B. Giebels

Active Galactic Nuclei under the scrutiny of CTA

Abstract Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer excellent conditions for efficient particle acceleration in internal and external shocks, turbulence, and magnetic reconnection events. The jets as well as particle accelerating regions close to the supermassive black holes (hereafter SMBH) at the intersection of plasma inflows and outflows, can produce readily detectable very high energy gamma-ray emission. As of now, more than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the present ground-based gamma-ray telescopes, which represents more than one third of the cosmic sources detected so far in the VHE gamma-ray regime. The future Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the VHE range by about one order of magnitude, shedding new light on AGN population studies, and AGN classification and unification schemes. CTA will be a unique tool to scrutinize the extreme high-energy tail of accelerated particles in SMBH environments, to revisit the central engines and their associated relativistic jets, and to study the particle acceleration and emission mechanisms, particularly exploring the missing link between accretion physics, SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an extremely rewarding observing program which will inform us about the inner workings and evolution of AGN. Furthermore these AGN are bright beacons of gamma-rays which will allow us to constrain the extragalactic infrared and optical backgrounds as well as the intergalactic magnetic field, and will enable tests of quantum gravity and other “exotic” phenomena.

Cascading on extragalactic background light

Context. High-energy γ-rays propagating in the intergalactic medium can interact with background infrared photons to produce e + e - pairs, resulting in the absorption of the intrinsic γ-ray spectrum. TeV observations of the distant blazar 1ES I 101-232 were thus recently used to put an upper limit on the infrared extragalactic background light density. Aims. The created pairs can upscatter background photons to high energies, which in turn may pair produce, thereby initiating a cascade. The pairs diffuse on the extragalactic magnetic field (EMF) and cascade emission has been suggested as a means for measuring its intensity. Limits on the IR background and EMF are reconsidered taking into account cascade emissions. Methods. The cascade equations are solved numerically. Assuming a power-law intrinsic spectrum, the observed 100 MeV-100 TeV spectrum is found as a function of the intrinsic spectral index and the intensity of the EMF. Results. Cascades emit mainly at or below 100 GeV. The observed TeV spectrum appears softer than for pure absorption when cascade emission is taken into account. The upper limit on the IR photon background is found to be robust. Inversely, the intrinsic spectra needed to fit the TeV data are uncomfortably hard when cascade emission makes a significant contribution to the observed spectrum. An EMF intensity around 10 -8 nG leads to a characteristic spectral hump in the GLAST band. Higher EMF intensities divert the pairs away from the line-of-sight and the cascade contribution to the spectrum becomes negligible.

Evidence for a cosmological effect in γ-ray spectra of BL Lacertae

We update the list of GeV-TeV extragalactic γ-ray sources using the two-year catalog from the Fermi Large-Area Telescope (LAT) and recent results from ground-based γ-ray telescopes. Breaks in the spectra between the high-energy (100 MeV 200 GeV) ranges, as well as their dependence on distance, are discussed in the context of absorption on the extragalactic background light (EBL). We calculate the size of the expected break using a model for the EBL and compare it to the data, taking into account systematic uncertainties in the measurements. We develop a novel Bayesian model to describe this dataset and use it to constrain two simple models for the EBL-induced breaks.

Preliminary results of the LAT Calibration Unit beam tests

The calibration strategy of the GLAST Large Area Telescope (LAT) combines analysis of cosmic ray data with accelerator particle beams measurements. An advanced Monte Carlo simulation of the LAT, based on the Geant4 package, was set up to reproduce the LAT response to such radiation and to benchmark the event reconstruction and the background rejection strategy before launch and during operation. To validate the LAT simulation, a massive campaign of beam tests was performed between July and November 2006, in parallel with the LAT integration and test, on the LAT Calibration Unit. This is a detector built with spare flight modules and flight‐like readout electronics, which was exposed to a large variety of beams, representing the whole spectrum of the signal that will be detected by the LAT, using the CERN and the GSI accelerator facilities. Beams of photons (0 – 2.5 GeV), electrons (1 – 300 GeV), hadrons (π and p, a few GeV – 100 GeV) and ions (C; Xe, 1.5 GeV/n) were shot through the CU to measure the phys...

The minijets-in-a-jet statistical model and the rms-flux correlation

The flux variability of blazars at very high energies does not have a clear origin. Flux variations on time scales down to the minute suggest that variability originates in the jet, where a relativistic boost can shorten the observed time scale, while the linear relation between the flux and its rms or the skewness of the flux distribution suggests that the variability stems from multiplicative processes, which are associated in some models with the accretion disk. We study the rms-flux relation and emphasize its link to Pareto distributions, characterized by a power-law probability density function. Such distributions are naturally generated within a minijets-in-a-jet statistical model, in which boosted emitting regions are isotropically oriented within the bulk relativistic flow of a jet. We prove that, within this model, the flux of a single minijet is proportional to its rms. This relation still holds when considering a large number of emitting regions, for which the distribution of the total flux is skewed and could be interpreted as being log-normal. The minijets-ina-jet statistical model reconciles the fast variations and the statistical properties of the flux of blazars at very high energies.

The camera of the fifth H.E.S.S. telescope. Part I: System description

In July 2012, as the four ground-based gamma-ray telescopes of the H.E.S.S. (High Energy Stereoscopic System) array reached their tenth year of operation in Khomas Highlands, Namibia, a fifth telescope took its first data as part of the system. This new Cherenkov detector, comprising a 614.5 m

Performance measurement of HARPO: A time projection chamber as a gamma-ray telescope and polarimeter

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First Nustar Observations of the Bl Lac-Type Blazar Pks 2155-304: Constraints on the Jet Content and Distribution of Radiating Particles

reflector with a highly pixellized camera in its focal plane, improves the sensitivity of the current array by a factor two and extends its energy domain down to a few tens of GeV. The present part I of the paper gives a detailed description of the fifth H.E.S.S. telescopes camera, presenting the details of both the hardware and the software, emphasizing the main improvements as compared to previous H.E.S.S. camera technology.

A Luminous and Isolated Gamma-Ray Flare from the Blazar B2 1215+30

Abstract We analyse the performance of a gas time projection chamber (TPC) as a high-performance gamma-ray telescope and polarimeter in the e + e − pair-creation regime. We use data collected at a gamma-ray beam of known polarisation. The TPC provides two orthogonal projections (x, z) and (y, z) of the tracks induced by each conversion in the gas volume. We use a simple vertex finder in which vertices and pseudo-tracks exiting from them are identified. We study the various contributions to the single-photon angular resolution using Monte Carlo simulations, compare them with the experimental data and find that they are in excellent agreement. The distribution of the azimuthal angle of pair conversions shows a bias due to the non-cylindrical-symmetric structure of the detector. This bias would average out for a long duration exposure on a space mission, but for this pencil-beam characterisation we have ensured its accurate simulation by a double systematics-control scheme, data taking with the detector rotated at several angles with respect to the beam polarisation direction and systematics control with a non-polarised beam. We measure, for the first time, the polarisation asymmetry of a linearly polarised gamma-ray beam in the low energy pair-creation regime. This sub-GeV energy range is critical for cosmic sources as their spectra are power laws which fall quickly as a function of increasing energy. This work could pave the way to extending polarised gamma-ray astronomy beyond the MeV energy regime.

From radio to TeV: the surprising spectral energy distribution of AP Librae

We report the first hard X-ray observations with NuSTAR of the BL Lac type blazar PKS 2155-304, augmented with soft X-ray data from XMM-Newton and gamma-ray data from the Fermi Large Area Telescope, obtained in April 2013 when the source was in a very low flux state. A joint NuSTAR and XMM spectrum, covering the energy range 0.5 - 60 keV, is best described by a model consisting of a log-parabola component with curvature beta = 0.3(+0.2,-0.1) and a (local) photon index 3.04 +/- 0.15 at photon energy of 2 keV, and a hard power-law tail with photon index 2.2 +/- 0.4. The hard X-ray tail can be smoothly joined to the quasi-simultaneous gamma-ray spectrum by a synchrotron self-Compton component produced by an electron distribution with index p = 2.2. Assuming that the power-law electron distribution extends down to the minimum electron Lorentz factor gamma_min = 1 and that there is one proton per electron, an unrealistically high total jet power L_p of roughly 10^47 erg/s is inferred. This can be reduced by two orders of magnitude either by considering a significant presence of electron-positron pairs with lepton-to-proton ratio of at least 30, or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor gamma_br1 of roughly 100. In either case, the jet composition is expected to be strongly matter-dominated.