The number of known extreme blazars is still quite low, hampering a detailed study of this class of sources. I will first review what has been available so far and then present the extreme blazar sample from the 3HSP catalogue (Chang, Arsioli, Giommi, Padovani, & Brandt, 2019), which includes 344 sources with nu_peak > 10^17 Hz. I will then discuss in some detail their observational properties, with a look towards future facilities.
In September 2017, the IceCube Neutrino Observatory detected an extremely-high-energy neutrino event, consisting of a muon coming from the bottom of the detector through the Earth, likely originated by a neutrino of energy of around 290 TeV. Promptly alerted, the Fermi LAT and MAGIC collaborations detected at more than 5σ a flare from the known gamma-ray blazar TXS 0506 +056, at a redshift ~0.34, within the region of sky consistent with the 50% probability region of the IceCube neutrino (of about one degree in size). The MAGIC detection allowed to determine a cutoff for the electromagnetic emission at ~400 GeV. The associated emission of gamma rays and neutrinos from the same source hints at an “hadronic mechanism” for the production of high energy cosmic rays.
In this paper we present a characterization of the photon fields and photon transfer in the vicinity of blazar TXS 0506+056 in order to characterize the properties of the emitting region. This was done under the assumptions of a true association between the detected neutrino and the active state blazar and that the neutrino and electromagnetic emission arise from the same spatial region.
The produced photon spectrum is expected to be similar to that of neutrinos, but photon energy is likely to degrade due to interactions, both with nearby fields and, along their path to Earth, with the extragalactic background light (EBL). Taking as a starting point the reported energy difference between the neutrino and electromagnetic components, a Monte-Carlo simulation of the evolution of an electromagnetic shower in the appropriate environment was implemented in order to extract the relevant features of the photon transport. The results can be used to constrain the size of the emitting region, a crucial parameter for many models of the electromagnetic emission of blazars.
The launch of Fermi produced a significant number of AGN detections to allow statistical treatment of their properties. It confirmed the
Blazar Sequence" established by EGRET and indicated a potentially novel one, namely that of theBlazar Divide" in FSRQs and BL Lacs according to their gamma-ray spectral index and luminosity. An MHD accretion wind model that describes the distribution of matter and magnetic fields in AGN over 5-6 decades in radius and provides a successful account of the Seyfert X-ray absorber properties, provides the vestiges of an account of the observed blazar classification in terms of a single parameter. We propose a model which reproduces in detail the broadband blazar spectra and their statistical properties in terms of the physical parameters of these MHD winds.
With the advent of the IXPE (Imaging X-ray Polarimetry Explorer) satellite
expected for launch in 2021, the study of the
X-ray polarization properties of several bright nearby sources
will become possible. Blazars are obvious candidates for X-ray
For instance, a high degree of X-ray polarization is expected from
high-peaked blazars (HBL) because synchrotron radiation in an ordered
is naturally polarized. Moreover, X-ray polarization observations
have the potential of discriminating between different scenarios for the
of the X-ray emission in low peaked blazars (LBL) objects.
In this talk, we discuss the perspectives of IXPE observations of blazars,
focusing on polarization modeling of different classes of BL Lac objects.
Among the various categories of Active Galactic Nuclei (AGN), Blazars are the most promising sources of TeV/PeV neutrinos, due to their high luminosity at very high-energy (VHE) range during the active periods. After the multi-wavelength observation of the TXS0506+056, with a extreme high energy (EHE) event of IceCube followed-up by a significant flaring emission registered by Fermi-LAT and MAGIC observatories, this hypothesis became even more solid. Here we present a sample of Bl-lacs and flat spectrum radio quasars (FSRQ), from the 3FHL and 3FGL catalogs, spatially connected with the high energy starting events (HESE) track-like type and EHE events, for the study of their variability as well as the luminosity during their flaring activity. A specific focus is dedicated to the extreme Blazar TXS0506+056 with the analysis of the HE duty cycle and the spectral energy distribution (SED) features. The variability and the luminosity of these extreme sources are then correlated, through a hadronic emission process, with the possibility to observe them with a VHE neutrino telescope.
The light emitted by all galaxies across the history of the Universe is encoded in the intensity of the extragalactic background light (EBL), the diffuse cosmic radiation field at ultraviolet, optical, and infrared wavelengths. The EBL is a source of opacity for high-energy γ rays via the photon-photon interaction (γγ → e+e−), leaving a characteristic attenuation imprint in the spectra of distant γ-ray sources. In this talk, I will report on an unprecedented measurement of the EBL using data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope, which has allowed us to derive the star-formation history of the Universe and estimate the number density of faint galaxies during the re-ionization epoch.
IXPE the Imaging X-ray Polarimetry explorer is the next SMEX NASA mission to be launched on 2021 with a large hardware contribution from ASI. It will re-open, after more than 40 years, the window of X-ray polarimetry for a number of celestial X-ray sources among almost all classes. In this talk we present the mission status and its astrophysical perspectives.