On the spine-layer scenario for the very high-energy emission of NGC 1275

Abstract

We discuss the γ-ray emission of the radiogalaxy NGC 1275 (the central galaxy of the Perseus Cluster), detected by Fermi-LAT (very-high energies) and MAGIC, in the framework of the 'spine-layer' scenario, in which the jet is assumed to be characterized by a velocity structure, with a fast spine surrounded by a slower layer. The existence of such a structure in the parsec scale jet of NGC 1275 has been recently proved through VLBI observations. We discuss the constraints that the observed spectral energy distribution imposes to the parameters and we present three alternative models, corresponding to three different choices of the angles between the jet and the line of sight (θv = 6°, 18° and 25°). While for the case with θv = 6° we obtain an excellent fit, we consider this solution unlikely, since such small angles seem to be excluded by radio observations of the large-scale jet. For θv = 25° the required large intrinsic luminosity of the soft (IR-optical) component of the spine determines a large optical depth for γ -rays through the pair production scattering γγ → e+e-, implying a narrow cut-offat ~50 GeV. We conclude that intermediate angles are required. In this case the low frequency and the high-energy emissions are produced by two separate regions and, in principle, a full variety of correlations is expected. The correlation observed between the optical and the γ-ray flux, close to linearity, is likely linked to variations of the emissivity of the spine. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.