Venue · Activities


Scientific Program – Abstracts

The H.E.S.S. extragalactic sky

Martin Raue (University of Hamburg) for the H.E.S.S. Collaboration

The H.E.S.S. Cherenkov telescope array, located on the southern hemisphere in Namibia, studies very high energy (VHE; E>100 GeV) gamma-ray emission from astrophysical objects. During its successful operations since 2002 more than 70 galactic and extra-galactic gamma-ray sources have been discovered. H.E.S.S. devotes over 300 hours of observation time per year to the observation of extra-galactic sources resulting in the discovery of over ten new sources, mostly AGNs, and in exciting physics result e.g. the discovery of very rapid variability during extreme flux outburst of PKS 2155-304, stringent limits on the density on the density of the extragalactic background light (EBL) in the near-infrared derived from the energy spectra of distant sources, or the discovery of short-term variability in the VHE emission from the radio galaxy M 87. With the recent launch of the Fermi satellite in 2008 new insights into the physics of AGNs at GeV energies emerged leading to discovery of several new extragalactic VHE sources. Multi-wavelength observations proof to be a powerful tool to investigate the production mechanism for VHE emission in AGNs. Here, new results from H.E.S.S. observations of extragalactic sources will be presented and their implications will be discussed.

VERITAS Very High Energy Gamma-ray Observations of Galaxies

Amy Furniss (University of California Santa Cruz)

Remarkable progress has been made in very high energy (VHE; E > 100 GeV) gamma-ray astrophysics in the last decade. The VHE source catalog has increased tenfold, with a wide variety of source classes, and includes more than 30 extragalactic objects. Currently, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) has made observations of numerous source candidates, both galactic and extragalactic, including more than 100 galaxies. The last 18 months of observations are especially noteworthy as they have been completed in a new array configuration, following the relocation of one of the four 12 meter telescopes. This new configuration has yielded a significant increase in the VERITAS sensitivity to gamma-ray sources above 100 GeV, resulting in several new source detections in the first few months of operations alone. To date, the VERITAS extragalactic source catalog includes various AGN, the radio galaxy M87 and the starburst galaxy M82 (the latter was detected using data obtained under the old array configuration). Highlights from the VERITAS extragalactic observation program will be presented, with a focus on the most recent discoveries.

MAGIC review

Elina Lindfors, Tuorla Observatory, University of Turku (on behalf of the MAGIC collaboration)

MAGIC is a system of two 17m Cherenkov telescopes, sensitive to very high energy (VHE) gamma-rays between 50 GeV up to several tens of TeV. MAGIC observations of Active Galactic Nuclei (AGN) have resulted in the discovery of many previously unknown VHE gamma-ray emitting sources adding also new source types to the list of extragalactic gamma-ray emitters as well as new constraints for the emission models. Many of the discoveries have been triggered by the optical high state of the source as reported by the Tuorla blazar monitoring program, in several cases the targets were also listed as promising candidates by the Fermi collaboration based on their flux and spectrum. In this talk I discuss the new discoveries as well as the new constraints for emission models of AGN derived from recent MAGIC observations.

A review of the multiwavelength studies on the blazars detected by AGILE

Immacolata Donnarumma (INAF-IASF Rome)

We report on the main results about gamma-ray blazars achieved by AGILE during almost 4 years spent in orbit. AGILE detected several flaring blazars, covering the entire sequence from FSRQ to HBL objects and, thanks to the rapid dissemination of our alerts, we have been able to collect multiwavelength data from radio to TeV energy bands thanks to the sinergy with other observatories such as GASP-WEBT, REM, Spitzer, Swift, RXTE, XMM, Suzaku, INTEGRAL, MAGIC, VERITAS. This set of simultaneous data allowed us to study the multiwavelength variability and to build up the spectral energy distribution of these blazars from radio to gamma-ray energy bands. The most relevant properties of our sample of blazars will be presented with a particular emphasis on the spectral thermal components, time lags,spectral trends (in X-rays and gamma-rays), jet geometry and acceleration mechanism at the inner portion of the jet itself.

XMM-Newton and SWIFT observations of High Frequency Peaked BL Lacs in the TeV era

Francesco Massaro (Smithsonian Astrophysical Observatory)

Most of TeV detected extragalactic sources are BL Lac objects. They belong to the subclass of ``high frequency peaked BL Lacs" (HBLs) exhibiting their spectral energy distributions with a lower energy peak in the X-ray band; this is interpreted as synchrotron emission. The X-ray spectra are generally curved, and well described in terms of a log-parabolic shape. In a previous investigation of TeV HBLs (TBLs) we found two correlations between their spectral parameters: (1) The synchrotron peak luminosity L_p increases with its peak energy E_p, (2) the curvature parameter b decreases as E_p increases. The first is consistent with the synchrotron scenario, while the second is expected with statistical/stochastic acceleration mechanisms for the emitting particles. Here we present an extensive X-ray analysis of a sample of HBLs observed with XMM Newton and SWIFT but not detected at TeV energies (NBLs), to compare their spectral behavior with that of TBLs. We investigate the distributions of their spectral parameters, providing an interpretation of both the E_p and b distributions in terms of systematic and stochastic acceleration processes. We also compare the X-ray spectral behavior of NBLs with that in gamma-rays and propose a criterion to select the best HBLs candidates for future TeV observations.

CTA: the next generation

Biland Adrian (ETH Zurich)

The Cherenkov Telescope Array (CTA) will become the next generation ground based gamma-ray observatory. The goal is to increase sensitivity and energy coverage by about one order of magnitude compared to current installations. In this talk I will present the current status of the project and discuss the possibilities for future observations of extragalactic sources.

New observational opportunities with the GAMMA-400 space observatory.

Sergey Suchkov (Lebedev Physical Institute)

After the Fermi/LAT gamma-ray and electron-positron results the additional problems have arisen. To solve them it is necessary a new instrument with wider energy range and improved characteristics (angular and energy resolutions, rejection factor). The GAMMA-400 gamma-ray telescope will correspond to these requirements. The best angular and energy resolutions, as well as identification capabilities (angular resolution ~0.01?, energy resolution ~1%, proton rejection factor ~106) will allow us to study the main galactic and extragalactic sources, diffuse gamma-ray background, gamma-ray bursts, to measure fluxes of electrons, positrons, and nuclei and give information on Dark Matter.

Gamma-ray Laue lenses under development for deep Blazar observations

Filippo Frontera (University of Ferrara, Physics Dept)

We will present the development status of Laue lenses at the Univerisity of Ferrara and science case related to this development. The Laue lenses we are developing have a broad passband (70/100-600 keV) and are particularly suitable for eventual deep studies of AGNs, in particular Blazars, whose spectra peak in the Laue lens passband.

Leptonic Models Overview

Charles Dermer, NRL

Hadronic Models Overview

Anita Reimer, University of Innsbruck

Radio Observations Overview

Matthias Kadler, Erlangen


Marc Türler, ISDC

The Caltech Blazar Monitoring Program: Status and Future Plans

Oliver King (Caltech)

The OVRO 40 m Telescope monitoring program currently carries out twice-weekly measurements of the 15 GHz flux density of nearly 1600 blazars and other AGN, including all those associated with northern (declination > -20 degrees) Fermi Large Area Telescope (LAT) detections and a preselected sample ideal for statistical studies. I will present some results from the program, describe the statistical method we have developed, and describe an upgrade to this program which will increase the sensitivity of the receiver and add the capability to measure linear polarization strength and position angle. I will also discuss a proposed program to provide high-cadence monitoring of the linear polarization of >100 gamma-ray loud blazars at optical wavelengths.

Gamma-ray properties and Faraday rotation in MOJAVE blazar jets

Talvikki Hovatta Purdue University

Monitoring Of Jets in Active galactic nuclei with the VLBA Experiments (MOJAVE) observes ~300 AGN at 15 GHz to monitor changes in total intensity and polarization at parsec scales. Since the launch of the Fermi satellite, we have added new gamma-ray bright sources to our radio-selected sample to enable us study the connection between gamma-ray and radio emission in AGN using statistically complete subsamples. I will present some preliminary results on our study of radio to gamma-ray correlation in these jets. Multi frequency polarimetry imaging data provide spatial information on the intrinsic magnetic field structure within the jet as well as on the amount of surrounding plasma. Observations at 8.1-15.3 GHz obtained during 2006 have yielded RM maps for 191 sources which have been used to derive the true magnetic field configuration in the jets, and to place limits on the plasma surrounding the jets. We find the core rotation measures in quasars to be significantly higher than in the BL Lac objects and to decrease with distance from the core. Our results support models in which BL Lacs have less material around their central engines, which in turn are important for models of gamma-ray production mechanisms. By comparing our observations with recent simulations in the literature, we find indications of toroidally-dominated large scale magnetic fields surrounding the jet in only a few sources in the whole sample.

Broad band radio monitoring of gamma-ray bright Blazars: the first four years of the F-GAMMA program

Ioannis Nestoras Max-Planck-Institut f¸r Radioastronomie

The F -GAMMA program is a coordinated effort of several observing facilities to understand the AGN/blazar phenomenon via a multi-frequency approach, especially in the era of Fermi-GST. Since 2007, we are performing this Fermi-dedicated monthly monitoring program and collecting (quasi-) simultaneous and high-precision broad-band (cm/mm/sub-mm) variability data for about 65 gamma-ray blazars. The participating observatories are the Effelsberg 100-m, the IRAM 30-m and the APEX 12-m telescopes.

Here, we present the first four years of observations, data analysis as well as the results obtained so far. In particular, the statistical properties of the sample are presented with emphasis on the variability/spectral characteristics including variability times scales, amplitudes, spectral evolution and corresponding physical parameters in comparison to some of their gamma-ray properties.

A phenomenological classification of radio cm/mm band spectral variability in gamma-ray blazars

Lars Fuhrmann Max-Planck-Institut fuer Radioastronomie, Bonn

The F-GAMMA program comprises a concise and regular multi-frequency monitoring program at cm/mm/sub-mm wavelength for a sample of about 65 Fermi-detected gamma-ray blazars. Despite the wealth of scenarios that have been proposed to explain the mechanisms that cause the observed variability in blazars, the F-GAMMA program has disclosed that all apparent multifold of variability patterns can be unified to only two fundamentally different fashions: (i) spectral evolution dominated variability where there exist strong evidence for a smooth evolution of the physical parameters that cause the spectrum to evolve, (ii) almost achromatic variability in which case the spectrum changes self-similarly with only a minor, almost negligible drift of the peak frequency. Here, we present these phenomenological findings using the first four years of F-GAMMA data. Our results imply that only two physical mechanisms produce the variability in our monitored sample of gamma-ray blazars.

Investigating the effects of finite resolution on observed transverse jet profiles

Eoin Murphy University College Cork

Both the emission properties and evolution of Active Galactic Nuclei (AGN) radio jets are dependent on the magnetic fields that thread them. Faraday Rotation gradients are a very important way of investigating these magnetic fields, and can provide information on the orientation and structure of the magnetic field in the immediate vicinity of the jet; for example, a toroidal or helical field component should give rise to a systematic gradient in the observed Faraday rotation across the jet, as well as characteristic intensity and polarization profiles. However, real observed radio images have finite resolution, usually expressed via convolution with a Gaussian beam whose size corresponds to the central lobe of the point source response function. This will tend to blur transverse structure in the jet profile, raising the question of how well a jet must be in the transverse direction in order to reliably detect transverse structure. We will present the results of simulated intensity, polarization and Faraday rotation images designed to directly investigate the effect of finite resolution on observed transverse jet structures.


Bagmeet Behera (DESY Zeuthen, Germany)

Flat Spectrum Radio Quasars have been established as a new VHE emitting blazar class in the last few years. Three FSRQs have been discovered till date, 3C 279 and PKS 1222+21 by MAGIC, and PKS 1510-089 by H.E.S.S.

The detection of VHE emission gives an unique perspective on the emission mechanism in FSRQs, which typically have steeper spectra in the gamma ray regime, as compared to BL Lac objects. These detections allow us to constrain the location of the blazar zone by intrinsic opacity arguments. Multiwavelength data on these objects reveal furthermore that a complex mechanism is responsible for the non thermal emission.

VHE emission is the key in unraveling the properties of the underlying particle population. In principle the larger redshift of these sources can help better understand the extragalactic background in the UV-IR regime. FSRQs will indeed be the most distant persistent sources of VHE emission that we can regularly monitor with the future generation of Cherenkov instruments.

Recent optically triggered AGN observations with the MAGIC telescope

Elina Lindfors (Tuorla Observatory, University of Turku, Finland) on behalf of the MAGIC collaboration

Since the beginning of its scientific operations MAGIC has been performing optically triggered observations of AGN. These observations have resulted in discoveries of new VHE gamma-ray emitting AGN, most recently of two high peaking BL Lac objects B3 2247+381 in October 2010 and 1ES 1215+303 in January 2011. In addition already known VHE gamma-ray emitters 3C279 and 1ES0806+524 have been observed in high optical state. In this talk I will report the results of the MAGIC and simultaneous optical and X-ray observations.

The first direct observational proof that the blazar zone lies inside the broad-line region

Juri Poutanen (University of Oulu)

GeV spectra of the brightest blazars detected by the Fermi Gamma-ray Space Telescope cannot be described by a simple power law model. A much better description is obtained with a broken power law, with the break energies of a few GeV. We show that the sharpness and the position of the breaks can be well reproduced by absorption of gamma-rays via photon--photon pair production on He II and H I Lyman recombination continuum and lines. This is the first direct observational proof that the blazar zone lies inside the broad-line region (BLR) within a few light-months from a super-massive black hole. This also implies that the jet is fully accelerated to relativistic velocities at 1000 Schwarschild radii from the black hole.

The Spectral Energy Distributions and some physical properties of Fermi bright blazars

Fan Li, Shanghai Astronomical Observatory

Using public Fermi/LAT and Swift/BAT observations with a sample of 70 blazars selected from the LAT Bright AGN Sample(LBAS) and a one-zone leptonic model, we fitted their SEDs and getted the distribution of the derived physical parameters. Studying their spectral properties, we find that FSRQs have many different physical characteristics with BL Lac objects such as magnetic field, break energy, Doppler factor, electron energy distribution, etc. Jet power of the sources are discussed in this paper. The SEDs allow us to estimate the total luminosity and write it as a function of observed radio or gamma-ray luminosity. We found the perfect equality relations between the total luminosity and the injected power of relativistic electron above break energy as measured in the comoving frame. This provides a strong evidence for the theories about the radiation cooling mechanism and the production of break power law of electron energy distribution. The anti-correlation between break energy and the energy density U_B+U_r is still valid, which support the existence of blazar sequence, but our result tends to give a similar slops for FSRQs and BL Lac objects. Combining the statistical results, we discussed the causes of break energy formation and the physical principle of blazar sequence. We prove that break energy is a boundary that the integral power of injected relativistic electrons is equal to the total radiation power above it, but is greater than the total power of radiation under it.

Implications on the blazar sequence from the Fermi bright blazars

Liang Chen, Shanghai Astronomical Observatory

We used the quasi-simultaneous spectra of Fermi bright blazars and Fermi detected narrow line Seyfert 1 (NLS1) to study the blazar sequence. The synchrotron peak luminosities ($L_{s}$) inverse correlate with the synchrotron peak frequencies ($\nu_{s}$), which is consistent with the blazar sequence. In addition to the correlation, there are some blazars showing low peak frequency low luminosity. To study the relation between these low $\nu_{s}$ low $L_{s}$ blazars and the blazar sequence, we present correlations between the parameter $L_{s}\nu_{s}^{1/4}$ and the ratio of Compton to synchrotron peak frequencies ($r_{Cs}\equiv\nu_{C}/\nu_{s}$) and the ratio of Compton to synchrotron luminosities ($CD\equiv L_{C}/L_{s}$). The results indicate that they correlate with each other. This do not support the idea that the low $\nu_{s}$ low $L_{s}$ blazars are sources with less beaming. Another possibility, as suggested by Ghisellini \& Tavecchio, is that these blazars have relative lower black hole masses. To test this, we collect the black hole masses of 30 blazars from archives, and find that the hole mass correlates with the parameter $L_{s}\nu_{s}^{0.44}$. Therefore, the black hole of low $\nu_{s}$ low $L_{s}$ blazars are statistically small. The NLS1s are thought to have lower black hole masses. We find that the four NLS1s detected by \emph{Fermi} have low $\nu_{s}$ and low $L_{s}$. This supports the above result.

Rapid TeV variability in Blazars: Proton-Synchrotron Radiation of blobs produced by Jet-Star Interaction

Maxim Barkov (MPI für Kernphysik, Heidelberg)

We propose a new self-consistent hydrodynamical model for description of ultra-short flares of TeV blazars by compact magnetized condensations (blobs) produced when red giant stars cross the jet close to the central black hole. Our study includes a hydrodynamical treatment of evolution of the envelope lost by the star in the jet, and its high energy nonthermal emission through different leptonic and hadronic radiation mechanisms. We show that the fragmented envelope of the star can be accelerated to Lorentz factors up to 100 and radiate effectively the available energy in gamma-rays predominantly through proton synchrotron radiation. The model can readily explain the minute-scale TeV flares on top of longer (typical time-scales of days) gamma-ray variability as observed from the blazar PKS 2155$-$304, and constrain the key parameters of sources such as the mass of the central black hole $M_{\rm BH}\approx 10^8 M_{\odot}$, the total jet power $L_{\rm j} \approx 1.7\times 10^{47} \ \rm erg/s$ and the Doppler factor of the gamma-ray emitting blobs $\delta\geq 50$.

Constraints given by the MAGIC discovery of the Flat Spectrum Radio Quasar PKS1222+21 in VHE Gamma rays

Josefa Becerra Gonzalez (Instituto de AstrofÌsica de Canarias)

The MAGIC telescopes discovered very high energy (VHE, E>100 GeV) gamma-ray emission coming from the distant Flat Spectrum Radio Quasar (FSRQ) PKS 1222+21 (4C +21.35, z=0.432). It is the second most distant VHE gamma-ray source, with well measured redshift, detected until now. The observation was performed on 2010 June 17 (MJD 55364.9) using the two 17 m diameter imaging Cherenkov telescopes on La Palma (Canary Islands, Spain). The MAGIC detection coincides with high energy MeV/GeV gamma-ray activity measured by the Large Area Telescope (LAT) on board the Fermi satellite. The averaged integral flux above 100 GeV is equivalent to 1 Crab Nebula flux. The VHE flux measured by MAGIC varies significantly within the 30 minutes of exposure implying a flux doubling time of about 10 minutes. The VHE and MeV/GeV spectra, corrected for the absorption by the extragalactic background light, can be described by a single power law with photon index 2.72 ± 0.34 between 3 GeV and 400 GeV, consistent with gamma-ray emission belonging to a single component in the jet. The absence of a spectral cutoff constrains the gamma-ray emission region to lie outside the broad line region, which would otherwise absorb the VHE gamma-rays. Together with the detected fast variability, this challenges present emission models from jets in FSRQs.

Multiple synchrotron self-Compton modeling of gamma-ray flares in 3C 279

Marc Türler (ISDC, University of Geneva, Switzerland)

The correlation often observed in blazars between optical-to-radio outbursts and gamma-ray flares suggests that the high-energy emission measured by Fermi is closely linked to the synchrotron component. As the latter is most likely emitted by shock waves propagating several parsecs away from the black hole, there is growing evidence that the seeds for inverse-Compton gamma-ray production must be synchrotron photons rather than optical radiation from the accretion disk, the dust torus or the broad-line region, which are probably too far away. For a low-energy peaking blazar like 3C 279, it has however been shown that such a synchrotron self-Compton (SSC) scenario is not able to account for the observed GeV gamma-ray emission. While this is true for single scatterings, the effect of multiple-inverse Compton orders has been considered recently to solve this apparent paradox. Here, we show for the first time a physical, self-consistent SSC modeling of a series of shock-waves in the jet of 3C 279. The analytic description of the high-energy emission from multiple inverse-Compton scatterings in the Klein-Nishina limit is found to reproduce well the observed gamma-ray spectrum of 3C 279 in flaring state. The implications for very-high energy emission in the TeV range and on the origin of the blazar sequence are discussed.

gamma-ray flaring emission in OJ287 location in the jet at more than 14pc from the SMBH

Iv·n Agudo Instituto de AstrofÌsica de AndalucÌa (CSIC)

We combine time-dependent multi-waveband flux and linear polarization observations with submilliarcsecond-scale polarimetric images at λ = 7 mm of the BL Lacertae type blazar OJ287 to locate the γ-ray emission in prominent flares in the jet of the source > 14 pc from the central engine. We demonstrate a highly significant correlation between the strongest γ-ray and millimeter-wave flares through Monte Carlo simulations. The two reported γñray peaks occurred near the beginning of two major millimeter-wave outbursts, each of which is associated with a linear polarization maximum at millimeter wavelengths. Our very long baseline array observations indicate that the two millimeter-wave flares originated in the second of two features in the jet that are separated by > 14 pc. The simultaneity of the peak of the higher-amplitude γ-ray flare and the maximum in polarization of the second jet feature implies that the γ-ray and millimeter-wave flares are cospatial and occur > 14 pc from the central engine. We also associate two optical flares, accompanied by sharp polarization peaks, with the two γñray events. The multi-waveband behavior is most easily explained if the γ-rays arise from synchrotron self Compton scattering of optical photons from the flares. We propose that flares are triggered by interaction of moving plasma blobs with a standing shock. The γ-ray and optical emission is quenched by inverse Compton losses as synchrotron photons from the newly shocked plasma cross the emission region. The millimeter-wave polarization is high at the onset of a flare, but decreases as the electrons emitting at these wavelengths penetrate less polarized regions. This work has been recently published in Agudo et al. (2011, ApJL, 726, L13).

Prominent polarized flares of the blazars AO 0235+164 and PKS 1510-089

Mahito Sasada (Hiroshima University)

We performed multi-color photometric and polarimetric monitorings of AO 0235+164 and PKS 1510-089. These two objects were in active states in 2008 and 2009, respectively. The fluxes of these blazars were about 10 times brighter than that in the quiescent states. In these active states, there were short-term flares, and the polarization degree rose simultaneously during these flares. Thus, there are positive correlations between the flux and the polarization degree of the objects. We compare these flares in AO 0235+164 and PKS 1510-089 with other short-term flares which were detected in 41 blazars. There was a positive correlation between the amplitudes of the flux and polarization degree of the short-term flares. It indicates that the short flares originates from the region where the magnetic field is aligned.

Gamma-ray variability and correlation properties of blazars observed with Fermi LAT.

Stefan Larsson Stockholm University

The Fermi Large Area Telescope provides long term systematic monitoring observations of the gamma-ray emission from blazars. The variability properties and the correlation with other wavelength bands are important clues for the evaluation of blazar models. We present results from timing and multiwavelength correlation analysis and discuss differences between different types of blazars and their implications.

Time lags of flaring AGN

Bjˆrn Eichmann TU Dortmund, Experimentelle Physik 5

The production mechanism of gamma rays by flaring AGN is still one of the main questions in High Energy Astroparticle Physics. A tool to distinguish the leptonic from the hadronic origin is to analyse time lags that occur within one flare event between photons of different wavelength and other particles like neutrinos. The present approach starts with the transport equation of the injected leptonic respectively hadronic particles and takes spatial diffusion and continuous energy losses into account. On the one hand, a relativistic electron pick-up is considered, that leads to Synchrotron, as well as Inverse Compton emission and on the other hand, a relativistic proton pick-up, which results in high energy photons and neutrinos by inelastic proton-proton collisions. The presentation ends up in the temporal development of the emergent photon and neutrino intensities of AGN flares in hadronic and leptonic interaction scenarios and gives useful predictions of flare durations and time lags between photons of different wavelength and high energy neutrinos.

The Blandford-Znajek process and the role of BH spin and accretion in powering relativistic jets in AGN

Laura Maraschi (INAF- Brera Observatory)

Based on simple considerations on the analytic expression of the electromagnetic power produced by the Blandford-Znajek (BZ) process we envisage a distinction between the roles of Black Hole spin and accretion in powering relativistic jets. We conjecture that for corotating disks, the BZ mechanism occurs only above a threshold in the adimensional BH spin parameter $a\simeq 0.5 $. Due to the limited range allowed for the spin parameter, the jet powers would then be tightly connected to the intensity of the magnetic flux advected towards the BH which can be related to the accretion rate. This approach allows to understand the existence of radio loud and radio quiet AGN with similar accretion disk properties (e.g. radio loud and radio quiet quasars) and at the same time the connection between jet power and accretion power, demonstrated most recently by the correlation of $\gamma$--ray luminosity and accretion disk luminosity found in the bright {\it Fermi} blazar sample.

Fermi and the Sequence

Jennifer Gupta Jodrell Bank Centre for Astrophysics, The University of Manchester

The Fermi LAT survey has dramatically increased the number of gamma-ray blazars known and means that for the first time, a large sample of blazars selected by the strength of their gamma-ray emission exists. We have cross-identified the blazars listed in the first Fermi-LAT catalog (1FGL) with the CRATES radio catalog. Using the 8.4 GHz flux density as a proxy for the jet power, we have computed Compton efficiencies, a measure of the ability of the jet to convert the power in the ultrarelativistic jet electrons into gamma-rays through the inverse Compton process. We have compared the Compton efficiencies of the two blazar subsets, BL Lacs and FSRQs, and find no evidence for different mean Compton efficiencies between the two classes, contrary to what might be expected from the blazar sequence. In addition to this, we also investigate the relationship between Compton efficiency and absolute magnitude within the FSRQs and find no correlation.

Fermi Gamma-ray Space Telescope Observations of the Gamma-ray Outburst from 3C 454.3 in November 2010

Benoit Lott CENBG/IN2P3/CNRS

The flat-spectrum radio quasar 3C 454.3 underwent an extraordinary 5-day gamma-ray outburst in November 2010 where the daily flux measured with the Fermi Large Area Telescope (LAT) at photon energies E>100 MeV reached (66+/-2) x 10^-6 ph cm^-2 s^-1. The 3-hr peak flux corresponded to an apparent isotropic luminosity of (2.1+/-0.2)x10^50 erg s^-1, the highest ever recorded from a blazar. In this talk we review the features of this exceptional event as measured by the Fermi LAT and discuss their implications.

Beamed Gamma Rays from Minijets

Krzysztof Nalewajko Nicolaus Copernicus Astronomical Center

Minijets are small-scale flows within magnetically-dominated relativistic jets driven by magnetic reconnection. We investigate radiative properties of minijets in the context of fast TeV flares detected in BL Lac objects. We adopt a relativistic Petschek reconnection framework, construct a multi-zone model taking into account numerous radiative effects and calculate broad-band spectra of integrated emission, paying particular attention to pair-production opacity. We show that observational constraints on the model parameters favour radiative interaction between oppositely-directed minijets. (based on arXiv:1007.3994)

Bottom-up modelling of gamma-ray blazars

Joni Tammi Aalto University Mets‰hovi Radio Observatory

I will discuss blazar modelling from the point of view of particles and low-energy emission. I will address certain problems inherent in many gamma-ray-modelling approaches focusing on the production and variability of the high-energy emission but that do not necessarily check the implications of the results against the distribution and behaviour of the source on lower energies.

I discuss modelling gamma-ray AGNs ìbottom upî, starting from the particles and emphasising the the role of accurate modelling of the primary synchrotron emission from radio to optical frequencies, in modelling the high-energy gamma radiation especially in the case of synchrotron self-Compton, but also for the external Compton scenario. I emphasise the role of multifrequency monitoring (from radio to gamma rays) -- especially the recent Planck observations of flat-spectrum radio sources -- as motivation for this kind of an approach.

I describe our bottom-up approach and the model in development, and discuss selected preliminary results, especially regarding the relevance of various often-omitted features such as non-instantaneous and non-power-law particle acceleration, hard particle spectra (s < 2), and low-frequency plasma effects.

MAGIC detection of Very High Energy Gamma-ray Emission from NGC1275 and IC310

Dorothee Hildebrand ETH Zurich

During observations of the central region of the Perseus Cluster, MAGIC detected Very High-Energy Gamma-ray (VHE) emission from its central galaxy NGC1275 as well as from IC310. IC310 is the first head-tail galaxy that is shown to emit VHE; this emission could be blazar-like or originate from the interaction of the jets with the intra-cluster medium. The classification of NGC1275 is currently disputed, leaving the VHE emission mechanism for this source also unclear. In this talk the latest MAGIC results will be presented.

Connection Between Optical and VHE Gamma-ray Emission in Blazar Jets

Riho Reinthal Tuorla Observatory

MAGIC has been performing optically triggered Target of Opportunity observations of flaring blazars since the beginning of its scientific operations. The alerts of flaring blazars originate from Tuorla blazar monitoring program, which started the optical monitoring of the candidate TeV blazars in 2002 and has now collected up to eight years of data on more than 50 blazars. These Target of Opportunity observations have resulted in the discovery of five new VHE gamma-ray emitting blazars (Mrk180, 1ES1011+496, S50716+714, B3 2247+381 and ON 325) and in addition the discoveries of BLLac and 3C279 were made during a high optical state. In this talk we present a detailed analysis of the optical light curves which are then compared to MAGIC observations of the same sources. We aim to answer the question "Is there a connection between optical and VHE gamma-ray high states in blazars or have we just been lucky?"

Multiwavelength Observations of HBL BL Lac object 1ES 1011+496 in Spring 2008

Riho Reinthal Tuorla Observatory

In this talk we present the results of MAGIC observations of 1ES1011+496 from 2008 and compare them to contemporaneous observations in other wavelengths (radio, optical, X-rays) and discuss their implications for blazar modeling. 1ES 1011+496 - an HBL BL Lac object discovered in VHE gamma-rays by the MAGIC telescope in 2007 during an optical outburst reported by the Tuorla Blazar Monitoring Program - was observed from March 4th to May 24th 2008 by MAGIC during a multiwavelength campaign which also included the Metsahˆvi, KVA, Swift and AGILE telescopes. The MAGIC observations span a total of 27.9 h of which ~20 h remained after quality cuts. The observations resulted in a strong detection at ~8 sigma significance level and with a mean flux and spectral index similar to those during the discovery.

Searching for extended Pair Halo emission with HESS

Lisa Fallon (Dublin Institute for Advanced Studies)

We have conducted a search for the giant Pair Halo structures which are inevitably formed around TeV sources due to interactions of very high energy gamma-rays with the Extragalactic Background Light (EBL). The resulting electron/positron pairs are Compton upscattered on photons of the 2.7 K Cosmic Microwave Background Radiation to produce a second generation of gamma-rays which again interact with the EBL; thus an electromagnetic cascade develops. If the magnetic fields on Mpc scales surrounding the central source are sufficiently strong (10^-11 G or more), electrons are effectively isotropised before interacting with radiation fields. In this case an extended halo is produced around the source. Using H.E.S.S. observations of Active Galactic Nuclei, including data from PKS 2155-304, 1ES 1101-232 and 1ES 0229+200, we have completed a detailed analysis of these sources. I will present and discuss the astrophysical implications of these results.

Three-year Swift/BAT survey of AGNs: reconciling theory and observations?

Davide Burlon (MPE Garching)

It is well accepted that unabsorbed as well as absorbed AGN are needed to explain the nature and the shape of the Cosmic X-ray background, even if the fraction of highly absorbed objects (dubbed Compton-thick sources) substantially still escapes detection. We derive and analyze the absorption distribution using a complete sample of AGN detected by SwiftñBAT in the first three years of the survey. The fraction of Compton-thick AGN represents only 4.6% of the total AGN population detected by SwiftñBAT. However, we show that once corrected for the bias against the detection of very absorbed sources the real intrinsic fraction of Compton-thick AGN is 20(+9−6)%. We proved for the first time (also in the BAT band) that the anti-correlation of the fraction of absorbed AGN and luminosity it tightly connected to the different behavior of the luminosity functions (XLFs) of absorbed and unabsorbed AGN. This points towards a difference between the two subsamples of objects with absorbed AGN being, on average, intrinsically less luminous than unobscured ones. Moreover the XLFs show that the fraction of obscured AGN might also decrease at very low luminosity. This can be successfully interpreted in the framework of a disk cloud outflow scenario as the disappearance of the obscuring region below a critical luminosity. Our results are discussed in the framework of population synthesis models and the origin of the Cosmic X-ray Background.

Do NLS1s have a beamed outflow? An unusual X-ray perspective for Mrk 766

Guido Risaliti, INAF - Arcetri Observatory

Recently, the detection of Gamma-ray emission from Narrow Line Seyfert 1s has suggested that these sources may emit beamed jets towards us. I report the results of a time-resolved spectral analysis of an XMM-Newton long observation of Mrk 766, showing that the X-ray source has been eclipsed several times by clouds with a cold, dense core, and a less dense, highly ionized tail. These clouds have blueshift velocities v>10^4 km/s (as measured through He-like iron absorption lines), and are therefore part of a strong outflow. These results provide new important constraints on the geometry and structure of this source, and may be relevant to understand the peculiarity of NLS1s as a class of AGNs.

Evidence for Helical Magnetic fields in Kiloparsec-Scale AGN Jets and the Action of a Cosmic Battery

Denise Gabuzda (University College Cork)

A search for transverse kiloparsec-scale gradients in Faraday rotation (FR) maps of extragalactic radio sources in the literature has yielded 7 AGNs displaying continuous, monotonic FR gradients across their jets, oriented roughly orthogonal to the local jet direction. The most natural interpretation of such transverse RM gradients is that they are caused by the systematic change in the line-of-sight components of helical magnetic fields associated with these jets. All the identified transverse FR gradients increase in the counterclockwise (CCW) direction on the sky relative to the centers of these AGNs. Taken together with the results of Contopoulos et al. (2009), who found predominance of clockwise (CW) transverse FR gradients across parsec-scale (VLBI) jets, this provides newevidence for preferred orientations of FR gradients due to helical jet magnetic fields, with a reversal from CW in the inner jets to CCW farther from the centers of activity. This can be explained by the "Poynting--Robertson cosmic-battery" mechanism, which can generate helical magnetic fields with a characteristic "twist", which are expelled with the jet outflows. If the Poynting--Robertson battery mechanism is not operating, an alternative mechanism must be identified, which is able to explain both the predominance of CW FR gradients on parsec scales and the change to a predominance of CCW FR gradients on kiloparsec scales.

MEM and CLEAN Imaging of 18-22cm VLBA Polarisation Observations of Compact Active Galactic Nuclei

Colm Coughlan (University College Cork)

We are in the process of obtaining VLBA polarisation data for the 135 MOJAVE-I Active Galactic Nuclei at four wavelengths in the 18-22cm band. Some results from the first 3 of 9 observations are presented. These observations enable studies of the evolution of the intensity and magnetic-field structures of these AGN jets as they propagate from parsec to kiloparsec scales, as well as studies of the thermal plasma present in the vicinity of the jets on these scales, manifest via Faraday rotation. A wide range of other multi-wavelength studies can also be carried out using these data. Both CLEAN and MEM (maximum entropy method) imaging techniques are used and the advantages of using both is demonstrated.

Faraday rotation in jets of AGN: the case of 3C120

Jose L. Gomez & Roca-Sogorb, Agudo, Marscher, Jorstad (Instituto de Astrofisica de Andalucia-CSIC)

The source of Faraday rotation in the jet of the radio galaxy 3C120 is analyzed through VLBA observations carried out between 1999 and 2010 at 86, 43, 22, 15, 12, 8, 5, 2, and 1.7 GHz. Uncorrelated changes in the linear polarization of the underlying jet emission and the Faraday rotation screen indicate that the emitting jet and the source of Faraday rotation are not closely connected physically and have different configurations for the magnetic field and/or kinematical properties. Furthermore, the existence of a region of enhanced rotation measure whose properties remain constant over three years requires a localized source of Faraday rotation, favoring a model in which a significant fraction of the rotation measure originates in foreground clouds.

These results, together with those obtained for other sources, are discussed in the context of the search for helical magnetic fields ñ and their implication in the jet formation and collimation processes ñ through Faraday rotation studies.

Kinetic studies of nonrelativistic parallel shocks

Jacek Niemiec (Institute of Nuclear Physics, Polish Academy of Sciences,Krakow, Poland)

Shock acceleration relies on the presence of magnetic field fluctuations that can scatter relativistic charged particles in the shock vicinity. We report on kinetic particle-in-cell studies of non-linear evolution of magnetic turbulence that arises upstream of the shock as well as at the shock itself. We will in particular address the relation between modes seen in the simulations and waves expected from a linear instability analysis, the efficiency of small-scale turbulence in scattering relativistic particles, and the feedback of high-energy particles on the microscopic instabilities that govern the formation of a shock. Preliminary results of large-scale PIC simulations of nonrelativistic parallel shock formation will also be presented.

The Use of Faraday Rotation Sign Maps as a Diagnostic for Helical Jet Magnetic Fields

Andrea Reichstein (UCC Cork)

We present maps of the sign of the Faraday Rotation measure [sign(RM)] obtained from multi-frequency radio observations on the Very Long Baseline Array (VLBA). Many of the Active Galactic Nuclei (AGN) considered have B-field structures with a central "spine" of B-field orthogonal to the jet

and/or a longitudinal B-field near one or both edges of the jet. This structure can plausibly be interpreted as being caused by a helical/toroidal jet magnetic field. Faraday Rotation is a rotation of the plane of polarization that occurs when the polarized radiation passes through a magnetized plasma. The sign of the RM is determined by the direction of the line-of-sight B-Field in the region causing the Faraday Rotation, and an ordered toroidal or helical magnetic field associated with an AGN jet will thus produce a distinctive bilateral distribution of positive and negative RMs across the jet. We present and discuss sign(RM) maps and their possible interpretation regarding the magnetic field geometries for several sources.

A hybrid Comptonization model for broad-band spectra of accreting supermassive black holes

Alexandra Veledina (University of Oulu)

Using simulations with the kinetic code that self-consistently models electron and photon distributions, we find that the power-law-like X-ray spectra in AGNs can be explained in terms of the SSC radiation of hybrid electron distribution, similarly to the hard/low state of GBHs. Under a very broad range of parameters the model predicts a rather narrow distribution of photon spectral slopes consistent with that observed from LINERs and Seyferts at luminosities less than 3 per cent of the Eddington luminosity. The entire infrared to X-ray spectrum of these objects can be described in terms of our model, suggesting tight correlation between the two energy bands. The recently found correlation between slope and the Eddington ratio at higher luminosities can be described by the increasing fraction of disc photons in the emitting region, which may be associated with the decreasing inner radius of the optically thick accretion disc. The softer X-ray spectra observed in narrow-line Seyfert galaxies may correspond to non-thermal Comptonization of the disc photons, predicting that no cutoff should be observed up to MeV energies in these sources, similarly to the soft-state GBHs.

Possible gamma-ray signals from low-luminosity active galactic nuclei

Hajime Takami (MPI for Physics, Munich)

Although it has been believed that the emission from an accretion disk and from a jet contributes to the spectral energy distribution of low-luminosity active galactic nucleus (LLAGN), the respective contribution of them is still an open question. Since a jet component is expected to associate with gamma-rays by emission mechanism similar to powerful blazars and radio galaxies, gamma-ray signals from LLAGNs can constrain a jet component and then play an important role to understand the physics of disk-jet connection. Here, we demonstrate the gamma-ray signals on the assumption that a jet component is dominated in X-ray bands, and discuss possible parameter sets on jets and the detectability of the gamma-rays. In some reasonable cases, Cherenkov Telescope Array can detect gamma-rays from a nearby LLAGN.

The average soft-gamma ray spectra of Radio-Loud and Radio-Quiet AGN

Claudio Ricci (ISDC Data Centre for Astrophysics)

I will present the results obtained from the deepest (with exposures up to ~100 Ms) study ever performed of the average hard X-ray/soft-gamma ray spectra of AGN. Our sample consists of ~300 AGN, of which 230 are radio quiet (RQ), and ~70 radio loud (RL). Using all the public INTEGRAL IBIS/ISGRI data, we produced the average 17-250 keV spectra of different classes of RQ and RL AGN, and compared their average characteristics with both a model-dependent and a model-independent analysis. One of the main results of our work is that mildly obscured objects (Nh~10^23 cm^-2) show a much stronger reflection component than less obscured objects. I will discuss in detail the differences and similarities between the hard X-ray/soft gamma-ray spectra of different classes of RQ and RL AGN, focusing on the influence of a jet component on the spectra. I will also discuss our results in the light of the unified model of AGN, of the dichotomy between RL and RQ objects, and their impact on the study of the Cosmic X-ray Background.

Probing the origin of VHE emission in M 87 with MWL observations

Martin Raue (University of Hamburg) for the H.E.S.S., MAGIC, VERITAS, and Fermi-LAT Collaborations and the M 87 MWL Monitoring Team

The large majority of extragalactic very high energy (VHE; E>100 GeV) sources belongs to the class of active galactic nuclei (AGN), in particular the BL Lac sub-class. AGNs are characterized by an extremely bright and compact emission region, powered by a super-massive black hole (SMBH) and an accretion disk, and relativistic outflows (jets) detected all across the electro-magnetic spectrum. In BL Lac sources the jet axis is oriented close to the line of sight, giving rise to a relativistic boosting of the emission. In radio galaxies, on the other hand, the jet makes a larger angle to the line of sight allowing to resolve the central core and the jet in great details. The giant radio galaxy M 87 with its proximity (16 Mpc) and its very massive black hole (3-6x10^9 M_solar) provides a unique laboratory to investigate VHE emission in such objects and thereby probe particle acceleration to relativistic energies near SMBH and in jets. M 87 has been established as a VHE emitter since 2005. The VHE emission displays strong variability on time-scales as short as a day. It has been subject of a large joint VHE and multi-wavelength (MWL) monitoring campaign in 2008, where a rise in the 43 GhZ VLBA radio emission of the innermost region (core) was found to coincide with a flaring activity at VHE. This had been interpreted as a strong indication that the VHE emission is produced in the direct vicinity of the SMBH black hole. In 2010 again a flare at VHE was detected triggering further MWL observations with the VLBA, Chandra, and other instruments. At the same time M 87 was also observed with the Fermi/LAT telescope at GeV energies and the European VLBI Network (EVN). In this contribution preliminary results from the campaign will be presented and their implications for VHE emission models will be discussed.

Variable TeV emission as a manifestation of jet formation in M87?

Amir Levinson (Tel Aviv University)

It is proposed that the variable TeV emission observed in M87 may be produced in a starved magnetospheric region, above which the outflow associated with the VLBA jet is established. It is shown that annihilation of MeV photons emitted by the radiative inefficient flow in the vicinity of the black hole, can lead to injection of seed charges on open magnetic field lines, with a density that depends sensitively on accretion rate, $n_\pm\propto\dot{m}^{4}$. For an accretion rate that corresponds to the inferred jet power, and to a fit of the observed SED by an ADAF model, the density of injected pairs is found to be smaller than the Goldreich-Julian density by a factor of a few. It is also shown that inverse Compton scattering of ambient photons by electrons (positrons) accelerating in the gap can lead to a large multiplicity, $\sim 10^3$, while still allowing photons at energies of up to a few TeV to freely escape the system. The estimated gap width is not smaller than $0.01 r_s$ if the density of seed charges is below the Goldreich-Julian value. The VHE power radiated by the gap can easily account for the luminosity of the TeV source detected by H.E.S.S. The strong dependence of injected pair density on accretion rate should render the gap emission highly intermittent. I shall also discuss briefly the application of this mechanism to Sgr A$^\star$.

Unbeamed gamma-rays from low luminosity AGNs

Andrzej Niedzwiecki (University of Lodz)

We present preliminary results for an alternative (to misaligned jet emission) scenario for FR I galaxies. We refine and extend previous works on tenuous, two-temperature accretion flows by including a self-consistent, fully GR description of both the dynamical and the radiative (both leptonic and hadronic) processes. We find that the hadronic (i.e. initiated by proton-proton interactions) processes dominate the formation of the overall SED in a flow surrounding a rapidly rotating black hole. The gamma-ray luminosities of such flows are sufficient to explain the Fermi/LAT observations of FR Is. We compare our modelling results to the observed, broad-band spectra of Cen A and M87.

GeV Observations of Star-forming Galaxies with the Fermi LAT

Keith Bechtol SLAC / Stanford / KIPAC

Recent detections of the starburst galaxies M82 and NGC 253 by gamma-ray telescopes suggest that galaxies rapidly forming massive stars are more luminous at gamma-ray energies compared to their quiescent relatives. Building upon those results, we study a sample of 69 dwarf, spiral, and luminous and ultraluminous infrared galaxies at photon energies 0.2--100 GeV using more than two years of data collected by the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope (Fermi). Measured fluxes from significantly detected sources and flux upper limits for the remaining candidates are used to explore the physics of cosmic rays in star-forming galaxies. We identify statistically significant correlations between both radio continuum luminosity and total infrared luminosity with gamma-ray luminosity which apply both to quiescent galaxies of the Local Group and low-redshift starburst galaxies. Using the relationship between infrared luminosity and gamma-ray luminosity, we estimate the contribution of star-forming galaxies to the isotropic diffuse gamma-ray background and find that this contribution is comparable to that of blazars in the GeV energy range.

Cosmic rays and star formation in galaxies.

Massimo Persic INAF+INFN Trieste, Italy

The CR energy densities in central regions of starburst galaxies, as inferred from radio and gamma-ray measurements of synchrotron and neutral-pion decay emission, are U = O(100) eV/cm3 -- i.e., about 2 orders of magnitude larger than around the Galactic center and in other mildly star forming galaxies. The discrepancy grows when more extreme starbursts, such as Arp 220, and more quiet environments, such as the Magellanic clouds, are considered. These very different energy density levels reflect a similar disparity in the respective SN rates observed in the two environments.

Search for neutrinos from AGNs with the ANTARES telescope

Juan Pablo GÛmez Gonz·lez Instituto de FÌsica Corpuscular (CSIC-UV)

The ANTARES detector is currently the largest deep-sea neutrino telescope in the Northern Hemisphere. Located on the bottom of the Mediterranean Sea, 40 km off the French coast, it consists of a three-dimensional array of 885 photomultiplier tubes which detect the Cherenkov light induced by the muons produced in the interaction of high energy cosmic neutrinos with the matter surrounding the detector. The main goal of ANTARES is the search for point-like sources. AGNs are among the most favoured candidate sources of high energy neutrinos. In this talk I will present the results of the search for point sources with the data gathered in 2007 and 2008, including the neutrino flux upper limits for the most promising AGNs and the map of Southern Hemisphere neutrino sky.

A bolometric approach to neutrino emission from AGN

Marlene Doert TU Dortmund

The origin of very high energy gamma-rays from AGN is still subject to debates. The ultimate way to distinguish leptonic from hadronic production processes is the detection of neutrinos from these sources. In a scenario in which the measured gamma-rays originate from the decay of neutral pions from proton interactions, the simultaneously produced charged pions would cause an accompanied emission of neutrinos in the course of their decay. In this presentation an approach for the prediction of the neutrino flux from AGN sources based on the measured flux in gamma-rays will be discussed. Subsequently its detectability with the IceCube detector will be investigated using Markarian 501 as an example TeV blazar. The prediction of the neutrino flux is based on the bolometric method, assuming that the energy emitted in VHE gamma-rays corresponds to the energy output in neutrinos. Paying attention to the fact that AGN are often highly variable sources, low state measurements as well as measurements taken during flaring activity are taken into account. The resulting predictions for neutrino fluxes will be presented and open questions will be discussed.

Ruling out EBL models - What can we learn from gamma-ray limits?

Tanja Kneiske (Universitaet Hamburg)

The study of extragalactic objects like blazars and galaxies requires a method to correct observed high energy gamma-ray data for extragalactic absorption due to photon photon pair production. Knowledge of the extragalactic background light (EBL) (at different redshifts) is essential for such a correction. On the other hand high energy gamma-ray data can be used to set limits on the EBL flux, ruling out EBL models. During the last few years many models describing the EBL have been published. I will review those models briefly, presenting their different and common features. In particular I will talk about the astrophysical implications if one or more of these EBL models are ruled out by high energy gamma-ray observations.

Semi-Analytic Modeling of the EBL and Consequences for Extragalactic Gamma-ray Spectra

Rudy Gilmore (SISSA)

Attenuation of high-energy gamma rays by pair-production with UV, optical and IR extragalactic background light (EBL) photons provides a link between the history of galaxy formation and high-energy astrophysics. I will present results from our latest semi-analytic models (SAMs), which employ all ingredients thought to be important to galaxy formation and evolution, as well as an improved method for reprocessing of starlight by dust to mid- and far-IR wavelengths. These SAMs are based upon a hierarchical structural formation scenario, and are successful in reproducing a large variety of observational constraints such as number counts, luminosity and mass functions, and color bimodality. Our fiducial model is based upon a WMAP5 cosmology and treats dust processing of starlight using templates based on Spitzer data. This model predicts a background flux considerably lower than found in optical and near-IR measurements that rely on subtraction of zodiacal and galactic foregrounds, and near the lower bounds set by number counts of resolvable sources at many wavelengths. I will conclude with a discussion of the implications of our work and how the science of gamma-ray astronomy will continue to help constrain cosmology. Our prediction of a low EBL flux suggests an optimistic future for further studies of distant gamma-ray sources.

Limits on the extragalactic background light from very high energy emitting blazars in the Fermi era

Manuel Meyer (University of Hamburg)

Very high energy (VHE) $\gamma$-rays from distant sources such as blazars are attenuated due to the interaction with photons of the extragalactic background light (EBL) in the ultraviolet to infra-red wavelength band. The EBL, thus, leaves an imprint on the observed energy spectra of such sources. With assumptions about the source physics it is possible to derive limits on the EBL energy density. In recent years, a wealth of new extragalactic VHE sources has been discovered by imaging Cherenkov telescopes such as HESS, MAGIC and VERITAS. Furthermore, the measurements of the Fermi satellite brought new insights into the intrinsic spectra of the sources at GeV energies. In this contribution new limits on the EBL are presented. The analysis follows the approach of Mazin \& Raue (2007) and it is extended to include the evolution of the EBL with redshift $z$ in order to account for the growing number of detected sources at $z\gtrsim 0.2$. First results are presented and new methods for constructing upper limits such as the comparison of the VHE spectra to the spectra measured by Fermi and the possibility of population studies are discussed.

On the transparency of the universe at very high energies

Manuel Meyer (University of Hamburg)

The transparency of the universe for very high energy (VHE, $E > 100$~GeV) photons from sources at cosmological distances is limited due to pair-production with low energy photons of the extra galactic background light (EBL) in the optical to infrared band. Here, we use all available energy spectra from VHE emitting active galactic nuclei to search for signatures of non-standard physics. A combined statistical study using a new approach shows evidence for a hardening of the intrinsic power laws at the transition from optical thin to optical thick regions. Additionally, we find a moderately significant $p$-value of the expected absorption effect.

Constraints on dark matter powered stars from the extragalactic background light

Martin Raue (University of Hamburg)

Authors: Martin Raue, Andreas Maurer, Peter Hauschildt, Dieter Horns, Tanja Kneiske, Dominik Elsaesser Recently, it has been proposed that self-annihilating dark matter could have a significant effect on the formation and development of the first stars in the universe. In such models, the energy release from self-annihilation of dark matter particles can provide the main power source for this class of young stellar objects called Dark Stars (DS). Their features (e.g. luminosity, temperature, lifetime) differ from normal Population III stars and therefore makes them distinguishable. Here, the contribution of DS to the extragalactic background light considering multiple initial parameters is calculated. By comparing our results with existing data of the EBL first observational limits on Dark Stars in the early universe can be derived.

Blazars lighthouses of the high-energy Universe and the origin of the Isotropic diffuse gamma-ray background

Marco Ajello (SLAC/KIPAC)

Fermi-LAT, with its unprecedented sensitivity, detected over a thousand point-like sources during the first year of operations. Most of the extragalactic sources are blazars detected over a wide range in redshift and luminosity. I will review the properties of the source populations detected by Fermi-LAT focusing in particular on the statistical properties of blazars. New results on the cosmological evolution and growth of FSRQs will be presented. Fermi data provide indications for the anti-hierarchical growth of FSRQs where more luminous objects form earlier in the history of the Universe while the less luminous, and bulk of the population, are more abundant at later epochs. At the far end of the spectrum and only ~2 Gyr from the Big Bang lie the FSRQs detected by Swift, among the most powerful objects ever observed. Powered by super-massive black holes with masses often in excess of 10 billion solar masses these objects pose a question on the mechanisms of formation of massive black holes in the high redshift Universe. Finally an up to date review of status of the origin of Isotropic Diffuse Gamma-ray Background will be presented.

Contribution from star-forming galaxies to the cosmic gamma-ray background radiation

Ryu Makiya (Kyoto University)

The origin of extragalactic gamma-ray background (EGRB) has been discussed for a long time and various sources have been discussed as possible contributors to EGRB, such as active galactic nuclei (AGNs), star-forming galaxies, galaxy clusters, structure formation, and dark matter annihilation. We present a new theoretical calculation of the contribution to the EGRB from star-forming galaxies based on a state-of-the-art model of hierarchical galaxy formation that is in quantitative agreement with a variety of observations of local galaxies as well as high redshifts. Gamma-ray luminosity and spectrum of galaxies are related to star formation rate, gas mass, and star formation mode (quiescent or starburst) of model galaxies using latest observed data of nearby galaxies. Our standard model predicts ~7% contribution from star-forming galaxies to the total EGRB flux recently reported by the Fermi Gamma-Ray Space Telescope. The sum of the known contributions from AGNs and star-forming galaxies can explain a large part of EGRB, with a remarkable agreement between the predicted model spectrum and observation.

Extragalactic Gamma-ray Background Radiation from Beamed and Unbeamed Active Galactic Nuclei

Yoshiyuki Inoue (Kyoto University)

The origin of the extragalactic gamma-ray background (EGRB) radiation has been a mystery in astrophysics for a long time. Recently the Fermi gamma-ray satellite (Fermi) has revealed that ~22% of EGRB would be explained by blazars, which are beamed active galactic nuclei (AGNs). The remaining ~78% of EGRB is still unknown. We estimate the contribution of gamma-ray loud radio galaxies, which are misaligned radio loud AGNs, to EGRB using the radio luminosity function of radio-loud AGNs with the correlation between the radio and gamma-ray luminosities. We find that ~25 % of EGRB will be explained by gamma-ray loud radio galaxy population. We would like to discuss further about the origin of EGRB by comparing the Fermi EGRB data with our studies on AGNs contribution to EGRB which are radio quiet AGNs, blazars, and the gamma-ray loud radio galaxies.