Active Galactic Nuclei
The Engines of Active Galaxies
Active Galactic Nuclei (AGN) are among the largest storehouses of
energy known in our cosmos. Consisting of an accreting supermassive
black hole from which a relativistic jet emerges, AGN represent one
third of the known VHE gamma-ray sources. Most of the detected objects
belong to the BL Lac or blazar class, in which the relativistic jet is
pointed almost directly towards Earth. The rapid variability of the
gamma-ray flux (as short as minute time scales) indicates that gamma-ray
production must occur close to the black hole, assisted by highly
relativistic motion of the jets resulting in time contraction when
viewed by an observer on Earth. Details of how the jets are launched or
even the types of particles of which they consist are poorly known.
Multi-wavelength observations with high temporal and spectral resolution
can help to distinguish between different scenarios, but this is at the
limit of the capabilities of current instruments. The sensitivity of
CTA, combined with simultaneous observations in other wavelengths will
provide the crucial advance in understanding how AGN work.
Studying the AGN Population
The detection of many more AGN opens the way to statistical studies
of the VHE AGN populations. The distribution in redshift (z) of known
and relatively nearby BL Lac peaks around z = 0.3. The large majority of
the population is found within z < 1, a range easily accessible with
CTA, thanks to its high sensitivity. CTA will therefore be able to
analyse blazar populations out to z ~ 2 in detail and the evolution of
AGN with redshift. This will enable the exploration of the relationship
between different types of blazars, and of the validity of unifying AGN
schemes.
Several scenarios have been proposed to explain the VHE emission of
blazars. However, none of them is fully self-consistent, and the current
data are not sufficient to firmly rule out or confirm a particular
mechanism. In the absence of a convincing global picture, a first goal
for CTA will be to constrain model-dependent parameters of blazars
within a given scenario, thereby ruling out some of the models or parts
of models. A second more difficult goal will be to distinguish between
the different remaining options and to firmly identify the dominant
radiation mechanisms. Detection of specific spectral features, breaks,
cut-offs, absorption or additional components, would be important here.
Exploring Rapid Variability
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| An extraordinary flare from the AGN PKS
2155-304 as detected with the HESS telescopes. Shown is the integral
flux above 200 GeV observed on July 28, 2006. The data are binned in 1
minute intervals. The horizontal dotted line represents the flux from
the Crab Nebula, the brightest steady source in the VHE gamma-ray sky.
(Aharonian et al., Astrophysical Journal (2007), 664, p. L71-L74,
available via arxiv.org/abs/0706.0797) |
CTA’s high sensitivity will mean that it is particularly good for
detecting transient phenomena, and the role of CTA as a timing explorer
will be decisive. For the brightest blazar flares, current instruments
are able to detect variability on the scales of several minutes, but it
is not clear if this is the minimum variability time in blazars. With
CTA, such flares should be detectable within seconds, rather than
minutes. Probing variability down to the shortest time scales will
significantly constrain acceleration and cooling times, instability
growth rates, and the time evolution of shocks and turbulences.
Resolving Radio Galaxies
Recently, radio galaxies have emerged as a new class of VHE emitting
AGN. Given the proximity of some of these objects to Earth and the
larger jet angle to the line of sight compared to BL Lac objects, the
outer and inner kiloparsec jet structures will be spatially resolved by
CTA. This will allow precise location of the main emission site and
searches for VHE radiation from large-scale jets and hot spots, besides
the central core and jets seen in radio VLBI images.
Extragalactic Magnetic Fields
The observation of VHE emission from distant objects and their
surroundings will also offer the unique opportunity to study
extragalactic magnetic fields at large distances. If the fields are
large, an electron-positron pair halo forms around AGNs, which CTA with
its high sensitivity and extended field of view should be capable of
detecting. For smaller magnetic field values, the effect of
electron-positron pair formation along the path to the Earth is seen
through energy-dependent time-delays of variable VHE emission, which
CTA with its excellent time resolution will be ideally suited to
measure.
Further Reading
Proceedings of 'AGN Physics in the CTA Era' held in Toulouse in May 2011.
Giebels et al., Active Galactic Nuclei and Gamma Rays; http://arxiv.org/abs/1005.2330
Wagner, Synoptic Studies of 17 Blazars Detected in Very High Energy Gamma Rays,
Monthly Notices of the Royal Astronomical Society (2008), 385, 1, p. 119-135; http://arxiv.org/abs/0711.3025
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An artist's impression of an active galaxy. (Image: J.J. van Ellinckhuijzen)
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