Very-high-energy quasars hint at ALPs
  One of the mysteries of very-high-energy (VHE) astrophysics is the
observation of flat spectrum radio quasars (FSRQs) above about 30 GeV, because
at those energies their broad line region should prevent photons produced by
the central engine to escape. Although a few astrophysical explanations have
been put forward, they are totally ad hoc. We show that a natural explanation
emerges within the conventional models of FSRQs provided that photon-ALP
oscillations take place inside the source for the model parameters within an
allowed range.
