Implications of axionic hair on shadow of M87*
  Detection of axion field can unfold intriguing facets of our universe in
several astrophysical and cosmological scenarios. In four dimensions, such a
field owes its origin to the completely anti-symmetric Kalb-Ramond field
strength tensor. Its invisibility in the solar system based tests compels one
to look for its signatures in the strong field regime. The recent observation
of the shadow of the supermassive black hole in the galaxy M87 ushers in a new
opportunity to test for the footprints of axion in the near horizon region of
black holes, where the gravity is expected to be strong. In this paper, we
explore the impact of axion on the black hole shadow and compare the result
with the available image of M87*. Our analysis indicates that axion which
violates the energy condition seems to be favored by observations. The
implications are discussed.
