Abstract
Radio loud active galactic nuclei are composed of different spatial features, each one characterized by different spectral properties in the radio band. Among them, blazars are the most common class of sources detected at gamma-rays by Fermi, and their radio emission is dominated by the flat spectrum compact core. In this contribution, we explore the connection between emission at high energy revealed by Fermi and at radio frequencies. Taking as a reference the strong and very highly significant correlation found between gamma rays and cm-λ radio emission, we explore the different behaviours found as we change the energy range in gamma rays and in radio, therefore changing the physical parameters of the zones involved in the emitted radiation. We find that the correlation weakens when we consider (1) gamma rays of energy above 10 GeV (except for high synchrotron peaked blazars) or (2) low frequency radio data taken by the Murchison Widefield Array; on the other hand, the correlation strengthens when we consider mm-λ data taken by Atacama Large Millimeter Array (ALMA).
Highlights
The basic arguments favouring the existence of such a correlation go from purely theoretical reasoning, i.e., that emission in both bands naturally arises through processes involving a population of relativistic particles, to observational matters, such as the fact that radio loud (RL) active galactic nuclei (AGN) dominate the census of gamma-ray catalogues; they made up about one half of the historical
98% of the RL AGNs in the third catalogue of AGN detected by Fermi-Large Area Telescope (LAT) (3LAC, [11]) belong to the subclass of blazars, which are RL AGNs whose jet axis is closely aligned with our line of sight; this indicates that relativistic beaming is a key element in both the radio and high energy emission, suggesting a strong connection between the two domains
We consider variations of the data analysis as gamma-ray or radio data of different wavelengths are considered: in Section 3, we focus on sources detected at energies E > 10 GeV; in Section 4, on sources detected at radio frequencies as low as 120 MHz [25] by the Murchison Widefield Array (MWA); in Section 5, on sources detected as high as 230 GHz with the Atacama Large Millimeter Array (ALMA)
Summary
The existence, significance, and interpretation of a correlation between radio and gamma-ray emission in extragalactic sources has attracted interest from several authors over the last few decades [1,2,3,4,5,6,7,8]. The gamma-ray variability time scales, in particular during flares, often reach short values, down to ∼1 h [15,16,17,18] This indicates that the high energy emission region must be extremely compact and optically thick at radio frequency, because of the synchrotron self-absorption (SSA) mechanism. Very High Energy (VHE, E > 100 GeV) gamma rays In this domain, observations are carried out targeted mode with imaging atmospheric Cherenkov telescopes (IACT), so that no systematic and in targeted mode with imaging atmospheric Cherenkov telescopes (IACT), so that no systematic unbiased survey exists. The 1FHL is based on three years of Fermi-LAT survey data and it is as uniform and it is as uniform and unbiased as possible It contains 514 sources, 76% of which are AGN and 13%.
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