F. Rieger

Formation of hard very-high energy spectra of blazars in leptonic models

The very high energy (VHE) {gamma}-ray spectra of some TeV blazars, after being corrected for absorption in the extragalactic background light (EBL), appear unusually hard, which poses challenges to conventional acceleration and emission models. We investigate the parameter space that allows the production of such hard TeV spectra within time-dependent leptonic models, both for synchrotron self-Compton and external Compton scenarios. In the context of the interpretation of very hard {gamma}-ray spectra, time-dependent considerations become crucial because even extremely hard, initial electron distributions can be significantly deformed due to radiative energy losses. We show that very steep VHE spectra can be avoided if adiabatic losses are taken into account. Another way to keep extremely hard electron distributions in the presence of radiative losses is to assume stochastic acceleration models that naturally lead to steady-state, relativistic, Maxwellian-type particle distributions. We demonstrate that in either case leptonic models can reproduce TeV spectra as hard as E{sub {gamma}} dN/dE{sub {gamma}}{proportional_to}E{gamma}. Unfortunately, this limits, to a large extent, the potential of extracting EBL from {gamma}-ray observations of blazars.

Fermi acceleration in astrophysical jets

Abstract 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 AGN, GRBs and microquasars, showing that multi-component powerlaw-type particle distributions are likely to occur.

Shear Acceleration in Relativistic Astrophysical Jets

We consider the acceleration of energetic particles by a velocity shear in the relativistic background flow containing scattering centers. Three possible acceleration sites for astrophysical jets are identified: (1) gradual velocity shear parallel to the jet axis, such as a velocity profile decreasing linearly outward with radial coordinates, (2) gradual velocity shear perpendicular to the jet axis, such as intrinsic jet rotation, and (3) nongradual and/or discontinuous longitudinal velocity shear at the jet-side boundary. We determine the characteristic acceleration timescales, specify the conditions for efficient acceleration, and discuss observational features with respect to each process. In particular, it is shown that in case 2 the higher energy emission is expected to be concentrated closer to the jet axis, while in cases 1 and 3 the higher energy particles are likely to be located near the edges of the jet, thus possibly leading to some form of limb brightening.

Variable TeV Emission as a Manifestation of Jet Formation in M87

It is proposed that the variable TeV emission observed in M87 may be produced in a starved magnetospheric region, above which the outflow associated with the VLBA jet is established. It is shown that annihilation of MeV photons emitted by the radiative inefficient flow in the vicinity of the black hole can lead to injection of seed charges on open magnetic field lines, with a density that depends sensitively on accretion rate, . For an accretion rate that corresponds to the inferred jet power, and to a fit of the observed spectral energy distribution by an ADAF model, the density of injected pairs is found to be smaller than the Goldreich-Julian (GJ) density by a factor of a few. It is also shown that inverse Compton scattering of ambient photons by electrons (positrons) accelerating in the gap can lead to a large multiplicity, ~103, while still allowing photons at energies of up to a few TeV to freely escape the system. The estimated gap width is not smaller than 0.01rs if the density of seed charges is below the GJ value. The very high energy power radiated by the gap can easily account for the luminosity of the TeV source detected by H.E.S.S. The strong dependence of injected pair density on accretion rate should render the gap emission highly intermittent. We also discuss briefly the application of this mechanism to Sgr A.

Variable VHE gamma-ray emission from non-blazar AGNs

Context. The observation of rapidly variable very high energy (VHE) gamma-rays from non-aligned active galactic nuclei (AGNs), as reported from M 87, proves challenging for conventional theoretical acceleration and emission models. Aims. Motivated by recent work on pulsar-type particle acceleration in M 87 (Neronov & Aharonian 2007, ApJ, 671, 85), we reexamine the centrifugal acceleration of particles by rotating jet magnetospheres in the vicinity of accreting supermassive black hole systems and analyze the energy constraints imposed for highly underluminous systems. Methods. The maximum Lorentz factor for centrifugally accelerated electrons in the presence of inverse Compton losses, and the associated characteristic variability time scale, are determined. Applications are presented for conditions expected to be present in the radio galaxy M 87, assuming accretion onto the central black hole to occur in an advection-dominated (ADAF) mode. Results. We show that for a highly underluminous source like M 87, centrifugally accelerated electrons may reach Lorentz factors up to γ ∼ (10 7 −10 8 ), allowing inverse Compton (Thomson) upscattering of sub-mm disk photons to the TeV regime. Upscattering of Comptonized disk photons results in a flat TeV spectrum Lν ∝ ν −αc with spectral index αc � 1.2. The characteristic variability time scale is of the order rL/c, which in the case of M 87 corresponds to � 1.7 d for a typical light cylinder radius of rL � 5rs. Conclusions. Centrifugal acceleration could provide a natural explanation for the challenging VHE emission features in M 87. Our results suggest that some advection-dominated accreting (non-blazar) AGNs could well be observable VHE emitting sources.

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.

On the Geometrical Origin of Periodicity in Blazar-type Sources

Periodicities in blazar light curves may be related to helical trajectories in extragalactic radio jets by differential Doppler boosting effects. We consider ballistic and nonballistic (i.e., radial) trajectories and discuss three possible periodic driving mechanisms for the origin of helical jet paths, namely, orbital motion in a binary black hole system, jet precession, and intrinsic jet rotation. It is shown that precessional-driven ballistic motion is unlikely to result in observable periods of less than several tens of years. We demonstrate that for nonballistic helical motion the observed period is generally strongly shortened relative to the real physical driving period because of light-travel time effects. Internal jet rotation may thus account for observed periods Pobs 10 days. Periodicity due to orbital-driven (nonballistic) helical motion, on the other hand, is usually constrained to periods of Pobs 10 days, while Newtonian-driven precession is unlikely to be responsible for periodicity on a timescale Pobs 100 days but may well be associated with periods of Pobs 1 yr.

A Microscopic Analysis of Shear Acceleration

A microscopic analysis of the viscous energy gain of energetic particles in (gradual) nonrelativistic 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) ∝ pα with α ≥ 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 p0, power-law differential particle number density spectra n(p) ∝ 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 are addressed.

Evidence for a Second Component in the High-energy Core Emission from Centaurus A?

We report on an analysis of Fermi Large Area Telescope data from four years of observations of the nearby radio galaxy Centaurus A (Cen A). The increased photon statistics results in a detection of high-energy (>100 MeV) gamma-rays up to 50 GeV from the core of Cen A, with a detection significance of about 44 sigma. The average gamma-ray spectrum of the core reveals evidence for a possible deviation from a simple power law. A likelihood analysis with a broken power-law model shows that the photon index becomes harder above E-b similar or equal to 4 GeV, changing from Gamma(1) = 2.74 +/- 0.03 below to Gamma(2) = 2.09 +/- 0.20 above. This hardening could be caused by the contribution of an additional high-energy component beyond the common synchrotron self-Compton jet emission. No clear evidence for variability in the high-energy domain is seen. We compare our results with the spectrum reported by H.E.S.S. in the TeV energy range and discuss possible origins of the hardening observed.

Deep observation of the giant radio lobes of Centaurus A with the Fermi Large Area Telescope

The detection of high-energy (HE) gamma-ray emission up to similar to 3 GeV from the giant lobes of the radio galaxy Centaurus A has been recently reported by the Fermi-LAT Collaboration based on ten months of all-sky survey observations. A data set more than three times larger is used here to study the morphology and photon spectrum of the lobes with higher statistics. The larger data set results in the detection of HE gamma-ray emission (up to similar to 6 GeV) from the lobes with a significance of more than 10 and 20 sigma for the north and the south lobe, respectively. Based on a detailed spatial analysis and comparison with the associated radio lobes, we report evidence for a substantial extension of the HE gamma-ray emission beyond the WMAP radio image for the northern lobe of Cen A. We reconstructed the spectral energy distribution (SED) of the lobes using radio (WMAP) and Fermi-LAT data from the same integration region. The implications are discussed in the context of hadronic and time-dependent leptonic scenarios.