Testing Theories of Gravity via Black Hole Shadows in Sgr A*

Upcoming sub-millimetre very-long-baseline-interferometry (VLBI) images of Sagittarius A* (Sgr A*) carried out by the Event Horizon Telescope (EHT) Collaboration are expected to provide critical evidence for the existence of a supermassive black hole at our galactic centre. In this work we assess our present ability to use EHT images to determine if they correspond to a Kerr black hole as predicted by Einstein's theory of general relativity or to a black hole in alternative theories of gravity. To this end, we perform general relativistic magnetohydrodynamic simulations and use general relativistic radiation transfer calculations to generate synthetic shadow images of a magnetized accretion flow onto a Kerr black hole and a non-rotating dilaton black hole, which we take as a representative solution of an alternative theory of gravity. Taking into account the VLBI configuration of the 2017 EHT observation of Sgr A*, we found that it could be difficult to distinguish between black holes from different theories of gravity, unless additional information were available.