arXiv subjects

Amir Levinson

Publications and source records attributed to Amir Levinson.

At least 55 records · Page 3Linked to original sources

arXiv2017

Existence of steady gap solutions in rotating black hole magnetospheres

Under conditions prevailing in certain classes of compact astrophysical systems, the active magnetosphere of a rotating black hole becomes charge-starved, giving rise to formation of a spark gap in which plasma is continuously produced. The plasma production process is accompanied by curvature and inverse Compton emission of gamma rays in the GeV-TeV band, that may be detectable by current and future experiments. The properties of the gap emission have been studied recently using a fully general relativistic model of a local steady gap. However, this model requires artificial adjustment of the electric current which is determined, in reality, by the global properties of the magnetosphere. In this paper we map the parameter regime in which steady gap solutions exist, using a steady-state gap model in Kerr geometry, and show that such solutions are allowed only under restrictive conditions that may not apply to most astrophysical systems. We further argue that even the allowed solutions are inconsistent with the global magnetospheric structure. We conclude that magnetospheric gaps are inherently intermittent, and point out that this may drastically change their emission properties.

astro-ph.HE↗

arXiv2017

Monte-Carlo simulations of relativistic radiation mediated shocks: I. photon rich regime

We explore the physics of relativistic radiation mediated shocks (RRMSs) in the regime where photon advection dominates over photon generation. For this purpose, a novel iterative method for deriving a self-consistent steady-state structure of RRMS is developed, based on a Monte-Carlo code that solves the transfer of photons subject to Compton scattering and pair production/annihilation. Systematic study is performed by imposing various upstream conditions which are characterized by the following three parameters: the photon-to-baryon inertia ratio $ξ_{u *}$, the photon-to-baryon number ratio $\tilde{n}$, and the shock Lorentz factor $γ_u$. We find that the properties of RRMSs vary considerably with these parameters. In particular, while a smooth decline in the velocity, accompanied by a gradual temperature increase is seen for $ξ_{u*} \gg 1$, an efficient bulk Comptonization, that leads to a heating precursor, is found for $ξ_{u*} \lesssim 1$. As a consequence, although particle acceleration is highly inefficient in these shocks, a broad non-thermal spectrum is produced in the latter case. The generation of high energy photons through bulk Comptonization leads, in certain cases, to a copious production of pairs that provide the dominant opacity for Compton scattering. We also find that for certain upstream conditions a weak subshock appears within the flow. For a choice of parameters suitable to gamma-ray bursts, the radiation spectrum within the shock is found to be compatible with that of the prompt emission, suggesting that subphotospheric shocks may give rise to the observed non-thermal features despite the absence of accelerated particles.

astro-ph.HE↗

arXiv2017

Limits on the growth rate of supermassive black holes at early cosmic epochs

The effect of AGN wind feedback on the accretion rate and mass evolution of supermassive black holes (SMBH) is considered. It is shown, under reasonable assumptions, that the rate at which gas can be supplied to a SMBH at the center of a young galaxy, is limited to $\sim 20 (σ/350~ {\rm km ~s^{-1}})^4~M_\odot/yr$ (where $σ$ is the velocity dispersion of the host bulge) by interaction of winds expelled from the innermost regions of the accretion flow with the gas in the bulge. This rate is independent of the black hole mass but is sensitive to the properties of the host bulge. It is further argued that the interaction of the wind and the inflowing gas in the bulge can strongly affect the structure of the accretion flow in the super-Eddington regime, potentially leading to highly super-Eddington accretion into the SMBH. About 300 Myr after the onset of the accretion phase, the AGN wind expels all the gas from the bulge and the accretion rate is strongly suppressed. This scenario is in remarkable agreement with recent observations, as it (i) reveals the origin of the maximal observed accretion rates, (ii) accounts for the inferred growth rate of SMBHs at high redshift (independent of their initial seeds masses), (iii) elucidates the decline in the accretion rate at lower redshifts, and (iv) explains the relation between $σ$ and the mass $M_{BH}$ of the central SMBH, measured in the local Universe.

astro-ph.GA↗

arXiv2017

Relativistic shock breakout from a stellar wind

We construct an analytic model for the breakout of a relativistic radiation mediated shock from a stellar wind, and exploit it to calculate the observational diagnostics of the breakout signal. The model accounts for photon escape through the finite optical depth wind, and treats the fraction of downstream photons escaping to infinity as an adiabatic parameter that evolves in a quasi-steady manner. It is shown that the shock is mediated by radiation even when a large fraction of the downstream photons escape, owing to self-generation and adjustment of opacity through accelerated pair creation. Relativistic breakout occurs at radii at which the total optical depth of the wind ahead of the shock is $\sim (m_e/m_p)Γ_{sh}$, provided that the local shock Lorentz factor $Γ_{sh}$ exceeds unity at this location. Otherwise the breakout occurs in the Newtonian regime. A relativistic breakout is expected in a highly energetic spherical explosion ($10^{52}-10^{53}$ erg) of a Wolf-Rayet star, or in cases where a smaller amount of energy ($\sim 10^{51}$ erg) is deposited by a jet in the outer layers of the star. The properties of the emission observed in such explosions during the relativistic breakout are derived. We find that for typical parameters about $10^{48}$ ergs are radiated in the form of MeV gamma-rays over a duration that can range from a fraction of a second to an hour. Such a signal may be detectable out to 10-100 Mpc by current gamma-ray satellites.

astro-ph.HE↗

arXiv2017

Dynamics of Relativistic Shock Waves Subject to a Strong Radiation Drag: Similarity Solutions and Numerical Simulations

We examine the effect of Compton drag on the dynamics of a relativistic shock wave. Similarity solutions describing a radiation-supported shock are obtained for certain profiles of the external radiation intensity and the density of the unshocked ejecta, and are compared with 1D numerical simulations of a blast wave expanding into an ambient medium containing isotropic seed radiation. Both the analytic model and the simulations indicate that under realistic conditions the radiation drag should strongly affect the dynamics and structure of the shocked layer. In particular, our calculations show significant strengthening of the reverse shock and weakening of the forward shock over time of the order of the inverse Compton cooling time. We conclude that the effect of radiation drag on the evolution of the emitting plasma should affect the resultant light curves and, conceivably, spectra of the observed emission during strong blazar flares.

astro-ph.HE↗

arXiv2017

Relativistic Rayleigh-Taylor Instability of a Decelerating Shell and its Implications for Gamma Ray Bursts

Global linear stability analysis of a self-similar solution describing the interaction of a relativistic shell with an ambient medium is performed. The solution is shown to be unstable to convective Rayleigh-Taylor modes having angular scales smaller than the causality scale. Longer wavelength modes are stable and decay with time. For modes of sufficiently large spherical harmonic degree $l$ the dimensionless growth rate scales as $\sqrt{l/Γ}$, where $Γ$ is the Lorentz factor of the shell. The instability commences at the contact interface separating the shocked eject a and shocked ambient gas and propagates to the shocks. The reverse shock front responds promptly to the in stability and exhibits rapidly growing distortions at early times. Propagation to the forward shock is slower, and it is anticipated that the region near the contact will become fully turbulent before the instability is communicated to the forward shock. The non-universality of the Blandford-McKee blast wave solution suggests that turbulence generated by the instability in the shocked ambient medium may decay slowly with time and may be the origin of magnetic fields over a long portion of the blast wave evolution. It is also speculated that the instability may affect the emission from the shocked ejecta in the early post-prompt phase of GRBs.

astro-ph.HE↗

arXiv2016

Reconfinement of highly magnetized jets: Implications for HST-1 in M87

Stationary features are occasionally observed in AGN jets. A notable example is the HST-1 knot in M87. Such features are commonly interpreted as re-confinement shocks in hydrodynamic jets or focusing nozzles in Poynting jets. In this paper we compute the structure and Lorentz factor of a highly magnetized jet confined by external pressure having a profile that flattens abruptly at some radius. We find the development of strong oscillations upon transition from the steeper to the flatter pressure profile medium. Analytic formula is derived for the location of the nodes of these oscillations. We apply the model to the M87 jet and show that if the jet remains magnetically dominated up to sub-kpc scales, then focusing is expected. The location of the HST-1 knot can be reconciled with recent measurements of the pressure profile around the Bondi radius if the jet luminosity satisfies $L_j\simeq10^{43}$ erg/s. However, we find that magnetic domination at the collimation break implies a Lorentz factor in excess of $10^2$, atypical to FRI sources. A much lower value of the asymptotic Lorentz factor would require substantial loading close to the black hole. In that case HST-1 may be associated with a collimation nozzle of a hydrodynamic flow.

astro-ph.HE↗

arXiv2016

The collimation of magnetic jets by disk winds

The collimation of a Poynting-flux dominated jet by a wind emanating from the surface of an accretion flow is computed using a semi-analytic model. The injection of the disk wind is treated as a boundary condition in the equatorial plane, and its evolution is followed by invoking a prescribed geometry of streamlines. Solutions are obtained for a wide range of disk wind parameters. It is found that jet collimation generally occurs when the total wind power exceeds about ten percents of the jet power. For moderate wind powers we find gradual collimation. For strong winds we find rapid collimation followed by focusing of the jet, after which it remains narrow over many Alfvén crossing times before becoming conical. We estimate that in the later case the jet's magnetic field may be dissipated by the current-driven kink instability over a distance of a few hundreds gravitational radii. We apply the model to M87 and show that the observed parabolic shape of the radio jet within the Bondi radius can be reproduced provided that the wind injection zone extends to several hundreds gravitational radii, and that its total power is about one third of the jet power. The radio spectrum can be produced by synchrotron radiation of relativistically hot, thermal electrons in the sheath flow surrounding the inner jet.

astro-ph.HE↗

arXiv2016

The effect of Compton drag on the dynamics of dissipative Poynting dominated flows: Implications for the unification of radio loud AGN

The dynamics of a dissipative Poynting dominated flow subject to a radiation drag due to Compton scattering of ambient photons by relativistic electrons accelerated in reconnecting current sheets is studied. It is found that the efficiency at which magnetic energy is converted to radiation is limited to a maximum value of $ε_c=3l_{dis}\,σ_0/4(σ_0+1)$, where $σ_0$ is the initial magnetization of the flow and $l_{dis}\le1$ the fraction of initial Poynting flux that can dissipate. The asymptotic Lorentz factor satisfies $Γ_\infty\geΓ_0(1+l_{dis}\,σ_0/4)$, where $Γ_0$ is the initial Lorentz factor. This limit is approached in cases where the cooling time is shorter than the local dissipation time. A somewhat smaller radiative efficiency is expected if radiative losses are dominated by synchrotron and SSC emissions. It is suggested that under certain conditions magnetic field dissipation may occur in two distinct phases: On small scales, asymmetric magnetic fields that are advected into the polar region and dragged out by the outflow dissipate to a more stable configuration. The dissipated energy is released predominantly as gamma rays. On much larger scales, the outflow encounters a flat density profile medium and re-collimates. This leads to further dissipation and wobbling of the jet head by the kink instability, as found recently in 3D MHD simulations. Within the framework of a model proposed recently to explain the dichotomy of radio loud AGN, this scenario can account for the unification of gamma-ray blazars with FRI and FRII radio sources.

astro-ph.HE↗

arXiv2015

Broadband extended emission in gravitational waves from core-collapse supernovae

Black holes in core-collapse of massive stars are expected to surge in mass and angular momentum by hyper-accretion immediately following their formation. We here describe a general framework of extended emission in gravitational waves from non-axisymmetric accretion flows from fallback matter of the progenitor envelope. It shows (a) a maximum efficiency in conversion of accretion energy into gravitational waves at hyper-accretion rates exceeding a critical value set by the ratio of the quadrupole mass inhomogeneity and viscosity with (b) a peak characteristic strain amplitude at the frequency $f_b=Ω_b/π$, where $Ω_b$ is the Keplerian angular velocity at which viscous torques equal angular momentum loss in gravitational radiation, with $h_{char}\propto f^{1/6}$ at $f f_b$. Upcoming gravitational wave observations may probe this scaling by extracting broadband spectra using time-sliced matched filtering with chirp templates, recently developed for identifying turbulence in noisy time series.

astro-ph.HE↗

arXiv2015

Blazar Flares from Compton Dragged Shells

We compute the dynamics and emission of dissipative shells that are subject to a strong Compton drag, under simplifying assumptions about the dissipation mechanism. We show that under conditions prevailing in blazars, substantial deceleration is anticipated on sub-parsec and parsec scales in cases of rapid dissipation. Such episodes may be the origin of some of the flaring activity occasionally observed in gamma ray blazars. The shape of the light curves thereby produced reflects the geometry of the emitting surface if the deceleration is very rapid, or the dynamics of the shell if the deceleration is delayed, or initially more gradual, owing, e.g., to continuous injection of energy and momentum.

astro-ph.HE↗

arXiv2014

On the origin of short GRBs with Extended Emission and long GRBs without associated SN

The Burst and Transient Source Experiment (BATSE) classifies cosmological gamma-ray bursts (GRBs) into short (less than 2 s) and long (over 2 s) events, commonly attributed to mergers of compact objects and, respectively, peculiar core-collapse supernovae. This standard classification has recently been challenged by the {\em Swift} discovery of short GRBs showing Extended Emission (SGRBEE) and nearby long GRBs without an accompanying supernova (LGRBN). Both show an initial hard pulse, characteristic of SGRBs, followed by a long duration soft tail. We here consider the spectral peak energy ($E_{p,i}$)-radiated energy $(E_{iso})$ correlation and the redshift distributions to probe the astronomical and physical origin of these different classes of GRBs. We consider {\em Swift} events of $15$ SGRBs, $7$ SGRBEEs, 3 LGRBNs and 230 LGRBs. The spectral-energy properties of the initial pulse of both SGRBEE and LGRBNs are found to coincide with those of SGRBs. A Monte Carlo simulation shows that the redshift distributions of SGRBs, SGRBEE and LGRBNs fall outside the distribution of LGRBs at 4.75$\,σ$, 4.67$\,σ$ and 4.31$σ$, respectively. A distinct origin of SGRBEEs with respect to LGRBs is also supported by the elliptical host galaxies of the SGRBEE events 050509B and 050724. This combined evidence supports the hypothesis that SGRBEE and LGRBNs originate in mergers as SGRBs. Moreover, long/soft tail of SGRB and LGRBNs satisfy the same $E_{p,i}-E_{iso}$ Amati-correlation holding for normal LGRBs. This fact points to rapidly rotating black holes as a common long-lived inner engine produced by different astronomical progenitors (mergers and supernovae).

astro-ph.HE↗

arXiv2014

Jet formation in GRBs: A semi-analytic model of MHD flow in Kerr geometry with realistic plasma injection

We construct a semi-analytic model for MHD flows in Kerr geometry, that incorporates energy loading via neutrino annihilation on magnetic field lines threading the horizon. We compute the double-flow structure for a wide range of energy injection rates, and identify the different operation regimes. At low injection rates the outflow is powered by the spinning black hole via the Blandford-Znajek mechanism, whereas at high injection rates it is driven by the pressure of the plasma deposited on magnetic field lines. In the intermediate regime both processes contribute to the outflow formation. The parameter that quantifies the load is the ratio of the net power injected below the stagnation radius and the maximum power that can be extracted magnetically from the black hole.

astro-ph.HE↗

arXiv2014

Sub-photospheric, radiation mediated shocks in GRBs: Multiple shock emission and the Band spectrum

We compute the time integrated, thermal emission produced by a series of radiation mediated shocks that emerge from the photosphere of a GRB outflow. We show that for a sufficiently broad distribution of shock strengths, the overall shape of the time integrated spectral energy distribution below the peak is a power law, $νE_ν\propto ν^α$, with a slope $1<α<2$. A substructure in the SED can also be produced in this model for certain choices of the shock train distribution. In particular, we demonstrate that our model can reproduce the double-peak SED observed in some bursts, in events whereby a strong shock is followed by a sequence sufficiently weaker ones.

astro-ph.HE↗

arXiv2013

Plasma injection and outflow formation in Kerr black holes

We discuss the role plasma injection plays in the formation of outflows in Kerr spacetime. Using a model for the double flow established in the polar region of a rotating black hole, we study the interplay between the different processes that can power the outflow. In particular, we find two types of flows with distinct properties that depend on the rate at which energy is deposited in the magnetosphere. We discuss the implications of this result for gamma ray bursts outflows.

astro-ph.HE↗

arXiv2013

Loaded magnetohydrodynamic flows in Kerr spacetime

The effect of mass and energy loading on the efficiency at which energy can be extracted magnetically from a Kerr black hole is explored, using a semi-analytic, ideal MHD model that incorporates plasma injection on magnetic field lines. We find a critical load below which the specific energy of the plasma inflowing into the black hole is negative, and above which it is positive, and identify two types of flows with distinct properties; at sub-critical loads a magnetic outflow is launched from the ergosphere, owing to extraction of the black hole spin energy, as originally proposed by Blandford and Znajek. At super-critical loads the structure of the flow depends on the details of the injection process. In cases where the injected plasma is relativistically hot, a pressure driven, double trans-magnetosonic flow is launched from a stagnation point located outside the ergosphere, between the inner and outer light cylinders. Some fraction of the energy deposited in the magnetosphere is then absorbed by the black hole and the rest emerges at infinity in the form of a relativistic outflow. When the injected plasma is cold an outflow may not form at all. We discuss the implications of our results to gamma ray bursts and active galactic nuclei.

astro-ph.HE↗

arXiv2013

Ultra-relativistic, neutrino driven flows in GRBs: A double transonic flow solution in Schwarzschild spacetime

The structure of a hydrodynamic, double transonic flow driven by neutrino annihilation in the polar region of a Schwarzschild black hole is computed for different energy deposition profiles. The requirement that both, the inflow into the black hole and the outflow to infinity pass smoothly through their sonic points fixes the stagnation radius and stagnation pressure. The asymptotic power of the outflow is shown to be the integral of the energy deposition rate above the stagnation radius. The outflow production efficiency depends on the energy deposition profile, and is generally higher for shallower profiles. Using recent calculations of the neutrino annihilation rate, we estimate that over 50 percents of the total energy deposited above the horizon can emerge in the form of a relativistic outflow at infinity. The continuous creation of plasma during the expansion of the outflow leads to generation of a large specific entropy. This has important implications for the prompt photospheric emission in GRBs.

astro-ph.HE↗

arXiv2013

Collimation and confinement of magnetic jets by external media

We study the collimation of a highly magnetized jet by a surrounding cocoon that forms as a result of the interaction of the jet with the external medium. We show that in regions where the jet is well confined by the cocoon, current-driven instabilities should develop over timescales shorter than the expansion time of the jet's head. We speculate that these instabilities would give rise to complete magnetic field destruction, whereby the jet undergoes a transition from high-to-low sigma above the collimation zone. Using this assumption, we construct a self-consistent model for the evolution of the jet-cocoon system in an ambient medium of arbitrary density profile. We apply the model to jet breakout in long GRBs, and show that the jet is highly collimated inside the envelope of the progenitor star, and is likely to remain confined well after breakout. We speculate that this strong confinement may provide a channel for magnetic field conversion in GRB outflows, whereby the hot, low-sigma jet section thereby produced is the source of the photospheric emission observed in many bursts.

astro-ph.HE↗