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Register a new userGamma-ray emission in radio galaxies under the VLBI scope -- I. Parsec-scale kinematics and high-energy properties of $gamma$-ray detected TANAMI radio galaxies
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In the framework of the TANAMI multi-wavelength and VLBI monitoring, we study the evolution of the parsec-scale radio emission in radio galaxies in the southern hemisphere and their relationship to the $gamma$-ray properties. In this first paper, we focus on Fermi-LAT-detected sources. We perform a kinematic analysis for five $gamma$-ray detected radio galaxies using multi-epoch 8.4 GHz VLBI images, deriving limits on intrinsic jet parameters. We analyzed Fermi-LAT data in order to study possible connections between the $gamma$-ray properties and the pc-scale jets of Fermi-LAT-detected radio galaxies, both in terms of variability and average properties. We discuss the individual source results and draw preliminary conclusions on sample properties including published VLBI results from the MOJAVE survey, with a total of fifteen sources. We find that the first $gamma$-ray detection of Pictor A might be associated with the passage of a new VLBI component through the radio core. For the peculiar AGN PKS 0521-36, we detect subluminal parsec-scale jet motions, and we confirm the presence of fast $gamma$-ray variability in the source down to timescales of 6 hours. We robustly confirm the presence of significant superluminal motion, up to $beta_{app}sim$3, in the jet of the TeV radio galaxy PKS 0625-35. Finally, we place a lower limit on the age of the Compact Symmetric Object (CSO) PKS 1718-649. We draw some preliminary conclusions on the relationship between pc-scale jets and $gamma$-ray emission in radio galaxies. We find that the VLBI core flux density correlates with the $gamma$-ray flux, as seen in blazars. On the other hand, the $gamma$-ray luminosity does not show any dependence on the core brightness temperature and core dominance, two indicators of Doppler boosting, suggesting that $gamma$-ray emission in radio galaxies is not driven by orientation-dependent effects.
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Following our study of the radio and high-energy properties of $gamma$-ray-emitting radio galaxies, here we investigate the kinematic and spectral properties of the parsec-scale jets of radio galaxies that have not yet been detected by Fermi-LAT. We take advantage of the regular VLBI observations provided by the TANAMI monitoring program, and explore the kinematic properties of six $gamma$-ray-faint radio galaxies. We include publicly available VLBI kinematics of $gamma$-ray-quiet radio galaxies monitored by the MOJAVE program and perform a Fermi-LAT analysis, deriving upper limits. We combine these results with those from our previous paper to construct the largest sample of radio galaxies with combined VLBI and $gamma$-ray measurements to date. We find superluminal motion up to $beta_mathrm{app}=3.6$ in the jet of PKS 2153$-$69. We find a clear trend of higher apparent speed as a function of distance from the jet core on scales of $sim10^5,R_s$, corresponding to the end of the collimation and acceleration zone in nearby radio galaxies. We find evidence of subluminal apparent motion in the jets of PKS 1258$-$321 and IC 4296, and no measurable motion for PKS 1549$-$79, PKS 1733$-$565 and PKS 2027$-$308. We compare the VLBI properties of $gamma$-ray-detected and undetected radio galaxies, and find significantly different distributions of median core flux density, and, possibly, of median core brightness temperature. We find a significant correlation between median core flux density and $gamma$-ray flux, but no correlation with typical Doppler boosting indicators such as median core brightness temperature and core dominance. Our study suggests that high-energy emission from radio galaxies is related to parsec-scale radio emission from the inner jet, but is not driven by Doppler boosting effects, in contrast to the situation in their blazar counterparts.
The parsec-scale radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been investigated using observations with the Very Long Baseline Array (VLBA). Comparisons between LAT and non-LAT detected samples were made using contemporaneous data. In total, 232 sources were used in the LAT-detected sample. This very large, radio flux-limited sample of active galactic nuclei (AGN) provides insights into the mechanism that produces strong gamma-ray emission. It has been found that LAT-detected BL Lac objects are very similar to the non-LAT BL Lac objects in most properties, although LAT BL Lac objects may have longer jets. The LAT flat spectrum radio quasars (FSRQs) are significantly different from non-LAT FSRQs and are likely extreme members of the FSRQ population. Contemporaneous observations showed a strong correlation, whereas no correlation is found using archival radio data. Most of the differences between the LAT and non-LAT populations are related to the cores of the sources, indicating that the gamma-ray emission may originate near the base of the jets (i.e., within a few pc of the central engine). There is some indication that LAT-detected sources may have larger jet opening angles than the non-LAT sources. Strong core polarization is significantly more common among the LAT sources, suggesting that gamma-ray emission is related to strong, uniform magnetic fields at the base of the jets of the blazars. Observations of sources in two epochs indicate that core fractional polarization was higher when the objects were detected by the LAT. Included in our sample are several non-blazar AGN such as 3C84, M82, and NGC 6251.
Using high-resolution radio imaging with VLBI techniques, the TANAMI program has been observing the parsec-scale radio jets of southern (declination south of -30{deg}) gamma-ray bright AGN simultaneously with Fermi/LAT monitoring of their gamma-ray emission. We present the radio and gamma-ray properties of the TANAMI sources based on one year of contemporaneous TANAMI and Fermi/LAT data. A large fraction (72%) of the TANAMI sample can be associated with bright gamma-ray sources for this time range. Association rates differ for different optical classes with all BL Lacs, 76% of quasars and just 17% of galaxies detected by the LAT. Upper limits were established on the gamma-ray flux from TANAMI sources not detected by LAT. This analysis led to the identification of three new Fermi sources whose detection was later confirmed. The gamma-ray and radio luminosities are related by $L_gamma propto L_r^{0.89+-0.04}$. The brightness temperatures of the radio cores increase with the average gamma-ray luminosity, and the presence of brightness temperatures above the inverse Compton limit implies strong Doppler boosting in those sources. The undetected sources have lower gamma/radio luminosity ratios and lower contemporaneous brightness temperatures. Unless the Fermi/LAT-undetected blazars are strongly gamma-ray-fainter than the Fermi/LAT-detected ones, their gamma-ray luminosity should not be significantly lower than the upper limits calculated here.
Cosmic-ray protons accumulate for cosmological times in clusters of galaxies as their typical radiative and diffusive escape times are longer than the Hubble time. Their hadronic interactions with protons of the intra-cluster medium generate secondary electrons, gamma-rays and neutrinos. We here estimate the contribution from clusters to the diffuse gamma-ray and neutrino backgrounds. We model the cluster population by means of their mass function, using a phenomenological luminosity-mass relation applied to all clusters, as well as a detailed semi-analytical model. Additionally, we consider observationally-motivated values for the cluster magnetic field. This is a crucial parameter since the observed radio counts due to synchrotron emission by secondary electrons need to be respected. For a choice of parameters respecting all current constraints, and assuming a spectral index of -2, we find that hadronic interactions in clusters contribute by less than 10% to the extragalactic gamma-ray background observed by Fermi and to the IceCube flux. They account for less than 1% for spectral indices <=-2. The neutrino flux observed by IceCube can be reproduced without violating constraints only if a very hard (and speculative) spectral index >-2 is adopted. However, this scenario is in tension with the IceCube data, which seem to suggest a spectral energy distribution of the neutrino flux that decreases with energy. In the case of proton-photon interactions, we find that very likely protons do not reach sufficiently high energies to produce neutrinos in clusters. We argue that our results are optimistic due to our assumptions, and that clusters of galaxies cannot give any relevant contribution to the extragalactic gamma-ray and neutrino backgrounds. Finally, we find that the cluster contribution to the angular fluctuations in the gamma-ray background is subdominant, less than 10%. [abridged]
The detection of gamma rays from a small number of Narrow Line Seyfert 1 galaxies by the LAT instrument onboard Fermi seriously challenged our understanding of AGN physics. Among the most important findings associated with their discovery has been the realisation that smaller-mass black holes seem to be hosted by these systems. Immediately after their discovery a radio multi- frequency monitoring campaign was initiated to understand their jet radio emission. Here the first results of the campaign are presented. The light curves and some first variability analyses are discussed, showing that the brightness temperatures and Doppler factors are moderate. The phenomenologies are typically blazar-like. The frequency domain on the other hand indicates intense spectral evolution and the variability patterns indicate mechanisms similar to those acting in the jets of BL Lacs and FSRQs. Finally, the linear polarisation also reveals the presence of a quiescent, optically thin jet in certain cases.
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