Location: Park 1.09

Q1 - Monday 14:00-15:40 (Matt Middleton)

Recent advances in relativistic reflection and reverberation


Andrew Fabian (University of Cambridge)


Exploring the Physics of Warped Accretion Disks with the Imaging X-ray Polarimetry Explorer (IXPE)


Quin Abarr (Washington University in St. Louis)

Many of the accretion disks of stellar mass black holes in X-ray binaries and supermassive black holes at the centers of Active Galactic Nuclei (AGN) are likely misaligned with the angular momentum of the distant accretion disk material. In such systems, the interplay of disk viscosity and general relativistic frame dragging is expected to cause the disk to warp or break into two or more distinct planes — this is called the Bardeen-Petterson effect. Recent general relativistic magnetohydrodynamic (GRMHD) simulations have found that this Bardeen-Petterson configuration is indeed possible, with the warp possibly occurring relatively close to the black hole. We discuss here the scientific prospects of stellar mass black hole and AGN observations with NASA’s upcoming Imaging X-ray Polarimetry Explorer (IXPE), based on our general relativistic ray tracing simulations of black holes with Bardeen-Petterson-type accretion disk configurations. We emphasize the importance to compliment the IXPE observations with spectro-temporal observations gathered with other satellites such as the X-ray telescope on the Neil Gehrels Swift Observatory and the Nuclear Spectroscopic Telescope Array (NuSTAR).

A relativistic disc model of Tidal Disruption Events


Andrew Mummery (Astrophysics, Oxford University)

An encounter between a passing star and a supermassive blackhole at the centre of a galaxy, a so-called tidal disruption event or TDE, may leave a debris disc that subsequently accretes onto the blackhole. These events and the subsequent evolution of their X-ray light curves comprise a particularly interesting application of relativistic disc theory. In this talk I will use a 1D disc evolution equation which is valid in the full Kerr geometry. The solutions of this equation can be analysed using modal techniques which are similar to those used to analyse the standard Newtonian evolution equation. I will summarise the properties of these solutions, drawing particular attention to the importance of the nature of the ISCO stress, which affects the global properties of the time dependent disc solutions. As a direct application of this theory I present fits to the evolving FUV and X-ray light curves of the particularly well observed TDE ASASSN-14li. This model provides a significantly better fit to the observed X-ray light curve of ASASSN-14li when compared to other models in the literature, while simultaneously fitting the late time (t > 200 days) FUV emission. The time-dependence of the evolving X-ray light curve can be understood with analytical techniques. I present a simple formula which describes the ASASSN-14li light curve extremely well, and will be of general use for fitting to future TDE observations.

Venturing beyond the ISCO: Mapping the extreme environments around black holes


Daniel Wilkins (KIPAC, Stanford University)

The immediate vicinities of black holes represent some of the most extreme environments in the Universe, where accreting material in its final moments before plunging through the event horizon powers some of the most luminous sources in the Universe; bright X-ray emitting coronae and vast jets launched close to the speed of light.

The advent of X-ray timing studies has revealed unprecedented amounts of information about the extreme environments around black holes. The reverberation of the coronal X-ray emission off the inner regions of the accretion disc allows the region outside the event horizon to be mapped. X-ray reverberation reveals the geometry and the dynamic nature of the corona. The structure of the corona is revealed, with the discovery of a collimated core, reminiscent of a jet embedded within an extended corona, along with how these components evolve to give rise to the extreme X-ray variability that is observed.

The next step is to detect and understand what happens to material after it crosses innermost stable circular orbit (ISCO), predicted by General Relativity where gravity is sufficiently strong that stable circular orbits cannot exist. General relativistic ray tracing simulations show how signatures of material inside the ISCO, plunging into the black hole, are manifested in observations of X-ray reverberation. Simulations reveal how emission specifically reverberating off of material in the plunging region may be detected with the next generation X-ray observatories.

The ability to directly detect the presence of an innermost stable orbit and plunging region would provide a unique test of general relativity in the strong field limit, only accessible around black holes. Probing the dynamics of material in the plunging region will reveal how the accretion flow behaves in its final moments and how it may launch jets, accelerate coronae and power some of the most extreme systems in the Universe.

Q2 - Monday 16:10-17:50 (Matt Middleton)

X-ray fluorescence from super-Eddington accreting black holes


Lars Lund Thomsen (Physics, The University of Hong Kong)

X-ray reverberation has proven to be a powerful tool capable of probing the innermost region of accretion disks around compact objects. Current theoretical effort generally assumes that the disk is geometrically thin, optically thick and orbiting with Keplerian speed. Thus, these models cannot be applied to systems where super-Eddington accretion happens because the thin disk approximation fails in this accretion regime. Furthermore, state-of-the-art numerical simulations show that optically thick winds are launched from the super-Eddington accretion disks, and thereby changing the reflection geometry significantly from the thin disk picture.
We carry out theoretical investigations on this topic by focusing on the Fe K-alpha fluorescent lines produced from super-Eddington disks, and show that their line profiles are shaped by the funnel geometry and wind acceleration. We also systematically compare the Fe line profiles from super-Eddington thick disks to those from thin disks, and find that the former are substantially more blueshifted and symmetric in shape. These results are consistent with the observed Fe K$alpha$ line from the jetted tidal disruption event, Swift J1644, in which a transient super-Eddington accretion disk was formed out of stellar debris. Therefore, careful analysis of the Fe K-alpha line profile can be used to identify systems undergoing super-Eddington accretion.

Magnetized advective disc-outflow symbiosis around black holes and its implications to ULXs and Blazars


Tushar Mondal (Astronomy and Astrophysics, Department of Physics, Indian Institute of Science, Bengaluru, India)

Matter from its environment, say, companion star or interstellar medium, spirals down towards the central black hole (BH) and forms accretion disc based on the outward transport of angular momentum. At the same time, the huge gravitational potential of a BH is released as heat, mechanical/magnetic energy, and radiation in the form of powerful outflows and/or jets. We initiate a magnetized, viscous, advective disc-outflow symbiosis around BHs to address the luminosity and spectral nature of several accreting systems with BHs spanning from stellar mass to supermassive scales. As an immediate observational consequence, we apply our model to explain some long standing issues for ultra-luminous X-ray sources (ULXs) for the first time. Most ULXs with steep power-law spectrum can be well explained through super-Eddington accretion phenomenon. Nevertheless the interpretation of a significant fraction of ULXs with a hard power-law component remains mysterious. Here we suggest that the observed hard-state ULXs are actually geometrically thick, highly magnetized, advective but sub-Eddington accretion flows orbiting stellar mass BHs and hence no need to incorporate the existence of the missing class of intermediate mass BHs, nor super-Eddington accretions. I will also discuss the Fermi blazar classifications based on our disc-outflow symbiotic model around supermassive BHs by controlling both the mass accretion rate and magnetic field strength. BL Lac objects are less luminous with harder spectra than flat-spectrum radio quasars (FSRQs) . We suggest that the BL Lacs are more optically thin
and magnetically dominated than FSRQs at the jet-footprint to explain their intrinsic γ-ray luminosities.


[1] Mondal T., Mukhopadhyay B., MNRAS, 486, 3465 (2019)
[2] Mondal T., Mukhopadhyay B., MNRAS Letters, 482, L24 (2019)
[3] Mondal T., Mukhopadhyay B., MNRAS, 476, 2396 (2018)

Thin and slim accretion disks at low mass accretion rates


Michal Bursa (ASU)

Thin disks long stand at the base of accretion disk theory, but due to model assumptions they can not accurately describe disks in high mass accretion rate regime, which is where slim disks do the job. Since the slim disk model, represented by the most advanced relativistic solution of Sadowski (2009), is just a generalization of the thin Novikov&Thorne (1974) model, the two are expected to give the same results at low mass accretion rates. Yet they seem to differ up to ~50% in the total radiative output, which can have a serious impact on black-hole spin measurements. The reason for the discrepancy seem to origin in the assumption the two models are based on and has only recently been identified.

The Birth of the First Supermassive Black Holes


Sam Patrick (ICG, University of Portsmouth)

Over 160 quasars have now been identified beyond z=6, the most distant reported at z=7.5. Explaining how these objects attained their masses this early by accretion onto PopIII remnants is difficult, so direct collapse is invoked to produce black hole seeds with the mass necessary to sustain rapid and continued growth. This occurs when a pristine halo reaches a sufficient virial mass to undergo atomic cooling via the Lyman-alpha channel. Using the Enzo hydrodynamical AMR code, we present the first simulations of this scenario to capture the collapse of the halo and the evolution of the subsequent accretion disk out to 2 Myr. We select halos with a range of assembly histories and compute accretion rates onto the central object which are used to calculate BH breakout masses.

Cavity in the hypernova ejecta of GRB 190114C


Gregory Vereshchagin (ICRANet, INAF)

Within the binary-driven hypernova 1 (BdHN 1) scenario, the gamma-ray burst GRB190114C originates in a binary system composed of a massive carbon-oxygen core, and a binary neutron star companion. As the CO core undergoes a supernova explosion with the creation of a new neutron star, hypercritical accretion occurs onto the companion binary neutron star until it exceeds the critical mass for gravitational collapse. The formation of a black hole captures baryons by enclosing them within its horizon, and thus a cavity around it. A further depletion of baryons in the cavity originates from the expansion of the electron-positron-photon plasma formed at the collapse. It is demonstrated here using an analytical model complemented by a hydrodynamic numerical simulation that part of the plasma is reflected off the walls of the cavity. The consequent outflow and its observed properties are shown to coincide with the featureless emission occurring in a time interval of duration, measured in the rest frame of the source, between 11 and 20 s of the GBM observation.

Q3 - Tuesday 14:00-15:40 (Matt Middleton)

New progress in observing neutron star jets


Nathalie Degenaar (Anton Pannekoek Institute for Astronomy, University of Amsterdam)

Accretion in different types of systems is universally connected to outflows in the form of jets, yet the physical process underlying their formation is poorly understood. A key approach to understanding jet formation is constraining the conditions in which jets are launched. The recent discovery of jets launched by strongly magnetic neutron stars open up new ways to explore jet launching. I will present recent observational results of a radio survey of such systems and what this implies for our understanding of jet formation.

Self-screening of magnetic field in a Kerr black hole


STEFANO CAMPION (ICRANet, Sapienza University of Rome)

It is well known that a Kerr black hole immersed in magnetic field generates electric field. We study combined effect of synchrotron radiation, magnetic pair production and acceleration of charged particles in external electric and magnetic fields and show that the created pairs induce magnetic field that screens the background one. These results are applied to the inner engine powering gamma-ray bursts in the Binary driven Hypernova scenario.

Equilibrium structure of charged perfect fluid around black hole


Audrey Trova (ZARM, University of Bremen)

Studies of equilibrium of toroidal structures of a perfect fluid are important to understand the physics of accretion disks in various systems (X-Ray binaries, AGN…). Our interest is about equilibrium of electrically charged-perfect fluid surrounding a rotating or non rotating compact object, embedded in magnetic field. The vertical and radial structure of the torus are influenced by the balance between the gravitational and the magnetic force. Previous study of rotating charged test fluid around a non rotating black hole showed that according to the spin of the black hole the existence of such structures change. We focus on orbiting structures in the equatorial plane, as single or double tori, and structures above as levitating tori. Our interest is about their existence, shape and how the various forces (electromagnetic, centrifugal and gravitational) influence their physics.

General relativistic magnetohydrodynamic dynamo in thick accretion disks: fully nonlinear simulations


Niccolò Tomei (Dipartimento di Fisica e Astronomia, Università degli studi di Firenze)

Large-scale, ordered magnetic fields are well known to play a key role in high-energy astrophysics, hence a natural question one needs to answer is how fields in sources such as compact stars or accretion disks are amplified from initial seed values. A mechanism that is believed to be fundamental in amplifying and supporting magnetic fields is known as mean-field dynamo. The turbulent magnetohydrodynamical regime allows the coupling between fluctuations in velocity and magnetic field. The result of this correlation leads to the formation of an electromotive force which is able to amplify the magnetic fields. Here, the mean-field dynamo is studied in the context of AGNs through General Relativistic MHD simulations in the fully non-linear regime. Combined with the differential rotation of the accreting disk, the process is able to produce an exponential growth of any initial magnetic field. Before reaching saturation we observe a secondary regime of exponential growth, where the turbulent magnetic field increases more slowly due to the accretion (dynamic regime). This model is also used to infer the thermal synchrotron emission by the magnetized plasma of the accreting matter of Sgr A* (the core of our Galactic center), the expected new target of Event Horizon Telescope (EHT). Assuming typical values of the numerical electron density, the model is able to reproduce the observed flux at the millimeter wavelengths confirming the capability of the dynamo to amplify the magnetic fields in a self-consistent way.

Q4 - Tuesday 16:10-17:50 (Matt Middleton)

Magnetic reconnection and particle acceleration in relativistic magnetospheres


Fabio Bacchini (Mathematics, Kathaolieke Universiteit Leuven)

Magnetic reconnection is likely a ubiquitous phenomenon taking place in high-energy astrophysical environments such as the magnetosphere of compact objects, like black holes and neutron stars. Here, the change in topology of the magnetic field liberates energy that is converted into heat and kinetic energy of plasma particles. Simulating such environments with magnetohydrodynamics (MHD) models is usually demanding due to the high magnetisation of the plasma, which causes numerical difficulties. Force-free (FF) models, which neglect the fluid’s contribution to the electromagnetic fields, are better suited for such situations, but fail at capturing the reconnection dynamics which is appropriately described by resistive MHD. Here, we present a coupling of resistive MHD and FF yielding a model able to efficiently capture both the highly magnetised regions of a magnetosphere and the reconnection sites. In such a model, we insert test particles evolved with a new algorithm, that allows for ignoring the gyro-motion when this takes place at sub-grid scales. We investigate the dynamics of such particles, accelerated in the reconnection zones.

Complex magnetic field topologies in core-collapse supernovae


Matteo Bugli (Astrophysics, CEA – Saclay)

The magnetic field is believed to play an important role in at least some core-collapse supernovae when its magnitude reaches 10^15 G, which is typical of the most magnetic neutron stars called magnetars. In the presence of fast rotation, such a strong magnetic field can drive powerful jet-like explosions if it has the large-scale coherence of a dipole. The topology of the magnetic field is, however, probably much more complex with strong multipolar and small-scale components and the consequences for the explosion are so far unclear. We investigate the effects of the magnetic field topology on the dynamics of core-collapse supernovae and the properties of the forming proto-neutron star (PNS) by comparing different multipolar orders and radial extents.
Using axisymmetric relativistic MHD simulations, we find that higher multipolar magnetic configurations lead to generally less energetic explosions, slower expanding shocks and less collimated outflows. Models with a low order multipolar configuration tend to produce more oblate PNS, which in some cases are surrounded by a rotationally supported toroidal structure of neutron-rich material. This change in the PNS shape can be directly associated with higher neutrino luminosities along the equatorial plane but smaller along the poles. Moreover, magnetic fields which are distributed on smaller angular scales produce more massive and faster rotating central PNS, suggesting that higher-order multipolar configurations tend to decrease the efficiency of the magnetorotational launching mechanism. Even if our dipolar models systematically display a far more efficient extraction of the rotational energy of the PNS, fields distributed on smaller angular scales are still capable of powering magnetorotational explosions and shape the evolution of the central compact object.

Off-axis Synchrotron Light Curves from Full-time-domain Moving-mesh Simulations of Jets from Massive Stars


Xiaoyi Xie (Mathematical Sciences, University of Southampton)

We present full-time-domain (FTD), moving-mesh, relativistic hydrodynamic simulations of jets launched from the center of a massive progenitor star and compute the resulting synchrotron light curves for observers at a range of viewing angles. We follow jet evolution from ignition inside the stellar center, propagation in the stellar envelope and breakout from the stellar surface, then through the coasting and deceleration phases. The jet compresses into a thin shell, sweeps up the circumstellar medium, and eventually enters the Newtonian phase. The jets naturally develop angular and radial structure due to hydrodynamical interaction with surrounding gas. The calculated synchrotron light curves cover the observed temporal range of prompt to late afterglow phases of long gamma-ray bursts. The on-axis light curves exhibit an early emission pulse originating in shock-heated stellar material, followed by a shallow decay and a later steeper decay. The off-axis light curves rise earlier than previously expected for top-hat jet models—on a timescale of seconds to minutes after jet breakout—and decay afterward. Sometimes the off-axis light curves have later rebrightening components that can be contemporaneous with SNe Ic-bl emission. Our calculations may shed light on the structure of GRB outflows in the afterglow stage. The off-axis light curves from FTD simulations advocate new light-curve templates for the search of off-axis/orphan afterglows.

The role of electric charge in relativistic accretion onto compact objects


Kris Schroven (Department of Galaxies and Planetary Systems, Astronomical Institute of the Czech Academy of Sciences)

Many high-luminosity phenomena in the observed universe can be traced back to accretion processes, in which electromagnetic fields play an important role. These fields are either produced within the accreted matter or they enter the stage as external fields like interstellar magnetic fields or fields, produced by the accreting object.
How does the presence of electric charges affect an accretion process? This question is tackled by means of analytical models. Realistically small electric net charges of an accreting black hole might affect the accretion of collisionless plasma noticeably. This is demonstrated by discussing the motion of charged test particles in Kerr-Newman spacetime and its ISCO solutions. Furthermore, we use the model of a charged thick accretion disc in an electromagnetic background field to study the influence of the accreting object’s spin on the existence and structure of thick accretion discs and polar clouds in the vicinity of a magnetic dipole field.

Q5 - Wednesday 14:00-15:40 (Matt Middleton)

Constraints on jet launching physics from rapid multiwavelength timing


Poshak Gandhi (University of Southampton)

Following almost 60 years of X-ray studies, we are at the threshold of a new era of fast multiwavelength timing studies of X-ray binaries. The optical and infrared regimes can directly measure the peak emission of the jet and hot flow in many accretion systems. When combined with simultaneous rapid (sub-second) X-ray observations, they can be a powerful tool to probe the accretion/outflow connection in ‘real-time’ and to measure key physical parameters of the various binary components. This field has long been handicapped by the lack of suitable fast detectors and the difficulty of multiwavelength coordination, but this is set to change with a multitude of new dedicated observatories now becoming operational. I will review advances made in this field, concentrating on results from multiwavelength observations of black hole binaries in the hard state. In particular, we have discovered a common (~0.1 light-second) elevation of the first optical jet base emission zone in three systems, suggesting that this could be a unifying characteristic. If so, this places important constraints on plasma speeds in internal shock models, and/or the locations of critical points in magnetohydrodynamic jet launching models.

The Role of Magnetic Field Geometry in the Evolution of Neutron Star Merger Accretion Disks


Ian Christie (CIERA (Northwestern University))

Neutron star mergers are unique multi-messenger laboratories of accretion, ejection, and r-process nucleosynthesis. Theoretically, however, our current understanding of such events is limited, especially of magnetic effects, such as the role the post-merger magnetic field geometry has on the evolution of merger remnant accretion disks. Through the use of 3D general relativistic magneto-hydrodynamic simulations, we investigate such effects while fully capturing mass accretion, ejection, and the production of relativistic jets, over time intervals exceeding several seconds. I will show that not only does an initially poloidal post-merger magnetic field geometry generate relativistic jets, but the more natural, purely toroidal post-merger geometry generates striped jets of alternating magnetic polarity, a result seen for the first time. Our simulated jet energies, durations, and opening angles for all magnetic configurations span the range of sGRB observations. Concurrent with jet formation, sub-relativistic winds, launched from the radially expanding accretion disk, provide efficient collimation of the relativistic jets and an observational window into the observed kilonova. In comparison to GW 170817/GRB 170817A, I will demonstrate that the blue kilonova component, although initially obscured by the red component, expands faster, outrunning the red component and becoming visible to off-axis observers.

Interaction between supernovae shells and relativistic jets


Dimitrios Millas (Mathematics, KU Leuven)

The W50-SS433 system is a well-known Galactic object, consisting of a supernova remnant (SNR) W50 and a relativistic jet from the X-ray binary SS433. Many observations reveal a non-spherical shape of the remnant and a clear “east-west” asymmetry that roughly corresponds to the mean axis of the jet. The morphology of the system is thus often attributed to the interaction between the jet and the remnant.
We report on the results of relativistic, hydrodynamic simulations in full 3D, simultaneously capturing the non-relativistic supernova blast and the relativistic jet, in order to better understand their interaction. This requires extensive use of adaptive mesh refinement in order to cover at least 4 orders of magnitude in size and follow the propagation of the jet. The density profile of the external (interstellar) medium is of particular importance, as it determines the timescale of the interaction and is closely related to the age of the jet and the observed shape of the remnant. A parametric study, in association with observations, can give an improved understanding of the interaction between W50 and SS433.

Estimating AGN jet parameters from observations of a jet shape break


Elena Nokhrina (Relativistic Astrophysics Laboratory, Moscow Institute of Physics and Technology)

The change in a jet boundary shape has been discovered first by Asada and Nakamura (2012) for M87. Now there are in total 14 nearby sources demonstrating the initially quasi-parabolic flow changing into the approximately conical one (Tseng et al. 2016, Nakahara et al. 2018, Hada et al. 2018, Nakahara et al. 2019, Kovalev et al. 2019). We propose that the change in a jet boundary shape is due to a transition of an outflow from the magnetically dominated to particle dominated (equipartition) regime. Within our model we are able to reproduce exactly the observed jet boundary shape behavior, assuming that the ambient pressure is due to Bondi accretion flow. Associating the jet shape break, obtained by our modelling, with the observed one, we are able to recover jet and central black hole properties: BH spin, total magnetic flux in a jet, an ambient pressure amplitude.

Q6 - Wednesday 16:10-17:50 (Matt Middleton)

Evolution of AGN jets from multiepoch core-shift studies


Aleksandr Plavin (Astro Space Center, Lebedev Physical Institute)

The apparent position of jet base (core) in radio-loud active galactic nuclei changes with frequency because of synchrotron self-absorption. Studying this `core shift` effect enables us to reconstruct properties of the jet regions close to the central engine. We report here results of core shift measurements in 40 AGNs observed with global VLBI at 2 and 8 GHz at multiple epochs from 1994 to 2016. The core shift is determined using a new automatic procedure to minimize possible biases, and these measurements are employed for examining temporal variability. We argue that the core shift variability is a common phenomenon, as established for 33 of 40 AGNs in this sample. Our analysis shows that the typical offsets between the core positions at 2 and 8 GHz are about 0.5 mas and they vary in time. Typical variability of the individual core positions is about 0.3 mas. The measurements show a strong dependence between the core position and its flux density, suggesting that changes in both are likely related to the nuclear flares injecting denser plasma into the flow. We determine that density of emitting relativistic particles significantly increases during these flares, while relative magnetic field changes less and in the opposite direction.

Acceleration of M87 Jet in the Black Hole Magnetosphere


Masaaki Takahashi (Physics and Astronomy , Aichi University of Education )

To understand the formation mechanism of relativistic jets in active galactic nuclei (AGNs), we consider
stationary trans-fast magnetosonic flows in a black hole magnetosphere. The questions to understand
about relativistic jets are how the relativistic jet is accelerated, how the collimated jet shape is obtained, and
where and how the energy conversion from magnetic energy to fluid kinetic energy in the jet flow occurs.
Recently, four important constraints on the M87 jet have been obtained by a series of VLBI observations.
That is, (1) the jet-width profile along the M87 jet by KaVA, (2) the radial profile of the M87 jet velocity by
KaVA. (3) the magnetization degree at the jet base of M87 by EHT, (4) the black hole mass for M87 by EHT.
Then, by using a stationary model of Tomimatsu & Takahashi (2003), which gives a trans-fast magnetosonic
flow solution easily without the regularity condition analysis at the magnetosonic surfaces in a black hole
magnetosphere, we extract the essence of these physical processes clearly.
We also extend the discussion for the general relativistic magnetohydrodynamic inflow/outflow model
(Takahashi & Tomimatsu 2008) and apply the solution of the distributions of plasma density, flow velocity
and magnetization to the M87 jet.

Numerical and statistical modelling approaches in the identification of precessing jets


Maya Horton (Centre for Astrophysics Research, University of Hertfordshire)

The detection of gravitational waves has opened up a new era in astrophysics through the confirmation of long-predicted mergers of binary black hole systems. Although mergers of supermassive black holes are not detectable with current technology, there is increasing interest in the search for signatures of coalescing black hole binaries. One of these signatures is predicted to be found in extragalactic radio jets, where precession characteristics can be observed in jet morphology. I show 3D simulations of a ballistic model to predict changes in jet structure depending on precession characteristics and light travel effects, and the application of the same model in a Bayesian context to identify potential jet paths in observed extragalactic sources, particularly the large and well-studied radio source Cygnus A.