On the origin of X-shaped radio galaxies

Abstract

After a brief, critical review of the leading explanations proposed for the small but important subset of radio galaxies showing an X-shaped morphology (XRGs) we propose a generalized model, based on the jet-shell interaction and spin-flip hypotheses. The most popular scenarios for this intriguing phenomenon invoke either hydrodynamical backflows and over-pressured cocoons or rapid jet reorientations, presumably from the spin-flips of central engines following the mergers of pairs of galaxies, each of which contains a supermassive black hole (SMBH). We confront these models with a number of key observations and thus argue that none of the models is capable of explaining the entire range of salient observational properties of XRGs, although some of the arguments raised in the literature against the spin-flip scenario are probably not tenable. We then propose here a new scenario which also involves galactic mergers but would allow the spin of the central engine to maintain its direction. Motivated by the detailed multi-band observations of the nearest radio galaxy, Centaurus A, this new model emphasizes the role of interactions between the jets and the shells of stars and gas that form and rotate around the merged galaxy and can cause temporary deflections of the jets, occasionally giving rise to an X-shaped radio structure. Although each of the models is likely to be relevant to a subset of XRGs, the bulk of the evidence indicates that most of them are best explained by the jet-shell interaction or spin-flip hypotheses.

Light-component spectrum of the primary cosmic rays in the multi-TeV region measured by the ARGO-YBJ experiment

“…Spectral differences between the proton and helium components might be related to different types of sources and acceleration environments [8,41]. The present analysis does not allow the determination of the individual proton and helium contributions to the measured flux, and shows a discrepancy with the spectra obtained at lower energies from direct measurements by the passive balloon-borne experiments JACEE and RUNJOB.…”

“…The evolution of the proton and helium spectra and their subtle differences can be an indication of a different population of cosmic ray sources or acceleration sites. In many current models, as, for instance, those proposed in [8,9], a scenario is envisaged, in which the explosion of normal supernovae directly into the interstellar medium and the explosion of massive stars into their former stellar wind, like that of Wolf-Rayet stars, are the main sources of cosmic rays up to the knee region. If elements heavier than hydrogen come mainly from the latter sources, their spectra are expected to be harder than the proton spectrum and extending to higher energies.…”

Cosmic ray acceleration in magnetic circumstellar bubbles

“…The similar conclusion about the efficient cosmic ray acceleration by supernova blast wave propagating in stellar winds of WR and O-stars and the prediction of two kinds of cosmic ray accelerating SNRs was made earlier by Biermann [42]. Later his model was used in several phenomenological models of Galactic cosmic rays [43,44].…”

123–321 models of classical novae

“…These two quantities are essential ingredients for the 321 approach. Within mixing-length theory 3 (Biermann 1932; see Böhm-Vitense 1958 and Cox & Giuli 1968, for early, detailed descriptions of the theory), the Schwarzschild criterion establishes that convective energy transport sets in whenever superadiabatic temperature gradients occur, ∇ > ∇ ad , where ∇ ≡ d ln T/d ln P.…”