arXiv subjects

Frank M. Rieger

Publications and source records attributed to Frank M. Rieger.

At least 37 records · Page 2Linked to original sources

arXiv2018

Radio Galaxies at VHE energies

Radio Galaxies have by now emerged as a new $γ$-ray emitting source class on the extragalactic sky. Given their remarkable observed characteristics, such as unusual gamma-ray spectra or ultrafast VHE variability, they represent unique examples to probe into the nature and physics of AGN in general. This review provides a compact summary of their observed characteristics at very high $γ$-ray energies (VHE; $> 100$ GeV) along with a discussion of their possible physics implications. A particular focus is given to a concise overview of fundamental concepts concerning the origin of variable VHE emission, including recent developments in black hole gap physics.

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arXiv2018

The complex gamma-ray behaviour of the Radio Galaxy M87

Context.In recent years, non-blazar Active Galactic Nuclei (AGN) such as Radio Galaxies have emerged as a highly instructive source class providing unique insights into high energy acceleration and radiation mechanisms. Aims.Here we aim at a detailed characterization of the high-energy (HE;>100 MeV) gamma-ray emission from the prominent radio galaxy M87. Methods.We analyze 8 years of Fermi-LAT data and derive the spectral energy distribution between 100 MeV and 300 GeV. We extract lightcurves and investigate the variability behaviour for the entire energy range as well as below and above 10 GeV. Results.Our analysis provides (i)evidence for HE gamma-ray flux variability and (ii)indications for a possible excess over the standard power-law model above Eb~10 GeV, similar to the earlier indications in the case of Cen A. When viewed in HE-VHE context, this is most naturally explained by an additional component dominating the highest-energy part of the spectrum. Investigation of the gamma-ray lightcurves suggests that the lower-energy (<10 GeV) component is variable on timescales of (at least) a few months. The statistics of the high energy component (>10 GeV) does not allow significant constraints on variability. We find indications, however, for spectral changes with time that support variability of the putative additional component and seem to favor jet-related scenarios for its origin capable of accommodating month-type variability. Conclusions.The current findings suggest that both the high-energy (> Eb) and the very high energy (VHE;>100 GeV) emission in M87 are compatible with originating from the same physical component. The variability behaviour at VHE then allows further constraints on the location and the nature of the second component. In particular, these considerations suggest that the VHE emission during the quiescent state originates in a similar region as during the flare.

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arXiv2018

The energy distribution of relativistic electrons in the kilo-parsec scale jet of M87 with Chandra

The X-ray emission from the jets in Active Galactic Nuclei (AGN) carries important information on the distributions of relativistic electrons and magnetic fields on large scales. We reanalyze archival Chandra observations on the jet of M87 from 2000 to 2016 with a total exposure of 1460 kiloseconds to explore the X-ray emission characteristics along the jet. We investigate the variability behaviours of the nucleus and the inner jet component HST-1, and confirm indications for day-scale X-ray variability in the nucleus contemporaneous to the 2010 high TeV gamma-ray state. HST-1 shows a general decline in X-ray flux over the last few years consistent with its synchrotron interpretation. We extract the X-ray spectra for the nucleus and all knots in the jet, showing that they are compatible with a single power-law within the X-ray band. There are indications of the resultant X-ray photon index to exhibit a trend, with slight but significant index variations ranging from $\simeq 2.2$ (e.g. in knot D) to $\simeq 2.4-2.6$ (in the outer knots F, A, and B). When viewed in a multi-wavelength context, a more complex situation is arising. Fitting the radio to X-ray spectral energy distributions (SEDs) assuming a synchrotron origin, we show that a broken power-law electron spectrum with break energy $E_b$ around $1~(300μG/B)^{1/2}$ TeV allows a satisfactorily description of the multi-band SEDs for most of the knots. However, in the case of knots B, C and D we find indications that an additional high energy component is needed to adequately reproduce the broadband SEDs. We discuss the implications and suggest that a stratified jet model may account for the differences.

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arXiv2017

Gamma-ray emission from the black hole's vicinity in AGN

Non-thermal magnetospheric processes in the vicinity of supermassive black holes have attracted particular attention in recent times. Gap-type particle acceleration accompanied by curvature and Inverse Compton radiation could in principle lead to variable gamma-ray emission that may be detectable with current instruments. We shortly comment on the occurrence of magnetospheric gaps and the realisation of different potentials. The detection of rapid variability becomes most instructive by imposing a constraint on possible gap sizes, thereby limiting extractable gap powers and allowing to assess the plausibility of a magnetospheric origin. The relevance of this is discussed for the radio galaxies Cen A, M87 and IC310. The detection of magnetospheric gamma-ray emission generally allows for a sensitive probe of the near-black-hole region and is thus of prime interest for advancing our understanding of the (astro)physics of extreme environments

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arXiv2017

Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei

The rapidly variable, very high-energy (VHE) gamma-ray emission from Active Galactic Nuclei (AGN) has been frequently associated with non-thermal processes occurring in the magnetospheres of their supermassive black holes. The present work aims to explore the adequacy of different gap-type (unscreened electric field) models to account for the observed characteristics. Based on a phenomenological description of the gap potential, we estimate the maximum extractable gap power $L_{gap}$ for different magnetospheric set-ups, and study its dependence on the accretion state of the source. $L_{gap}$ is found to be in general proportional to the Blandford-Znajek jet power $L_{BZ}$ and a sensitive function of gap size $h$, i.e. $L_{gap} \sim L_{BZ} (h/r_g)^β$, where the power index $β\geq 1$ is dependent on the respective gap-setup. The transparency of the black hole vicinity to VHE photons generally requires a radiatively inefficient accretion environment and thereby imposes constraints on possible accretion rates, and correspondingly on $L_{BZ}$. Similarly, rapid variability, if observed, may allow to constrain the gap size $h\sim c Δt$. Combining these constraints, we provide a general classification to assess the likelihood that the VHE gamma-ray emission observed from an AGN can be attributed to a magnetospheric origin. When applied to prominent candidate sources these considerations suggest that the variable (day-scale) VHE activity seen in the radio galaxy M87 could be compatible with a magnetospheric origin, while such an origin appears less likely for the (minute-scale) VHE activity in IC310.

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arXiv2016

Gamma-Rays from Non-Blazar AGN

Non-blazar Active Galactic Nuclei (AGN) have emerged as a new gamma-ray emitting source class on the extragalactic sky and started to deepen our understanding of the physical processes and the nature of AGN in general. The detection of Narrow Line Seyfert 1 galaxies in the Fermi-LAT energy regime, for example, offers important information for our understanding of jet formation and radio-loudness. Radio galaxies, on the other hand, have become particularly interesting at high (HE) and very high (VHE) gamma-ray energies. With their jets not directly pointing towards us (i.e. misaligned), they offer a unique tool to probe into the nature of the fundamental (and often hidden) physical processes in AGN. This review highlights and discusses some of the observational and theoretical progress achieved in the gamma-ray regime during recent years, including the evidence for unexpected spectral hardening in Centaurus A and extreme short-term variability as seen in IC 310 and M87.

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arXiv2013

TeV Astronomy

With the successful realization of the current-generation of ground-based detectors, TeV Astronomy has entered into a new era. We review recent advances in VHE astronomy, focusing on the potential of Imaging Atmospheric Cherenkov Telescopes (IACTs), and highlight astrophysical implications of the results obtained within recent years.

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arXiv2012

On the origin of very high energy gamma-rays from radio galaxies

Radio galaxies have emerged as a new gamma-ray emitting source class on the extragalactic sky. With their jets misaligned, i.e. not directly pointing towards us, they offer a unique tool to probe some of the fundamental (and otherwise hidden) non-thermal processes in AGN. This contribution briefly summarizes the observed characteristics of the four radio galaxies detected so far at very high energies (VHE). Given its prominence, particular attention is given to the origin of the variable VHE emission in M87. We discuss some of the theoretical progress achieved for this source within recent years highlighting, amongst others, the relevance of magnetospheric particle acceleration and emission models.

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arXiv2012

Probing the central black hole in M87 with gamma-rays

Recent high-sensitivity observation of the nearby radio galaxy M87 have provided important insights into the central engine that drives the large-scale outflows seen in radio, optical and X-rays. This review summarizes the observational status achieved in the high energy (HE;<100 GeV) and very high energy (VHE; >100 GeV) gamma-ray domains, and discusses the theoretical progress in understanding the physical origin of this emission and its relation to the activity of the central black hole.

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arXiv2011

Non-thermal Processes in Black-Hole-Jet Magnetospheres

The environs of supermassive black holes are among the universe's most extreme phenomena. Understanding the physical processes occurring in the vicinity of black holes may provide the key to answer a number of fundamental astrophysical questions including the detectability of strong gravity effects, the formation and propagation of relativistic jets, the origin of the highest energy gamma-rays and cosmic-rays, and the nature and evolution of the central engine in Active Galactic Nuclei (AGN). As a step towards this direction, this paper reviews some of the progress achieved in the field based on observations in the very high energy domain. It particularly focuses on non-thermal particle acceleration and emission processes that may occur in the rotating magnetospheres originating from accreting, supermassive black hole systems. Topics covered include direct electric field acceleration in the black hole's magnetosphere, ultra-high energy cosmic ray production, Blandford-Znajek mechanism, centrifugal acceleration and magnetic reconnection, along with the relevant efficiency constraints imposed by interactions with matter, radiation and fields. By way of application, a detailed discussion of well-known sources (Sgr A*; Cen A; M87; NGC1399) is presented.

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arXiv2011

Rapid VHE variability in blazars

Active Galactic Nuclei (AGN) are known to show significant variability over a wide frequency range. We review observational results on the variability characteristics of blazars in the very high energy (VHE) domain, focusing on recent findings of rapid VHE variability and evidence for an underlying multiplicative driving process in PKS 2155-304. We explore a physical scenario where the variability is assumed to arise due to accretion disk fluctuations transmitted to the jet, and discuss its implications for the central powerhouse.

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arXiv2010

Short-term VHE variability in blazars: PKS 2155-304

Context: The $γ$-ray blazar PKS 2155-304 has attracted considerable attention because of its extreme TeV variability characteristics during an exceptional flaring period in 2006. Among the observed key findings are (i) a minimum variability timescale as short as $\sim 200$ sec and (ii) highly variable TeV emission, which in the frequency interval [$10^{-4}$ Hz, $10^{-2}$ Hz] can be described by a log-normal distribution and suggests an underlying multiplicative (and not additive) process. Aims: Simultaneously accounting for these findings appears difficult within conventional approaches. Following earlier suggestions for the TeV blazar Mkn 501, we explore a possible scenario where PKS 2155-304 is supposed to harbor a supermassive binary black hole system and where the observed TeV variability is dominated by emission from the less massive black hole. Methods: We analyze the constraints on the very high energy (VHE) source imposed by the observed variability characteristics and the integrated VHE luminosity output, and discuss its implications for a binary black hole system. Results: We show that for a secondary mass of $m_{\rm BH} \sim 10^7 M_{\odot}$, fluctuations in the disk accretion rate that feed the jet could account for the observed red-noise type variability process down to frequencies of $\sim 10^{-2}$ Hz. Jet curvature induced by orbital motion, on the other hand, could further relax constraints on the intrinsic jet speeds. Conclusions: Because a binary system can lead to different (yet not independent) periodicities in different energy bands, a longterm (quasi-) periodicity analysis could offer important insights into the real nature of the central engine of PKS~2155-304.

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arXiv2009

Cosmic Ray Acceleration in Active Galactic Nuclei - On Centaurus A as a possible UHECR Source

We discuss a representative selection of particle acceleration mechanisms believed to be operating in Active Galactic Nuclei. Starting from direct electrostatic field acceleration in the vicinity of the black hole up to Fermi-type particle acceleration in the jet and beyond, possible efficiency constraints on the energization of ultra-high energy cosmic rays (UHECR) are evaluated. When paradigmatically applied to Cen A, the following results are obtained: (i) Proton acceleration to energies of $E_c = 5\times 10^{19}$ eV and beyond remains challenging and most likely requires the operation of an additional mechanism capable of boosting energetic seed protons up by a factor of $\sim$ten. It is argued that shear acceleration along the large-scale jet in Cen A could be a promising candidate for this. (ii) Heavier elements, like iron nuclei, are more easily accelerated (by, e.g., shocks or direct electrostatic fields) and may not need additional boosting to reach $E \geq E_c$; (iii) If Cen A indeed proves to be an UHECR source, the cosmic ray composition might thus be expected to become heavier above energies of a few times $10^{19}$ eV.

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arXiv2009

On particle acceleration and very high energy gamma-ray emission in Crab-like pulsars

The origin of very energetic charged particles and the production of very high-energy (VHE) gamma-ray emission remains still a challenging issue in modern pulsar physics. By applying a toy model, we explore the acceleration of co-rotating charged particles close to the light surface in a plasma-rich pulsar magnetosphere and study their interactions with magnetic and photon fields under conditions appropriate for Crab-type pulsars. Centrifugal acceleration of particles in a monopol-like magnetic field geometry is analyzed and the efficiency constraints, imposed by corotation, inverse Compton interactions and curvature radiation reaction are determined. We derive expressions for the maximum particle energy and provide estimates for the corresponding high-energy curvature and inverse Compton power outputs. It is shown that for Crab-like pulsars, electron Lorentz factor up to $γ\sim 10^7$ can be achieved, allowing inverse Compton (Klein-Nishina) up-scattering of thermal photons to TeV energies with a maximum luminosity output of $\sim10^{31}$ erg/s. Curvature radiation, on the other hand, will result in a strong GeV emission output of up to $\sim(10^{34}-10^{35})$ erg/s, quasi-exponentially decreasing towards higher energies for photon energies below $\sim 50$ GeV. Accordingly to the results presented only young pulsars are expected to be sites of detectable VHE $γ$-ray emission.

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arXiv2006

Supermassive binary black holes among cosmic gamma-ray sources

Supermassive binary black holes (SBBHs) are a natural outcome of galaxy mergers. Here we show that low-frequency ($f \leq 10^{-6}$ Hz) quasi-periodic variability observed from cosmic blazar sources can provide substantial inductive support for the presence of close ($d \lppr 0.1$ pc) 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. This is illustrated for a binary model where optical longterm periodicity is related to accretion disk interactions, radio periodicity to Newtonian jet precession, and periodicities in the high energy bands to the orbital motion of the jet. We use the observed periodicities to constrain the properties for a sample of SBBH candidates including OJ 287 and AO 0235+16, and discuss the results within the context of jet activity and binary evolution.

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arXiv2006

A microscopic analysis of shear acceleration

A microscopic analysis of the viscous energy gain of energetic particles in (gradual) non-relativistic shear flows is presented. We extend previous work and derive the Fokker-Planck coefficients for the average rate of momentum change and dispersion in the general case of a momentum-dependent scattering time $τ(p) \propto p^α$ with $α\geq 0$. We show that in contrast to diffusive shock acceleration the characteristic shear acceleration timescale depends inversely on the particle mean free path which makes the mechanism particularly attractive for high energy seed particles. Based on an analysis of the associated Fokker-Planck equation we show that above the injection momentum $p_0$ power-law differential particle number density spectra $n(p) \propto p^{-(1+ α)}$ are generated for $α>0$ if radiative energy losses are negligible. We discuss the modifications introduced by synchrotron losses and determine the contribution of the accelerated particles to the viscosity of the background flow. Possible implications for the plasma composition in mildly relativistic extragalactic jet sources (WATs) are addressed.

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arXiv2006

Fermi acceleration in astrophysical jets

We consider the acceleration of energetic particles by Fermi processes (i.e., diffusive shock acceleration, second order Fermi acceleration, and gradual shear acceleration) in relativistic astrophysical jets, with particular attention given to recent progress in the field of viscous shear acceleration. We analyze the associated acceleration timescales and the resulting particle distributions, and discuss the relevance of these processes for the acceleration of charged particles in the jets of AGNs, GRBs and microquasars, showing that multi-component powerlaw-type particle distributions are likely to occur.

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arXiv2005

Particle Acceleration in Gamma-Ray Burst Jets

Gradual shear acceleration of energetic particles in gamma-ray burst (GRB) jets is considered. Special emphasis is given to the analysis of universal structured jets, and characteristic acceleration timescales are determined for a power-law and a Gaussian evolution of the bulk flow Lorentz factor $γ_b$ with angle $ϕ$ from the jet axis. The results suggest that local power-law particle distributions may be generated and that higher energy particles are generally concentrated closer to the jet axis. Taking several constraints into account we show that efficient electron acceleration in gradual shear flows, with maximum particle energy successively decreasing with time, may be possible on scales larger than $r \sim 10^{15}$ cm, provided the jet magnetic field becomes sufficiently weak and/or decreases rapidly enough with distance, while efficient acceleration of protons to ultra-high energies $> 10^{20}$ eV may be possible under a wide range of conditions.

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