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Amir Levinson

Publications and source records attributed to Amir Levinson.

At least 91 records · Page 5Linked to original sources
arXiv2006

High-Energy Aspects of Astrophysical Jets

Various aspects of the high-energy emission from relativistic jets associated wi th compact astrophysical systems are reviewed. The main leptonic and hadronic processes responsible for the production of high-energy $γ$-rays, very-high energy neutrinos and ultra-high energy cosmic rays are discussed. Relations between the $γγ$ pair production and photomeson production opacities are derived, and their consequences for the relative emission of $γ$-rays and neutrinos are examined. The scaling of the size and location of the various emission zones and other quantities with black hole mass and dimensionless luminosity is elucidated. The results are applied to individual classes of objects, including blazars, microquasars and gamma-ray bursts. It is concluded that if baryons are present in the jet at sufficient quantities, then under optimal conditions most systems exhibiting relativistic jets may be detectable by upcoming neutrino telescopes. An exception is the class of TeV blazars, for which $γ$-ray observations imply neutrino yields well below detection limit.

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arXiv2006

Neutrino-Driven Mass Loading of GRMHD Outflows

A GRMHD model of disk outflows with neutrino-driven mass ejection is presented. The model is used to calculate the structure of the outflow in the sub-slow magnetosonic region and the mass loading of the outflow, under conditions anticipated in the central engines of gamma-ray bursts. It is concluded that magnetic launching of ultra-relativistic polar outflows is in principle possible along low inclination field lines (with respect to the symmetry axis), provided the neutrino luminosity is sufficiently low, $L_ν\simlt10^{52}$ erg s$^{-1}$.

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arXiv2006

The Planets-Capture Model of V838 Monocerotis

The planets capture model for the eruption of V838 Mon is discussed. We used three methods to estimate the location where the planets were consumed. There is a nice consistency for the results of the three different methods, and we find that the typical stopping / slowing radius for the planets is about 1Ro. The three peaks in the optical light curve of V838 Mon are either explained by the swallowing of three planets at different radii or by three steps in the slowing down process of a single planet. We discuss the other models offered for the outburst of V838 Mon, and conclude that the binary merger model and the planet/s scenario seem to be the most promising. These two models have several similarities, and the main differences are the stellar evolutionary stage, and the mass of the accreted material. We show that the energy emitted in the V838 Mon event is consistent with the planets scenario. We suggest a few explanations for the trigger for the outburst and for the double structure of the optical peaks in the light curve of V838 Mon.

astro-ph↗
arXiv2006

Are Gamma-ray Bursts Universal?

It is noted that the Liang-Zhang correlation can be accounted for with the viewing angle interpretation proposed earlier. The Ghirlanda correlation, recently generalized by Nava et al (2006) to a wind profile, can be accounted for by the viewing angle interpretation accordingly generalized to a wind profile. Most of the scatter in the spectra and time-integrated brightness in $γ$-ray bursts (GRB) can thus be accounted for by variation in two parameters, 1) the viewing angle and 2) the jet opening angle, with very little variation in any other intrinsic parameters. The scatter in apparent isotropic equivalent fluence and other parameters is reduced by a factor of order 30 when each of these parameters is considered. Possible difficulties with alternative explanations are briefly discussed. It is also noted that the relative scatter in the Amati and Ghirlanda correlations suggests certain conclusions about the inner engine.

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arXiv2006

The planets capture model of V838 Monocerotis: conclusions for the penetration depth of the planet/s

V838 Mon is the prototype of a new class of objects. Understanding the nature of its multi-stage outburst and similar systems is challenging. So far, several scenarios have been invoked to explain this group of stars. In this work, the planets-swallowing model for V838 Mon is further investigated, taking into account the findings that the progenitor is most likely a massive B type star. We find that the super-Eddington luminosity during the eruption can explain the fast rising times of the three peaks in the optical light curve. We used two different methods to estimate the location where the planets were consumed. There is a nice agreement between the values obtained from the luminosities of the peaks and from their rising time scale. We estimate that the planets were stopped at a typical distance of one solar radius from the center of the host giant star. The planets-devouring model seems to give a satisfying explanation to the differences in the luminosities and rising times of the three peaks in the optical light curve of V838 Mon. The peaks may be explained by the consumption of three planets or alternatively by three steps in the terminal falling process of a single planet. We argue that only the binary merger and the planets-swallowing models are consistent with the observations of the new type of stars defined by V838 Mon.

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arXiv2006

General Relativistic, Neutrino-Assisted MHD winds - Theory and Application to GRBs. I. Schwarzschild Geometry

(short version) - A model for GRMHD disk outflows with neutrino-driven mass ejection is developed,and employed to calculate the structure of the outflow in the sub-slow magnetosonic region and the mass loading of the outflow, under conditions anticipated in the central engines of gamma-ray bursts. The dependence of the mass flux on the conditions in the disk, on magnetic field geometry, and on other factors is carefully examined for a range of neutrino luminosities expected in hyperaccreting black holes. The fraction of neutrino luminosity that is being converted to kinetic energy flux is shown to be a sensitive function of the effective neutrino temperature at the flow injection point, and the shape of magnetic field lines in the sub-slow region, but is practically independent of the strength of poloidal and toroidal magnetic fields. We conclude that magnetic launching of ultra-relativistic polar outflows from the innermost parts of the disk is in principle possible provided the neutrino luminosity is sufficiently low, $L_ν\simlt10^{52}$ erg s$^{-1}$ or so. The conditions found to be optimal for the launching of an ultra-relativistic jet are also the conditions favorable for large neutron-to-proton ratio in the disk.

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arXiv2005

A Complete Survey of the Transient Radio Sky and Implications for Gamma-Ray Bursts, Supernovae, and other Relativistic Explosions

We had previously reported on a survey for radio transients, used to set an upper limit on the number of orphan gamma-ray burst (GRB) radio afterglows, and thus a lower limit on the typical GRB beaming factor. Here we report radio and optical follow-up observations of these possible transients, achieving the first full characterization of the transient radio sky. We find that only two source are likely to be real radio transients, an optically obscured radio supernova (SN) in the nearby galaxy NGC 4216, and a source not associated with a bright host galaxy, which is too radio luminous to be a GRB afterglow. We speculate that this may be a flare from a peculiar active galactic nucleus, or a burst from an unusual Galactic compact object. We place an upper limit of 65 radio transients above 6 mJy over the entire sky at the 95% confidence level. The implications are as follows. First, we derive a limit on the typical beaming of GRBs; we find f_b^{-1} >~ 60, ~5 times higher than our earlier results. Second, we impose an upper limit on the rate of events that eject >~ 10^{51} erg in unconfined relativistic ejecta, whether or not accompanied by detectable emission in wavebands other than the radio. Our estimated rate, <=1000/y/Gpc, is about two orders of magnitude smaller than the rate of core-collapse SNe (and type Ib/c events in particular), indicating that only a minority of such events eject significant amounts of relativistic material, which are required by fireball models of long-soft GRBs. Finally, we show that future wider and/or deeper radio variability surveys are expected to detect numerous orphan radio GRB afterglows. Our survey also illustrates the great potential of sensitive surveys with new instruments to revolutionize the study of nearby SNe (abridged).

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arXiv2005

The gravitational-wave spectrum of a non-axisymmetric torus around a rapidly spinning black hole

The gravitational-wave spectrum emitted by a non-axisymmetric torus rotating at an angular velocity $Ω_T$, is derived in terms of a structure function representing a combination of sausage-tilt modes in the torus in the limit of an incompressible fluid. The analysis of the gravitational-wave spectrum is then applied to a model proposed recently, in which a highly magnetized torus interacts with a stellar mass, Kerr black hole via poloidal field lines that connect the torus and the horizon. The deformation of the torus results from global magnetic instabilities when the magnetic field strength inside the torus exceeds a few times $10^{15}$ Gauss. The dynamics of the system is illustrated using a non-MHD toy model. It is found that, quite generally, most of the gravitational-wave energy emitted is in the frequency range of sensitivity of LIGO and Virgo.

astro-ph↗
arXiv2005

Energy Extraction from a Kerr Black Hole - An Ultimate Power Source in the Universe?

It is widely believed that some classes of high-energy transients may be powered by the rotational energy of a rapidly spinning black hole. The energy extraction mechanism commonly discussed involves macroscopic magnetic fields that are produced by currents flowing in a disk or torus surrounding the black hole. In spite of large efforts that have led to impressive progress in our understanding of the physics of magnetized flows in Kerr geometry, several important issues remain unresolved: the nature of the load in the Blandford-Znajeck model, the causality and stability of a force-free magnetosphere, global current closure, and the role of boundary conditions on the channels through which the extracted energy is released. This chapter provides an account of recent work that addresses some of those open questions.

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arXiv2005

The Difference Between the Amati and Ghirlanda Relations

It is pointed out that the beaming correction commonly inferred from the achromatic breaks in the afterglow light curve is biased in situations where the isotropic equivalent energy is affected by factors other than the spread in opening angles. In particular, it underestimates the beaming factor of sources observed off-axis. Here we show that both the slopes and scatters in the Amati and Ghirlanda relations, and the difference between them, are quantitatively consistent with a model proposed recently, in which the $E_{iso}-ν_{peak}$ relation, as originally derived by Amati et al., is due to viewing angle effects. The quantitative difference between them confirms the relations between opening angle and break time suggested by Frail et al.

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arXiv2005

Large-Amplitude, Pair-Creating Oscillations in Pulsar and Black Hole Magnetospheres

A time-dependent model for pair creation in a pulsar magnetosphere is developed. It is argued that the parallel electric field that develops in a charge-starved region (a gap) of a pulsar magnetosphere oscillates with large amplitude. Electrons and positrons are accelerated periodically and the amplitude of the oscillations is assumed large enough to cause creation of upgoing and downgoing pairs at different phases of the oscillation. With a charge-starved initial condition, we find that the oscillations result in bursts of pair creation in which the pair density rises exponentially with time. The pair density saturates at $N_\pm\simeq E_{0}^2/(8πm_ec^2Γ_{\rm thr})$, where $E_0$ is the parallel electric field in the charge-starved initial state, and $Γ_{\rm thr}$ is the Lorentz factor for effec tive pair creation. The frequency of oscillations following the pair creation burst is given roughly by $ω_{\rm osc}=eE_0/(8m_ecΓ_{\rm thr})$. A positive feedback keeps the system stable, such that the average pair creation rate balances the loss rate due to pairs escaping the magnetosphere.

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arXiv2004

Polarization of GRB by scattering off relativistically moving material: Compton sailing and high polarization

The polarization of gamma ray emission scattered off the baryon rich material that collimates a GRB fireball and the coasting speed of the irradiated matter are calculated numerically for different geometries of the radiation source and the collimating wall. It is shown that when the scattering material is Compton sailing, the direction of maximum polarization is quite generally well within the beaming cone of scattered radiation. As a result, the probability for observing bright highly polarized GRB's increases considerably, provided the Lorentz factor of the coasting matter is not well below 30, and the scattered radiation is highly polarized even when the beam thickness is large compared to $1/Γ$. It is suggested that correlation between polarization and intensity could provide clues as to whether energy flows from matter to photons or the reverse.

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arXiv2004

On time evolution and causality of force-free black hole magnetospheres

The rotational energy of a Kerr black hole is often invoked as the free energy source that powers compact astrophysical systems An important question concerning the energy extraction mechanism is whether a Kerr black hole can communicate stresses to a distant load via a surrounding, force-free magnetosphere, and whether such structures are stable. In this paper we address this question. We first re-examine the properties of short wavelength force-free waves, and show that contrary to earlier claims, fast magnetosonic disturbances do affect the global electric current flowing in the system, even in flat spacetime; beyond the light cylinder the perturbed charge density is on the order of the perturbed Goldreich-Julian charge density. This implies that the fast magnetosonic surface is a causal wind boundary. We then go on to study the evolution of a force-free system driven by a spinning-up black hole, by solving the time dependent Maxwell equations near the horizon using perturbation method. We find that the electromagnetic field, current and charge density grow exponentially in the linear regime. We conclude that after time $t=2M\ln(1/2α^2)$, where $α$ is the lapse function, the evolution will reach an adiabatic stage, whereby the solution is to a good approximation the static one, with the angular momentum of the hole being the adiabatic parameter. This implies that black hole magnetospheres are stable. A brief discussion on the relation between global current closure and causality in force-free systems is given at the end.

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arXiv2003

Gravitational radiation from gamma-ray bursts as observational opportunities for LIGO and VIRGO

Gamma-ray bursts are believed to originate in core-collapse of massive stars. This produces an active nucleus containing a rapidly rotating Kerr black hole surrounded by a uniformly magnetized torus represented by two counter-oriented current rings. We quantify black hole spin-interactions with the torus and charged particles along open magnetic flux-tubes subtended by the event horizon. A major output of Egw=4e53 erg is radiated in gravitational waves of frequency fgw=500 Hz by a quadrupole mass-moment in the torus. Consistent with GRB-SNe, we find (i) Ts=90s (tens of s, Kouveliotou et al. 1993), (ii) aspherical SNe of kinetic energy Esn=2e51 erg (2e51 erg in SN1998bw, Hoeflich et al. 1999) and (iii) GRB-energies Egamma=2e50 erg (3e50erg in Frail et al. 2001). GRB-SNe occur perhaps about once a year within D=100Mpc. Correlating LIGO/Virgo detectors enables searches for nearby events and their spectral closure density 6e-9 around 250Hz in the stochastic background radiation in gravitational waves. At current sensitivity, LIGO-Hanford may place an upper bound around 150MSolar in GRB030329. Detection of Egw thus provides a method for identifying Kerr black holes by calorimetry.

gr-qc↗
arXiv2003

Polarization of GRB via scattering off a relativistic sheath

GRB fireballs could be ensheathed by a relativistic, baryon-enriched outflow that would in selected directions scatter a polarized beam of gamma rays. The strong polarization recently announced by RHESSI is attainable by the scattering mechanism, though not most of the time. The scattering mechanism, when acting in addition to the synchrotron mechanism, could circumvent some of the predictions implied by a purely synchrotron origin for the polarization. The time profile of the scattered component could differ from that of the primary gamma rays.

astro-ph↗
arXiv2002

Theory and astrophysical consequences of a magnetized torus around a rapidly rotating black hole

(Abbrev.) We analyze the topology, lifetime, and emissions of a torus around a black hole formed in hypernovae and black hole-neutron star coalescence. The torus is ab initio uniformly magnetized, represented by two counter oriented current-rings, and develops a state of suspended accretion against a "magnetic wall" around the black hole. Magnetic stability of the torus gives rise to a new fundamental limit EB/Ek<0.1 for the ratio of poloidal magnetic field energy-to-kinetic energy. The lifetime of rapid spin of the black hole is effectively defined by the timescale of dissipation of black hole-spin energy in the horizon, and satisfies T= 40s (MH/7MSun)(R/6MH)^4(0.03MH/MT) for a black hole of mass MH surrounded by a torus of mass MT and radius R. The torus converts a major fraction Egw/Erot=0.1 into gravitational radiation through a finite number of multipole mass-moments, and a smaller fraction into MeV neutrinos and baryon-rich winds. At a source distance of 100Mpc, these emissions over N=2e4 periods give rise to a characteristic strain amplitude \sqrt{N}hchar=6e-21. We argue that torus winds create an open magnetic flux-tube on the black hole, which carries a minor and standard fraction Ej/Erot=1e-3 in baryon-poor outflows to infinity. We identify this baryon poor output of tens of seconds with GRBs with contemporaneous and strongly correlated emissions in gravitational radiation, conceivably at multiple frequencies. Ultimately, this leaves a black hole binary surrounded by a supernova remnant.

astro-ph↗
arXiv2002

Orphan GRB radio afterglows: Candidates and constraints on beaming

The number of orphan radio afterglows associated with gamma-ray bursts (GRBs) that should be detected by a flux limited radio survey, is calculated. It is shown that for jetted GRBs this number is smaller for smaller jet opening angle (theta), contrary to naive expectation. For a beaming factor f_b^{-1}=(theta^2/2)^{-1} = 500, roughly the value inferred by Frail et al. (2001) from analysis of afterglow light curves, we predict that between several hundreds to several thousands orphan radio afterglows should be detectable (over all sky) above 1 mJy at GHz frequencies at any given time. This orphan population is dominated by sources lying at distances of a few hundred Mpc, and having an age of ~1 yr. A search for point-like radio transients with flux densities greater than 6 mJy was conducted using the FIRST and NVSS surveys, yielding a list of 25 orphan candidates. We argue that most of the candidates are unlikely to be radio supernovae. However, the possibility that they are radio loud AGNs cannot be ruled out without further observations. Our analysis sets an upper limit for the all sky number of radio orphans, which corresponds to a lower limit f_b^{-1}>10 on the beaming factor. Rejection of all candidates found in our search would imply f_b^{-1}>100. This, and the fact that some candidates may indeed be radio afterglows, strongly motivate further observations of these transients.

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