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We analyze the active-galaxy correlation reported in 2007 by the Pierre Auger Collaboration. The signal diminishes if the correlation-function approach (counting all "source–event" pairs and not only "nearest neighbors") is used, suggesting that the correlation may reveal individual sources and not their population. We analyze available data on physical conditions in these individual correlated sources and conclude that acceleration of protons to the observed energies is hardly possible in any of these galaxies, while heavier nuclei would be deflected by the Galactic magnetic field thus spoiling the correlation. Our results question the Auger interpretation of the reported anisotropy signal but do not contradict to its explanation with intermediate-mass nuclei accelerated in Cen A.

The high-frequency-peaked BL Lac PKS 2155-304 is one of the brightest and best-studied VHE gamma-ray sources in the southern hemisphere. Since 2002, the High Energy Stereoscopic System (H.E.S.S.) has monitored this source and found it to be in an unusually high state in July 2006. On the nights of 28 and 30 July, two major outbursts occurred, with peak fluxes ~ 80 times the usual values and well-resolved structures varying on time scales of ~ 200 s. Here, we report on spectral variability studies of VHE data of the first flare and show first results of the H.E.S.S. observations during the second flare night.

On the night of 29–30 July 2006, a major γ-ray outburst occurred from PKS 2155-304, which was observed simultaneously with H.E.S.S., Chandra and the Bronberg optical telescopes. An overall 6–8 hours of simultaneous uninterrupted coverage was obtained. Here we report the first preliminary results of this exceptional dataset. The source showed correlated variability between the X-ray and VHE bands, with no lags. The spectra evolve with similar patterns. Huge VHE variations (~ 22×) are accompanied only by small-amplitude X-ray and optical variations (factor 2 and 15% respectively). The source has shown for the first time in an HBL a large Compton dominance (L_C/L_S ~ 10), and a cubic relation between VHE and X-ray flux variations, during a decaying phase. These results challenge all known "standard" scenarios for the blazar emission.

We present the results from multiwavelength campaigns of three powerful gamma-ray quasars, PKS 1510-089, RBS 315 and Swift J0746.3+2548, recently organized with Suzaku. The Suzaku observation provided one of the highest S/N X-ray spectra ever reported between 0.3 and 50 keV. For these quasars, the X-ray spectrum is well represented by an extremely hard power-law with photon index Γ ≃ 1.2, but is augmented by an additional soft component apparently below 1 keV for PKS 1510-089, whereas a strong deficit of soft photons is observed in RBS 315. We model the broadband spectra of these powerful quasars and argue that the power of the jet is dominated by protons but with the number of electrons/positrons exceeding the number of protons by a factor ≃ 10. We also argue that an extremely hard X-ray spectra may result from a double power-law form of the injected electrons, with the break energy γ_br ≃ 1000 corresponding to the anticipated threshold of diffusive shock acceleration.

The quasar 3C 279 was the target of an extensive multiwavelength monitoring campaign from January through April 2006, including an optical-IR-radio Whole Earth Blazar Telescope (WEBT) campaign and Target of Opportunity X-ray and soft γ-ray observations with Chandra and INTEGRAL in mid-January 2006, with additional X-ray coverage by RXTE and Swift XRT as well as independent very-high-energy (VHE) γ-ray observations by MAGIC, which led to the first-ever reported tentative detection of a quasar at VHE γ-rays. In this paper we summarize the results of the WEBT campaign.

The source exhibited substantial variability of optical flux and spectral shape, with a characteristic time scale of a few days. The variability patterns throughout the optical BVRI bands were very closely correlated with each other, while there was no obvious correlation between the optical and radio variability. In intriguing contrast to other (in particular, BL Lac type) blazars, we find a lag of shorter-wavelength behind longer-wavelength variability throughout the RVB wavelength ranges, with a time delay increasing with increasing frequency. Spectral hardening during flares appears delayed with respect to a rising optical flux. This, in combination with the very steep IR-optical continuum spectral index of α_o ~ 1.5 – 2.0, may indicate a highly oblique magnetic field configuration near the base of the jet, leading to inefficient particle acceleration and a very steep electron injection spectrum. An alternative explanation through a slow (time scale of several days) acceleration mechanism would require an unusually low magnetic field of B < 0.2 G, about an order of magnitude lower than inferred from previous analyses of simultaneous SEDs of 3C 279 and other FSRQs with similar properties.

Multi-frequency (4.6, 5, 5.5, 8, 8.8, 13, 15, 22 & 43 GHz) polarization observations of six "blazars" were obtained on the American Very Long Baseline Array (VLBA) over a 24-hr period on 2 July 2006. Observing at several frequencies, separated by short and long intervals, enabled reliable determination of the distribution of Faraday rotation on a range of scales. In all cases the magnitude of the RM increases in the higher frequency observations, implying that the electron density and/or magnetic field strength is increasing as we get closer to the central engine. After correcting for Faraday rotation, the polarization orientation in the jet is either parallel or perpendicular to the jet direction. A transverse rotation measure (RM) gradient was detected in the jet of 0954+658, providing evidence for the presence of a helical magnetic field surrounding the jet. For three of the sources (0954+658, 1418+546, 2200+420), the sign of the RM in the core region changes in different frequency-intervals, indicating that the line-of-sight component of the magnetic field is changing with distance from the base of the jet. We suggest an explanation for this in terms of bends in a relativistic jet surrounded by a helical magnetic field; where there is no clear evidence for pc-scale bends, the same effect can be explained by an accelerating/decelerating jet.

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.

M 87 is the first extragalactic source detected in the TeV γ-ray domain that is not a blazar, its large scale jet not being aligned to the line of sight. We present here a multi-blob synchrotron self-Compton model accounting explicitly for large viewing angles and moderate Lorentz factors as inferred from magnetohydrodynamic simulations of jet formation, motivated by the detection of M 87 at very high energies (VHE; E > 100 GeV). Predictions are presented for the very high-energy emission of active galactic nuclei with extended optical or X-ray jet, which could be misaligned blazars but still show some moderate beaming. We include predictions for 3C 273, Cen A and PKS 0521–36.

In powerful cosmic nonthermal radiation sources with dominant magnetic-field self generation, the generation of magnetic fields at almost equipartition strength by relativistic plasma instabilities operates as fast as the acceleration or injection of ultra-high energy radiating electrons and hadrons in these sources. Consequently, the magnetic field strength becomes time-dependent and adjusts itself to the actual kinetic energy density of the radiating electrons in these sources. This coupling of the magnetic field and the magnetic field energy density to the kinetic energy of the radiating particles changes both the intrinsic temporal evolution of the relativistic particle energy spectrum after injection and the synchrotron and synchrotron self-Compton emissivities.

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.

Spectral line profiles produced in an outflow near a neutron star or a black hole can be strongly influenced by gravitational redshifting and by Doppler shifting due to a global motion of plasma. We consider a scenario in which a resonant absorption in a spectral line takes place in the outflowing plasma within several tens of Schwarzschild radii from a compact object. The main goal of this work is to show that under certain conditions a combination of the gravitational redshifting and Doppler blue/redshifting may produce line profiles which can be considered as "fingerprints" of the gravitational field of the compact object, much as P-Cygni profiles are "fingerprints" of stellar winds.

We introduce the use of a well-known parameter, the Alfvén Radius, R_A, as a new tool to discern whether an X-ray binary system may undergo a microquasar phase, i.e. ejecting relativistic particles orthogonal to the accretion disk. We study what we call the basic condition, R_A/R_* = 1 in its dependency on the magnetic field strength and the mass accretion rate. With this basic condition we establish under which combination of parameters any class of accreting neutron stars could become a microquasar instead of confining disk-material down to the magnetic poles and creating the two emitting caps typical for an X-ray pulsar. In the case of black-hole accreting binaries we equate the magnetic field pressure to the plasma pressure in the last stable orbit (i.e. R_A/R_LSO = 1) and we get upper limits for the magnetic field strength as a function of the mass accretion rate and the black hole mass.

The current very high energy (VHE; E > 100 GeV) experiments have tremendously increased the number of detected extragalactic sources. We present a synchrotron self-Compton modeling tour of the active galactic nuclei currently established as VHE emitters so far, and investigate possible correlations among the intrinsic and derived parameters.

The leptonic model has become the standard method of fitting the multiwavelength spectra of blazars. In what follows we review the basic premises of the model, giving emphasis on the evolution of its key ideas and assumptions.

We have compared the parsec-scale jet linear polarization properties of the Fermi LAT-detected and non-detected sources in the complete flux-density-limited (MOJAVE-1) sample of highly beamed AGN. Of the 123 MOJAVE sources, 30 were detected by the LAT during its first three months of operation. We find that during the era since the launch of Fermi, the unresolved core components of the LAT-detected jets have significantly higher median fractional polarization at 15 GHz. This complements our previous findings that these LAT sources have higher apparent jet speeds, brightness temperatures and Doppler factors, and are preferentially found in higher activity states.

We investigate the spectral energy distribution (SED) of Centaurus A resulting from a steady compact acceleration region, located close to the central black hole, where both leptonic and hadronic relativistic populations arise. We present here results of such a model, where we have considered synchrotron radiation by primary electrons and protons, inverse Compton scattering, and gamma-ray emission originated by the inelastic hadronic interactions between relativistic protons and cold nuclei within the jets. Photo-meson production by relativistic hadrons were also taken into account, as well as the effects of secondary particles injected by all interactions. The internal and external absorption of gamma-rays is shown to be of great relevance to shape the observable SED, which was also recently constrained by the results of Fermi and HESS.

We present a sequence of 12 monthly polarimetric multi-frequency VLBA observations of the radio galaxy 3C 120. The motion of multiple superluminal components allows the mapping of the polarization structure along most of the jet and across its width, revealing a coherent in time Faraday screen and RM-corrected polarization angles. Gradients in Faraday rotation and degree of polarization across the jet are observed, together with a localized region of high rotation measure superposed on this structure. This is explained as produced by the presence of a helical magnetic field in a two-fluid jet model, consisting of an inner emitting jet and a sheath containing nonrelativistic electrons. Interaction of the jet with the external medium would explain the confined region of enhanced Faraday rotation.

Modeling implications of recent VERITAS discoveries of Intermediate BL Lac Objects (IBLs) are presented. Leptonic jet models for the IBLs W Comae (z = 0.102) and 3C 66A (z = 0.444) are, in principle, viable with only synchrotron and synchrotron self-Compton (SSC) components, but more plausible parameters can be achieved including an external infrared radiation field as source for Compton upscattering to produce the observed VHE gamma-ray emission. The unknown redshift of PKS 1424+240 makes a theoretical interpretation difficult. A pure SSC model seems to be sufficient to represent its SED, and modeling results favor a low redshift of z ≲ 0.1.