arXiv subjects

Amir Levinson

Publications and source records attributed to Amir Levinson.

At least 73 records · Page 4Linked to original sources

arXiv2012

Observational signatures of sub-photospheric radiation mediated shocks in the prompt phase of GRBs

A shock that form below the photosphere of a GRB outflow is mediated by Compton scattering of radiation advected into the shock by the upstream fluid. The characteristic scale of such a shock, a few Thomson depths, is larger than any kinetic scale involved by several orders of magnitudes, hence, unlike collisionless shocks, radiation mediated shocks cannot accelerate particles to non-thermal energies. The spectrum emitted by a shock that emerges from the photosphere of a GRB jet, reflects the temperature profile downstream of the shock, with a possible contribution at the highest energies from the shock transition layer itself. We study the properties of radiation mediated shocks that form during the prompt phase of GRBs, and compute the time integrated spectrum emitted by the shocked fluid following shock breakout. We show that the time integrated emission from a single shock exhibits a prominent thermal peak, with the location of the peak depending on the shock velocity profile. We also point out that multiple shock emission can produce a spectrum that mimics a Band spectrum.

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arXiv2011

Electromagnetic priors for black hole spindown in searches for gravitational-waves from supernovae and long GRBs

Some core-collapse supernovae appear to be hyper-energetic, and a subset of these are aspherical and associated with long GRBs. We use observations of electromagnetic emission from core-collapse supernovae and GRBs to impose constraints on their free energy source as a prior to searches for their gravitational wave emission. We review these events based on a finite efficiency for the conversion of spin energy to magnetic winds powering supernovae. We find that some of the hyper-energetic events cannot be powered by the spindown of rapidly rotating proto-neutron stars by virtue of their limited rotational energy. They can, instead, be produced by the spindown of black holes providing a distinct prospect for gravitational-wave emission of interest to LIGO, Virgo, and the LCGT.

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arXiv2011

Variable TeV emission as a manifestation of jet formation in M87?

It is proposed that the variable TeV emission observed in M87 may be produced in a starved magnetospheric region, above which the outflow associated with the VLBA jet is established. It is shown that annihilation of MeV photons emitted by the radiative inefficient flow in the vicinity of the black hole, can lead to injection of seed charges on open magnetic field lines, with a density that depends sensitively on accretion rate, $n_\pm\propto\dot{m}^{4}$. For an accretion rate that corresponds to the inferred jet power, and to a fit of the observed SED by an ADAF model, the density of injected pairs is found to be smaller than the Goldreich-Julian density by a factor of a few. It is also shown that inverse Compton scattering of ambient photons by electrons (positrons) accelerating in the gap can lead to a large multiplicity, $\sim 10^3$, while still allowing photons at energies of up to a few TeV to freely escape the system. The estimated gap width is not smaller than $0.01 r_s$ if the density of seed charges is below the Goldreich-Julian value. The VHE power radiated by the gap can easily account for the luminosity of the TeV source detected by H.E.S.S. The strong dependence of injected pair density on accretion rate should render the gap emission highly intermittent. We also discuss briefly the application of this mechanism to Sgr A$^\star$.

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arXiv2011

Sub-Photospheric Emission from Relativistic Radiation Mediated Shocks in GRBs

It is proposed that the prompt emission observed in bursts that exhibit a thermal component originates from relativistic radiation mediated shocks that form below the photosphere of the GRB outflow. It is argue that such shocks are expected to form in luminous bursts via collisions of shells that propagate with moderate Lorentz factors $Γ\lesssim 500$. Faster shells will collide above the photosphere to form collisionless shocks. We demonstrate that in events like GRB 090902B a substantial fraction of the explosion energy is dissipated below the photosphere, in a region of moderate optical depth $τ\lesssim300$, whereas in GRB 080916C the major fraction of the energy dissipates above the photosphere. We show that under conditions anticipated in many GRBs, such relativistic radiation mediated shocks convect enough radiation upstream to render photon production in the shock transition negligible, unlike the case of shock breakout in supernovae. The resulting spectrum, as measured in the shock frame, has a relatively low thermal peak, followed by a broad, nonthermal component extending up to the KN limit.

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arXiv2010

Relativistic jets at high energies

A recent progress in the study of $γ$-ray jets is reviewed, with a focus on some theoretical interpretations of the VHE emission from M87, and possibly other misaligned blazars; the connection betwe en the GeV breaks exhibited by bright LAT blazars and opacity sources in the broad line region; the consequences of the detection of GeV emission from GRBs to models of magnetic outf lows; and the implications of the thermal emission observed is some GRBs to dissipation of the outflow bulk energy.

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arXiv2010

Interaction of a magnetized shell with an ambient medium: limits on impulsive magnetic acceleration

The interaction of relativistic magnetized ejecta with an ambient medium is studied for a range of structures and magnetization of the unshocked ejecta. We particularly focus on the effect of the ambient medium on the dynamics of an impulsive, high-sigma shell. It is found that for sufficiently high values of the initial magnetization $σ_0$ the evolution of the system is significantly altered by the ambient medium well before the shell reaches its coasting phase. The maximum Lorentz factor of the shell is limited to values well below $σ_0$; for a shell of initial energy $E=10^{52}E_{52}$ ergs and size $r_0=10^{12}T_{30}$ cm expelled into a medium having a uniform density $n_i$ we obtain $Γ_{\rm max}\simeq180(E_{52}/T_{30}^3 n_i)^{1/8}$ in the high sigma limit. The reverse shock and any internal shocks that might form if the source is fluctuating are shown to be very weak. The restriction on the Lorentz factor is more severe for shells propagating in a stellar wind. Intermittent ejection of small sub-shells doesn't seem to help, as the shells merge while still highly magnetized. Lower sigma shells start decelerating after reaching the coasting phase and spreading away. The properties of the reverse shock then depend on the density profiles of the coasting shell and the ambient medium. For a self-similar cold shell the reverse shock becomes strong as it propagates inwards, and the system eventually approaches the self-similar solution derived recently by Nakamura \& Shigeyama.

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arXiv2010

Jets on all scales

A brief overview of jets and their central drivers is presented, with a focus on accreting black hole systems. In particular, scaling relations that elucidate some basic properties of the engines are derived, and the implications for the associated outflows are discussed. The kinematics and dynamics of relativistic jets in various systems and the dissipation of their bulk energy is considered, with an emphasis on consequences of recent observations. Also considered is the interaction of the jets with their environment. Comments on multi-messenger probes are made at the end.

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arXiv2009

Convective Instability of a Relativistic Ejecta Decelerated by a Surrounding Medium: An Origin of Magnetic Fields in GRBs?

Global linear stability analysis of a self-similar solution describing a relativistic shell decelerated by an ambient medium is performed. The system is shown to be subject to the convective Rayleigh-Taylor instability, with a rapid growth of eigenmodes having angular scale much smaller than the causality scale. The growth rate appears to be largest at the interface separating the shocked ejecta and shocked ambient gas. The disturbances produced at the contact interface propagate in the shocked media and cause nonlinear oscillations of the forward and reverse shock fronts. It is speculated that such oscillations may affect the emission from the shocked ejecta in the early afterglow phase of GRBs, and may be the origin of the magnetic field in the shocked circum-burst medium.

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arXiv2009

Recollimation and Radiative Focusing of Relativistic Jets: Applications to Blazars and M87

Recent observations of M87 and some blazars reveal violent activity in small regions located at relatively large distances from the central engine. Motivated by these considerations, we study the hydrodynamic collimation of a relativistic cooling outflow using a semi-analytical model developed earlier. We first demonstrate that radiative cooling of the shocked outflow layer can lead to a focusing of the outflow and its reconfinement in a region having a very small cross-sectional radius. Such a configuration can produce rapid variability at large distances from the central engine via reflections of the converging recollimation shock. Possible applications of this model to TeV blazars are discussed. We then apply our model to M87. The low radiative efficiency of the M87 jet renders focusing unlikely. However, the shallow profile of the ambient medium pressure inferred from observations, results in extremely good collimation that can explain the reported variability of the X-ray flux emitted from the HST-1 knot.

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arXiv2009

Probing Cosmic Accelerators Using VHE Gamma Rays and UHE cosmic rays

The $γ$-ray emission observed in several classes of Galactic and extragalactic astrophysical sources appears to be linked to accreting black holes and rotational powered neutron stars. These systems are prodigious cosmic accelerators, and are also potential sources of the UHE cosmic rays detected by several experiments and VHE neutrinos. We review a recent progress in our understanding of these objects, and demonstrate how recent and future observations can be employed to probe the conditions in the sources.

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arXiv2008

Relativistic Flows in TeV Blazars

Rapid variability of the TeV emission in several blazars implies a central black hole mass $M_{BH}<10^8M_\odot$, appreciably smaller than the values estimated from the $M_{BH}-L_{bulg}$ relation, and Doppler factors for the $γ$-ray emitting fluid much larger than those associated with radio patterns. We discuss the conditions in the central engine required to account for the short timescales and large luminosities observed, and propose some explanations for the inferred kinematics of the source on various scales.

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arXiv2008

Relativistic Photon Mediated Shocks

A system of equations governing the structure of a steady, relativistic radiation dominated shock is derived, starting from the general form of the transfer equation obeyed by the photon distribution function. Closure is obtained by truncating the system of moment equations at some order. The anisotropy of the photon distribution function inside the shock is shown to increase with increasing shock velocity, approaching nearly perfect beaming at upstream Lorentz factors $Γ_{-}>>1$. Solutions of the shock equations are presented for some range of upstream conditions. These solutions are shown to converge as the truncation order is increased.

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arXiv2007

Collimation and Radiative Deceleration of Jets in TeV AGNs

We consider some implications of the rapid X-ray and TeV variability observed in M87 and the TeV blazars. We outline a model for jet focusing and demonstrate that modest radiative cooling can lead to recollimation of a relativistic jet in a nozzle having a very small cross-sectional radius. Such a configuration can produce rapid variability at large distances from the central engine and may explain recent observations of the HST-1 knot in M87. Possible applications of this model to TeV blazars are discussed. We also discuss a scenario for the very rapid TeV flares observed with HESS and MAGIC in some blazars, that accommodates the relatively small Doppler factors inferred from radio observations.

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arXiv2007

Hydrodynamic Collimation of Relativistic Outflows: Semianalytic Solutions and Application to Gamma-Ray Bursts

A model is developed for the confinement and collimation of a baryon poor outflow by its surrounding medium. Both, confinement by kinetic pressure of a static corona, and confinement by the ram pressure of a supersonic wind emanating from a disk surrounding the inner source are considered. Solutions are presented for the structure of the shocked layers of a deflected baryon poor jet (BPJ) and exterior wind. The dependence of the opening angle of the BPJ on the parameters of the confining medium are carefully examined. It is found that the BPJ shock may either converge to the symmetry axis or diverge away from it, depending on the opening angle of the BPJ injection cone. In the latter case the inner flow exhibits a non-uniform structure, consisting of an ultra-relativistic core containing the unshocked BPJ enveloped by the slower, shocked BPJ layer. The implications of our results to the prompt GRB emission are briefly discussed.

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arXiv2007

On the Origin of Rapid Flares in TeV Blazars

The rapid variability of the VHE emission reported for some TeV blazars implies Doppler factors well in excess of those inferred from superluminal motions and unification schemes. We propose that those extreme flares may result from radiative deceleration of blobs on scales where local dissipation occurs. The minimum jet power estimated from the resolved synchrotron emission on VLBI scales appears to be consistent with this model. It is shown that if the energy distribution of nonthermal electrons accelerated locally in the blob is reasonably flat, then a background radiation field having a luminosity in the range 10$^{41}-10^{42}$ erg s$^{-1}$ can give rise to a substantial deceleration of the blob, but still be transparent enough to allow the TeV $γ$-rays thereby produced to escape the system.

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arXiv2007

Phenomenology of Gamma-Ray Jets

We discuss some phenomenological aspects of $γ$-ray emitting jets. In particular, we present calculations of the $γ$-sphere and $π$-sphere for various target photon fields, and employ them to demonstrate how $γ$-ray observations at very high energies can be used to constraint the Doppler factor of the emitting plasma and the production of VHE neutrinos. We also consider the implications of the rapid TeV variability observed in M87 and the TeV blazars, and propose a model for the very rapid TeV flares observed with HESS and MAGIC in some blazars,that accommodates the relatively small Doppler factors inferred from radio observations. Finally, we briefly discuss the prospects for detecting VHE neutrinos from relativistic jets.

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arXiv2007

Structure and Nuclear Composition of General Relativistic, Magnetohydrodynamic Outflows from Neutrino-Cooled Disks

We compute the structure and degree of neutronization of general relativistic magnetohydrodynamic (GRMHD) outflows originating from the inner region of neutrino-cooled disks. We consider both, outflows expelled from a hydrostatic disk corona and outflows driven by disk turbulence. We show that in outflows driven thermally from a static disk the electron fraction quickly evolves to its equilibrium value which is dominated by neutrino capture. Those outflows are generally proton rich and, under certain conditions, can be magnetically dominated. They may also provide sites for effective production of 56Ni. Centrifugally driven outflows and outflows driven by disk turbulence, on the other hand, can preserve the large in-disk neutron excess. Those outflows are, quite generally, subrelativistic by virtue of the large mass flux driven by the additional forces.

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arXiv2007

Gamma Ray and Neutrino Emission as a Probe of Relativistic Jets

Constraints on the dynamics, dissipation, and production of VHE neutrinos in relativistic jets are derived using opacity calculations and VHE $γ$-ray observations. In particular, it is demonstrated how rapid variability of the $γ$-ray emission at very high energies ($> 100$ GeV) can be used to map the location of the $γ$-spheres, to derive lower limits on the Doppler factor of the $γ$-ray emission zone, and to constrain the photopion production opacity. The apparent discrepancy between jet Lorentz factors inferred from superluminal motions and source statistics in the TeV blazars and those derived from the $γ$-ray emission is discussed. The relation to the high-energy emission from the HST1 knot in M87 is briefly mentioned. Estimates of neutrino yields in upcoming neutrino telescopes are given for various sources. It is shown that for TeV blazars, the rapid variability of the TeV emission implies neutrino yields well below detection limit.

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