International Journal of Modern Physics D

After a brief overview of the main properties of blazars, some significant results from recent multiwavelength observations are summarized, in the context of the jet physics.

The total set of the 14 active galactic nuclei detected by MAGIC so far includes well-studied bright blazars like Mkn 501, the giant radio galaxy M 87, but also the distant flat-spectrum radio quasar 3C 279, and an intriguing gamma-ray source in the 3C 66A/B region, whose energy spectrum is not compatible with the expectations from 3C 66A. Besides scheduled observations, so far MAGIC succeeded in discovering TeV gamma rays from three blazars following triggers from high optical states. I report selected highlights from recent MAGIC observations of extragalactic TeV gamma-ray sources, emphasizing and discussing the new physics insights the MAGIC observations were able to contribute.

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).

Theory predicts a neutrino signal is possible from jets from active galactic nuclei (AGN) if there is hadronic acceleration in the jets. Flaring activity in X-rays and γ-rays from blazars could indicate similar activity in neutrinos in the jets. Short duration flares, on the scale of hours, could allow for narrow time windows to be searched for a Neutrino signal above the background of atmospheric neutrinos. A wavelet technique will be run over orbit-by-orbit Swift BAT (Burst Alert Telescope) data to identify flaring periods to accurately determine start and end times. The wavelet technique is notable as it accounts for both uneven time sampling and varying error bars across a time series.

The ANTARES underwater neutrino telescope has been completed in May 2008 and is now taking data continuously. Thanks to its very good angular resolution (better than 0.3° for neutrinos with energy above 10 TeV) ANTARES is especially suited for the search of astrophysical point-like sources of high energy neutrinos. Data taken with a limited detector (5 out of 12 lines) between January and December 2007 have been analyzed to look for a possible neutrino excess from a list of prospective neutrino sources. In the case of transient sources, like GRBs, the short duration of the expected neutrino signal can be exploited to enhance the signal to noise ratio. ANTARES strategy for both steady and transient point-like sources is discussed. The methodology adopted and the results obtained are shown.

Despite many advances in AGN physics, observationally as well as theoretically, the relativistic matter composition of extragalactic jets remains concealed. In contrast to the so called leptonic AGN emission models, hadronic models consider relativistic protons to be present in those sources, in addition to the relativistic electrons and positrons, and possibly cold material. Here I briefly discuss most processes that are commonly believed to characterize the so-called hadronic AGN emission models, and these include leptonic as well as hadronic processes. As an application predictions for the neutrino output during the TeV-flare of 3C 279 in 2006 are provided.

This is a summary of recent modeling results of three newly detected TeV blazars of the intermediate BL Lac object (IBL) and flat-spectrum radio quasar (FSRQ) sub-classes: W Comae, 3C 66A, and 3C 279. For model fits to the spectral energy distributions of W Comae and 3C 66A, a non-negligible contribution from a source of soft radiation external to the jets of these blazars is strongly preferred over pure synchrotron-self-Compton or synchrotron-proton-blazar models, respectively. This indicates that IBLs are truly intermediate between the "classical" TeV blazars (high-frequency-peaked BL Lac objects) and FSRQs. The VHE γ-ray detection of the FSRQ 3C 279 poses severe challenges for leptonic models, and seems to favor a hadronic origin of the high-energy emission. However, even a hadronic model suffers from problems with extreme energy requirements.

We examine several constraints on the physical parameters in active galactic nuclei and gamma-ray burst, originating from the assumption that these sources are efficient in converting their energy into gamma-rays. It is shown that in most cases these constraints may be reformulated in terms of bounds on the jet Lorentz factor, for which both the lower and the upper limits can be evaluated.

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.

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.

We discuss the relevance of gamma-ray observations of galaxy clusters, to unveil several issues that are relevant for Cosmology and Astro-Particle Physics of cosmic structures: the nature of Dark Matter, the origin of cosmic rays and the impact of black holes.

We present the observations of Gamma-Ray Bursts (GRB) by the Fermi Large Area Telescope (LAT). In the first six months of the mission four GRB were detected at energies greater than 100 MeV by the LAT. The spectral and temporal properties of the three events detected within the LAT field of view, particulary with respect to the low-energy emission detected by the GBM, will be outlined.

I discuss some recent results on the prompt emission of gamma-ray bursts, in particular the jet and radiation properties of the naked-eye burst GRB 080319b, based on Swift and related observations. I then discuss the recent observations by the Fermi satellite of GRB 080916C, the resulting constraints for the bulk Lorentz factor determinations, and the highest lower limit on the quantum gravity energy scale obtained so far.

Gamma-ray bursts (GRBs) are the possible sources of ultra-high-energy cosmic rays (UHECRs), because of the extreme values of ultra-relativistic velocities and strong magnetic fields. In this paper we review the validity of GRBs as sources of UHECRs, and introduce our recent detailed studies of gamma-rays and neutrinos due to UHECRs utilizing comprehensive Monte Carlo codes. Future observations such as Fermi, ground-based telescopes and other facilities may provide constraints on the UHECR acceleration efficiency in GRBs. We also show a preliminary result of neutrino detection efficiency from a GRB with JEM-EUSO.

With their narrow emission window gamma-ray bursts (GRBs) are among the most promising objects for the first identification of high-energy cosmic neutrinos. If a considerable fraction of the ultra-high energy cosmic rays is indeed produced in GRBs, IceCube, which is now more than half-way completed, should be able to detect the associated neutrinos in the next few years. Furthermore, optical follow-up observations of neutrino multiplets will enhance IceCube's sensitivity to choked GRBs which do not produce a gamma-ray signal.

Gamma-ray Bursts (GRBs) are relativistic cosmological beacons of transient high energy radiation whose afterglows span the electromagnetic spectrum. Theoretical expectations of correlated neutrino emission position GRBs at an astrophysical nexus for a metamorphosis in our understanding of the Cosmos. This new dawn in the era of experimental (particle) astrophysics and cosmology is afforded by current facilities enabling the novel astronomy of high energy neutrinos, in concert with unprecedented electromagnetic coverage. In that regard, GRBs represent a compelling scientific theme that may facilitate fundamental breakthroughs in the context of Swift, Fermi and IceCube. Scientific synergy will be achieved by leveraging the combined sensitivity of contemporaneous ground-based and satellite observatories, thus optimizing their collective discovery potential. Hence, the advent of GRB multi-messenger astronomy may cement an explicit connection to fundamental physics, via nascent cosmic windows, throughout the next decade.

During their acceleration, both UHECR nuclei and UHECR protons may interact with the radiation field present in the accelerator region leading to photo-disintegration and photo-pion production interactions respectively. This leads to a close connection between the degree of disintegration of UHECR nuclei and the UHE neutrino flux produced by a source. Upon escaping the accelerator, UHECR protons go on to generate a secondary UHE photon flux through photo-pion interactions with the cosmic microwave background (CMB) photons, These secondary UHE photons produced (via neutral pion decay) will themselves also interact with the cosmic radio background (CRB) photons. This leads to a second connection between UHE photons and nearby source of UHECR protons. We hear investigate both these connections to draw conclusions on what we can learn about UHECR sources.

Ultra high energy cosmic ray events presently show a spectrum, which we interpret here as galactic cosmic rays due to a starburst, in the radio galaxy Cen A which is pushed up in energy by the shock of a relativistic jet. The knee feature and the particles with energy immediately higher in galactic cosmic rays then turn into the bulk of ultra high energy cosmic rays. This entails that all ultra high energy cosmic rays are heavy nuclei. This picture is viable if the majority of the observed ultra high energy events come from the radio galaxy Cen A, and are scattered by intergalactic magnetic fields across much of the sky.

This paper discusses the correlation reported in 2008 by the Pierre Auger Observatory (PAO) of the arrival directions of the highest energy cosmic rays with active galactic nuclei (AGN). It is argued that these correlating AGN do not have the power required to be the sources of ultra-high energy protons. This current PAO dataset is further shown to disfavor giant radio-galaxies (both Fanaroff–Riley type I and II) as sources of ultra-high energy protons. The current data thus likely point to the local large scale structure, in which the actual sources of ultra-high energy cosmic rays camouflage. Finally, it is shown that the last gamma-ray burst in Centaurus A could explain, through rescattering on the Cen A lobes, the apparent cluster of events in this direction.

Point source searches with neutrino telescopes like IceCube are normally restricted to one hemisphere, due to the selection of up-going events as a way of rejecting the atmospheric muon background. In this work we show that the down-going region above the horizon can be included in the search by suppressing the background through energy-sensitive selection procedures. This approach increases the reach to the EeV regime of the signal spectrum, which was previously not accessible due to the absorption of neutrinos with energies above a PeV inside the Earth. We present preliminary results of this analysis, which for the first time includes up-going as well as down-going muon events in a combined approach. We used data collected with IceCube in a configuration of 22 strings. No significant excess above the atmospheric background is observed. While other analyses provided results for the Northern hemisphere, this new approach extends the field of view to a large part of the southern sky, which was previously not covered with IceCube.

We calculated for the nearest active galactic nucleus (AGN), Centaurus A (Cen A), the flux of high energy cosmic rays and of accompanying secondary photons and neutrinos expected from hadronic interactions in the source. We used as two basic models for the generation of ultrahigh energy cosmic rays (UHECR) shock acceleration in the radio jet and acceleration in the regular electromagnetic field close to the core of the AGN, normalizing the UHECR flux to the observations of the Auger experiment. Here we compare the previously obtained photon fluxes with the recent data reported by the Fermi LAT and H.E.S.S. collaborations. In the case of the core model, we find good agreement both for the predicted spectral shape and the overall normalization between our earlier results and the H.E.S.S. observations for a primary proton energy dN/dE ∝ E^-α with α ~ 2 or smaller. A broken power law with high-energy part α = -2.7 leads to photon fluxes in excess of the Fermi measurements. The energy spectrum of the photon fluxes obtained by us for the jet scenario is in all cases at variance with the H.E.S.S. and Fermi observations.

We show that at wavelengths comparable to the length of the shower produced by an Ultra-High Energy cosmic ray or neutrino, radio signals are an extremely efficient way to detect these particles. First results are presented of an analysis of 20 hours of observation data for NuMoon project using the Westerbork Synthesis Radio Telescope to search for short radio pulses from the Moon.

The IceCube observatory is currently being contructed at the South Pole. Scheduled to be completed in 2011, it already offers a large increase in sensitivity over its predecessor, the AMANDA detector. This increase is expected to improve significantly the search for extraterrestrial neutrinos which are currently under way. One of the key searches at the IceCube observatory is for a diffuse flux of high energy extraterrestrial muon neutrinos, in excess of that observed from cosmic-ray induced atmospheric neutrinos. In this paper, we will review results obtained with the AMANDA and early IceCube detectors as well as give an outlook on upcoming analyses.

In clusters of galaxies, accretion and merger shocks are potential accelerators of high energy protons, as well as intracluster active galactic nuclei. We discuss the possibility that protons from cluster shocks make a significant contribution to the observed cosmic rays in the energy range between the second knee at ~10^17.5 eV and the ankle at ~10^18.5 eV. The accompanying neutrino and gamma-ray signals could be detectable by upcoming telescopes such as IceCube/KM3Net and CTA, providing a test of this scenario as well as a probe of cosmic-ray confinement properties in clusters.

ANTARES is a neutrino telescope under the Mediterranean Sea, in a site 40 km off the French coast at a depth of 2475 m. It is an array of 12 lines equipped with 884 photomultipliers. The detection mechanism relies on the observation of the Cherenkov light emitted by charged leptons produced by neutrinos interacting in the water and ground surrounding the detector. First studies of the detector performances and preliminary results on reconstruction of atmospheric muons and neutrinos are presented, with the expected sensitivity for a diffuse flux of high energy neutrinos.

We investigate the consequences of higher dimension Lorentz violating, CPT even kinetic operators that couple standard model fields to a non-zero vector field in an Effective Field Theory framework. Comparing the ultra-high energy cosmic ray spectrum reconstructed in the presence of such terms with data from the Pierre Auger Observatory allows us to establish stringent bounds on O(E/M_Pl)² suppressed violations of Lorentz invariance.

Clusters of galaxies are believed to contain a significant population of cosmic rays. From the radio and probably hard X-ray bands it is known that clusters are the spatially most extended emitters of non-thermal radiation in the Universe. Due to their content of cosmic rays, galaxy clusters are also potential sources of VHE (> 100 GeV) gamma rays. Recently, the massive, nearby cluster Abell 85 has been observed with the H.E.S.S. experiment in VHE gamma rays with a very deep exposure as part of an ongoing campaign. No significant gamma-ray signal has been found at the position of the cluster. The non-detection of this object with H.E.S.S. constrains the total energy of cosmic rays in this system. For a hard spectral index of the cosmic rays of -2.1 and if the cosmic-ray energy density follows the large scale gas density profile, the limit on the fraction of energy in these non-thermal particles with respect to the total thermal energy of the intra-cluster medium is 8% for this particular cluster. This value is at the lower bounds of model predictions.

The discovery of distant sources of very high energy (VHE) γ-rays with hard energy spectra enabled to derive strong upper limits on the density of the extragalactic background light (EBL). These limits are close to the lower limits derived from deep source counts. A recent re-dertemination of the EBL contribution from resolved sources at 3.6 μm finds a higher EBL density, which is claimed to be in conflict with the assumptions utilized to derive the EBL upper limits from VHE spectra. Here, it is shown that is possible to recover the canonical Γ ~ 1.5 intrinsic spectra for such a higher EBL density.

This paper presents a gamma-ray down-going induced muon analysis combining the Gamma-ray bursts Coordinates Network (which announces a possible GRB event) with a blind strategy. The goal of this analysis is to find an astrophysical source through the ability of distinguishing between a real alert (signal) and a fake one (background), through comparisons between the position of an Antares event and the position of the alert, leading us to a better understanding of gamma-ray bursts.

Flaring activity of Active Galactic Nuclei (AGN) in VHE γ-ray astronomy is observed on timescales from minutes to years and can be explained either by the interaction of relativistic jets with the surrounding material or by imprints of the central engine, like temporal modulation caused by binary systems of supermassive black holes. The key to answer those questions lies in combining 24/7 monitoring with short high sensitivity exposures as provided by the third generation γ-ray astronomy instruments like MAGIC, VERITAS and H.E.S.S. The long-term observations can be provided by a global network of small robotic Cherenkov telescopes.¹ As a first step, we are currently setting up a dedicated Cherenkov telescope, which will carry out joint observations with the Whipple 10 m telescope for AGN monitoring. The new telescope will be designed for low costs but high performance by upgrading one of the former HEGRA telescopes, still located at the MAGIC site on the Canary Island of La Palma (Spain). The main novelties will be its robotic operation and a novel camera type, resulting in a greatly improved sensitivity and a lower energy threshold.

Internally rotating jets are expected to be present in a number of astrophysical sources including AGNs. Here we consider the acceleration of energetic charged particles within such flows and discuss the role of shear and centrifugal effects for efficient particle energization. We show that in the case of significant rotation, centrifugal effects could play an important role and facilitate efficient acceleration near the spine of the jet. We point out that shear acceleration could be particularly interesting in the context of hadronic models.

Many of the astrophysical sources and violent phenomena observed in our Universe are potential emitters of gravitational waves (GWs) and high-energy neutrinos (HENs). A network of GW detectors such as LIGO and Virgo can determine the direction/time of GW bursts while the IceCube and ANTARES neutrino telescopes can also provide accurate directional information for HEN events. Requiring the consistency between both, totally independent, detection channels shall enable new searches for cosmic events arriving from potential common sources, of which many extra-galactic objects.

Blazars are effective emitters of γ-rays with spectra extending to GeV and TeV energies. In the case of distant TeV blazars, γ-rays undergo severe absorption due to interactions with diffuse intergalactic radiation fields resulting in significant deformations of the initial (source) spectra. Even for the lowest possible level of the Extragalactic Background Light (EBL), the absorption-corrected γ-ray spectra from some TeV blazars appear unusually hard, which cannot be explained within the standard particle acceleration models of blazars. The simplest and most natural solution of this problem seems to us the realization of internal absorption of γ-rays in narrow (e.g. Planckian type) radiation fields in the immediate vicinity of the γ-ray production region.