arXiv subjects

Amir Levinson

Publications and source records attributed to Amir Levinson.

At least 37 records · Page 2Linked to original sources

arXiv2021

The response of black hole spark gaps to external changes: A production mechanism of rapid TeV flares?

We study the response of a starved Kerr black hole magnetosphere to abrupt changes in the intensity of disk emission and in the global magnetospheric current, by means of 1D general relativistic particle-in-cell simulations. Such changes likely arise from the intermittency of the accretion process. We find that in cases where the pair production opacity contributed by the soft disk photons is modest, as in, e.g., M87, such changes can give rise to delayed, strong TeV flares, dominated by curvature emission of particles accelerated in the gap. The flare rise time, and the delay between the external variation and the onset of the flare emitted from the outer gap boundary, are of the order of the light crossing time of the gap. The rapid, large amplitude TeV flares observed in M87 and, perhaps, other AGNs may be produced by such a mechanism.

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arXiv2021

GRMHD simulations of BH activation by small scale magnetic loops: Formation of striped jets and active coronae

We have performed a series of numerical experiments aimed at studying the activation of Kerr black holes (BHs) by advection of small scale magnetic fields. Such configurations may potentially give rise to the formation of quasi-striped Blandford-Znajek jets. It can also lead to enhanced dissipation and generation of plasmoids in current sheets formed in the vicinity of the BH horizon, which may constitute a mechanism to power the hard X-ray emission seen in many accreting BH systems (a la lamppost models). Our analysis suggests that formation of quasi-striped jets with significant power may be possible provided loops with alternating polarity having sizes larger than $\sim 10 r_g$ or so can be maintained (either form sporadically or advected from outside) at a radius $\lesssim 10^2 r_g$. This conclusion is consistent with recent results of general relativistic force-free simulations. We also find that the accretion dynamics exhibits cyclic behaviour in MAD states, alternating between high accretion phases and quenched accretion phases during which the magnetosphere becomes force-free out to radii $\gtrsim 10r_g$. We suggest that such a behaviour should lead to notable variations of the observed luminosity and image of the inner disc (BH shadow image). Finally, we find that the transition between accreted loops on the BH gives rise to the formation of current sheets and energetic plasmoids on the jet boundary during intermittent periods when the jet becomes inactive, in addition to an equatorial current sheet that forms during peaks in the jet activity.

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arXiv2021

Intermittent hydrodynamic jets in collapsars do not produce GRBs

Strong variability is a common characteristic of the prompt emission of gamma-ray bursts (GRB). This observed variability is widely attributed to an intermittency of the central engine, through formation of strong internal shocks in the GRB-emitting jet expelled by the engine. In this paper we study numerically the propagation of hydrodynamic jets, injected periodically by a variable engine, through the envelope of a collapsed star. By post-processing the output of 3D numerical simulations, we compute the net radiative efficiency of the outflow. We find that all intermittent jets are subject to heavy baryon contamination that inhibits the emission at and above the photosphere well below detection limits. This is in contrast to continuous jets that, as shown recently, produce a highly variable gamma-ray photospheric emission with high efficiency, owing to the interaction of the jet with the stellar envelope. Our results challenge the variable engine model for hydrodynamic jets, and may impose constraints on the duty cycle of GRB engines. If such systems exist in nature, they are not expected to produce bright gamma-ray emission, but should appear as X-ray, optical and radio transients that resemble a delayed GRB afterglow signal.

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arXiv2021

Intermittent mildly magnetized jets as the source of GRBs

Gamma-ray bursts (GRBs) are powered by relativistic jets that exhibit intermittency over a broad range of timescales - from $ \sim $ ms to seconds. Previous numerical studies have shown that hydrodynamic (i.e., unmagnetized) jets that are expelled from a variable engine are subject to strong mixing of jet and cocoon material, which strongly inhibits the GRB emission. In this paper we conduct 3D RMHD simulations of mildly magnetized jets with power modulation over durations of 0.1 s and 1 s, and a steady magnetic field at injection. We find that when the jet magnetization at the launching site is $σ\sim 0.1$, the initial magnetization is amplified by shocks formed in the flow to the point where it strongly suppresses baryon loading. We estimate that a significant contamination can be avoided if the magnetic energy at injection constitutes at least a few percent of the jet energy. The variability timescales of the jet after it breaks out of the star are then governed by the injection cycles rather than by the mixing process, suggesting that in practice jet injection should fluctuate on timescales as short as $ \sim 10 $ ms in order to account for the observed light curves. Better stability is found for jets with shorter modulations. We conclude that for sufficiently hot jets, the Lorentz factor near the photosphere can be high enough to allow efficient photospheric emission. Our results imply that jets with $ 10^{-2} < σ< 1 $ injected by a variable engine with $ \sim 10 $ ms duty cycle are plausible sources of long GRBs.

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arXiv2020

Spherical shocks in a steep density gradient of expanding media

We study the propagation of a Newtonian shock in a spherically symmetric, homologously expanding ejecta. We focus on media with a steep power-law density profile of the form $ρ\propto t^{-3}v^{-α}$, with $α>5$, where $v$ is the velocity of the expanding medium and $t$ is time. Such profiles are expected in the leading edge of supernovae ejecta and sub-relativistic outflows from binary neutron star mergers. We find that such shocks always accelerate in the observer frame and lose causal contact with the bulk of the driver gas, owing to the steep density profile. However, the prolonged shock evolution exhibits two distinct pathways: In one, the shock strength diminishes with time until the shock eventually dies out. In the other, the shock strength steadily increases, and the solution approaches the self-similar solution of a shock is a static medium. By mapping the parameter space of shock solutions, we find that the evolutionary pathways are dictated by $α$ and by the initial ratio between the shock velocity and the local upstream velocity. We find that for $α<ω_c$ ($ω_c \approx 8$), the shock always decays, and that for $α>ω_c$ the shock may decay or grow stronger depending on the initial value of the velocity ratio. These two branches bifurcate from a self-similar solution derived analytically for a constant velocity ratio. We analyze properties of the solutions that may have an impact on the observational signatures of such systems, and assess the conditions required for decaying shocks to break out from a finite medium.

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arXiv2020

Plasma kinetic effects in relativistic radiation mediated shocks

Fast shocks that form in optically thick media are mediated by Compton scattering and, if relativistic, pair creation. Since the radiation force acts primarily on electrons and positrons, the question arises of how the force is mediated to the ions which are the dominant carriers of the shock energy. It has been widely thought that a small charge separation induced by the radiation force generates electric field inside the shock that decelerates the ions. In this paper we argue that, while this is true in sub-relativistic shocks which are devoid of positrons, in relativistic radiation mediated shocks (RRMS), which are dominated by newly created $e^+e^-$ pairs, additional coupling is needed, owing to the opposite electric force acting on electrons and positrons. Specifically, we show that dissipation of the ions energy must involve collective plasma interactions. By constructing a multi-fluid model for RRMS that incorporates friction forces, we estimate that momentum transfer between electrons and positrons (and/or ions) via collective interactions on scales of tens to thousands proton skin depths, depending on whether friction is effective only between $e^+e^-$ pairs or also between pairs and ions, is sufficient to coupe all particles and radiation inside the shock into a single fluid. This leaves open the question whether in relativistic RMS particles can effectively accelerate to high energies by scattering off plasma turbulence. Such acceleration might have important consequences for relativistic shock breakout signals.

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arXiv2020

Comprehensive analysis of magnetospheric gaps around Kerr black holes using 1D GRPIC simulations

Spark gaps are likely the source of plasma in active black hole (BH) magnetospheres. In this paper, we present results of 1D general-relativistic particle-in-cell simulations of a starved BH magnetosphere with a realistic treatment of inverse Compton scattering and pair production, for a broad range of conditions, run times longer than in previous studies, and different setups. We find that following the initial discharge, the system undergoes gradual evolution over prolonged time until either, restoring the vacuum state or reaching a state of quasi-periodic oscillations, depending on the spectral shape and luminosity of the ambient radiation field. The oscillations occur near the null charge surface in cases where the global magnetospheric current is in the direction defined by the product of the asymptotic Goldreich-Julian charge density and the radial velocity, while they occur near the boundary of the simulation box when it is the opposite direction (return current). Their amplitude and the resultant luminosity of TeV photons emitted from the gap depend sensitively on the conditions; for the cases studied here the ratio of TeV luminosity to the Blandford-Znajek power ranges from $10^{-5}$ to $10^{-2}$, suggesting that strong flares may be generated by moderate changes in disk emission. We also examined the dependence of the solution on the initial number of particles per cell (PPC) and found convergence for PPC of about 50 for the cases examined. At lower PPC values the pair multiplicity is found to be artificially high, affecting the solution considerably.

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arXiv2020

Monte-Carlo simulations of fast Newtonian and mildly relativistic shock breakout from a stellar wind

Strong explosion of a compact star surrounded by a thick stellar wind drives a fast ($>0.1c$) radiation mediated shock (RMS) that propagates in the wind, and ultimately breaks out gradually once photons start escaping from the shock transition layer. In exceptionally strong or aspherical explosions the shock velocity may even be relativistic. The properties of the breakout signal depend on the dynamics and structure of the shock during the breakout phase. Here we present, for the first time, spectra and lightcurves of the breakout emission of fast Newtonian and mildly relativistic shocks, that were calculated using self-consistent Monte-Carlo simulations of finite RMS with radiative losses. We find a strong dependence of the $νF_ν$ peak on shock velocity, ranging from $\sim 1$ keV for $v_s/c=0.1$ to $\sim 100$ keV for $v_s/c=0.5$, with a shift to lower energies as losses increase. For all cases studied the spectrum below the peak exhibits a nearly flat component ($F_ν\sim ν^0$) that extends down to the break frequency below which absorption becomes important. This implies much bright optical/UV emission than hitherto expected. The computed lightcurves show a gradual rise over tens to hundreds of seconds for representative conditions. The application to SN 2008D/XRT 080109 and the detectability limits are also discussed. We predict a detection rate of about one per year with eROSITA.

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arXiv2020

Physics of radiation mediated shocks and its applications to GRBs, supernovae, and neutron star mergers

The first electromagnetic signal observed in different types of cosmic explosions is released upon emergence of a shock created in the explosion from the opaque envelope enshrouding the central source. Notable examples are the early emission from various types of supernovae and low luminosity GRBs, the prompt photospheric emission in long GRBs, and the gamma-ray emission that accompanied the gravitational wave signal in neutron star mergers. In all of these examples, the shock driven by the explosion is mediated by the radiation trapped inside it, and its velocity and structure, that depend on environmental conditions, dictate the characteristics of the observed electromagnetic emission at early times, and potentially also their neutrino emission. Much efforts have been devoted in recent years to develop a detailed theory of radiation mediated shocks in an attempt to predict the properties of the early emission in the aforementioned systems. These efforts are timely in view of the anticipated detection rate of shock breakout candidates by upcoming transient factories, and the potential detection of a gamma-ray flash from shock breakout in neutron star mergers like GW170817. This review aims at providing a comprehensive overview of the theory and applications of radiation mediated shocks, starting from basic principles. The classification of shock solutions, which are governed by the conditions prevailing in each class of objects, and the methods used to solve the shock equations in different regimes will be described, with particular emphasis on the observational diagnostics. The applications to supernovae, low-luminosity GRBs, long GRBs, neutron star mergers, and neutrino emission will be highlighted.

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arXiv2019

Monte-Carlo simulations of relativistic radiation mediated shocks: II. photon starved regime

Radiation mediated shocks (RMS) play a key role in shaping the early emission observed in many transients. In most cases, e.g., shock breakout in supernovae, llGRBs and neutron star mergers, the upstream plasma is devoid of radiation, and the photons that ultimately reach the observer are generated predominantly inside and downstream of the shock. Predicting the observed spectrum requires detailed calculations of the shock structure and thermodynamic state that account properly for the shock microphysics. We present results of self-consistent Monte-Carlo simulations of photon-starved RMS, that yield the shock structure and emission for a broad range of shock velocities, from sub-relativistic ($β_{sh} = 0.1$) to highly relativistic ($Γ_{sh} = 20$). Our simulations confirm that in relativistic RMS the immediate downstream temperature is regulated by exponential pair creation, ranging from $50$ keV at $β_{sh}=0.5$ to $200$ keV at $Γ_{sh}=20$. At lower velocities the temperature becomes sensitive to the shock velocity, with $kT\sim 0.5$ keV at $β_{sh}=0.1$. We also confirm that in relativistic shocks the opacity is completely dominated by newly created pairs, which has important implications for the breakout physics. We find the transition to pair dominance to occur at $β_{sh}=0.5$ roughly. In all cases examined, the spectrum below the $νF_ν$ peak has been found to be substantially softer than the Planck distribution. This has important implications for the optical emission in fast and relativistic breakouts, and their detection. The applications to GRB 060218 and GRB 170817A are discussed.

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arXiv2019

Prospects for multi-messenger extended emission from core-collapse supernovae in the Local Universe

Multi-messenger emissions from SN1987A and GW170817/GRB170817A suggest a Universe rife with multi-messenger transients associated with black holes and neutron stars. For LIGO-Virgo, soon to be joined by KAGRA, these observations promise unprecedented opportunities to probe the central engines of core-collapse supernovae (CC-SNe) and gamma-ray bursts. Compared to neutron stars, central engines powered by black hole-disk or torus systems may be of particular interest to multi-messenger observations by the relatively large energy reservoir $E_J$ of angular momentum, up to 29\% of total mass in the Kerr metric. These central engines are expected from relatively massive stellar progenitors and compact binary coalescence involving a neutron star. We review prospects of multi-messenger emission by catalytic conversion of $E_J$ by a non-axisymmetric disk or torus. Observational support for this radiation process is found in a recent identification of ${\cal E}\simeq (3.5\pm1)\%M_\odot c^2$ in Extended Emission to GW170817 at a significance of 4.2\,$σ$ concurrent with GRB170817A. A prospect on similar emissions from nearby CC-SNe justifies the need for all-sky blind searches of long duration bursts by heterogeneous computing.

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arXiv2019

Multi-messenger Extended Emission from the compact remnant in GW170817

GW170817/GRB170817A probably marks a double neutron star coalescence. Extended Emission $t_s\simeq (0.67\pm0.03)$\,s post-merger shows an estimated energy output ${\cal E}\simeq (3.5\pm1)\%M_\odot c^2$ determined by response curves to power-law signal injections, where $c$ is the velocity of light. It provides calorimetric evidence for a rotating black hole of $\sim 3M_\odot$, inheriting the angular momentum $J$ of the merged hyper-massive neutron star in the immediate aftermath of GW170817 following core-collapse about or prior to $t_s$. Core collapse greatly increases the central energy reservoir to $E_J\lesssim 1M_\odot c^2$, accounting for ${\cal E}$ even at modest efficiencies in radiating gravitational waves through a non axisymmetric thick torus. The associated multi-messenger output in ultra-relativistic outflows and sub-relativistic mass-ejecta is consistent with observational constraints from the GRB-afterglow emission of GRB170817A and accompanying kilonova.

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arXiv2019

BALQSO Spectra Explained by Shock Disruption of Galactic Clouds

Blue-shifted Broad Absorption Lines (BALs) detected in quasar's spectra are indicative of AGN outflows. We show, using 2D hydrodynamical simulations, that disruption of interstellar clouds by a fast AGN wind can lead to formation of cold, dense high speed blobs that give rise to broad absorption features in the transmission spectrum of the AGN continuum source. For a wind velocity of $0.1 c$ and sufficiently high cloud density ($n_{c}$ < $10^4$ cm$^{-3}$, depending on size), absorption troughs with velocities up to about $3000$ km s$^{-1}$ can be produced. For slower winds and/or lower cloud density the anticipated velocity of the absorbing clouds should be smaller.

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arXiv2019

High efficiency photospheric emission entailed by formation of a collimation shock in gamma-ray bursts

The primary dissipation mechanism in jets of gamma-ray bursts (GRBs), and the high efficiency of the prompt emission are long standing issues. One possibility is strong collimation of a weakly magnetized relativistic jet by the surrounding medium, which can considerably enhance the efficiency of the photospheric emission. We derive a simple analytic criterion for the radiative efficiency of a collimated jet showing that it depends most strongly on the baryon loading. We confirm this analytic result by 3D numerical simulations, and further find that mixing of jet and cocoon material at the collimation throat leads to a substantial stratification of the outflow as well as sporadic loading, even if the injected jet is uniform and continuous. One consequence of this mixing is a strong angular dependence of the radiative efficiency. Another is large differences in the Lorentz factor of different fluid elements that lead to formation of internal shocks. Our analysis indicates that in both long and short GRBs a prominent photospheric component cannot be avoided when observed within an angle of a few degrees to the axis, unless the asymptotic Lorentz factor is limited by baryon loading at the jet base to $Γ_\infty <100$ (with a weak dependence on outflow power). Photon generation by newly created pairs behind the collimation shock regulates the observed temperature at $\sim 50~θ_0^{-1}$ keV, where $θ_0$ is the initial jet opening angle, in remarkable agreement with the observed peak energies of prompt emission spectra. Further consequences for the properties of the prompt emission are discussed at the end.

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arXiv2019

The Spectrum of a Fast Shock Breakout from a Stellar Wind

The breakout of a fast ($>0.1 c$), yet sub-relativistic shock from a thick stellar wind is expected to produce a pulse of X-rays with a rise time of seconds to hours. Here, we construct a semi-analytic model for the breakout of a sub-relativistic, radiation-mediated shock from a thick stellar wind, and use it to compute the spectrum of the breakout emission. The model incorporates photon escape through the finite optical depth wind, assuming a diffusion approximation and a quasi-steady evolution of the shock structure during the breakout phase. We find that in sufficiently fast shocks, for which the breakout velocity exceeds about $0.1c$, the time-integrated spectrum of the breakout pulse is non-thermal, and the time-resolved temperature is expected to exhibit substantial decrease (roughly by one order of magnitude) during breakout, when the flux is still rising, because of the photon generation by the shock compression associated with the photon escape. We also derive a closure relation between the breakout duration, peak luminosity, and characteristic temperature that can be used to test whether an observed X-ray flare is consistent with being associated with a sub-relativistic shock breakout from a thick stellar wind or not. We also discuss implications of the spectral softening for a possible breakout event XRT 080109/SN 2008D.

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arXiv2018

Radio Galaxies at VHE energies

Radio Galaxies have by now emerged as a new $γ$-ray emitting source class on the extragalactic sky. Given their remarkable observed characteristics, such as unusual gamma-ray spectra or ultrafast VHE variability, they represent unique examples to probe into the nature and physics of AGN in general. This review provides a compact summary of their observed characteristics at very high $γ$-ray energies (VHE; $> 100$ GeV) along with a discussion of their possible physics implications. A particular focus is given to a concise overview of fundamental concepts concerning the origin of variable VHE emission, including recent developments in black hole gap physics.

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arXiv2018

Numerical simulations of AGN wind feedback on black hole accretion: probing down to scales within the sphere of influence

Several processes may limit the accretion rate onto a super-massive black hole (SMBH). Two processes that are commonly considered (e.g., for sub-grid prescriptions) are Bondi-Hoyle-Lyttleton accretion and the Eddington limit. A third one is AGN wind feedback. It has been long suggested that such a wind feedback regulates the final SMBH mass, however, it has been shown recently that AGN winds can also regulate the average accretion rate at a level consistent with observations of high redshift AGNs. In this paper we study the effect of wind feedback on the accretion rate using 2D, high resolution hydrodynamic simulations, that incorporate a self-consistent wind injection scheme and resolves the SMBH sphere of influence. Two different cases are explored and compared: one in which the initial gas density is uniform, and one in which it has an isothermal sphere profile. We also compare simulations with and without cooling. Our main finding is that for reasonable parameters, AGN feedback always limits the accretion rate to be far below the Bondi-Hoyle-Lyttleton limit. For typical wind parameters and a uniform ISM densities of the order $1 {cm}^{-3}$, the accretion rate is found to be several orders of magnitudes smaller than that inferred in large samples of high redshift AGNs. On the other hand, the accretion rate obtained for initially isothermal density profile is found to be consistent with the observations, particularly when cooling is included. Furthermore, it roughly scales as $σ^{5}$ with the velocity dispersion of the bulge, in accord with the $M-σ$ relation.

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arXiv2018

Particle-in-cell simulations of pair discharges in a starved magnetosphere of a Kerr black hole

We investigate the dynamics and emission of a starved magnetospheric region (gap) formed in the vicinity of a Kerr black hole horizon, using a new, fully general relativistic particle-in-cell code that implements Monte Carlo methods to compute gamma-ray emission and pair production through the interaction of pairs and gamma rays with soft photons emitted by the accretion flow. It is found that when the Thomson length for collision with disk photons exceeds the gap width, screening of the gap occurs through low-amplitude, rapid plasma oscillations that produce self-sustained pair cascades, with quasi-stationary pair and gamma-ray spectra, and with a pair multiplicity that increases in proportion to the pair production opacity. The gamma-ray spectrum emitted from the gap peaks in the TeV band, with a total luminosity that constitutes a fraction of about $10^{-5}$ of the corresponding Blandford-Znajek power. This stage is preceded by a prompt discharge phase of duration $\sim r_g/c$, during which the potential energy initially stored in the gap is released as a flare of curvature TeV photons. We speculate that the TeV emission observed in M87 may be produced by pair discharges in a spark gap.

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