2,072 research outputs found
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.Comment: Invited review paper for the 6th Symposium on High Energy Gamma-Ray
Astronomy, to appear in AIP Conference Proc., 11 pages, 6 figure
Gamma-Ray Astrophysics in the Time Domain
The last few years have seen gamma-ray astronomy maturing and advancing in
the field of time-domain astronomy, utilizing source variability on timescales
over many orders of magnitudes, from a decade down to a few minutes and
shorter, depending on the source. This review focuses on some of the key
science issues and conceptual developments concerning the timing
characteristics of active galactic nuclei (AGN) at gamma-ray energies. It
highlights the relevance of adequate statistical tools and illustrates that the
developments in the gamma-ray domain bear the potential to fundamentally deepen
our understanding of the nature of the emitting source and the link between
accretion dynamics, black hole physics, and jet ejection.Comment: review paper; accepted for publication in Galaxies, Special Issue
Monitoring the Non-Thermal Universe; 17 pages, 11 figure
Periodic variability and binary black hole systems in blazars
We consider the periodic modulation of emission from jets in blazar-type
sources. A differential Doppler boosting origin, associated with the helical
motion of a radiating component, is analyzed for different periodic driving
sources including orbital motion and jet precession in a binary black hole
system (BBHS). We emphasize that for non-ballistic helical motion classical
travel time effects can lead to strong shortening effects, such that the
observed period may be a factor smaller than the underlying
driving period, where denotes the bulk Lorentz factor of the jet
flow. The relevance of the above noted scenarios is discussed for the BL Lac
object AO 0235+16.Comment: 6 pages, 1 figure; contribution to: High Energy Gamma-Ray Astronomy:
2nd International Symposium (Heidelberg, July 2004); AIP Conf. Proceedings,
eds. F. A. Aharonian, H J. Voelk, and D. Horn
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.Comment: 6 pages, 3 figures. Accepted to the Proceedings of the 25th Texas
Symposium on Relativistic Astrophysics (Heidelberg, 2010
Radio Galaxies at VHE energies
Radio Galaxies have by now emerged as a new -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 -ray energies (VHE; 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.Comment: Invited review article, submitted to Galaxies; review, 21 pages, 14
figures; small typos corrected and references fixed to match accepted versio
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 environmentsComment: Talk presented at the 7th Fermi Symposium, Garmisch-Partenkirchen,
October 201
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
with angle 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 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 eV may be
possible under a wide range of conditions.Comment: 11 pages, 3 figures; ApJ Letters accepted; final version: small typos
corrected in order to match published versio
Gap-type Particle Acceleration in the Magnetospheres of Rotating Supermassive Black Holes
The detection of rapidly variable gamma-ray emission in active galactic
nuclei has generated renewed interest in magnetospheric particle acceleration
and emission scenarios. In order to explore its potential, we study the
possibility of steady gap acceleration around the null surface of a rotating
black hole magnetosphere. We employ a simplified (1D) description along with
the general relativistic expression of Gauss's law, and we assume that the gap
is embedded in the radiation field of a radiatively inefficient accretion flow.
The model is used to derive expressions for the radial distribution of the
parallel electric field component, the electron and positron charge density,
the particle Lorentz factor, and the number density of -ray photons. We
integrate the set of equations numerically, imposing suitable boundary
conditions. The results show that the existence of a steady gap solution for a
relative high value of the global current is in principle possible if charge
injection of both species is allowed at the boundaries. We present gap
solutions for different choices of the global current and the accretion rate.
When put in context, our results suggest that the variable very high energy
-ray emission in M87 could be compatible with a magnetospheric origin.Comment: 20 pages, 11 figures; ApJ accepted; minor typos fixed to match
published versio
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