Observational signatures from horizonless black shells imitating
  rotating black holes
  In arXiv:1705.10172 it was proposed that string theory replaces Schwarzschild
black holes with horizonless thin shells with an AdS interior. In this paper we
extend the analysis to slowly rotating black holes, solving the
Israel-Lanczos-Sen junction conditions for a rotating shell composed of stringy
matter to determine the metric. Outside of the shell we find a vacuum solution
that differs from Kerr with a 32% larger quadrupole moment. We discuss the
observational consequences and explore the possibility to distinguish between a
black shell and a black hole. Promising methods include imaging of the black
hole at the center of the Milky Way using the Event Horizon Telescope,
precision measurements of stars in close orbits around the central black hole,
and future observations of colliding super massive black holes using the space
based gravitational wave observatory LISA.
