Narrow Results

Most of the electromagnetic output of blazars (BL Lac objects and Flat Spectrum Radio Quasars) comes out in the γ–ray band, making the Large Area Telescope [0.1–100 GeV] onboard the Fermi satellite and the Cherenkov telescopes crucial for gather crucial data and thus to understand their physics. These data are complemented by the observations of the Swift satellite in the X–ray and optical–UV bands, and by ground based optical and radio telescopes. This rich coverage of the spectrum allows a robust modelling, from which important trends start to emerge. In powerful sources we see the contribution of the accretion disk that, once modeled, give us the black hole mass and the accretion rate. Even when not directly visible, the disk luminosity can be derived through the broad emission lines. Therefore we start to know the jet power, the disk luminosity, and the black hole mass, 3 crucial ingredients if we want to draw a general scenario. At the start, jets are believed to be magnetically dominated. And yet, on the scale where they emit most of their luminosity, their power is already in the form of kinetic energy of particles. Relativistic jets are formed for a very broad range of the disk luminosity, from close to Eddington down to at least 10^-4 Eddington. Their power correlates with the accretion rate, and can be even more powerful than the accretion disk luminosity.

The problem of the hard (intrinsic) γ-ray source spectra in TeV blazars is examined. We show that relativistic Maxwell-like particle distributions provide a suitable interpretation for the inferred hard, very high energy (VHE) photon spectra. We discuss the potential of these distributions to produce broader and softer spectra if multiple zones contribute to the observed emission. We show that the dominance of one component could lead to a VHE flaring state with hard spectral features, like the one observed for Mkn 501 in July 2009.

More than fifty extragalactic very high energy (VHE; E > 100 GeV) sources have been found using ground-based imaging atmospheric Cherenkov telescopes, about twenty of which have been discovered using the H.E.S.S. (High Energy Stereoscopic System) experiment based in Namibia. Even though BL Lac objects are the dominant class of VHE detected extragalactic objects, other types of sources (starburst galaxies, radio galaxies or flat spectrum radio quasars) begin to emerge. A review of the extragalactic sources studied with H.E.S.S. is given, with an emphasis on new results.

Diffusive shock acceleration (DSA) at relativistic shocks is likely to be an important acceleration mechanism in various astrophysical jet sources, including radio-loud AGN. An important recent development for blazar science is the ability of Fermi-LAT data to pin down the power-law index of the high energy portion of emission in these sources, and therefore also the index of the underlying non-thermal particle population. This diagnostic potential was not possible prior to Fermi launch, when gamma-ray information was dominated by the highly-absorbed TeV band. This paper highlights how multiwavelength spectra including X-ray band and Fermi data can be used to probe diffusive acceleration in relativistic, oblique, MHD shocks in blazar jets. The spectral index of the non-thermal particle distributions resulting from Monte Carlo simulations of DSA, and the fraction of thermal particles accelerated to non-thermal energies, depend sensitively on the particles' mean free path scale, and also on the magnetic field obliquity to the shock normal. We investigate self-consistently the radiative synchrotron/Compton signatures of the resulting thermal and non-thermal particle distributions. Important constraints on the frequency of particle scattering and the level of field turbulence are identified for the blazar AO 0235+164. The possible interpretation that turbulence levels decline with remoteness from jet shocks, and a significant role for non-gyroresonant diffusion, are discussed.

We present the neutrino and UHECR spectra obtained from a detailed fitting of the spectral energy distribution (SED) of Mrk 421 (March 2001) using two variations of the leptohadronic model. In particular, while the low-energy component (optical to X-rays) of the SED is fitted by synchrotron emission of primary electrons in both models, the high-energy one (GeV-TeV gamma-rays) is synchrotron emission attributed either to ultra-high energy protons (LHs model) or to secondary electrons produced by the decay of charged pions (LHπ model). In the LHπ case we find that the produced neutrino spectra are sharply peaked at E_ν ~ 30 PeV with a peak flux slightly below the IC-40 sensitivity limit for Mrk 421. In the LHs model, on the other hand, the neutrino spectra fall well outside the PeV energy range, but the calculated E ~ 30 EeV — UHECR flux at earth is close to that observed by HiresI, Telescope Array and Pierre Augere experiments.

After updating the status of the measurements of the cosmic neutrino flux by the IceCube experiment, we summarize the observations of the first identified source of cosmic rays and speculate on the connection between the two observations.

PKS 1510-089 (z = 0.361), one of only a handful of flat spectrum radio quasars detected in the very high energy (VHE, E > 100 GeV) γ-rays, is known for its flux variability and complex multiwavelength behaviour. VHE observations by H.E.S.S. and MAGIC in May 2016 detected an unprecedented flare, both in intensity and in the shortness of its variability timescale. The flare lasted less than 48 hours, during which time the flux reached about 80 per cent of the Crab Nebula flux above 200 GeV. In addition, the intranight variability of this source was detected for the first time. Simultaneous observations in high energy (HE, E > 100 MeV) γ-rays performed with Fermi-LAT and optical R-band performed with ATOM show behaviour not consistent with simple simultaneous brightening in all bands. While a significant hardening of the spectrum is visible in HE, the flux increased only moderately. A simultaneous rise in daily-averaged optical flux was seen in the R-band. However, the intranight R-band flux evolution shows two prominent peaks, while only one is visible in the VHE range. These intriguing features of the flare will be presented in detail. We will also discuss possible explanations for the observed emission.

Different observers have estimated drastically different bulk Lorentz factors and Doppler factors for the jets of the TeV blazars, depending on the method used. On the one hand, rapid variability at high energies on timescales of a few minutes and fits to the spectral energy distributions (SEDs) imply large Doppler factors approaching 100. On the other hand, jet morphologies, kinematics, and brightness temperatures measured from parsec-scale imaging with the Very Long Baseline Array (VLBA) imply at most modest Doppler boosting. Such modest Doppler boosting is also suggested by unification arguments. This so-called Doppler crisis may indicate the presence of structured jets in these sources, with different regions of different Lorentz factors, for example, a fast spine and a slower layer. We discuss the constraints on such structured jet models from our latest observations of these sources with the VLBA.