, and non-magnetized pre-collapse stars, and discuss the observational implications for lGRB jets.

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arXiv2025

Relativistic MHD simulations of merging and collapsing stars and effects on GRB transient

Compact binary mergers and the collapse of massive stars can produce intense transients observable across high-energy wavelengths. Events such as gamma-ray bursts and kilonova emissions are often accompanied by gravitational wave detections, making them crucial sources for multimessenger astrophysics. To explore these phenomena theoretically, state-of-the-art approaches of General Relativistic magnetohydrodynamic simulations are used. We present recent findings from our simulations, and discuss observational consequences of the stellar/post-merger environment on the gamma ray burst prompt emission properties.

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arXiv2025

Radiation pressure instability: from heart-beat states in black hole binary systems to Quasars and Changing-Look AGN

Radiation-pressure instability was identified soon after the seminal classical accretion disk models of Shakura-Sunyaev and Novikov-Thorne, yet its full implications remain an active area of investigation. These models form the backbone of our understanding of accretion onto compact objects and successfully describe the phenomenology of black hole and neutron star X-ray binaries, as well as luminous active galactic nuclei (AGN), in the regime of high mass accretion rates. At luminosities approaching a significant fraction of the Eddington limit (L/LEdd > 0.1), standard thin disks are predicted to become thermally unstable due to the dominance of radiation pressure. This prediction has found empirical support in several Galactic stellar-mass black hole systems, where the instability manifests as quasi-periodic, large-amplitude luminosity oscillations, so-called "heartbeat states", and has been proposed as a driver of observed signatures of deterministic chaos in accretion-driven light curves. The scope of radiation-pressure-induced variability extends beyond stellar-mass black holes: both black holes across mass scales and accreting neutron stars can exhibit related behavior, though the presence of a boundary layer in neutron stars adds complexity and offers a unique laboratory for testing the interplay between accretion dynamics and the central object. On extragalactic scales, the instability has been invoked to explain the duty cycles and apparent short lifetimes of radio-loud AGN, as well as the dramatic spectral-state transitions seen in Changing-Look AGN. (...)

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arXiv2025

The impact of disk outflows on the structure of short GRB jets at large scales

Short Gamma-Ray Bursts (GRBs) are known to be associated with binary neutron star (NSNS) or black hole-neutron star (BHNS) mergers. The detection of gravitational wave and its associated electromagnetic counterparts GW/GRB 170817A has shown that interactions between relativistic jets and mildly relativistic ejecta influence observed radiation. Previous studies simulated a uniform jet propagating through a homologously expanding wind, however, jets and disk outflows are launched together during accretion, making the interaction more complex. We investigate how the disk wind impacts jet propagation at distances $r\sim 10^8\,-\,10^{11}~$cm. We are using two-dimensional special relativistic hydrodynamical simulations. As initial conditions, we remap the outflows from general relativistic magnetohydrodynamical simulations of BH accretion disks that represent post-merger NSNS or BHNS remnants. We account for wind stratification and r-process nucleosynthesis, which alter the pressure profile from that of an ideal gas in the initial conditions. We found that a) self-consistent wind pressure leads to significant changes in the jet collimation and cocoon expansion; b) the angular structure of thermal and kinetic energy components in the jets, cocoons, and winds differ with respect to simple homologous models; c) the temporal evolution of the structure reveals conversion of thermal to kinetic energy being different for each component in the system (jet, cocoon, and wind); d) dynamical ejecta alters the interaction between jets and disk winds. Our results show that the jet and cocoon structure is shaped by the accretion disk wind that alters the effect of dynamical ejecta and may have an impact on the observed afterglow emission.

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arXiv2025

Theoretical Modelling of Gamma-Ray Burst 090510

Gamma-ray bursts detected at high energies provide valuable insights into the emission mechanisms behind these still puzzling enigmatic events. In this study, we focus on GRB 090510, which is an unusual short GRB exhibiting plateau emission observed by the Fermi-LAT. Using the general relativistic magnetohydrodynamic code (HARM), we aim to infer the key properties of this GRB, such as the jet opening angle, the energetics, the Lorentz Gamma factor, the jet structure and its variability, and the progenitor parameters of the compact binary system. We explored both the 2D and 3D models and estimated the variability timescales. Our findings show that the predicted jet opening angle is within $88\%$ of the observed upper limit from observations, and the energetics are in general agreement with observed values when accounting for the evolution of jet opening angle with redshift. This work establishes the foundation for ongoing exploration, which will further align the theoretical model simulations with observational data.

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arXiv2024

Broad band spectral modeling of M87 nucleus

We study spectra produced by weakly accreting black hole (BH) systems using the semi-analytic advection dominated accretion flow (ADAF) model and the general-relativistic magentohydrodynamic (GRMHD) simulation. We find significant differences between these two approaches related to a wider spread of the flow parameters as well as a much steeper radial distribution of the magnetic field in the latter. We apply these spectral models to the broad-band spectral energy distribution (SED) of the nucleus of M87 galaxy. The standard (in particular, one-dimensional) formulation of the ADAF model does not allow to explain it; previous claims that this model reproduces the observed SED suffer from an inaccurate treatment of the Compton process. The spectra based on GRMHD simulation are in a much better agreement with the observed data. In our GRMHD model, in which we assumed the BH spin $a=0.9$, bulk of radiation observed between the millimeter and the X-ray range is produced in the disk area within 4 gravitational radii from the BH. In this solution, the synchrotron component easily reproduces the spectral data between the millimeter and the UV range, and the Compton component does not violate the X-rays constraints, for accretion rates not exceeding 0.01 Msun/year and a relatively strong magnetic field, with the plasma $β\sim 1$ in the region where radiation is produced. However, the Compton component cannot explain the observed X-ray spectrum. Instead, the X-ray spectrum can be reproduced by a high energy tail of the synchrotron spectrum if electrons have a hybrid energy distribution with a $\sim 5$ per cent nonthermal component.

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arXiv2024

Chaotic magnetic disconnections trigger flux eruptions in accretion flows channeled onto magnetically saturated Kerr black holes

Magnetized accretion flow onto a black hole (BH) may lead to accumulation of poloidal magnetic flux across its horizon, which for high BH spin can power far-reaching relativistic jets. The BH magnetic flux is subject to a saturation mechanism by means of magnetic flux eruptions involving relativistic magnetic reconnection. Such accretion flows have been described as magnetically arrested (MAD) or choked (MCAF). The main goal of this work is to describe the onset of relativistic reconnection and initial development of magnetic flux eruption in accretion flow onto magnetically saturated BH. We analyze the results of 3D ideal GRMHD numerical simulations in the Kerr metric, starting from weakly magnetized geometrically thick tori rotating either prograde or retrograde. We integrate large samples of magnetic field lines in order to probe magnetic connectivity with the BH horizon. The boundary between magnetically connected and disconnected domains coincides roughly with enthalpy equipartition. The geometrically constricted innermost part of the disconnected domain develops a rigid structure of magnetic field lines - rotating slowly and insensitive to the BH spin orientation. The typical shape of innermost disconnected lines is a double spiral converging to a sharp inner tip anchored at the single equatorial current layer. The footpoints of magnetic flux eruptions are found to zip around the BH along with other azimuthal patterns. Magnetic flux eruptions from magnetically saturated accreting BHs can be triggered by minor density gaps in the disconnected domain, resulting from chaotic disconnection of plasma-depleted magnetospheric lines. Accretion flow is effectively channeled along the disconnected lines towards the current layer, and further towards the BH by turbulent cross-field diffusion. Rotation of flux eruption footpoints may contribute to the variability of BH crescent images.

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arXiv2024

What Is the Black Hole Spin in Cyg X-1?

We perform a detailed study of the black hole spin of Cyg X-1, using accurate broad-band X-ray data obtained in the soft spectral state by simultaneous NICER and NuSTAR observations, supplemented at high energies by INTEGRAL data. We use the relativistic disk model kerrbb together with different models of the Comptonization high energy tail and the relativistically-broadened reflection features. Unlike most previous studies, we tie the spin parameters of the disk and relativistic broadening models, thus combining the continuum and reflection methods of spin determination. We also consider a likely increase of the disk color correction due to a partial support of the disk by large scale magnetic fields. We find that such models yield the spin parameter of $a_*= 0.87^{+0.04}_{-0.03}$ if the disk inclination is allowed to be free, with $i= 39^{+1}_{-1}$ degree. Assuming $i=27.5$ degree, as determined by optical studies of the binary, worsens the fit, but leads to similar values of the spin, $a_*= 0.90^{+0.01}_{-0.01}$. In addition, we consider the presence of a warm Comptonization layer on top of the disk, motivated by successful modeling of soft X-ray excesses in other sources with such a model. This dramatically lowers the spin, to $a_*\lesssim 0.1$, consistent with the spin measurements from black-hole mergers. On the other hand, if the natal spin of Cyg X-1 was low but now $a_*\approx 0.9$, a period of effective super-critical accretion had to take place in the past. Such accretion could be facilitated by photon advection, as proposed for ultraluminous X-ray sources.

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arXiv2024

What we can learn about compact binary mergers from their kilonova signals?

Compact binary mergers are sources of gravitational waves, and can be accompanied by electromagnetic signals. We discuss the possible features in the kilonova emissions which may help distinguish the black hole - neutron star mergers from the binary neutron stars. In addition, the amount of ejected material may depend on whether the system undergoes the creation of a transient hyper-massive, differentially rotating neutron star. In this context, the numerical simulations of post-merger systems and their outflows are important for our understanding of the nature of short GRB progenitor systems. In this article, we present a suite of GR MHD simulations performed by the CTP PAS astrophysics group, to model the neutrino driven disk winds and their contribution to the kilonova emissions. The contribution of the disk wind to the jet collimation and variability is also briefly discussed.

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arXiv2024

Black hole outflows initiated by a large-scale magnetic field

Accreting black hole sources show variable outflows at different mass scales. For instance, in the case of galactic nuclei, our own galactic center Sgr A* exhibits flares and outbursts in the X-ray and infrared bands. Recent studies suggest that the inner magnetospheres of these sources have a pronounced effect on such emissions. Accreting plasma carries the frozen-in magnetic flux along with it down to the black hole horizon. During the in-fall, the magnetic field intensifies and it can lead to a magnetically arrested state. We investigate the competing effects of inflows at the black hole horizon and the outflows developed in the accreting plasma due to the action of magnetic field in the inner magnetosphere and their implications. We start with a spherically symmetric Bondi-type inflow and introduce the magnetic field. In order to understand the influence of the initial configuration, we start the computations with an aligned magnetic field with respect to the black hole rotation axis. Then we proceed to the case of magnetic fields inclined to the black hole rotation axis. We employ the 2D and 3D versions of HARM code for the aligned field models while using the 3D version for the inclined field and compare the results of computations against each other. We observe how the magnetic lines of force start accreting with the plasma while an equatorial intermittent outflow develops and goes on pushing some material away from the black hole partially along the equatorial plane, and partly ejecting it out of the plane in the vertical direction. In consequence, the accretion rate also fluctuates. The black hole spin direction prevails at later stages and it determines the flow geometry near the event horizon, whereas on larger scales the flow geometry stays influenced by the initial inclination of the field.

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arXiv2024

Viscous torque in turbulent magnetized AGN accretion disks and its effects on EMRI's gravitational waves

The merger of supermassive black holes (SMBHs) produces mHz gravitational waves (GW), which are potentially detectable by future Laser Interferometer Space Antenna (LISA). Such binary systems are usually embedded in an accretion disk environment at the centre of the active galactic nucleus (AGN). Recent studies suggest the plasma environment imposes measurable imprints on the GW signal if the mass ratio of the binary is around q $ \sim10^{-4}-10^{-3}$. The effect of the gaseous environment on the GW signal is strongly dependent on the disk's parameters, therefore it is believed that future low-frequency GW detections will provide us with precious information about the physics of AGN accretion disks. We investigate this effect by measuring the viscous torque via modelling the evolution of magnetized tori around the primary massive black hole. Using GRMHD HARM-COOL code, we perform 2D and 3D simulations of weakly-magnetized thin accretion disks, with a possible truncation and transition to advection-dominated accretion flow (ADAF). We study the angular momentum transport and turbulence generated by magnetorotational instability (MRI). We quantify the disk's effective alpha viscosity and its evolution over time. We apply our numerical results to quantify the relativistic viscous torque on a hypothetical low-mass secondary black hole via a 1D analytical approach, and we estimate the GW phase shift due to the gas environment.

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arXiv2024

Lense-Thirring Precession after a Supermassive Black Hole Disrupts a Star

An accretion disk formed around a supermassive black hole (SMBH) after it disrupts a star is expected to be initially misaligned with respect to the black hole's equatorial plane. This misalignment induces relativistic torques (the Lense-Thirring effect) on the disk, causing the disk to precess at early times, while at late times the disk aligns with the black hole and precession terminates. Here, using high-cadence X-ray monitoring observations of a TDE, we report the discovery of strong, quasi-periodic X-ray flux and temperature modulations from a TDE. These X-ray modulations are separated by 17.0$^{+1.2}_{-2.4}$ days and persist for roughly 130 days during the early phase of the TDE. Lense-Thirring precession of the accretion flow can produce this X-ray variability, but other physical mechanisms, such as the radiation-pressure instability, cannot be ruled out. Assuming typical TDE parameters, i.e., a solar-like star with the resulting disk extending at-most to so-called circularization radius, and that the disk precesses as a rigid body, we constrain the disrupting black hole's dimensionless spin parameter to be 0.05<|a|<0.5.

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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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