Variability Models of Gamma-Ray Blazars

A large fraction of the AGNs detected by EGRET exhibit strong variability in many wavebands- from radio to the highest energy gamma-rays observed. The origin of the transient emission in blazars is presumably associated with relativistic jets. In this talk plausible variability mechanisms that may operate in blazars are discussed.Comment: 4 pages. To Appear in the Proceedings of the Turku Workshope on "BL Lac Phenomenon

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 $\gamma$-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.Comment: 8 pages, prc. "Blazar variability across the electromagnetic spectrum", Palaiseau, France, minor change

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 $\gamma$-ray observations. In particular, it is demonstrated how rapid variability of the $\gamma$-ray emission at very high energies ($> 100$ GeV) can be used to map the location of the $\gamma$-spheres, to derive lower limits on the Doppler factor of the $\gamma$-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 $\gamma$-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.Comment: 6 pages; to appear in "Extragalactic Jets: Theory and Observation from Radio to Gamma Ray", eds. T.A. Rector and D.S. De Young, ASP conf. pro

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 $\sqrt{l/\Gamma}$, where $\Gamma$ 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.Comment: 25 pages, post referee versio

TeV Emission by Ultra-High Energy Cosmic Rays in Nearby, Dormant AGNs

The curvature radiation produced by particles accelerating near the event horizon of a spinning supermassive black hole, threaded by externally supported magnetic field lines is considered. It is shown that light nuclei suffer catastrophic curvature losses that limit the maximum energy they can attain to values well below that imposed by the maximum potential difference induced by the black hole dynamo, unless the curvature radius of magnetic field lines largely exceeds the gravitational radius. It is further shown that the dominant fraction of the rotational energy extracted from the black hole is radiated in the TeV band. Given the observed flux of ultra-high energy cosmic rays, and the estimated number of nearby supermassive black holes, it is expected that if dormant AGNs are the sources of UHECRs, as proposed recently by Boldt & Ghosh, then they should also be detectable at TeV energies by present TeV experiments.Comment: 4 pages, uses REVTEX, submitted to Phys. Rev. Let

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 $\epsilon_c=3l_{dis}\,\sigma_0/4(\sigma_0+1)$, where $\sigma_0$ 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 $\Gamma_\infty\ge\Gamma_0(1+l_{dis}\,\sigma_0/4)$, where $\Gamma_0$ 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.Comment: 6 pages, 3 figures, submitted to MNRA

On the Viewing Angle Dependence of Blazar Variability

Internal shocks propagating through an ambient radiation field, are subject to a radiative drag that, under certain conditions, can significantly affect their dynamics and, consequently, the evolution of the beaming cone of emission produced behind the shocks. The resultant change of the Doppler factor combined with opacity effects leads to a strong dependence of the variability pattern produced by such systems, specifically, the shape of the light curves and the characteristics of correlated emission, on viewing angle. One implication is that objects oriented at relatively large viewing angles to the observer should exhibit a higher level of activity at high synchrotron frequencies (above the self-absorption frequency) and at gamma-ray energies below the threshold energy to pair production, than at lower (radio/millimeter) frequencies.Comment: 6 pages, accepted by MNRA

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 Alfv\'en 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.Comment: 11 pages, 5 figures, submitted to MNRA

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.Comment: 6 pages, 2 figures, to appear in the proceedings of the HEPRO 4 meetin

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.Comment: To appear in PRD. Minor changes after proof corrections. Figure 3 replace