CTAO Science
Active galactic nuclei
Ultra-relativistic jets powered by Super Massive Black Holes (SMBHs) in the core of AGNs are the most impressive particle accelerators in the Universe. We do not know what triggers the formation of jets and do not yet understand the details of particle acceleration. As most of the jet power can be emitted in the gamma-ray band, CTAO will provide key information to answer the questions: How do black holes transfer rotational energy to jets? What is the structure of jets? Can they accelerate cosmic-rays to ZeV energies? How to explain the dynamics of jets and their variability? What is the role of mildly jetted AGNs and hidden accelerators.
Most massive galaxies of the Universe probably host an SMBH in their core but only about 1% of them are active i.e. are efficiently converting gravitational energy of in-falling matter into radiation. Active Galactic Nuclei (AGNs) probably share a common grand-design with different environments and viewing angle explaining their different observational properties. AGNs with a high accretion rate are probably surrounded by an accretion disk, capable of efficiently converting gravitational energy to ultraviolet and X-ray radiation close to the black hole. Some of these disks launch high-velocity (relativistic) winds. Stars and clouds of gas are orbiting around, the latter producing broad emission optical lines. Molecules are evaporated close to the central regions and are found at a distance, forming a torus-shaped distribution of dust clouds. These clouds hide the active nucleus for a large fraction of the viewing directions and thermalise the radiated gravitational energy in the infrared. In most galaxies, the accretion rate, and the gas density, are so low that a disk cannot form. Gas is then advected in the black hole converting a small fraction of the gravitational energy to light.
SMBHs with high and low-accretion rates can launch jets accelerating particles. Jets are predominantly detected in the radio via the synchrotron emission of the accelerated electrons, up to very large distances from the black hole. Jets are probably formed when the spin energy of the black hole can be extracted to accelerate particles to relativistic energies via magnetic eld lines stretched close to the black-hole horizon. Because of relativistic effects, most of the jet power is focused in a narrow cone in the jet direction.
SMBHs with jets pointing towards the direction of the Earth appear very bright and rapidly variable. They are named blazars and are bright from the radio to the VHE gamma-rays. Their spectral energy distribution (SED) shows a low-energy radio-optical synchrotron component and a gamma-ray component interpreted as inverse Compton emission of the same electrons in purely leptonic scenarios. High-energy gamma rays can also be emitted from the interactions of ultra-relativistic cosmic rays, potentially accelerated in the jets, with the ambient radiation or matter. These interactions would also produce particle showers and in particular high-energy neutrinos travelling along the jet (hadronic models).
On 22 Sep. 2017, a high-energy neutrino was identified by the IceCube neutrino telescope at the South Pole from the direction of a blazar, TXS 0506+056, flaring in the gamma-ray band as observed also by Fermi-LAT and MAGIC. A coincident observation of gamma-rays and neutrinos represents a smoking gun for the identification of CR sources, as in their interactions with ambient gas or matter they produce both. Later on, a more significant excess of neutrinos was identified from that direction by scientists at the DPNC/UniGe using historical data when the blazar was flaring in the radio and optical but not in gamma-rays. This triggered alternate structured jet models with multiple zones of particle and radiation production. Future more sensitive detections of multi-messenger transient activities in blazars by CTAO will shed light on the nature of phenomena powering high-energy activity in AGN jets.
The observed variability of the SED of AGNs and blazars is confronted with models involving leptonic and hadronic scenarios, with complex patterns entangling acceleration mechanisms and jet geometry, with the latter impacting the long-term trends and the former responsible for short timescales.
Acceleration processes, such as shock-in-jet models, are considered where inhomogeneities, evolving due to hydrodynamic instabilities, produce relativistic shocks traversing the jet ow. These accelerate particles through diffusive shock acceleration and produce mid/short timescale variability at all wavelengths. This could not explain very fast variability episodes, observed at TeV energies, with timescales of minutes. Hence, they triggered the attention on processes, such as magnetic reconnection, and modern particle-in-cell simulations, which have been able to reproduce spectra typical of Fermi acceleration and close to the observed spectral curvature and cutoffs. Long-term timescales require taking into account geometrical factors impacting on the beaming, such as changes in the jet orientation and bending and the transparency of the emitting region. The order-of-magnitude improvement of sensitivity that will be available with CTAO will allow to see details of the puzzling fast variability of the gamma-ray emission and to determine its minimal time scale.
The integration of CTAO into the network of communicating observatories will allow triggering multi-messenger observations of fast-variable gamma-ray flares.During the science commissioning operation, in Aug. 2021, the CTAO LST-1 telescope detected a highly significant flare from the blazar BL Lac.
With an analysis reconstructing gamma-rays using the full detector waveform of every camera pixel, scientists at the DPNC/UniGe could detect the are down to an energy of 20 GeV. Several telescopes in stereoscopic mode will detect such flares to even lower energies, where the number of photons is larger. On the time span of CTAO operations, it will be possible to combine and cross-correlate its data with those of the next generation of observatories: SKAO in the radio, Vera Rubin Observatory, E-ELT and Euclid in the visible, the proposed Athena or AMEGO in the X-rays, the neutrino telescopes IceCube-Gen2 and KM3NET, and the upgrades of Pierre Auger and Telescope Array and its successors from space and ground for UHECRs.
It is conjectured that jetted AGNs are responsible for the acceleration of the UHECRs, the highest-energy particles reaching ZeV energies. UHECRs could induce the high-energy neutrinos that contribute to the puzzling astrophysical diffuse neutrino signal discovered by IceCube a decade ago at energies above 100 TeV.
During energy densities in the Universe. The diffuse gamma-ray energy density connects with the neutrino ux and, at even higher energies, with the UHECR flux. Actually, only a part of the cosmic neutrino flux can be due to jetted AGNs as searches for neutrinos from stacked Fermi blazars set upper limits at 20-50% contribution level, depending on the injection CR spectral index assumed for all of them. It is also noted that the cosmic neutrino flux overcomes the Waxman and Bahcall (WB) upper limit calculated as if UHECRs are injected from extragalactic sources transparent to UHECRs and neutrinos.
Additionally, gamma-ray bursts are also disfavoured as UHECR sources due to the null results of coincident searches with neutrinos. The magnetic fields in their jets might be too high for protons to be able to generate UHECRs or their baryon loading too low. All this evidence hints at an important role of calorimetric systems that can overcome the WB limit, as all the hadrons lose their energy inside the accelerating region, TeV radiation is absorbed and only neutrinos escape, but not UHECRs
One of such system, NGC 1068, first emerged as a candidate calorimetric neutrino source in a search performed at DPNC/UniGe. This close-by Seyfert II AGN, hosting a starburst region and a very mildly relativistic jet, is identified by IceCube with more than 70 neutrinos with a significance of more than 4σ to be of astrophysical origin. NGC 1068 shows a clear excess of gamma-ray emission up a few tens of GeV in Fermi-LAT data initially pointed out by scientists at the Astronomy Department/UniGe, and interpreted as inverse Compton emission from the AGN on the starburst low-energy photons [arXiv:1008.5164]. On the other hand, gamma rays are not detected by MAGIC in TeV gamma rays. The emerging spectrum is therefore of leptonic origin, contrasting with the IceCube detection. The absence of a gamma-ray counterpart to neutrinos triggered AGN corona models, where neutrinos are accelerated in the region surrounding the horizon of the SMBH. NGC 1068 is a composite system and there might be also contributions of other acceleration processes such as AGN-driven winds from the accretion disk and/or shock acceleration from the starburst region from wind bubbles with consequent proton-proton interactions. Such a composite object will be an interesting target for the CTAO LSTs to explore the interplay between the AGN and starburst winds.
Future frontier of AGN studies
The current generation of IACTs detected the blazars closer to us and a few not-jetted AGNs. CTAO will massively expand the sizes of the VHE detected blazar and AGN source samples, and it will uncover aspects of the cosmological evolution of the blazar population, by detecting high-redshift sources. This will open the possibility of a systematic study leading to a possible relation between the observed characteristics of the VHE signals and AGN populations. There might be an evolutionary relation between blazar sub-classes, the understanding of which is limited by the still small sample of VHE-detected blazars. The increased statistics will allow to determine the model parameters that derfine the efficiency of particle acceleration in one or another type of AGN and what triggers, quenches, or enhances the acceleration process.
Even though the CTAO angular resolution is not enough to image the vicinity of SMBHs, it is still possible to "zoom" into the region close to it if the gamma-ray signal is magni ed by the effect of gravitational lensing of a massive galaxy. In this case, the micro-lensing of the AGN emission by stars in the lensing galaxy passing close to the line-of-sight, temporarily magni es the ux from the very compact region in the AGN central engine. This provides a possibility to measure the size of the gamma-ray emitting source. CTAO will have enough sensitivity to detect such micro-lensing and locate the gamma-ray source within the AGN in the lensed sources.