Outflows from Naked Singularities, Infall through the Black Hole Horizon: Hydrodynamic Simulations of Accretion in the Reissner-Nordström Space-Time.

We performed the first simulations of accretion onto the compact objects in the Reissner-Nordström (RN) space-time. The results obtained in general relativity are representative of those for spherically symmetric naked singularities and black holes in a number of modified gravity theories. A possible application of these calculations is to the active galactic nuclei with their powerful jets and outflows. It is now possible to compare the results of such simulations with the accreting supermassive objects in our own Milky Way and a nearby galaxy: observations of the core regions of galactic nuclei (Sgr A* and M87) performed with unprecedented resolution by the Event Horizon Telescope (EHT) collaboration allow fairly direct tests of the space-time metric of the central compact object. In this context we present general-relativistic hydrodynamical simulation results of accretion from an orbiting accretion torus (with a cusp) onto a RN black hole and a RN naked singularity. The results could not be more different for the two cases. For a black hole, just as in the familiar Kerr case, matter overflowing the cusp plunges into the black hole horizon. For the naked singularity, the accreting matter forms an inner structure of toroidal topology and leaves the system via powerful outflows. It is an open question whether this inner structure can give rise to an image quantitatively similar to the ones reported by EHT for M87 and Sgr A*.