X-ray evidence of an obscured nucleus in the type 2 Seyfert galaxy Mkn3

Abstract

SEYFERT galaxies comprise the majority of active galactic nuclei (AGNs), in which a bright compact nucleus is thought to be powered by matter accreting onto a massive black hole. Seyfert galaxies are classified as type 1 or 2 according to the presence or absence of broad emission lines in the optical spectrum. The high velocities indicated by the broad lines in Seyfert 1 galaxies are taken to be good evidence of a compact, massive object, as are the strong and variable hard X-ray sources that are also generally observed in these objects. In contrast, Seyfert 2 galaxies possess neither of these characteristics, so the theory that they too have an accreting massive black hole is less compelling. Since the discovery¹ by spectropolarimetry of a 'hidden' Seyfert 1 nucleus in the prototypical Seyfert 2, NGC1068, the long-standing hope that the two classes may be unified has been revived. Here we present evidence from observations by the Ginga satellite that another Seyfert 2, Mkn3, has the X-ray spectral signature of a hidden type 1 nucleus.