arXiv subjects

A. Reimer

Publications and source records attributed to A. Reimer.

69 records · Page 4Linked to original sources

arXiv2010

The Connection between Radio and Gamma-ray Emission in Active Galactic Nuclei

Radio and gamma-ray emission from active galactic nuclei (AGN) are thought to share a common origin, related to the ejection phenomena in the vicinity of supermassive black holes. Thanks to its sensitivity, surveying capability, and broad energy range, the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope has permitted us to discover and characterize a huge number of extragalactic gamma-ray sources. Similarly to what was found by EGRET, these sources are typically associated with blazars, characterized by significant radio emission and flat spectrum. The radio luminosity distribution is extended over 7 orders of magnitudes, with flat spectrum radio quasars clustered at higher powers and BL Lacs more scattered; the average spectral index is consistent with alpha=0, although a few remarkable sources have alpha>0.5. A comparison of the radio flux density and the gamma-ray photon flux is presented, although claims on its significance require a detailed discussion and Monte Carlo simulations which will be presented in a future paper.

astro-ph.CO↗

arXiv2009

The new surprising behaviour of the two "prototype" blazars PKS 2155-304 and 3C 279

Recent VHE observations have unveiled a surprising behaviour in two well-known blazars at opposite sides of the blazar sequence. PKS 2155-304 have shown for the first time in an HBL a large Compton dominance, high gamma-ray luminosities and a cubic relation between X-ray and VHE fluxes. 3C 279 is the first FSRQ detected at VHE. The high luminosity required to overcome the significant absorption caused by the BLR emission cannot be easily reconciled with the historical and quasi-simultaneous SED properties. Both cases shed a new light on the structure and ambient fields of blazars. Contrary to previous claims, it is also shown that 3C 279 --as any FSRQ-- cannot in general provide robust constraints on the EBL.

astro-ph.CO↗

arXiv2009

Parameter constraints for high-energy models of colliding winds of massive stars: the case WR 147

We explore the ability of high energy observations to constrain orbital parameters of long period massive binary systems by means of an inverse Compton model acting in colliding wind environments. This is particular relevant for (very) long period binaries where orbital parameters are often poorly known from conventional methods, as is the case e.g. for the Wolf-Rayet (WR) star binary system WR 147 where INTEGRAL and MAGIC upper limits on the high-energy emission have recently been presented. We conduct a parameter study of the set of free quantities describing the yet vaguely constrained geometry and respective effects on the non-thermal high-energy radiation from WR 147. The results are confronted with the recently obtained high-energy observations and with sensitivities of contemporaneous high-energy instruments like Fermi-LAT. For binaries with sufficient long periods, like WR 147, gamma-ray attenuation is unlikely to cause any distinctive features in the high-energy spectrum. This leaves the anisotropic inverse Compton scattering as the only process that reacts sensitively on the line-of-sight angle with respect to the orbital plane, and therefore allows the deduction of system parameters even from observations not covering a substantial part of the orbit. Provided that particle acceleration acts sufficiently effectively to allow the production of GeV photons through inverse Compton scattering, our analysis indicates a preference for WR 147 to possess a large inclination angle. Otherwise, for low inclination angles, electron acceleration is constrained to be less efficient as anticipated here.

astro-ph.HE↗

arXiv2008

A Hard X-Ray View of Two Distant VHE Blazars: 1ES 1101-232 and 1ES 1553+113

TeV blazars are known as prominent nonthermal emitters across the entire electromagnetic spectrum with their photon power peaking in the X-ray and TeV bands. If distant, absorption of gamma-ray photons by the extragalactic background light (EBL) alters the intrinsic TeV spectral shape, thereby affecting the overall interpretation. Suzaku observations for two of the more distant TeV blazars known to date, 1ES 1101-232 and 1ES 1553+113, were carried out in 2006 May and July, respectively, including a quasi-simultaneous coverage with the state-of-the-art Cerenkov telescope facilities. We report on the resulting data sets with emphasis on the X-ray band and set in context to their historical behavior. During our campaign, we did not detect any significant X-ray or gamma-ray variability. 1ES 1101-232 was found in a quiescent state with the lowest X-ray flux ever measured. The combined XIS and HXD PIN data for 1ES 1101-232 and 1ES 1553+113 clearly indicate spectral curvature up to the highest hard X-ray data point (~30 keV), manifesting as softening with increasing energy. We describe this spectral shape by either a broken power law or a log-parabolic fit with equal statistical goodness of fits. The combined 1ES 1553+113 very high energy spectrum (90-500 GeV) did not show any significant changes with respect to earlier observations. The resulting contemporaneous broadband spectral energy distributions of both TeV blazars are discussed in view of implications for intrinsic blazar parameter values, taking into account the gamma-ray absorption in the EBL.

astro-ph↗

arXiv2007

VHE gamma-rays from Westerlund 2 and implications for the inferred energetics

The H.E.S.S. collaboration recently reported the discovery of VHE gamma-ray emission coincident with the young stellar cluster Westerlund 2. This system is known to host a population of hot, massive stars, and, most particularly, the WR binary WR20a. Particle acceleration to TeV energies in Westerlund 2 can be accomplished in several alternative scenarios, therefore we only discuss energetic constraints based on the total available kinetic energy in the system, the actual mass loss rates of respective cluster members, and implied gamma-ray production from processes such as inverse Compton scattering or neutral pion decay. From the inferred gamma-ray luminosity of the order of 10^35 erg/s, implications for the efficiency of converting available kinetic energy into non-thermal radiation associated with stellar winds in the Westerlund 2 cluster are discussed under consideration of either the presence or absence of wind clumping.

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arXiv2007

Clumping effects on non-thermal particle spectra in massive star systems

Observational evidence exists that winds of massive stars are clumped. Many massive star systems are known as non-thermal particle production sites, as indicated by their synchrotron emission in the radio band. As a consequence they are also considered as candidate sites for non-thermal high-energy photon production up to gamma-ray energies. The present work considers the effects of wind clumpiness expected on the emitting relativistic particle spectrum in colliding wind systems, built up from the pool of thermal wind particles through diffusive particle acceleration, and taking into account inverse Compton and synchrotron losses. In comparison to a homogeneous wind, a clumpy wind causes flux variations of the emitting particle spectrum when the clump enters the wind collision region. It is found that the spectral features associated with this variability moves temporally from low to high energy bands with the time shift between any two spectral bands being dependent on clump size, filling factor, and the energy-dependence of particle energy gains and losses.

astro-ph↗

arXiv2007

GeV photon absorption in cosmologically evolving quasar environments

The question of a possible redshift-dependence of gamma-ray absorption due to gamma-gamma pair production of jet photons in the accretion disk and BLR radiation field in strong-line quasars is investigated. For this relevant aspects of cosmological black hole and quasar evolution are applied to the expected pair production opacity of GeV-photons in those sources. I demonstrate that for positively evolving and non-evolving accretion rates over cosmological time, detectable gamma-ray optical depths originating within the AGN system will show a redshift-dependence in the LAT energy range with larger opacity from sources at higher redshifts. This introduces ambiguities in the interpretation of spectral absorption features, and complicates approaches for estimating the evolution of the extragalactic background light by probing the gamma-ray horizon when using blazars whose external photon fields are involved in gamma-ray production.

astro-ph↗

arXiv2007

The redshift-dependence of gamma-ray absorption in the environments of strong-line AGN

The case of gamma-ray absorption due to photon-photon pair production of jet photons in the external photon environment like accretion disk and broad-line region radiation field of gamma-ray loud active galactic nuclei (AGN) that exhibit strong emission lines is considered. I demonstrate that this ''local opacity'', if detected, will almost unavoidably be redshift-dependent in the sub-TeV range. This introduces non-negligible biases, and complicates approaches for studying the evolution of the extragalactic background light with contemporary GeV instruments like e.g. the Gamma-ray Large Area Space Telescope (GLAST), etc., where the gamma-ray horizon is probed by means of statistical analysis of absorption features (e.g. Fazio-Stecker relation, etc.) in AGN spectra at various redshifts. It particularly applies to strong-line quasars where external photon fields are potentially involved in gamma-ray production.

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arXiv2005

Non-thermal high-energy emission from colliding winds of massive stars

Colliding winds of massive star binary systems are considered as potential sites of non-thermal high-energy photon production. This is motivated merely by the detection of synchrotron radio emission from the expected colliding wind location. Here we investigate the properties of high-energy photon production in colliding winds of long-period WR+OB-systems. We found that in the dominating leptonic radiation process anisotropy and Klein-Nishina effects may yield spectral and variability signatures in the gamma-ray domain at or above the sensitivity of current or upcoming gamma-ray telescopes. Analytical formulae for the steady-state particle spectra are derived assuming diffusive particle acceleration out of a pool of thermal wind particles, and taking into account adiabatic and all relevant radiative losses. For the first time we include their advection/convection in the wind collision zone, and distinguish two regions within this extended region: the acceleration region where spatial diffusion is superior to convective/advective motion, and the convection region defined by the convection time shorter than the diffusion time scale. The calculation of the Inverse Compton radiation uses the full Klein-Nishina cross section, and takes into account the anisotropic nature of the scattering process. This leads to orbital flux variations by up to several orders of magnitude which may, however, be blurred by the geometry of the system. The calculations are applied to the typical WR+OB-systems WR 140 and WR 147 to yield predictions of their expected spectral and temporal characteristica and to evaluate chances to detect high-energy emission with the current and upcoming gamma-ray experiments. (abridged)

astro-ph↗

arXiv2005

Neutrino emission in the hadronic Synchrotron Mirror Model: the "orphan" TeV flare from 1ES 1959+650

A challenge to standard leptonic SSC models are so-called orphan TeV flares, i.e. enhanced very high energy (VHE) gamma-ray emission without any contemporaneous X-ray flaring activity, that have recently been observed in TeV-blazars (e.g., 1ES 1959+650). In order to explain the orphan TeV flare of 1ES 1959+650 observed in June 2002, the co-called hadronic synchrotron mirror model has been developed. Here, relativistic protons are proposed to exist in the jet, and interact with reflected electron-synchrotron radiation of the precursor SSC flare. If the reflector is located in the cloud region, time shifts of several days are possible between the precursor and the orphan flare. The external photons, blueshifted in the comoving jet frame, are able to excite the Δ(1232)-resonance when interacting with protons of Lorentz factors γ'_p~10^3-10^4. The decay products of this resonance include charged pions which, upon decay, give rise to neutrino production during the orphan flare. In this paper we calculate the expected neutrino emission for the June 4, 2002, orphan TeV flare of 1ES 1959+650. We compare our results with the recent observations of AMANDA-II of a neutrino event in spatial and temporal coincidence with the orphan flare of this blazar. We find that the expected neutrino signal from the hadronic synchrotron mirror model is insufficient to explain the observed neutrino event from the direction of 1ES 1959+650.

astro-ph↗

arXiv2004

The Coma Cluster at gamma-ray energies: multifrequency constraints

The Coma cluster exhibits evidence for a high-energetic non-thermal particle population. At frequencies >1 GHz recent radio halo observations confirm a significant spectral steepening of the volume-integrated emission. We calculate the volume-averaged high-energy spectrum due to inverse Compton scattering off the CMB radiation field and non-thermal bremsstrahlung according to an exponential cutoff in the electron spectrum as deduced from the radio observations. The synchrotron radiation from secondary pairs, created from the decay of charged mesons produced in hadronic pp-interactions, is found to set significant constraints on the energy content of relativistic hadrons in Coma. This limits the maximum flux at high energies. Our findings support a low ratio of relativistic hadron to thermal energy density. Predictions for Coma's high energy emission are discussed in the light of current and expected abilities of upcoming gamma-ray instruments.

astro-ph↗

arXiv2004

Modeling the Multiwavelength Spectra and Variability of BL Lacertae in 2000

BL Lacertae was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. In this paper, we are using leptonic and hadronic jet models to fit the observed broadband spectra and spectral variability patterns. We start out with global spectral models. Subsequently, we investigate various flaring scenarios for comparison with the observed short-term variability. For our leptonic jet model, we find that the short-term variability, in particular the optical and X-ray spectral variability, can be best represented with a flaring scenario dominated by a spectral-index change of the spectrum of ultrarelativistic electrons injected into the jet. Based on this result, a detailed model simulation, reproducing the observed optical and X-ray spectral variability and broadband SED of BL Lacertae simultaneously, is presented. Our leptonic modeling results are compared to fits using the hadronic synchrotron-proton blazar (SPB) model. That model can reproduce the observed SEDs of BL Lacertae in a scenario with muon-synchrotron dominated high-energy emission. It requires a significantly higher magnetic field than the leptonic model (~ 40 G vs. ~ 2 G in the leptonic model) and a lower Doppler factor associated with the bulk motion of the emission region (D ~ 8 vs. D ~ 18 in the leptonic model). The hadronic model predicts a significantly larger > 100 GeV flux than the leptonic models, well within the anticipated capabilities of VERITAS and MAGIC.

astro-ph↗

arXiv2004

M87 as a misaligned Synchrotron-Proton Blazar

The giant radio galaxy M87 is usually classified as a Fanaroff-Riley class I source, suggesting that M87 is a mis-aligned BL Lac object. Its unresolved nuclear region emits strong non-thermal emission from radio to X-rays which has been interpreted as synchrotron radiation. In an earlier paper we predicted M87 as a source of detectable gamma ray emission in the context of the hadronic Synchrotron-Proton Blazar (SPB) model. The subsequent tentative detection of TeV energy photons by the HEGRA-telescope array would, if confirmed, make it the first radio galaxy to be detected at TeV-energies. We discuss the emission from the unresolved nuclear region of M87 in the context of the SPB model, and give examples of possible model representations of its non-simultaneous spectral energy distribution. The low-energy component can be explained as synchrotron radiation by a primary relativistic electron population that is injected together with energetic protons into a highly magnetized emission region. We find that the gamma-ray power output is dominated either by mu/pi-synchrotron or proton synchrotron radiation depending on whether the primary electron synchrotron component peaks at low or high energies, respectively. The predicted gamma-ray luminosity peaks at around 100 GeV at a level comparable to that of the low-energy hump, and this makes M87 a promising candidate source for the newly-commissioned high-sensitivity low-threshold Cherenkov telescopes H.E.S.S., VERITAS, MAGIC and CANGAROO III. (abridged)

astro-ph↗

arXiv2002

TeV gamma rays and cosmic rays from the nucleus of M87, a mis-aligned BL Lac object

The unresolved nuclear region of M87 emits strong non-thermal emission from radio to X-rays. Assuming this emission to originate in the pc scale jet aligned at $θ\sim 30^\circ$ to the line of sight, we interpret this emission in the context of the Synchrotron Proton Blazar (SPB) model. We find the observed nuclear jet emission to be consistent with M87 being a mis-aligned BL Lac Object and predict gamma-ray emission extending up to at least 100 GeV at a level easily detectable by GLAST and MAGIC, and possibly by VERITAS depending on whether it is high-frequency or low-frequency peaked. Predicted neutrino emission is below the sensitivity of existing and planned neutrino telescopes. Ultra-high energy neutrons produced in pion photoproduction interactions decay into protons after escaping from the host galaxy. Because energetic protons are deflected by the intergalactic magnetic field, the protons from the decay of neutrons emitted in all directions, including along the jet axis where the Doppler factor and hence emitted neutron energies are higher, can contribute to the observed ultra-high energy cosmic rays. We consider the propagation of these cosmic ray protons to Earth and conclude that M87 could account for the observed flux if the extragalactic magnetic field topology were favourable.

astro-ph↗

arXiv2002

Predictions of the High-Energy Emission from BL Lac objects: The Case of W Comae

Spectral fitting of the radio through hard X-ray emission of BL Lac objects has previously been used to predict their level of high-energy (GeV - TeV) emission. In this paper, we point out that such spectral fitting can have very large uncertainties with respect to predictions of the VHE emission. This is demonstrated with the example of W Comae. We show that the best currently available contemporaneous optical - X-ray spectrum of W Comae still allows for a large range of possible parameters, resulting in drastically different > 40 GeV fluxes. We find that all acceptable leptonic-model fits to the optical - X-ray emission of W Comae predict a cut-off of the high-energy emission around ~ 100 GeV. We suggest that detailed measurements and analysis of the soft X-ray variability of W Comae may be used to break the degeneracy in the choice of possible fit parameters, and thus allow a more reliable prediction of the VHE emission from this object. Using the available soft X-ray variability measured by BeppoSAX, we predict a > 40 GeV flux from W Comae of ~ (0.4 - 1) * 10^{-10} photons/(cm^2 s) with no significant emission at E > 100 GeV for a leptonic jet model. We compare our results concerning leptonic jet models with detailed predictions of the hadronic Synchrotron-Proton Blazar model. This hadronic model predicts > 40 GeV fluxes very similar to those found for the leptonic models, but results in > 100 GeV emission which should be clearly detectable with future high-sensitivity instruments like VERITAS. Thus, we suggest this object as a promising target for VHE gamma-ray and co-ordinated broadband observations to distinguish between leptonic and hadronic jet models for blazars.

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