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On the radio and GeV-TeV gamma-ray emission connection in Fermi blazars

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 Added by Rocco Lico
 Publication date 2017
  fields Physics
and research's language is English




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The Fermi-LAT revealed that the census of the gamma-ray sky is dominated by blazars. Looking for a possible connection between radio and gamma-ray emission is a central issue for understanding the blazar physics, and various works were dedicated to this topic. However, while a strong and significant correlation was found between radio and gamma-ray emission in the 0.1-100 GeV energy range, the connection between radio and very high energy (VHE, E>0.1 TeV) emission is still elusive. The main reason is the lack of a homogeneous VHE sky coverage, due to the operational mode of the imaging atmospheric Cherenkov telescopes. With the present work we aim to quantify and assess the significance of the possible connection between high-resolution radio emission, on milliarcsecond scale, and GeV-TeV gamma-ray emission in blazars. For achieving our goal we extract two large and unbiased blazar samples from the 1FHL and 2FHL Fermi catalogs, above 10 GeV and 50 GeV, respectively. To investigate how the correlation evolves as the gamma-ray energy increases, we perform the same analysis by using the 0.1-300 GeV 3FGL gamma-ray energy fluxes. When we consider the 0.1-300 GeV gamma-ray energy range, we find a strong and significant correlation for all of the blazar sub-classes. Conversely, when we consider the gamma-ray emission above 10 GeV the correlation with the radio emission vanishes, with the exception of the blazar sub-class of high synchrotron peaked objects.



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Blazars are a sub-category of radio-loud active galactic nuclei with relativistic jets pointing towards to the observer. They are well-known for their non-thermal variable emission, which practically extends over the whole electromagnetic spectrum. Despite the plethora of multi-wavelength observations, the issue about the origin of the $gamma$-ray and radio emission in blazar jets remains unsettled. Here, we construct a parametric leptonic model for studying the connection between the $gamma$-ray and radio emission in both steady-state and flaring states of blazars. Assuming that relativistic electrons are injected continuously at a fixed distance from the black hole, we numerically study the evolution of their population as it propagates to larger distances while losing energy due to expansion and radiative cooling. In this framework, $gamma$-ray photons are naturally produced at small distances (e.g. $10^{-3}$ pc) when the electrons are still very energetic, whereas the radio emission is produced at larger distances (e.g. $1$ pc), after the electrons have cooled and the emitting region has become optically thin to synchrotron self-absorption due to expansion. We present preliminary results of our numerical investigation for the steady-state jet emission and the predicted time lags between $gamma$-rays and radio during flares.
142 - L. Costamante 2012
Fermi-LAT spectra at high energies (HE, 0.1-100 GeV) are often extrapolated to very high energies (VHE, >100 GeV) and considered either a good estimate or an upper limit for the blazars intrinsic VHE spectrum. This assumption seems not well justified, neither theoretically nor observationally. Besides being often softer, observations do indicate that spectra at VHE could be also harder than at HE, even when adopting the limit of Gamma=1.5. Results based on such straightforward GeV-TeV extrapolations are in general not reliable, and should be considered with caution.
Since mid-2007 we have carried out a dedicated long-term monitoring programme at 15 GHz using the Owens Valley Radio Observatory 40 meter telescope. One of the main goals of this programme is to study the relation between the radio and gamma-ray emission in blazars and to use it as a tool to locate the site of high energy emission. Using this large sample of objects we are able to characterize the radio variability, and study the significance of correlations between the radio and gamma-ray bands. We find that the radio variability of many sources can be described using a simple power law power spectral density, and that when taking into account the red-noise characteristics of the light curves, cases with significant correlation are rare. We note that while significant correlations are found in few individual objects, radio variations are most often delayed with respect to the gamma-ray variations. This suggests that the gamma-ray emission originates upstream of the radio emission. Because strong flares in most known gamma-ray-loud blazars are infrequent, longer light curves are required to settle the issue of the strength of radio-gamma cross-correlations and establish confidently possible delays between the two. For this reason continuous multiwavelength monitoring over a longer time period is essential for statistical tests of jet emission models.
We compare the gamma-ray photon flux variability of northern blazars in the Fermi/LAT First Source Catalog with 37 GHz radio flux density curves from the Metsahovi quasar monitoring program. We find that the relationship between simultaneous millimeter (mm) flux density and gamma-ray photon flux is different for different types of blazars. The flux relation between the two bands is positively correlated for quasars and does no exist for BLLacs. Furthermore, we find that the levels of gamma-ray emission in high states depend on the phase of the high frequency radio flare, with the brightest gamma-ray events coinciding with the initial stages of a mm flare. The mean observed delay from the beginning of a mm flare to the peak of the gamma-ray emission is about 70 days, which places the average location of the gamma-ray production at or downstream of the radio core. We discuss alternative scenarios for the production of gamma-rays at distances of parsecs along the length of the jet
The Fermi Large Area Telescope (LAT) revealed that blazars, representing the most extreme radio-loud active galactic nuclei (AGN) population, dominate the census of the gamma-ray sky, and a significant correlation was found between radio and gamma-ray emission in the 0.1-100 GeV energy range. However, the possible connection between radio and very high energy (VHE, E>0.1 TeV) emission still remains elusive, owing to the lack of a homogeneous coverage of the VHE sky. The main goal of this work is to quantify and assess the significance of a possible connection between the radio emission on parsec scale measured by the very long baseline interferometry (VLBI) and GeV-TeV gamma-ray emission in blazars, which is a central issue for understanding the blazar physics and the emission processes. We investigate the radio VLBI and high energy gamma-ray emission by using two large and unbiased AGN samples extracted from the first and second Fermi-LAT catalogs of hard gamma-ray sources detected above 10 GeV (1FHL) and 50 GeV (2FHL). For comparison, we perform the same correlation analysis by using the 0.1-300 GeV gamma-ray energy flux provided by the third Fermi-LAT source catalog. We find that the correlation strength and significance depend on the gamma-ray energy range with a different behavior among the blazar sub-classes. Overall, the radio and gamma-ray emission above 10 GeV turns out to be uncorrelated for the full samples and for all of the blazar sub-classes with the exception of high synchrotron peaked (HSP) objects, which show a strong and significant correlation. On the contrary, when 0.1-300 GeV gamma-ray energies are considered, a strong and significant correlation is found for the full blazar sample as well as for all of the blazar sub-classes. We interpret and explain this correlation behavior within the framework of the blazar spectral energy distribution properties.
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