Matter under extreme conditions
Active Galactic Nuclei
The MeV energy range is crucial for our understanding of the processes driving active galactic nuclei (AGN). First of all, the energetics of blazars are not well-understood. This is mainly due to the fact that the flat spectrum radio quasars (FSRQs), the most energetic compact objects in our universe, have their peak emission in the MeV range. Thus, only through observations in this range will we finally be able to verify emission models for this type of object. It is essential to study the emission processes close to the maximum energy output in the SED. For the inverse Compton branch, this is situated in the MeV range for nearly all powerful blazars. Only studying the peak of the emission allows for significant constraints on the total energy output. It also offers the possibility to directly investigate the correlation between the synchrotron branch, which dominates the emission of blazars below the X-ray range, and the inverse Compton component. Only observations in the MeV range will allow for the study of the position of the peak of the emission and its strength. Gamma-ray observations above 100 MeV have given us, up to now, only hints of the full picture as they are measuring the tail of the inverse Compton branch.
CGRO/COMPTEL detected 14 blazars (which were also detected by EGRET) and Cen A. Twelve of these objects were flat spectrum radio quasars, and only two BL Lac-type blazars have been detected: Mkn 421 and PKS 0716+714 (Collmar 2006). OSSE data provided information mostly in the 50-150 keV energy band, and again, only the Seyfert galaxies Cen A and NGC 4151 are significantly detectable above 150 keV. To estimate the number of AGN that DUAL will detect, one can extrapolate the observed source density in the hard X-ray band as measured by e.g. Swift/BAT and INTEGRAL/IBIS and from the γ-rays as determined by Fermi/LAT. Using the number counts of Swift/BAT blazars (Ajello et al. 2009), one expects a density of BL Lac objects in the sky of 0.001 AGN/deg2 and about 0.01 AGN/deg2 for the bright FSRQ blazars at the DUAL sensitivity limit. An even higher number of AGN is expected when extrapolating from the Fermi/LAT detected FSRQs (Abdo et al. 2009), with about 0.035 AGN/deg2 after a three year DUAL mission. Thus, of the order of 1,000 AGN will be detected by DUAL, out of which about 10% will be BL Lac objects.
As for most objects, at least for the FSRQs, we will measure their emission at the peak of the spectral energy distribution. It will be possible for the first time to determine to the total energy output of beamed sources in the Universe. This will answer questions like:
•How much energy is injected into the interstellar and intergalactic medium by blazars?
•How important is particle injection for the star burst evolution in the host galaxies?
•How do AGN contribute to the life cycle of matter?
•What sources dominate the so far unresolved γ-ray background?
With source numbers similar to those detected by Fermi/LAT within the first two years of its mission, and with an increase in source numbers by a factor of ~60 compared to the most sensitive previous telescope in the MeV range, some of the DUAL detections of AGN will be surprising. As in the Fermi/LAT energy band, non-beamed sources, such as starburst and Seyfert galaxies, might be detectable because of high-energy photons emitted in shocks of outflowing material interacting with the interstellar medium
