Extra light fermions in E6-inspired models and the 3.5 keV X-ray line signal

We propose a scenario in which extra light fermions in an E-6-inspired U(1) extension of the standard model constitute the dark matter, as a simple variation of our model for dark radiation presented in 2010. Interestingly, for the light fermions of mass about 7 keV, we naturally obtain a desired mixing angle to explain the recently discovered 3.5 keV X-ray line signal through the radiative decay into active neutrinos and photons with a lifetime in the range of 10(27)-10(28) seconds. (C) 2014 The Authors. Published by Elsevier B.V.