Discovery of a strong soft X-ray excess in MKN 335 - evidence for an accretion disc ?

Abstract

Exosat observations of Mkn 335 reveal a hard power-law spectrum above about 1 keV, typical for Seyfert 1 galaxies, but with unusually strong variability on time-scales of about 1-2 hr. In addition, an intense soft X-ray component is found to dominate the overall spectrum of Mkn 335 below about 0.6 keV. Both soft and hard components increased in strength by a factor about 6 between 1983 November and 1984 December. These observations are discussed in terms of models in which an accretion disc feeds a central massive black hole. It is found that radiation pressure only allows a thermal origin for the soft X-radiation if the black hole mass is less than about 10 to the 7th solar masses, as suggested also by the observed variability time-scale.