arXiv subjects

Andreas Zech

Publications and source records attributed to Andreas Zech.

31 records · Page 2Linked to original sources

arXiv2017

The relativistic jet of the gamma-ray emitting narrow-line Seyfert 1 galaxy 1H 0323+342

The detection of several radio-loud narrow-line Seyfert 1 (NLS1) galaxies by the Fermi Gamma-Ray Space Telescope hints at the existence of a rare, new class of gamma-ray emitting active galactic nuclei with low black hole masses. Like flat spectrum radio quasars (FSRQs), their gamma-ray emission is thought to be produced via the external Compton mechanism whereby relativistic jet electrons upscatter a photon field external to the jet, e.g. from the accretion disc, broad line region (BLR) and dusty torus, to higher energies. Here we study the origin of the gamma-ray emission in the lowest-redshift candidate among the currently-known gamma-ray emitting NLS1s, 1H 0323+342, and take a new approach. We observationally constrain the external photon field using quasi-simultaneous near-IR, optical and X-ray spectroscopy. Applying a one-zone leptonic jet model, we simulate the range of jet parameters for which this photon field, when Compton scattered to higher energies, can explain the gamma-ray emission. We find that the site of the gamma-ray emission lies well within the BLR and that the seed photons mainly originate from the accretion disc. The jet power that we determine, $1.0 \times 10^{45}$ erg s$^{-1}$, is approximately half the accretion disc luminosity. We show that this object is not simply a low-mass FSRQ, its jet is intrinsically less powerful than predicted by scaling a typical FSRQ jet by black hole mass and accretion rate. That gamma-ray emitting NLS1s appear to host underpowered jets may go some way to explaining why so few have been detected to date.

astro-ph.GA↗

arXiv2017

Gammapy: high level data analysis for extragalactic science cases with the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) observatory will probe the non-thermal universe above 20 GeV up to several hundreds of TeV with a significant improvement in sensitivity and angular resolution compared to current experiments. Its outstanding capabilities will allow to increase the number of extragalactic cosmic accelerators detected at very high energy (VHE) and therefore to better constrain the population of VHE accelerators and the gamma-ray absorption processes in the intergalactic medium. For the first time in the history of imaging atmospheric Cherenkov telescopes (IACTs), CTA will be an open observatory and high-level data will be made available to the astronomical community. Gammapy is an open-source Python package developed by the Cherenkov telescope community that provides tools to simulate the gamma-ray sky and analyse IACT data. The versatile architecture of, and steady user contributions to Gammapy enable a large variety of high-level data analyses. Examples of Gaammapy applications are presented, particularly in the context of extragalactic science with CTA.

astro-ph.HE↗

arXiv2017

The Giant Radio Array for Neutrino Detection (GRAND): Present and Perspectives

The Giant Radio Array for Neutrino Detection (GRAND) aims at detecting ultra-high energy extraterrestrial neutrinos via the extensive air showers induced by the decay of tau leptons created in the interaction of neutrinos under the Earth's surface. Consisting of an array of $\sim10^5$ radio antennas deployed over $\sim 2\times10^5\,\rm {km}^2$, GRAND plans to reach, for the first time, an all-flavor sensitivity of $\sim1.5\times10^{-10} \,\rm GeV\, cm^{-2} \,s^{-1}\, sr^{-1}$ above $5\times10^{17}$ eV and a sub-degree angular resolution, beyond the reach of other planned detectors. We describe here preliminary designs and simulation results, plans for the ongoing, staged approach to the construction of GRAND, and the rich research program made possible by GRAND's design sensitivity and angular resolution.

astro-ph.IM↗

arXiv2017

Expected signatures from hadronic emission processes in the TeV spectra of BL Lac objects

The wealth of recent data from imaging air Cherenkov telescopes (IACTs), ultra-high energy cosmic-ray experiments and neutrino telescopes have fuelled a renewed interest in hadronic emission models for gamma-loud blazars. We explore physically plausible solutions for a lepto-hadronic interpretation of the stationary emission from high-frequency peaked BL Lac objects (HBLs). The modelled spectral energy distributions are then searched for specific signatures at very high energies that could help to distinguish the hadronic origin of the emission from a standard leptonic scenario. By introducing a few basic constraints on parameters of the model, such as assuming the co-acceleration of electrons and protons, we significantly reduced the number of free parameters. We then systematically explored the parameter space of the size of the emission region and its magnetic field for two bright gamma-loud HBLs, PKS 2155-304 and Mrk 421. For all solutions close to equipartition between the energy densities of protons and of the magnetic field, and with acceptable jet power and light-crossing timescales, we inspected the spectral hardening in the multi-TeV domain from proton-photon induced cascades and muon-synchrotron emission inside the source. Very-high-energy spectra simulated with the available instrument functions from the future Cherenkov Telescope Array (CTA) were evaluated for detectable features as a function of exposure time, source redshift, and flux level. Over a large range of model parameters, the spectral hardening due to internal synchrotron-pair cascades, the "cascade bump", should be detectable for acceptable exposure times with the future CTA for a few nearby and bright HBLs.

astro-ph.HE↗

arXiv2015

Monte Carlo Studies of the GCT Telescope for the Cherenkov Telescope Array

The GCT is an innovative dual-mirror solution proposed for the small-size telescopes for CTA, capable of imaging primary cosmic gamma-rays from below a TeV to hundreds of TeV. The reduced plate scale resulting from the secondary optics allows the use of compact photosensors, including multi-anode photomultiplier tubes or silicon photomultipliers. We show preliminary results of Monte Carlo simulations using the packages CORSIKA and Sim_telarray, comparing the relative performance of each photosensor type. We also investigate the effect of the secondary optics in terms of optical performance, image resolution and camera response. With the ongoing commissioning of the prototype structure and camera, we present the preliminary expected performance of GCT.

astro-ph.IM↗

arXiv2014

Equipartition Gamma-Ray Blazars and the Location of the Gamma-Ray Emission Site in 3C 279

Blazar spectral models generally have numerous unconstrained parameters, leading to ambiguous values for physical properties like Doppler factor delta or fluid magnetic field B'. To help remedy this problem, a few modifications of the standard leptonic blazar jet scenario are considered. First, a log-parabola function for the electron distribution is used. Second, analytic expressions relating energy loss and kinematics to blazar luminosity and variability, written in terms of equipartition parameters, imply delta, B', and the principal electron Lorentz factor gamma'_pk. The external radiation field in a blazar is approximated by Ly alpha radiation from the broad line region (BLR) and ~0.1 eV infrared radiation from a dusty torus. When used to model 3C 279 SEDs from 2008 and 2009 reported by Hayashida et al. (2012), we derive delta ~ 20-30, B' ~ few G, and total (IR + BLR) external radiation field energy densities u ~ 0.01 - 0.001 erg/cm^3, implying an origin of the gamma-ray emission site in 3C 279 at the outer edges of the BLR. This is consistent with the gamma-ray emission site being located at a distance R <~ Gamma^2 c t_{var} ~ 0.1 (Γ/30)^2 (t_{var}/10^4 s) pc from the black hole powering 3C 279's jets, where t_{var} is the variability time scale of the radiation in the source frame, and at farther distances for narrow-jet and magnetic_reconnection models. Excess >~ 5 GeV gamma-ray emission observed with Fermi LAT from 3C 279 challenge the model, opening the possibility of hadronic origins of the emission. For low hadronic content, absolute jet powers of ~10% of the Eddington luminosity are calculated.

astro-ph.HE↗

arXiv2013

Constraining the parameter space of the one-zone synchrotron-self-Compton model for GeV-TeV detected BL Lac objects

The one-zone synchrotron-self-Compton (SSC) model aims to describe the spectral energy distribution (SED) of BL Lac objects via synchrotron emission by a non-thermal population of electrons and positrons in a single homogeneous emission region, partially upscattered to gamma-rays by the particles themselves. The model is usually considered as degenerate, given that the number of free parameters is higher than the number of observables. It is thus common to model the SED by choosing a single set of values for the SSC-model parameters that provide a good description of the data, without studying the entire parameter space. We present here a new numerical algorithm which permits us to find the complete set of solutions, using the information coming from the detection in the GeV and TeV energy bands. The algorithm is composed of three separate steps: we first prepare a grid of simulated SEDs and extract from each SED the values of the observables; we then parametrize each observable as a function of the SSC parameters; we finally solve the system for a given set of observables. We iteratively solve the system to take into account uncertainties in the values of the observables, producing a family of solutions. We present a first application of our algorithm to the typical high-frequency-peaked BL Lac object 1RXS J101015.9-311909, provide constraints on the SSC parameters, and discuss the result in terms of our understanding of the blazar emitting region.

astro-ph.HE↗

arXiv2013

Gamma-ray blazars near equipartition and the origin of the GeV spectral break in 3C 454.3

Observations performed with the Fermi-LAT telescope have revealed the presence of a spectral break in the GeV spectrum of flat-spectrum radio quasars (FSRQs) and other low- and intermediate-synchrotron peaked blazars. We propose that this feature can be explained by Compton scattering of broad-line region (BLR) photons by a non-thermal population of electrons described by a log-parabolic function. We consider in particular a scenario in which the energy densities of particles, magnetic field, and soft photons in the emitting region are close to equipartition. We show that this model can satisfactorily account for the overall spectral energy distribution of the FSRQ 3C 454.3, reproducing the GeV spectal cutoff due to Klein-Nishina effects and a curving electron distribution.

astro-ph.HE↗

arXiv2012

A mixed lepto-hadronic scenario for PKS 2155-304

The models developed to describe the spectral energy distribution (SED) of blazars can be divided into leptonic or hadronic scenarios, according to the particles responsible for the high-energy component. We have developed a new stationary code which computes all the relevant leptonic and hadronic processes, permitting the study of both leptonic and hadronic scenarios in a consistent way. Interestingly, mixed lepto-hadronic scenarios (in which both components contribute to the high energy emission) naturally arise in this framework. We present the first application to the well known BL Lac object PKS 2155-304.

astro-ph.HE↗

arXiv2011

Lepto-hadronic modelling of blazar emission

The characteristic double-bumped spectral energy distribution (SED) of blazars is explained by either leptonic or hadronic models. In the former, Inverse Compton emission dominates the emission of the high energy bump, while proton synchrotron emission and proton-gamma interactions dominate it in the latter. We present a new stationary lepto-hadronic code that evaluates both the leptonic and the hadronic interactions. Apart from the modelling of the SED produced in a leptonic or hadronic model, the code permits the study of interesting mixed lepto-hadronic scenarios, where both processes contribute significantly to the high energy bump. A first application to data from the high frequency peaked BL Lac object PKS 2155-304 is discussed.

astro-ph.CO↗

arXiv2006

Recent results from the Pierre Auger Observatory

The goal of the Pierre Auger Observatory is to determine the still unknown nature and origin of ultra-high energy cosmic rays. The study of these elusive particles probes astrophysical sites of particle acceleration as well as fundamental interactions at energies unattainable in accelerator facilities. Auger combines two observational techniques, a large surface array and air fluorescence detectors, to observe the extended air showers generated in the atmosphere by cosmic rays. This hybrid observation yields an excellent resolution and allows for important cross-checks. The Auger South site, located in Mendoza (Argentina), is now nearing completion, with 60% of its surface array and three out of its four fluorescence detectors in operation. First results on the energy spectrum measurement, the search for anisotropies in arrival directions and the upper limit on the photon fraction are discussed.

astro-ph↗

arXiv2005

Studies of Systematic Uncertainties in the HiRes-II Measurement of the UHECR Spectrum

We present studies of systematic uncertainties in the measurement of the ultra-high energy cosmic ray (UHECR) spectrum with the FADC detector of the High Resolution Fly's Eye experiment (HiRes-II). One source of uncertainties lies in the simulation of the energy dependent aperture of the air fluorescence detector. We study the impact of changes in the energy spectrum and composition that are used as input to the aperture simulation. We also compare aperture estimates for two different hadronic interaction models - QGSJet and SIBYLL. Systematic uncertainties may further be introduced by the modeling of the aerosol component of the atmosphere. We have repeated the HiRes-II monocular analysis using an atmospheric database with hourly entries instead of our measurement of the average aerosol content. We will discuss changes in reconstructed energies and in the resulting spectrum.

astro-ph↗

arXiv2004

A Measurement of the UHECR Spectrum with the HiRes FADC Detector

We have measured the energy spectrum of ultra-high energy cosmic rays (UHECR) with the HiRes FADC detector (HiRes-2) in monocular mode. A detailed Monte Carlo simulation of the detector response to air showers has been used to calculate the energy dependent acceptance of the air fluorescence detector. The measured spectrum complements the measurement by the HiRes-1 detector down to lower energies. Systematic effects of the assumed input spectrum and composition on the aperture are presented, as well as systematics due to the atmosphere.

astro-ph↗