Polarisation studies of Magnetically Arrested Disk emission
The Event Horizon Telescope (EHT) collaboration produced the first ever image of a black hole released in 2019. Now for the first time astronomers have been able to measure polarisation, a signature of magnetic fields, close to the edge of a black hole. The M87 image shows the polarised view of the black hole in the core of this galaxy. The orientation of polarisation vectors is related to the magnetic field around the shadow of the black hole.
We calculated theoretical polarimetric images of the magnetically arrested disk (MAD) accretion model.
The plots show a result of 3D long term numerical simulation with β=30. Synchrotron emission models assume the ion to electron temperature ratio, Rh=1, and nearly face-on viewing angle, i=160°. The left-most panel shows density distribution at the equatorial plane together with projected magnetic field lines. Second, third and fourth panels display the total intensity (Stokes I), linear and circular polarimetric images.
Calculations made with our version of the code HARM (Janiuk et al. 2018, SCFE, 5, 86) and post-processed with ray-tracing code iPole for covariant, polarized radiative transport (Moscibrodzka & Gammie 2018). See details in Moscibrodzka, Janiuk & de Laurentis (2021, MNRAS 475, 43).
Jet formation and magnetic-driven outflows
Figure: The 2D profile of the ratio of gas pressure to magnetic pressure and magnetic filed contour lines of the magnetized disk with neutrino cooling at t=0.03s.
Neutrino-cooled accretion flow around a spinning black hole, produced by a compact binary merger, is a promising scenario for jet formation and magnetic-driven outflows. Based on GW170817 gravitational wave detection by LIGO-Virgo and observation of the EM counterparts, this model can explain the central engine of the short duration gamma ray bursts and kilonova radiation. Using the open-source GRMHD HARM_COOL code, we study the magnetically-driven evolution of accretion disks with realistic equation of state in the fixed curved space-time background, with different disk configurations. We mainly focus on measuring the outflows’ properties such as composition, velocity and mass to estimate the kilonova’s luminosity. We also investigate how the initial disk parameters can affect the features of the post-merger outflows by varying disk mass and black hole spin. We choose the initial disk mass in the range of [0.01, 0.3] solar mass to match with a realist post-merger remnant disks from a black hole-neutron star or neutron star-neutron star binary system.
Structure and time variability studies of jets from accreting black hole sources
Figure: 3D jet structure from one of our models represented by the contours of jet energetics parameter at an evolved time of the simulation (James et al. 2022)
Variable accretion flows are a common phenomena in astrophysical black hole sources. Such sources are found at different mass scales, from the cores of active galactic nuclei, e.g. in radio loud objects such as blazars, to gamma ray bursts. Both the gamma-ray bursts and blazars often have a relativistic jet pointing towards or at a small angle from our line of sight. In such sources, the variability of the inflow can be transmitted to the properties of outflows. Observational studies have shown correlations between the observed jet variability time-scales and Lorentz factor of the emitted jet. Motivated by these observational properties, we investigate the relation between the central engine properties and the variability of the jet outflows by means of numerical general relativistic magnetohydrodynamic (GRMHD) simulations. We perform axisymmetric 2D simulations as well as non-axisymmetric 3D simulations of the evolution of a central engine composed of a magnetized torus around a Kerr black hole that results in the launching of a non-uniform jet. The simulations are performed in a general relativistic framework. We probe the jet energetics at chosen locations along the jet direction and we measure the jet time variability. Thus we try to understand the time variability properties of the jet emission by comparing our results with the observations. We also investigate the angular distribution profile of such jets as they have a more complex structure than a simple top hat. These studies are relevant in the better underastanding of jet emission observed in gamma-ray bursts as well as in active galactic nuclei since we are able to scale our results with the black hole mass.
Jet launching mechanisms studies using BHAC code simulations on the MESA-evolved stars
Illustration of a numerical grid with an adaptive mesh, used in the code BHAC with a result of one of our testing simulations for stellar collapse. Left panel shows the density distribution in the compact stellar code, as extracted from MESA code. Middle panel shows the adaptive grid shape, used to dynamically model evolution of the inner regions of this core. Right panel shows the resolved region of the innermost few tens of gravitational radii (in units of M) around the central black hole inside the collapsar. Streamlines in this panel present the configuration of turbulent magnetic fields in the disk, accompanied with open field lines in the funnel region
Teaching activities in our group
Summer 2022
This summer, in July-September, 2022, our group members co-supervised four projects of the students of the University of Warsaw. The projects were part of our program of internships.
Mr. Adam Gonstal, 4th year student of physics at FUW, studied several magnetized accretion disk simulations in order to estimate the post-merger outflow mass. He measured the unbound mass using different criteria in the literature. He also studied the energy budget of the accretion disks, and investigated the anti-dynamo theorem in our 2D simulations to quantify how much magnetic field dissipated as a result of the axisymmetric assumption.
Figure: The evolution of the volume-averaged specific internal energy, thermal energy, and magnetic energy versus time. The energy of the cold matter is estimated from polytropic equation of state.
Mr Jakub Szyndler, 3rd year student of astronomy at OAUW, investigated the equilibrium torus solution, as proposed by Fishbone & Moncrief (1976) for a barotropic fluid around the Kerr black hole. He compared the exact analytical solution with numerically integrated setup, used as the initial condition for our MHD simulations. He checked if a numerical solution at t=0 satisfies the condition dθ Tθθ + dr Trθ = 0. The result is within numerical accuracy. He was also verifying the procedure for calculation of magnetic field components, in the poloidal loop configuration given by the vector potential.
Figure: The angular distribution of magnetic field components at t=0, at the radius of pressure maximum, r=12. The poloidal field is reconstructed, within numerical accuracy.
Mr. Szymon Bójko, student of 2nd year of physics at Warsaw University, conducted a project on the observed features of gamma ray burst signals observed by the Swift satellite. He performed data analysis for a sample of about 700 bursts. He fitted the exponential decay profiles to their pulses shape, parameterized by two quantities, tau and alpha, defining the timescale and slope of the decay. It is found, that the GRBs apparently form two populations, for which the tau-alpha correlation is found (with slopes steeper than -2), and those for which no such correlation is present (flatter slopes)
Figure: The results of fitting analysis to the time profiles of ~700 GRBs observed by Swift satellite. Three different cases are compared: with a constant parameter B>0 (left column), with any constant B value fitted (middle), and without a constant.
Mr. Mateusz Kapusta, student of 3rd year of physics at Warsaw University, investigated the structure of magnetic field and effects of reconnections in the innermost regions of the accretion disk in a magnetically dominated accretion state (MAD).
Figure: Visualization of magnetic field lines, threading the horizon of the black hole and an accretion disk around it. Plot is based on the data from an evolved state of 3D GR MHD simulation.
