VLBA observations of radio faint<i>Fermi</i>-LAT sources above 10 GeV

Lico, Rocco; Giroletti, M.; Orienti, M.; D’Ammando, F.

doi:10.1051/0004-6361/201628775

Cited by 16 publications

(21 citation statements)

References 38 publications

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“…This is a key observational result that in turn implies peak bulk Lorentz factors of order 4 in the parsec-scale radio emitting portions of these jets. This rather low value for the bulk Lorentz factor is entirely consistent with that found by other VLBI estimates, such as the radio core brightness temperatures (Paper I; Lister et al 2011;Lico et al 2016).…”

Section: Superluminal Componentssupporting

confidence: 91%

“…These observations include the low brightness temperatures of the VLBI cores (e.g., Lister et al 2011;Piner & Edwards 2014;Lico et al 2016), the absence of rapid superluminal motions of the jet components (e.g., Kharb et al 2008;Piner et al 2010;Tiet et al 2012;Lico et al 2012;Lister et al 2016), and others including the radio variability, core dominance, and jet morphology (see Piner et al 2008 for a complete discussion). This discrepancy between the Doppler factors derived at different wavebands for the HBL blazar class was dubbed the 'Doppler crisis' by Tavecchio (2006).…”

Section: Introductionmentioning

confidence: 99%

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Multi-epoch VLBA Imaging of 20 New TeV Blazars: Apparent Jet Speeds

Piner

Edwards

2018

ApJ

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We present 88 multi-epoch Very Long Baseline Array (VLBA) images (most at an observing frequency of 8 GHz), of 20 TeV blazars, all of the HBL class, that have not been previously studied at multiple epochs on the parsec scale. From these 20 sources, we analyze the apparent speeds of 43 jet components that are all detected at four or more epochs. As has been found for other TeV HBLs, the apparent speeds of these components are relatively slow. About two-thirds of the components have an apparent speed that is consistent (within 2σ) with no motion; and some of these components may be stationary patterns whose apparent speed does not relate to the underlying bulk flow speed. In addition, a superluminal tail to the apparent speed distribution of the TeV HBLs is detected for the first time, with eight components in seven sources having a 2σ lower limit on the apparent speed exceeding 1c. We combine the data from these 20 sources with an additional 18 sources from the literature to analyze the complete apparent speed distribution of all 38 TeV HBLs that have been studied with VLBI at multiple epochs. The highest 2σ apparent speed lower limit considering all sources is 3.6c. This suggests that bulk Lorentz factors of up to about 4, but probably not much higher, exist in the parsec-scale radio emitting regions of these sources, consistent with estimates obtained in the radio by other means such as brightness temperatures. This can be reconciled with the high Lorentz factors estimated from the high-energy data if the jet has velocity structures consisting of different emission regions with different Lorentz factors. In particular, we analyze the current apparent speed data for the TeV HBLs in the context of a model with a fast central spine and a slower outer layer.

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Section: Superluminal Componentssupporting

confidence: 91%

Section: Introductionmentioning

confidence: 99%

Multi-epoch VLBA Imaging of 20 New TeV Blazars: Apparent Jet Speeds

Piner

Edwards

2018

ApJ

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“…Being the available RFC 5 GHz VLBI flux densities consistent with those at 8 GHz (their average spectral index is 0.0 ± 0.1), we use either 5 GHz or 8 GHz RFC flux densities for our analysis. For those sources not included in the RFC we use the 5 GHz very long baseline array flux densities reported in Lico et al (2016).…”

Section: Sample Selection and Constructionmentioning

confidence: 99%

Exploring the connection between radio and GeV-TeV γ-ray emission in the 1FHL and 2FHL AGN samples

Lico

Giroletti

Orienti

et al. 2017

A&A

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Context. With the advent of the Fermi Large Area Telescope (LAT) it was revealed that blazars, representing the most extreme radioloud active galactic nuclei (AGN) population, dominate the census of the γ-ray sky, and a significant correlation was found between radio and γ-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. Aims. 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 γ-ray emission in blazars, which is a central issue for understanding the blazar physics and the emission processes in these objects. Methods. We investigate the radio VLBI and high energy γ-ray emission by using two large and unbiased AGN samples extracted from the first and second Fermi-LAT catalogs of hard γ-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 γ-ray energy flux provided by the third Fermi-LAT source catalog (3FGL). We assess the correlation statistical significance by means of a method based on permutations of the luminosities, by taking into account the various observational biases, which may apparently enhance or spoil any intrinsic correlation. Results. We find that the correlation strength and significance depend on the γ-ray energy range with a different behavior among the blazar sub-classes. Overall, the radio and γ-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 γ-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. Conclusions. We interpret and explain this correlation behavior within the framework of the blazar spectral energy distribution properties. In the most powerful blazars, in general of low synchrotron peaked type, the high energy emission component peaks at energies lower that those sampled by the LAT. On the contrary, in HSP blazars the part of the high energy spectrum affected by cooling effects is well beyond the energy range sampled by the LAT, showing a rising spectrum both in the 3FGL and 1FHL/2FHL energy ranges.

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“…which several works have highlighted to be rather peculiar [14,15,21,22,36]. Lico et al [23] have band), the sample shows a strong and highly significant correlation, with r = 0.73 and P chance < 10 −6 ,…”

mentioning

confidence: 84%

“…Lico et al [36] have presented new 5 GHz VLBI images for the less studied objects, proposing also associations for some of the UGS. Thanks to these new images, it is now possible to discuss the radio-gamma correlation for the sample (in particular, the northern subset) without any observational bias.…”

Section: Radio-gamma Ray Connection Between Vlbi and E > 10 Gev Datamentioning

confidence: 99%

The Radio/Gamma-Ray Connection from 120 MHz to 230 GHz

et al. 2016

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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).

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VLBA observations of radio faintFermi-LAT sources above 10 GeV

Cited by 16 publications

References 38 publications

Multi-epoch VLBA Imaging of 20 New TeV Blazars: Apparent Jet Speeds

Multi-epoch VLBA Imaging of 20 New TeV Blazars: Apparent Jet Speeds

Exploring the connection between radio and GeV-TeV γ-ray emission in the 1FHL and 2FHL AGN samples

The Radio/Gamma-Ray Connection from 120 MHz to 230 GHz

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