Active Galactic Nuclei
An artist's impression of an active galaxy. (Image: J.J. van Ellinckhuijzen)
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
|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.
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