Narrow Results

It has been hypothesized recently that core collapse supernovae are triggered by mildly relativistic jets following observations of radio properties of these explosions. Association of a jet, similar to a gamma-ray burst jet but only slower, allows shock acceleration of particles to high energy and non-thermal neutrino emission from a supernova. Detection of these high energy neutrinos in upcoming kilometer scale Cherenkov detectors may be the only direct way to probe inside these astrophysical phenomena as electromagnetic radiation is thermal and contains little information. Calculation of high energy neutrino signal from a simple and slow jet model buried inside the pre-supernova star is reviewed here. The detection prospect of these neutrinos in water or ice detector is also discussed in this brief review. Jetted core collapse supernovae in nearby galaxies may provide the strongest high energy neutrino signal from point sources.

A synthesis of the present knowledge on gamma-ray emission from the magnetosphere of a rapidly rotating neutron star is presented, focusing on the electrodynamics of particle accelerators. The combined curvature, synchrotron, and inverse-Compton emission from ultra-relativistic positrons and electrons, which are created by two-photon and/or one-photon pair creation processes, or emitted from the neutron-star surface, provide us with essential information on the properties of the accelerator — electric potential drop along the magnetic field lines. A new accelerator model, which is a mixture of traditional inner gap and outer gap models, is also proposed, by solving the Poisson equation for the electrostatic potential together with the Boltzmann equations for particles and gamma-rays in the two-dimensional configuration and two-dimensional momentum spaces.

Recent high-sensitivity observation of the nearby radio galaxy M87 has provided important insights into the central engine that drives the large-scale outflows seen in radio, optical and X-rays. This review summarizes the observational status achieved in the high energy (HE < 100 GeV) and very high energy (VHE > 100 GeV) gamma-ray domains, and discusses the theoretical progress in understanding the physical origin of this emission and its relation to the activity of the central black hole.

The direct detection of annihilation products in cosmic rays offers an alternative way to search for supersymmetric dark matter particles candidates. The study of the spectrum of gamma-rays, antiprotons and positrons offers good possibilities to perform this search in a significant portion of the Minimal Supersymmetric Standard Model parameters space. In particular the EGRET team have seen a convincing signal for a strong excess of emission from the galactic center that have not easily explanation with standard processes. We will review the achievable limits with the experiment GLAST taking into accounts the LEP results and we will compare this method with the antiproton and positrons experiments, the direct underground detection and with future experiments at LHC.

The extragalactic gamma-ray sky is dominated by two classes of sources: Gamma-Ray Bursts (GRBs) and radio loud active galactic nuclei whose jets are pointing at us (blazars). We believe that the radiation we receive from them originates from the transformation of bulk relativistic energy into random energy. Although the mechanisms to produce, collimate and accelerate the jets in these sources are uncertain, it is fruitful to compare the characteristics of both classes of sources in search of enlightening similarities. I will review some general characteristics of radio loud AGNs and GRBs and I will discuss the possibility that both classes of sources can work in the same way. Finally, I will discuss some recent exciting prospects to use blazars to put constraints on the cosmic IR-Optical-UV backgrounds, and to use GRBs as standard candles to measure the Universe.

The observational data for the Galactic source Cygnus X-3 collected with the SHALON mirror Cherenkov telescope are presented. The Cygnus X-3 binary have been regularly observed since 1995 with the average γ-ray flux F (E > 0.8 TeV) = (6.8 ± 0.7) × 10^-13cm^-2s^-1. The flux in year 2003 was (1.79 ± 0.33) × 10^-12cm^-2s^-1. Earlier, in 1997, an increase of the flux was also observed.

The gamma-quantum emitting objects in our Galaxy are the supernova remnants and binaries. Preliminary analysis of the data for γ-ray sources Tycho Brage and Geminga obtained with the SHALON γ-ray telescope is briefly discussed. The energy spectra of γ's from the Geminga and Tycho's sources, F (E > 0.8 TeV) ∝ E^k are found to be harder than the spectrum from Crab Nebula. The expected π⁰-decay γ-ray flux extends up to ~ 30 TeV, whereas the inverse Compton γ-ray flux has a cutoff above a few TeV. Hence, the detection of γ rays at energies ~ 10 – 30 TeV provides a hint of their hadronic origin.

The new distant metagalactic γ-sources 1739+522 (z = 1.375) and 3c454.3 (z = 0.857) are detected at energies E > 0.8 TeV with the fluxes (0.53 ± 0.10) × 10^-12cm^-2s^-1 and (0.43 ± 0.13) × 10^-12cm^-2s^-1, respectively. The γ-ray spectra and fluxes of known blazars Mkn 421, Mkn 501 and distant flat-spectrum radio quasars 1739+522 and 3c454.3 are presented.

Various aspects of the high-energy emission from relativistic jets associated with 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.

We consider that electromagnetic pulses produced in the jets of this innermost part of the accretion disk accelerate charged particles (protons, ions, electrons) to very high energies via wakefield acceleration, including energies above 10${}^{20}$ eV for the case of protons and nucleus and 10${}^{12-15}$ eV for electrons by electromagnetic wave-particle interaction. Thereby, the wakefield acceleration mechanism supplements the pervasive Fermi’s stochastic acceleration mechanism (and overcomes its difficulties in the highest energy cosmic ray generation). The episodic eruptive accretion in the disk by the magneto-rotational instability gives rise to the strong Alfvenic pulses, which acts as the driver of the collective accelerating pondermotive force. This pondermotive force drives the wakes. The accelerated hadrons (protons and nuclei) are released to the intergalactic space to be ultra-high energy cosmic rays. The high-energy electrons, on the other hand, emit photons to produce various non-thermal emissions (radio, IR, visible, UV, and gamma-rays) of active galactic nuclei in an episodic manner, giving observational telltale signatures.

The crucial observation on the occurrence of subpulses (overtones) in the Power Spectral Density of the August 27 (1998) event from SGR1900+14, as discovered by BeppoSAX,⁷ has received no consistent explanation in the context of the competing theories to explain the SGRs phenomenology: the magnetar and accretion-driven models. Based on the ultra-relativistic, ultracompact X-ray binary model introduced in the accompanying paper,²⁰ I present here a self-consistent explanation for such an striking feature. I suggest that both the fundamental mode and the overtones observed in SGR1900+14 stem from pulsations of a massive white dwarf (WD). The fundamental mode (and likely some of its harmonics) is excited because of the mutual gravitational interaction with its orbital companion (a NS, envisioned here as point mass object) whenever the binary Keplerian orbital frequency is a multiple integer number (m) of that mode frequency.²⁸ Besides, a large part of the powerful irradiation from the fireball-like explosion occurring on the NS (after partial accretion of disk material) is absorbed in different regions of the star driving the excitation of other multipoles,^25,26 i.e., the overtones (fluid modes of higher frequency). Part of this energy is then reemitted into space from the WD surface or atmosphere. This way, the WD lightcurve carries with it the signature of these pulsations inasmuch the way as it happens with the Sun pulsations in Helioseismology. It is shown that our theoretical prediction on the pulsation spectrum agrees quite well with the one found by BeppoSAX,⁷ a feature confirmed by numerical simulations (Montgomery & Winget 2000).

The evidence is reviewed that the primary form of energy that escapes to infinity from gamma-ray bursts (GRBs) is gamma-rays, and/or Poynting flux, and that the kinetic energy in ultrarelativistic baryons is a secondary component resulting from acceleration of baryons by radiation pressure near or beyond the photosphere. This could account for several observed characteristics of observed GRB spectra and light curves, such as the typical peak photon energy, the correlation of this peak with apparent GRB energy, and the profiles and spectral lagging of GRB subpulses.

The radio galaxy M87 has recently been found to be a rapidly variable TeV emitting source. We analyze the implications of the observed TeV characteristics and show that it proves challenging to account for them within conventional acceleration and emission models. We discuss a new pulsar-type scenario for the origin of variable, very high energy (VHE) emission close to the central supermassive black hole and show that magneto-centrifugally accelerated electrons could efficiently Compton upscatter sub-mm ADAF disk photons to the TeV regime, leading to VHE characteristics close to those observed. This suggests, conversely, that VHE observations of highly under-luminous AGNs could provide an important diagnostic tool for probing the conditions prevalent in the inner accretion disk of these sources.

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 H.E.S.S. and MAGIC in some blazars, that accommodates the relatively small Doppler factors inferred from radio observations.

Supercriticality of the same kind as that in a nuclear pile can take place in high-energy astrophysical objects producing a number of impressive effects. For example, it could cause an explosive release of the energy of a cloud of ultrarelativistic protons into radiation. More certainly, supercriticality should be responsible for energy dissipation of very energetic relativistic fluids such as ultrarelativistic shocks in gamma-ray bursts and jets in active galactic nuclei (AGNs). In this case, the photon breeding process operates. It is a kind of converter mechanism with the high-energy photons and e⁺e^- pairs converting into each other via pair production and inverse Compton scattering. Under certain conditions, which should be satisfied in powerful AGNs, the photon breeding mechanism becomes supercritical: the high-energy photons breed exponentially until their feedback on the fluid changes its velocity pattern. Then the system comes to a self-adjusting near-critical steady state. Monte-Carlo simulations with detailed treatment of particle propagation and interactions demonstrate that a jet with a Lorentz factor Γ ≈ 20 can radiate away up to a half of its total energy, and for Γ = 40 the radiation efficiency can be up to 80 per cent. Outer layers of the jet decelerate down to a moderate Lorentz factor 2–4, while the spine of the jet has a final Lorentz factor in the range 10–20 independent of the initial Γ. Such sharp deceleration under the impact of radiation must cause a number of interesting phenomena such as formation of internal shocks and an early generation of turbulence.

Photon breeding in relativistic jets involves multiplication of high-energy photons propagating from the jet into the external environment and back, with the conversion into electron-positron pairs. The exponential growth of the energy density of these photons is a supercritical process powered by the bulk energy of the jet. The efficient deceleration of the jet outer layers creates a structured jet morphology with a fast spine and slow sheath. In initially fast and high-power jets even the spine can be decelerated efficiently leading to very high radiative efficiencies of conversion of the jet bulk energy into radiation. The decelerating, structured jets have angular distribution of radiation significantly broader than that predicted by a simple blob model with a constant Lorentz factor. This reconciles the discrepancy between the high Doppler factors determined by the fits to the spectra of TeV blazars and the low apparent velocities observed at VLBI scales as well as the low jet Lorentz factors required by the observed statistics and luminosity ratio of Fanaroff-Riley I radio galaxies and BL Lac objects. Photon breeding produces a population of high-energy leptons in agreement with the constraints on the electron injection function required by spectral fits of the TeV blazars. Relativistic pairs created outside the jet and emitting gamma-rays by the inverse Compton process might explain the relatively high level of TeV emission from the misaligned jet in the radio galaxies. The mechanism reproduces basic spectral features observed in blazars including the blazar sequence (shift of the spectral peaks towards lower energies with increasing luminosity). The mechanism is very robust and can operate in various environments characterized by the high photon density.

The detection of TeV gamma-rays from LS 5039 and the binary pulsar PSR B1259–63 by HESS, and from LS I +61 303 and the stellar-mass black hole Cygnus X-1 by MAGIC, provides clear evidence of very efficient acceleration of particles to multi-TeV energies in X-ray binaries. These observations demonstrate the richness of nonthermal phenomena in compact galactic objects containing relativistic outflows or winds produced near black holes and neutron stars. I review here some of the main observational results on very high energy (VHE) γ-ray emission from X-ray binaries, as well as some of the proposed scenarios to explain the production of VHE γ-rays. I put special emphasis on the flare TeV emission, suggesting that the flaring activity might be a common phenomena in X-ray binaries.

LS I +61 303 is a puzzling object detected from radio up to very high-energy gamma-rays. Variability has recently been observed in its high-energy emission. The object is a binary system, with a compact object and a Be star as primary. The nature of the secondary and the origin of the gamma-ray emission are not clearly established at present. Recent VLBA radio data have been used to claim that the system is a Be/neutron star colliding wind binary, instead of a microquasar. We review the main views on the nature of LS I +61 303 and present results of 3D SPH simulations that can shed some light on the nature of the system. Our results support an accretion powered source, compatible with a microquasar interpretation.

M 87 is the first and brightest radio galaxy detected in the TeV regime. It is the closest extragalactic object showing variability and the only one that does not have its jet pointing toward the line of sight. The structure of the M 87 jet is spatially resolved in X-ray, optical and radio observations. Time correlation between the TeV flux and emission at other wavelengths provides a unique opportunity to localize the VHE emission process occurring in active galaxy nuclei. For 10 years, M87 has been monitored in the TeV band by imaging air Cherenkov telescopes (IACT) as well as in X-ray, optical and radio bands. In 2008, the three main IACTs, H.E.S.S./MAGIC/VERITAS, coordinated their observations in a joint campaign. In February, high TeV activities with rapid flares have been detected. Contemporaneously, M 87 was observed with high resolution instruments in the X-ray (Chandra) and Radio band (VLBA).

Cygnus X-1 was the first X–ray source widely accepted to be a black hole candidate, and remains among the most studied astronomical objects in its class. The detection of nonthermal radio, hard X–rays and gamma-rays reveals the fact that this kind of objects are capable of accelerating particles up to very high energies.

In order to explain the electromagnetic emission from Cygnus X-1 in the low-hard state we present a model of a black hole corona with both relativistic lepton and hadron content. We characterize the corona as a two-temperature hot plasma plus a mixed nonthermal population in which energetic particles interact with magnetic, photon and matter fields. Our calculations include the radiation emitted by secondary particles (pions, muons and electron–positron pairs). Finally, we take into account the effects of photon absorption. We compare the results obtained from our model with data of Cygnus X-1 obtained by the COMPTEL instrument.