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The Texas Symposia on Relativistic Astrophysics had their origins in a couple of astounding scientific discoveries and a demographic accident. A sequence of additional discoveries over the next 15 or so years (pulsars, X‐ray binaries, the microwave background, gamma ray bursts….) perpetuated the series, while the inventory of topics thought appropriate for discussion first expanded and then contracted down to a now fairly rigid set, which is easily discerned from the titles of the rest of the papers in these proceedings, while the past inventory has included complex molecules, large telescopes, and at least a few planets. We explore here portions of what happened from 1st Texas in Dallas in December 1963 to the 25th gathering in Heidelberg in 2010, including the parade of “hot” topics, some demographic and scientometric developments, and a few probably authentic anecdotes. Changes in the sources of funding, nationalities of speakers and participants, and gender balance reflect (sometimes with considerable...

In order to revolutionize the current understanding of the high energy universe, a number of new X‐ray missions are being developed and planned. Among them, e‐ROSITA/SRG, NuSTAR, ASTROSAT, GEMS and ASTRO‐H will be realized in the next decade. And then, much larger missions, such as IXO, have been proposed for the 2020’s. NuSTAR and ASTRO‐H will open up completely new field of spatial studies of non‐thermal emission above 10 keV by hard X‐ray telescopes. They will also uniquely allow mapping of the spatial extent of the hard X‐ray emission in diffuse sources, thus tracing the sites of particle acceleration in structures ranging in size from clusters of galaxies down to supernova remnants. Multi‐wavelength spectra by ASTROSAT and ASTRO‐H are indispensable to understand physical processes in high energy phenomena, such as particle acceleration in the Universe. Imaging spectroscopy with an energy resolution

Galaxy clusters, as the largest well defined objects in our Universe, form an integral part of the cosmic large‐scale structure. Their formation and evolution is therefore tightly coupled to the evolution of the Universe as a whole, which makes clusters important probes to test cosmological models. Both, the cluster abundance quantified by the mass function and the cluster spatial distribution can be used as sensitive probes to constrain cosmological parameters. This use of galaxy clusters is illustrated by means of the X‐ray detected galaxy cluster samples from the ROSAT All‐Sky Survey. One of the current limitations of using galaxy clusters for cosmological tests is the uncertain link between observable cluster properties with the cluster mass, which enters cosmological modeling. Large progress has been made recently, however, in understanding the statistics of galaxy cluster structure and the scaling relations between cluster observables and cluster gravitational mass in particular for X‐ray observatio...

Extragalactic relativistic jets are engines able to carry out to large distances a huge amount of power, not only in the form of radiation, but especially in the form of kinetic energy of matter and fields. As such, they can be thought as one of the most efficient engines of Nature, perhaps even more efficient than accretion. We are starting to disclose these features through a detailed study of their properties, made possible by the analysis of the energy band where they emit most of their electromagnetic output, namely the gamma-ray band. That is why the observations by the Fermi satellite and by the ground based Cherenkov telescopes are crucial to understand extragalactic jets. At the start, we believe they are 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. The spectral properties of bright sources show a trend, controlled mainly by the bolometric apparent luminosity. With improved sensitivity, and the detection of weaker sources, we can explore the idea that the spectral trends are a result of the same physical quantities controlling the emission of non-jetted sources: the black hole mass and the accretion rate. This is based on recent results on sources showing a thermal component in their spectrum, besides a non-thermal continuum. That the jet power should be linked to accretion is intriguing. Most of the apparent diversity of extragalactic radio sources can then be understood on the basis of the viewing angle, controlling the relativistic Doppler boosting of the emission, the black hole mass and the accretion rate., Comment: 17 pages, 8 figures, invited plenary talk at the Texas symposium, Heidelberg, December 2010

This is an update on a previous, more extended conference review text [61], with emphasis on outstanding problem areas. The role of dissipation of magnetic energy in accelerating the flow is discussed, and its importance for explaining high Lorentz factors. The transition between disk and outflow is one of the least understood parts of the magnetic theory; its role in setting the mass flux in the wind, in possible modulations of the mass flux, and the problems in treating it realistically are discussed. Current views on most of these problems are still strongly influenced by the restriction to 2 dimensions (axisymmetry) in previous analytical and numerical work; 3‐D effects likely to be important are suggested. An interesting problem area is the nature and origin of the strong, preferably highly ordered magnetic fields known to work best for jet production. The presence or absence of such fields may well be the ‘second parameter’ governing not only the presence of jets but also the X‐ray spectra and timing behavior of X‐ray binaries.

Cosmic ray acceleration and magnetic amplification in supernova remnants is shortly reviewed. The results on the modeling of broadband electromagnetic emission from supernova remnants are presented.

Many uncertainties in cosmic ray experiments will be eliminated with more measurements about interaction between nucleons in the Large Hadron Collider (LHC) experiment. Cosmic ray experiments will enter an era marked by the high precision of measurements carried out by combining multipurpose instruments on very large scales. They have been putting strict constraints on models of cosmic ray acceleration at sources and propagation through the universe. Measurements of electrons, all kinds of nuclei, even gamma rays and neutrinos will enhance the opportunity of breakthrough in solving the mystery of origin of cosmic rays or even finding the source of the dark matter. A fundamental problem in cosmic ray experiments is the energy scale of ground based experiments for ultra high energy cosmic rays. A unique and straightforward approach is to match the energy spectra of nuclei measured by space or balloon borne experiments with what measured by ground based detectors at high altitudes and propagate the scale up to the cut‐off of the spectrum, so that complete, consistent, unique and reliable measurements will be performed on their energy spectrum, composition and anisotropy.

Looking deep into the Universe through powerful telescopes, we can see images of the Universe when it was younger because of the finite time it takes light to travel to us from distant sources. Existing data sets include an image of the Universe when it was 0.4 million years old (in the form of the cosmic microwave background), as well as images of individual galaxies when the Universe was older than a billion years. But there is a serious challenge: in between these two epochs was a period when the Universe was dark, stars had not yet formed, and the cosmic microwave background no longer traced the distribution of matter. And this is precisely the most interesting period, when the primordial soup evolved into the rich zoo of objects we now see.The observers are moving ahead along several fronts. The first involves the construction of large infrared telescopes on the ground and in space, that will provide us with new photos of the first galaxies. Current plans include ground‐based telescopes which are 24–...

We discuss cosmic‐ray acceleration in Galactic and extragalactic sources and transport in cosmic magnetic fields. Discussion includes the acceleration in supernova shocks, the collective effects of cosmic rays in the Galaxy, the nature of “knee” in cosmic ray spectrum, the transition to extragalactic component, and the acceleration by AGN jets.

Stationary knots observed in many AGN jets can be explained in terms of a reconfinement shock that forms when relativistic flow of the jet matter collides with the external medium. The position of these knots can be used, together with information on external pressure profile, to constrain dynamical parameters of the jet. We present a semi‐analytical model for the dynamical structure of reconfinement shocks, taking into account exact conservation laws both across the shock surface and in the zone of the shocked jet matter. We show that, due to the transverse pressure gradient in the shock zone, the position of the reconfinement is larger than predicted by simple models. A portion of kinetic energy is converted at the shock surface to internal energy, with efficiency increasing strongly with both bulk Lorentz factor of the jet matter and the jet half‐opening angle. Our model may be useful as a framework for modeling non‐thermal radiation produced within the stationary features.

Quasars remain one of the most powerful probes of the early Universe, providing measurements and constraints for re‐ionization, early structure formation, and early chemical enrichment epochs [1], [2], [3]. The obscured phase of QSOs evolution, as well as their accretion history, can be best followed by observing QSOs bright in MeV gamma‐rays [4]. By analysing resonant absorption troughs in spectral energy distribution of flaring QSOs one can measure the (baryonic) absorbing column, e.g. baryonic content, and metallicity of the QSO host galaxy [5], while the flare strength will give information on the accretion rate of the QSO powering supermassive black hole. By measuring the baryonic absorbing column for QSOs at different redshifts one can follow the early obscured evolution of AGN, as well as the evolution of the IGM transmission at the end of the cosmic re‐ionization epoch, while absorption by metals constrain the early chemical evolution of the IGM. These possibilities can be explored in the near fut...

From January to April 2001 the active galactic nucleus Markarian 421 showed a persistent high‐flux state and frequent flares with unprecedentedly rapid rise and fall times in the very‐high energy domain. We analyze the flare rise and fall times in a large 56‐night data sample with rather large statistics obtained with the HEGRA telescope CT1. A fine time‐structure analysis reveals typical rise and fall doubling times of about 20–30 minutes. The shortest detected rise and fall doubling times are 10–15 minutes. The central black‐hole mass of Mkn 421 implies that this rapid variability cannot be explained by objects moving around its last stable orbit.

Gamma‐ray bursts are observed to emit at least up to GeV energies, and their photon spectrum at the source is expected to extend up to TeV, due to either or both leptonic and hadronic mechanisms. I review some recent developments in GRB phenomenology in the light of Swift and other sub‐MeV measurement, as well as recent theoretical work. I discuss then the acceleration of cosmic rays in GRB, which can extend to GZK energies, and the possibility of acceleration in GRB‐related or other hypernovae. In both, synchrotron and inverse Compton, as well as hadronic processes, can lead to GeV‐TeV gamma‐rays measurable by GLAST, AGILE, or ACTs, providing useful probes of the burst physics and model parameters. Photo‐meson and pp interactions also produce neutrinos at energies ranging from sub‐TeV to EeV, which are targets for experiments such as IceCube, ANITA and KM3NeT.

We consider the opacity of TeV gamma rays emitted by the blazar PKS 2155‐304 resulting from pair production on soft photons emitted by the accretion disk. If the accretion disk emits radiation at anything like the flux implied by the standard Shakura‐Sunyaev model then the gamma rays must have been emitted from of order 100 gravitational radii from the black hole. If the disk radiation is significantly suppressed as a result of gravitational power being channelled into a disk wind then it is possible that the TeV gamma rays could have been emitted from within a few gravitational radii of the black hole.

The absorption of γ‐rays in the intergalactic medium due to the EBL (Extragalactic Background Light) causes the observed blazar spectrum to be fainter and softer than their intrinsic state. It could thus be expected to see an effective spectral‐softening trend with redshift. No such trend is evident in the sample of VHE blazars currently observed.To check which distributions of the properties of the parent blazar population could reproduce the observations, various simulations are done. The resulting subsamples that satisfy a generic detection criterion for the current generation of ACTs (Atmospheric Cherenkov Telescope) are checked to identify whether any inherent correlations (of spectral properties with redshift) are required to explain the current observations.

We investigate the centrifugally driven curvature drift instability to study how field lines twist close to the light cylinder surface of an AGN, through which the free motion of AGN winds can be monitored. By studying the dynamics of the relativistic MHD flow close to the light cylinder surface, we derive and solve analytically the dispersion relation of the instability by applying a single particle approach based on the centrifugal acceleration. Considering the typical values of AGN winds, it is shown that the timescale of the curvature drift instability is far less than the accretion process timescale, indicating that the present instability is very efficient and might strongly influence processes in AGN plasmas.

The Advanced Gamma‐Ray Imaging System (AGIS) is a concept for the next generation observatory in ground‐based very high energy gamma‐ray astronomy. Design goals are ten times better sensitivity, higher angular resolution, and a lower energy threshold than existing Cherenkov telescopes. Simulations show that a substantial improvement in angular resolution may be achieved if the pixel diameter is reduced to the order of 0.05 deg, i.e. two to three times smaller than the pixel diameter of current Cherenkov telescope cameras. At these dimensions, photon detectors with smaller physical dimensions can be attractive alternatives to the classical photomultiplier tube (PMT). Furthermore, the operation of an experiment with the size of AGIS requires photon detectors that are among other things more reliable, more durable, and possibly higher efficiency photon detectors. Alternative photon detectors we are considering for AGIS include both silicon photomultipliers (SiPMs) and multi‐anode photomultipliers (MAPMTs). H...

The current generation of Imaging Atmospheric telescopes (IACTs) has demonstrated the power of this observational technique, providing high sensitivity and an angular resolution of ∼0.1° per event above an energy threshold of ∼100 GeV. Planned future arrays of IACTs such as AGIS or CTA are aiming at significantly improving the angular resolution. Preliminary results have shown that values down to ∼1′ might be achievable. Here we present the results of Monte‐Carlo simulations that aim to exploring the limits of angular resolution for next generation IACTs and investigate how the resolution can be optimised by changes to array and telescope parameters such as the number of pixel in the camera, the field of view of the camera, the angular pixel size, the mirror size, and also the telescope separation.

High energy and very high energy (VHE; >10 GeV) γ‐rays are expected to be produced from the vicinity of super‐massive black holes (SMBH). In the magnetosphere of a spinning black hole acceleration of particles will take place in the field gaps. VHE emission from these particles is feasible via several processes. Systems surrounded by a low photon field will allow the emission to escape. In order to predict the level of emission of these systems, the mass of the central object and the intensity of the magnetic field need to be known. An estimate of the magnetic field is obtained here using the connection between BH magnetosphere and the magnetized accretion disk. Predicted fluxes and energy domain of the corresponding VHE emission are discussed.

Gamma‐ray absorption due to photon‐photon pair production of γ‐ray jet photons travelling in the external photon environment of a binary supermassive black hole system is considered. We investigate to what extent the location of a gamma‐ray emitter exposed to the anisotropic radiation field of both accretion disks affects the gamma‐ray opacity. The simple model analysed in this paper consists of a primary black hole surrounded by a standard accretion disk in symbiosis with a relativistic jet. The secondary black hole located at a given position nearby has a similar mass (or smaller) and is surrounded by a viscous Shakura‐Sunyaev small accretion disk. We show that for standard parameters of the accretion disks and a separation distance between the two black holes of 0.05 to 0.1 pc (close system), there is still a significant γ‐ray absorption from the secondary disk.

Radio and X‐ray observations towards the VHE gamma‐ray source HESS J1813‐178, initially dubbed as a “dark accelerator,” suggested its association with the SNR G12.8‐0.0. Subsequent XMM and Chandra observations confirmed earlier ASCA findings of an extended, center‐filled X‐ray source, not resembling the contours of the radio‐detected supernova remnant (SNR). Morphological and spectral resemblance of the X‐ray emission seen at the center of the shell of SNR G12.8‐0.0 with a Pulsar Wind Nebula (PWN) suggest HESS J1813‐178 is one of the increasing number of objects classified as “composite SNR.” In March 2007, 65 ksec Suzaku observations were taken, and we are able to confirm the previously reported characteristics of the X‐ray counterpart from HESS J1813‐178 as a diffuse hard X‐ray source central to the radio‐shell of SNR G12.8‐0.0.

In the unification scheme, radio quasars and FR II radio galaxies come from the same parent population, but viewed at different angles. Based on the Comptonization models for the γ‐ray emission from active galactic nuclei (AGNs), we estimate the number of radio quasars and FR II radio galaxies to be detected by the Gamma‐Ray Large Area Space Telescope (GLAST) using the luminosity function (LF) of their parent population derived from the flat‐spectrum radio quasar (FSRQ) LF. We find that ∼1200 radio quasars will be detected by GLAST, if the soft seed photons for Comptonization come from the regions outside the jets. We also consider the synchrotron self‐Comptonization(SSC) model, and find it unlikely to be responsible for γ‐ray emission from radio quasars. We find that no FR II radio galaxies will be detected by GLAST. Our results show that most radio AGNs to be detected by GLAST will be FSRQs (∼99% for the external Comptonization model, EC model), while the remainder (∼1%) will be steep‐spectrum radio qua...

We present the spectral characteristics of a sample of Intermediate‐Luminosity Broad‐Line Radio Galaxies in X‐rays, optical and radio. Here, we focus on the radio spectra acquired with the 100 m radio telescope in Effelsberg between 2.6 GHz and 32 GHz. These measurements reveal different spectral shapes urging for radio imaging that would disclose the source morphology. Comparing them with the X‐ray spectra acquired with XMM‐Newton, we find that sources with steep radio spectrum are heavily obscured whereas flat spectrum ones appear unabsorbed in accordance with unified scheme.

PSR B1259—63 is a 48 ms radio pulsar in a highly eccentric 3.4 year orbit with a Be star. Unpulsed γ‐ray, X‐ray and radio emission is observed from the binary system and the collision of the pulsar wind with the wind of Be star plays a crucial role in the generation of the observed emission. 2007 periastron passage was observed in unprecedented details with Suzaku, Swift, XMM‐Newton and Chandra. We present here the results of this campaign, compare them with the previ observations and discuss the physical implications.

The currently operating phase I of the imaging atmospheric Cherenkov telescope array H.E.S.S. consists of four telescopes, each with a 107 m2 reflector, which are arranged on the corners of a square of 120 m side length. At the moment, the collaboration is building a much larger 600 m2 telescope in the center of the array. This will not only increase the sensitivity in the currently accessible energy regime, but will in addition lower the energy threshold of the system. The H.E.S.S. II telescope will have a parabolic reflector of 36 m focal length consisting of 850 individual mirrors, which can be automatically aligned, similar to the H.E.S.S. I telescopes. The drive system will have a repositioning speed of 100 deg/min in elevation and 200 deg/min in azimuth and achieve an online tracking accuracy of ∼1 arcmin and an offline absolute pointing accuracy of ∼10 arcsec. This proceeding contribution presents details on the technical implementation of the telescope mechanics, optics and drive system.

We outline here preliminary performance estimates of an array of modest‐sized (24 m2) Cherenkov imaging telescopes for multi‐TeV γ‐ray astronomy. We have considered an example sparse array of 9 cells of 5 telescopes arranged in a sparsely separated grid, for which the intercell spacing is 1.5 km, and each cell has a side telescope spacing of 500 m. Cosmic‐ray background rejection makes use of scaled Hillas‐based parameters, and additionaly, we incorporate direction reconstruction exploiting the time development of Cherenkov images, aiming to improve the imaging method at large core distances (typically >200 m). In the energy range we focus on at present, 10 to 100 TeV, this array provides energy flux sensitivities approaching 3×10−13 erg cm−2 s−1, representing a factor 5 to 10 improvement over H.E.S.S. A low altitude (0.2 km) site also provides an improved collection area vs. that obtained at a medium altitude (1.8 km). These preliminary results serve as useful guide in designing a telescope array for multi‐TeV astronomy and further improvements in analysis and array design are under investigation.

The high kinetic energy outflowing in the jets of microquasars is delivered to the surrounding interstellar medium. This energy input can lead to the formation of bow shocks and cocoons that may emit from radio to gamma‐ray energies. Evidence and hints of emission from jet/medium interactions have been reported for some sources, but little has been done regarding the theoretical modelisation of the resulting non‐thermal emission. We have developed an analytical model for the interaction of microquasar jets with their surroundings based on those successfully applied in AGNs. Focusing on the adiabatic phase of the growing structures, we give estimations of the expected luminosities at high and very high energies, and study the detectability with present (MAGIC, HESS, VERITAS and GLAST) and future (CTA) gamma‐ray observatories.

The confinement of a relativistic cooling jet by the pressure and inertia of a surrounding matter is considered. It is demonstrated that, under certain conditions, even modest radiative losses behind the reconfinement shock can lead to a substantial focusing of the jet. The applications to blazars and M87 are discussed. Emphasize is given to the possibility that these recollimation nozzles may be the sources of the highly variable radiation observed in those objects.

Supermassive binary black holes (SBBHs) are a natural outcome of hierarchical galaxy mergers, yet phenomeno‐logical evidence for the existence of close (separation d⩽0.1 pc) SBBHs, that are able to coalesce within a Hubble time, still remains ambiguous. We show that low‐frequency quasi‐periodic variability observed from (jet‐aligned) radio‐loud Active Galactic Nuclei (AGNs) can provide substantial inductive support for the presence of close SBBHs at their centers: It is argued on physical grounds that such close binary systems are likely to give rise to different (although not independent) periodicities in the radio, optical and X‐ray/TeV regime, and, hence that detection of appropriate period ratios significantly corroborates the SBBH interpretation. We point out that a binary framework may further allow to reconcile some of the apparently divergent central mass estimates inferred from host galaxy and fast high energy variability observations, and in particular allow to relax minimum variability source c...

The next generation of ground-based gamma-ray observatories, such as e.g. CTA, will consist of about 50-100 telescopes, and cameras with in total ~100000 to ~200000 channels. The telescopes of the core array will cover and effective area of ~ 1 km2 and will be possibly accompanied by a large halo of smaller telescopes spread over about 10 km2 . In order to make maximum use of the stereoscopic approach, a very flexible inter-telescope trigger scheme is needed which allows to couple telescopes that located up to ~1 km apart. The development of a cost effective readout scheme for the camera signals exhibits a major technological challenge. Here we present ideas on a new asynchronous inter-telescope trigger scheme, and a very cost-effective, high-bandwidth frontend to backend data transfer system, both based on standard Ethernet components and an Ethernet front-end interface based on mass production standard FPGAs., 4 pages, 3 figures ; modified version of article to be published in the proceedings of the "4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy 2008"

Next generation ground based γ‐ray observatories such as AGIS1 and CTA2 are expected to cover a 1 km2 area with 50–100 imaging atmospheric Cherenkov telescopes. The stereoscopic view ol air showers using multiple view points raises the possibility to use a topological array trigger that adds substantial flexibility, new background suppression capabilities and a reduced energy threshold. In this paper we report on the concept and technical implementation of a fast topological trigger system, that makes use of real time image processing of individual camera patterns and their combination in a stereoscopic array analysis. A prototype system is currently under construction and we discuss the design and hardware of this topological array trigger system.

We recorded 600,000 (EASs) from Dec. 2006 to Oct. 2007 with a small array of water Cherenkov detectors. We found zenith and azimuth distributions of these events that confirm previous distributions from array of scintillator detectors. The study on EGRET sources are under consideration, besides, we have improved the size of our detectors to yield the best efficiency. The best shape for each detector will be a tank with 40 cm in diameter and 60 cm in height with a PMT in the center of lid.

The Milky Way has been estabished to emit gamma rays. These gamma rays are presumably dominated by decays of neutral pions, although inverse Compton scatterings and bremsstrahlung also contribute. It is plausible that other galaxies can be diffuse sources of gamma rays in a similar manner. Starburst galaxies are particularly interesting to study as they are expected to have much higher cosmic-ray fluxes and interstellar matter densities. The neutral pions are created in cosmic-ray interactions with interstellar matter. Presented here is an overview of the recent work by Karlsson and co-workers on proton-proton interactions and the resulting secondary particle inclusive cross sections and angular distributions. This model can be used to calculated the $\pi^{0}$ component of the gamma-ray yield and spectrum from a starburst galaxy. The yield is expected to increase significantly (30% to 50%) and the spectrum to be harder than the incident proton spectrum., Comment: 4 pages, 4 figures, submitted for the Proceedings of the 4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, July 7-11, 2008, in Heidelberg, Germany

We estimate the radiation produced at high energies by the cluster of galaxies Abell 3376, which is characterized by double opposite, giant ringlike radio structures. These recently detected radio relics support the presence of relativistic particles that are assumed to be accelerated in‐situ by a diffusive shock mechanism. We calculate the spectral energy distribution resulting from the most relevant non‐thermal processes (synchrotron radiation, inverse Compton (IC) scattering, relativistic Bremsstrahlung, and inelastic proton‐proton collisions). From our specific model, we find that the major radiative component at high energies is IC scattering. Hadronic interactions would yield a minor contribution to the overall non‐thermal emission, but would dominate at ultra‐high energies. The nearby cluster Abell 3376 might be detectable at gamma‐rays by GLAST satellite and Cherenkov arrays.

We report X‐ray imaging of the powerful FR II radio galaxy 3C 353 using the Chandra X‐ray Observatory. 3C 353’s two 4”‐wide and 2’‐long jets allow us to study in detail the internal structure of the large‐scale relativistic outflows at both radio and X‐ray photon energies with the sub‐arcsecond spatial resolution provided by the VLA and Chandra instruments. In a 90 ks Chandra observation, we have detected X‐ray emission from most radio structures in 3C 353, including the nucleus, the jet and the counterjet, the terminal jet regions (hotspots), and one radio lobe. We show that the detection of the X‐ray emission associated with the radio knots and counterknots, which is most likely non‐thermal in origin, puts several crucial constraints on the X‐ray emission mechanisms in powerful large‐scale jets of quasars and FR II sources. In particular, we show that this detection is inconsistent with the inverse‐Compton model proposed in the literature, and instead implies a synchrotron origin of the X‐ray jet photons. We also find that the width of the X‐ray counterjet is possibly narrower than that measured in radio bands, that the radio‐to‐X‐ray flux ratio decreases systematically downstream along the jets, and that there are substantial (kpc‐scale) offsets between the positions of the X‐ray and radio intensity maxima within each knot, whose magnitudes increase away from the nucleus. We discuss all these findings in the wider context of the physics of extragalactic jets, proposing some particular though not definitive solutions or interpretations for each problem. In general, we find that the synchrotron X‐ray emission of extragalactic large‐scale jets is not only shaped by the global hydrodynamical configuration of the outflows, but is also likely to be very sensitive to the microscopic parameters of the jet plasma. A complete, self‐consistent model for the X‐ray emission of extragalactic jets still remains elusive.

The recent, extreme episodes of variability detected from Blazars by the leading atmospheric Cerenkov experiments motivate the development and application of specialized statistical techniques that enable the study of this rich data set to its furthest extent. The identification of the shortest variability timescales supported by the data and the actual variability structure observed in the light curves of these sources are some of the fundamental aspects being studied, that answers can bring new developments on the understanding of the physics of these objects and on the mechanisms of production of VHE gamma‐rays in the Universe. Some of our efforts in studying the time variability of VHE sources involve the application of dynamic programming algorithms to the problem of detecting change‐points in a Poisson sequence. In this particular paper we concentrate on the more primary issue of the applicability of counting statistics to the analysis of time‐series on VHE gamma‐ray astronomy.

One of the key scientific objectives of the new generation high energy instruments is the quest for signatures from the extragalactic background light (EBL) at UV/optical/IR energies and its evolution by means of photon‐photon absorption over extragalactic distances.I will discuss the various methods proposed, and biases that may be introduced when studying the evolution of the EBL with capable γ‐ray observatories like e.g. GLAST or CTA, where the γ‐ray horizon is probed by means of statistical analysis of absorption features in AGN spectra at various redshifts. In particular, the effect of the redshift‐dependence of “local opacity” in γ‐ray loud quasars due to possible γ‐ray absorption through photon‐photon pair production of jet photons in the external photon environments (accretion disk, broad‐line region radiation field) on evolutionary studies of the EBL is highlighted.

Self‐annihilating dark matter contributes to the extra galactic very high‐energy γ‐ray background. This contribution is expected to be anisotropic following the density distribution of non‐baryonic dark matter. We explore the possibilities to search for these anisotropies with present and future ground‐based gamma‐ray experiments like H.E.S.S., MAGIC, or CTA. A multipole‐expansion of simulated events is used to investigate the sensitivity for anisotropies detectable with narrow field of view observations.

We present a data set derived from ∼50 ksec continuous Suzaku observations and covered with quasi‐simultaneous TeV‐observations (HESS, MAGIC) of two of the more distant TeV‐blazars detected to date: 1ES 1101‐232 and 1ES 1553+113. Both sources are found in a non‐variable state with combined XIS‐PIN spectra indicating downward curvature up to several tens of keV. 1ES 101–232 was found in a quiet state with the lowest X‐ray flux ever measured. We discuss the contemporaneous broadband spectral energy distribution (SED) of both sources and implications from absorption in the EBL for the redshift of 1ES 1553+113.

Blazar observations in VHE Gamma‐rays show intensity variations on time scales of minutes to years, most frequently with variability times of about one day. They could be caused by the interaction of relativistic jets with the surroundings, but also carry the signature of internal processes of the central engine, possibly binary systems of supermassive black holes. Ultimately, long‐term monitoring with 24‐hour coverage is needed in addition to the shorter high sensitivity exposures provided by telescopes such as MAGIC, VERITAS and H.E.S.S., in order to study the physical origin of such flaring activity. This can be achieved with a global network of small robotic Cherenkov telescopes. As a first step, we plan to set up a fully dedicated small Cherenkov telescope and carry out joint observations with the Whipple 10 m monitor telescope. The new low cost, but high performance telescope will be the upgrade of one of the former HEGRA telescopes, still located at the Observatorio del Roque de los Muchachos on the Canarian Island of La Palma (Spain). The most important novelties will be its robotic operation and novel camera, greatly improving its sensitivity and lowering the energy threshold.

With the MAGIC experiment, the software development of the MAGIC Analysis and Reconstruction Software (MARS) has been started. In the past years, the modular concept was improved and enhanced by a full automation concept. Both together build a user‐friendly and modular framework fulfilling the requirements of future observatories like small monitoring telescopes or high‐sensitivity instruments. To better distinguish this enhanced development from the MARS version further developed as a MAGIC specialized analysis, we have given it the name MARS (Now: Modular Analysis and Reconstruction Software)—Cherenkov Observatory Edition (CheObs). We will show that the existing concept, together with the latest improvements and the automation concept, is well suited as a framework for future Cherenkov observatories.

An increase in quantum efficiency in photodetectors could result in a proportional reduction in the area of atmospheric Cherenkov telescopes and an even larger reduction in cost. We report on the development of high quantum efficiency, high gain, UV/blue photon‐counting detectors based on AlGaN/InGaN photocathode heterostructures grown by molecular beam epitaxy. This research could eventually result in nearly ideal light detectors with a number of distinct advantages over existing technologies for numerous applications in high‐energy physics and particle astrophysics. Potential advantages include much lower noise detection, better stability and radiation resistance than other cathode structures, high VUV sensitivity and very low radioactive background levels for deep underground experiments, and high detection efficiency for the detection of individual VUV‐visible photons. We are also developing photocathodes with intrinsic gain, initially improving the detection efficiency of hybrid semiconductor‐vacuum tube devices and eventually leading to a new type of all‐solid‐state photomultiplier device.

In this work we use a simple linear analysis of the GRBs pulse shape around maximum to have an estimation for the k‐correction of temporal lags. We find a simple result for high energy GRBs.

FERMI (formely GLAST) will shortly provide crucial information on relativistic particles in galaxy clusters. We discuss non-thermal emission in the context of general calculations in which relativistic particles (protons and secondary electrons due to proton-proton collisions) interact with MHD turbulence generated in the cluster volume during cluster mergers. Diffuse cluster-scale radio emission (Radio Halos) and hard X-rays are produced during massive mergers while gamma ray emission, at some level, is expected to be common in galaxy clusters., 4 pages, 2 Figure, proc. of the 4th Heidelberg International Symposium on High Energy Gamma-ray Astronomy

The Advanced Gamma‐ray Imaging System (AGIS) is a US‐led concept for a next‐generation instrument in ground‐based very‐high‐energy gamma‐ray astronomy. The most important design requirement for AGIS is a sensitivity of about 10 times greater than current observatories like Veritas, H.E.S.S or MAGIC. We present results of simulation studies of various possible designs for AGIS. The primary characteristics of the array performance, collecting area, angular resolution, background rejection, and sensitivity are discussed.

The Galactic gamma‐ray source HESS J1838‐069 was discovered with H.E.S.S. in the 2004 galactic plane survey. The source is extended and is among the brightest H.E.S.S. survey sources. To identify the nature of this source, its central part was observed by the Chandra X‐ray Observatory. The morphological and spectral characteristics of two possible counterparts, AX J1838.0‐0655 and AX J1837.3‐0652, are presented here. From the morphology and spectra, both sources are combatible with pulsar wind nebula scenarios. However, because of the high absorption column, the nebula around AX J1838.0‐0655 might also be explained by a scattering halo around the central point source. We compare the Chandra data to expectations in this scenario.

We present simultaneous optical (BVRI) and X‐ray (RXTE PCA) data on the quasar 3C279 from the day of the recent VHE detection by MAGIC and discuss the implications of the snap‐shot spectral energy distribution (SED) for leptonic jet models of blazars. A one‐zone synchrotron‐self‐Compton origin of the SED up to VHE γ‐rays can be ruled out. The VHE emission could, in principle, be interpreted as Compton upscattering of external radiation (e.g., from the broad‐line regions) in a one‐zone leptonic model. However, such an interpretation would require either an unusually low magnetic field of B∼0.03 G, or (in order to achieve approximate equipartition between magnetic field at B∼0.25 G and relativistic electrons) an unrealistically high Doppler factor of Γ∼140. In addition, such a model fails to reproduce the observed X‐ray flux. We therefore conclude that a simple one‐zone, homogeneous leptonic jet model is not able to plausibly reproduce the SED of 3C279 including the recently detected VHE γ‐ray emission. Thi...