Faint $\gamma$-ray sources at low-redshift: the radio galaxy IC 1531

Abstract

We present a multi-wavelength study of IC 1531 (z=0.02564), an extragalactic radio source associated with the $\gamma$-ray object 3FGL J0009.9-3206 and classified as a blazar of uncertain type in the Third Fermi Large Area Telescope AGN Catalog (3LAC). A core-jet structure, visible in radio and X-rays, is enclosed within a $\sim$220 kpc wide radio structure. The morphology and spectral characteristics of the kiloparsec jet in radio and X-rays are typical of Fanaroff-Riley type I galaxies. The analysis of the radio data and optical spectrum and different diagnostic methods based on the optical, infrared and $\gamma$-ray luminosities also support a classification as a low-power radio galaxy seen at moderate angles ($\theta=$10$^\circ$-20$^\circ$). In the framework of leptonic models, the high-energy peak of the non-thermal nuclear spectral energy distribution can be explained in terms of synchrotron-self-Compton emission from a jet seen at $\theta\sim$15$^\circ$. Similarly to other misaligned AGNs detected by Fermi, the required bulk motion is lower ($\Gamma_{\rm bulk}=$4) than the values inferred in BL Lac objects, confirming that, because of the de-boosting of emission from the highly-relativistic blazar region, these nearby systems are valuable targets to probe the existence of multiple sites of production of the most energetic emission in the jets.