Abstract
Context.In March 2018, the detection by VERITAS of very-high-energy emission (VHE > 100 GeV) from 3C 264 was reported. This is the sixth, and second most distant, radio galaxy ever detected in the TeV regime.Aims.In this article we present a radio and X-ray analysis of the jet in 3C 264. We determine the main physical parameters of the parsec-scale flow and explore the implications of the inferred kinematic structure for radiative models of thisγ-ray emitting jet.Methods.The radio data set is comprised of VLBI observations at 15 GHz from the MOJAVE program, and covers a time period of about two years. Through a segmented wavelet decomposition method (WISE code), we estimated the apparent displacement of individual plasma features; we then performed a pixel-based analysis of the stacked image to determine the jet shape. The X-ray data set includes all available observations from theChandra, XMM, andSwiftsatellites, and is used, together with archival data in the other bands, to build the spectral energy distribution (SED).Results.Proper motion is mostly detected along the edges of the flow, which appears strongly limb brightened. The apparent speeds increase as a function of distance from the core up to a maximum of ∼11.5 c. This constrains the jet viewing angle to assume relatively small values (θ ≲ 10°). In the acceleration region, extending up to a de-projected distance of ∼4.8 × 104Schwarzschild radii (∼11 pc), the jet is collimating (r ∝ z0.40 ± 0.04), as predicted for a magnetically-driven plasma flow. By assuming that the core region is indeed magnetically dominated (UB/Ue > 1), the SED and the jet power can be well reproduced in the framework of leptonic models, provided that the high-energy component is associated to a second emitting region. The possibility that this region is located at the end of the acceleration zone, either in the jet layer or in the spine, is explored in the modeling.
Highlights
Over the past ten years, the gamma-ray sky has been thoroughly probed by the Fermi-Large Area Telescope (LAT), and active galactic nuclei (AGN) can be firmly defined as the dominant class among γ-ray emitters (Ackermann et al 2015)
Due to the bias introduced by integrating the spectrum to calculate the flux, the present data are insufficient to attest the statistical significance of a correlation between Γ and flux
Motivated by the recent detection of 3C 264 in the TeV regime (Mukherjee 2018), we have conducted a multiband study of its misaligned jet, combining very-long-baseline interferometry (VLBI) radio images with high frequency data collected by different satellites (Chandra, XMM, Swift, Fermi-LAT) and adopting the inferred properties to model the broadband spectral energy distribution (SED)
Summary
Over the past ten years, the gamma-ray sky has been thoroughly probed by the Fermi-Large Area Telescope (LAT), and active galactic nuclei (AGN) can be firmly defined as the dominant class among γ-ray emitters (Ackermann et al 2015). In addition to the multizone leptonic models, hadronic, and lepto-hadronic models (e.g., Mannheim & Biermann 1992; Dar & Laor 1997; Böttcher 2005; Mastichiadis et al 2013), which attribute the high-energy SED component to proton-synchrotron or to proton-photon interactions, provide valid alternative solutions for reproducing the TeV emission in blazars and in radio galaxies (Aharonian 2000; Mücke et al 2003; Aharonian et al 2005; Protheroe et al 2003; Petropoulou et al 2014). In this article we investigate the radio and X-ray properties of the jet in 3C 264, and we use the inferred parameters to model the broadband SED This radio source is well studied on subkiloparsec and larger scales, but the innermost regions of its jet, where the most energetic processes are likely to take place, are still largely unexplored.
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