Relativistic jets are ubiquitous on many different systems. From stellar-size objects such as X-ray binaries (XRBs) and gamma-ray bursts (GRBs) to billion times larger such as in active galactic nuclei (AGNs). Similarly, the inferred Lorentz factors of the jets range from mildly relativistic to ultra-relativistic with Gamma ~1000 in some GRBs. Despite decades of research, the structure,...
Shock waves are abundant in astrophysical sources, and are responsible for much of the electromagnetic emission that we observe from many sources. Therefore, the shock dynamics can significantly affect the observable signatures, and are important to account for when interpreting the observations. I will first discuss a planar collision between two cold shells, which may be relevant either for...
I will present cuHARM, a general relativistic radiation magnetohydrodynamic solver optimized to exploit exascale computing facililties. After describing the core numerical strategy for multi-node and multi-GPU setups, I will detail how radiation and its feedback on the dynamics are modeled. In cuHARM, the specific intensity is discretized in space and momentum, and is evolved through the...
Recent observations indicate that stripped-envelope core-collapse supernovae are often surrounded by dense circumstellar material (CSM). Motivated by this, we develop an analytic model to systematically study the dynamics of long gamma-ray burst (LGRB) jet propagation in various CSM environments. We derive a general expression for the jet head velocity ($\beta_{\rm h}$) and breakout time...
Recent observations of peculiar gamma-ray bursts (GRBs), such as GRB 211211A and GRB 230307A, challenge the traditional view that hyper-accreting black holes power these events. Instead, key signatures—temporal, spectral, and kilonova features—suggest millisecond magnetars as central engines, formed in compact star mergers. This talk highlights recent progress in understanding magnetar-driven...
In this work, I will presentthe t-distributed stochastic neighbour embedding (t-SNE), a machine-learning technique, to classify GRBs. We present the results for GRBs observed until 2022 July by the Swift/ BAT (Burst Alert Telescope)instrument in all its energy bands. We show the effects of varying the learning rate and perplexity parameters, as well as the benefit of preprocessing the data...
In this talk, I will overview the results of recent simulations of black holes that are hungry, yet picky: they end up ejecting most of the gas they have access to. I will discuss the factors that affect their picky-ness and make the connection to the production of relativistic collimated outflows, or jets.
"Classical" accretion disks are geometrically thin, radiatively efficient and mechanized by turbulent viscosity. Yet, many observational and theoretical issues challenge this paradigm. Realistic quasar disks may be fed from cold, highly magnetized gas complexes, which can result in magnetically dominated disks that accrete extremely quickly. I will present horizon-scale simulations of...
Observations of the 3C120 jet indicate that this jet is likely pair-dominated [Zdziarski, et al. 2022]. This result implies strong production of the electron-positron plasma in the system. The currently accepted model of pair production involves an electromagnetic cascade near the base of the jet. Numerically solving the model equations one shows that the cascade indeed forms and can populate...
ULXs are non-nuclear extragalactic sources that emit X-rays at luminosities exceeding $10^{39}\, erg/s$. One of the most accepted models to explain the extraordinary luminosity of ULXs is the super-Eddington accretion onto stellar-mass compact objects. This model with the central object of a neutron star revived interest in 2014, after the discovery of the neutron star-like pulsation period in...
We apply machine learning techniques to model the multi-wavelength emission of the extremely bright GRB 210822A using the AFTERGLOWPY library. This approach allows us to estimate the observer angle $\theta_{obs}$, the initial energy $E_0$, the electron index $p$, the thermal energy fractions in electrons ($\epsilon_{e}$) and in the magnetic field ($\epsilon_{B}$), the efficiency $\chi$, and...
Accretion disks are essential for understanding the dynamics of gas around black holes. The magnetically arrested disk (MAD) state, where the magnetic flux near the event horizon becomes saturated, has garnered significant attention following observations of supermassive black holes in M87 and Sagittarius A* by the Event Horizon Telescope (EHT) collaboration, which suggest that this is the...
In this talk, I will present the wide range of multi-messenger signals expected from gravitational wave (GW) sources across the frequency bands of all current and future GW detectors. I will begin at high frequency, discussing compact object binary mergers and massive stellar death. I will present some novel results on signatures expected from these latter events, which call into question our...
Collapsars are known to be the origin of GRB jets, black hole populations, and even potentially important r-process production sites in the early universe. In this talk, I will demonstrate how we can study the central engines of collapsars and jets, and establish the natal properties of their black holes. In particular, I will discuss how collapsar black holes acquire the strong magnetic...
GRBs from collapsars have been studied by imposing jets at intermediate scales beyond the iron core region while exploring a wide range of parameters, such as luminosity and central engine duration. However, these conditions should be validated by studying jets launched directly from the central engine to show a global picture of the jet propagation inside and outside of the progenitor star....
In recent years, the Magnetically Arrested Disc (MAD) model of accretion flows onto spinning black holes has gained significant attention due to its consistency with several observations, including those of Sgr A∗ and M87∗ . Such discs support powerful relativistic jets and episodic magnetic flux eruptions powering high-energy flares, which were also found to impact the structure of the inner...
This study focuses on the analysis of relativistic jets in collapsars with a self-gravitating stellar envelope. In our simulations the initial mass of the black hole is three solar masses, while the stellar envelope mass is twenty-five solar masses. Therefore, self-gravity cannot be neglected in the analysis. We compare two models—with and without self-gravity—under identical initial...
Rotating plasma forms an accretion disk around compact objects such as black holes and neutron stars. The gravitational energy released during accretion is converted into the kinetic energy, internal energy, and radiation energy. A portion of the converted energy is ejected into interstellar space via outflows and radiation. However, the detailed structure of the accretion disk and the...
Neutrinos play a crucial role in determining fluid dynamics and nucleosynthesis in core-collapse supernova (CCSN) and binary neutron star merger (BNSM). The neutrino kinetics which governs their transport in phase space, interactions with matter, and flavor conversions (or neutrino oscillations) is essential to develop realistic models of CCSN and BNSM. Accurate determination of neutrino...
Radiation-driven outflows play a crucial role in extracting mass and angular momentum from binary systems undergoing rapid mass transfer at super-Eddington rates. We study this process by conducting three-dimensional radiation hydrodynamical simulations of mass-transferring BH binary systems. Our simulations show that super-Eddignton mass transfer leads to a significant mass loss from the...
I will present the results of our fully general relativistic magnetohydrodynamic simulations of accretion flows onto spinning supermassive black hole binary mergers. Supermassive binary black hole systems are formed after galaxy collisions and they are powerful sources of gravitational waves that will be detected by the future LISA mission. In our simulations we investigated the dynamics of...
When Neutron Stars are in orbit with a companion star, they can
capture the outer layers of the latter. This new matter can burn
unstably on the surface of the star and explode in bright
X-ray flashes called the Type I Bursts.
In this talk I will show results of MHD modelling of the flame during
the Type I Bursts and discuss its link to observations and to the
properties of the neutron...
I will discuss the issues related with calculating the magnetic field structure in the interior of neutron stars, and how numerical simulations can be used to asses the stability and evolution of such configurations.
In particular I will focus on instabilities and on the development of turbulence. I will show new results in which the turbulence is resolved, and discuss it's impact on...
The internal magnetic field topology and equilibrium configurations of neutron stars are thought to play a fundamental role in determining the nature and strength of astrophysical phenomena. We model the development of the super strong magnetic fields in neutron stars using the General Relativistic MagnetoHydroDynamic (GRMHD) code AthenaK. In this talk, I will present the long-term evolutions...
The geometry of the X-ray emission region in accreting black holes has been a subject of debate for over three decades. Despite extensive spectral and timing data, no consensus has emerged on the structure of these regions. The launch of the Imaging X-ray Polarimetry Explorer (IXPE) at the end of 2021 marked a major advancement in X-ray astronomy, as it is the first satellite specifically...
Very long baseline interferometry observations can now resolve event-horizon angular scales for at least 2 supermassive black holes, M87 and Sagittarius A. What is more, these observations give us access to resolved polarimetry, that constitute a particularly powerful tool for the diagnostic of the accretion flow and magnetic fields in the compact region. I will discuss how the polarization...
We explore off-equatorial acceleration of electrically charged matter near a magnetized black hole with the aim of understanding the boundaries between the regions of stable, plunging, and escaping motion. As a generalisation of the Innermost Stable Circular Orbit (ISCO), the concept of the radius of the Innermost Stable Spherical Orbit (ISSO) determines the inner rim of inclined...
We present first results from Spritz, a general-relativistic magnetohydrodynamics (GRMHD) code developed for high-precision simulations of binary neutron star (BNS) mergers using nuclear equations of state. Spritz is designed with a focus on robustness and accuracy, incorporating high-order shock-capturing schemes and support for tabulated equations of state, allowing for the inclusion of...
It has been known since the 70’s that accretion disks are unstable above an accretion rate of ~10% Eddington. Yet, despite the several accreting systems known, only a handful of stellar mass black holes have been showing clear signs of accretion disk instabilities. Through an unprecedented multi-wavelength campaign of a highly accreting neutron star, it has been recently shown that the whole...
The energy released in the collapse of the core of a massive star are believed to produce a wide range of astrophysical transients including core-collapse (type Ib, Ic, II) supernovae and long duration gamma-ray bursts. A number of engines have been proposed to extract the energy released in the collapse and power an energetic explosion. All of the proposed engines are likely to occur in...
In this talk, I will report on some of the largest (in terms of simulation domain size) and longest (in terms of duration) 3D general relativistic radiation magnetohydrodynamic simulations of super-critical accretion onto black holes. The simulations are all set for a rapidly rotating ($a_* = 0.9$), stellar-mass ($M_\mathrm{BH} = 6.62 M_\odot$) black hole. The simulations vary in their target...
We performed the first simulations of accretion onto the compact objects in the Reissner-Nordström (RN) space-time. The results could not be more different for the two cases. For a black hole, just as in the familiar Kerr case, matter overflowing the cusp plunges into the black hole horizon. For the naked singularity, the accreting matter forms an inner structure of toroidal topology and...
We present preliminary results of our attempt to build a physically consistent model for the active nucleus of the galaxy M87, based on the GRMHD simulations. Our model simultaneously reproduces the broad-band spectrum and intensity maps, offering a unified explanation of these observations. In our solution, most of the radiation observed at frequencies above 100 GHz originates from the inner...
We present a new pseudo-Newtonian potential for the gravity around a
Reissner–Nordström naked singularity and perform numerical simulations of matter encircling such object. Simulations with our potential reproduce exactly the radial dependence of the Keplerian orbital frequency, with the orbital angular velocity vanishing at the zero gravity radius and showing a maximum at 4/3 of that...
I will present magnetohydrodynamic (MHD) simulations of a binary black hole (BBH) system embedded within a magnetized active galactic nucleus (AGN) disk, aiming to explore the accretion dynamics and the formation of outflows. Unlike traditional models that assume a circumbinary accretion disk around the binary, our approach allows the interaction between the binary system and the surrounding...
Developments in observing technology have produced the first event-horizon-scale images of accreting supermassive black holes. The interpretation of such observations relies on sizable libraries of synthetic data produced from general-relativistic magnetohydrodynamic (GRMHD) simulations. This approach has provided considerable insight into these systems, but also suffers from some limitations....
The time-variable (in particular pulsed-) emission from black holes and neutron stars holds key information on the nature of curved spacetime and ultra dense nuclear matter. However, due to the complex plasma dynamics at play, the interpretation of the observed signals is difficult. I will present some recent work on modeling general relativistic magnetohydrodynamic processes around black...
Black hole X-ray binaries (BHXRBs) and Active Galactic Nuclei (AGN) transition through a series of accretion states in a well-defined order. The accretion states, each associated with different luminosities, spectral and variability characteristics, quasi periodic oscillations (QPOs) and outflow properties, are thought to be triggered by physical changes in the accretion disk around the...
Relativistic jets are powerful collimated outflows from accreting compact objects, especially spinning black holes. Jets, as well as their associated mechanisms of energy dissipation and particle acceleration, can be investigated by global or local numerical simulations using methods like general-relativistic magneto-hydro-dynamics (GRMHD), particle-in-cell (PIC), etc. This presentation...
X-ray binaries hosting compact objects exhibit various variabilities in their X-ray spectra, often attributed to thermoviscous instabilities within their accretion disks. In particular, short-term variabilities , believed to arise from radiation pressure instabilities (RPIs) from the inner regions of the disk, have been well studied in black hole (BH) X-ray binaries. Recent observations...
