Constraining alternatives to the Kerr black hole

Understand

The recent observation of the shadow of the supermassive compact object M87$^*$ by the Event Horizon Telescope (EHT) collaboration has opened up a new window to probe the strong gravity regime.

• In this paper, we study shadows cast by two viable alternatives to the Kerr black hole, and compare them with the shadow of M87$^{*}$.
• The first alternative is a horizonless compact object (HCO) having radius $r_0$ and exterior Kerr geometry.
• The second one is a rotating generalisation of the recently obtained one parameter ($r_0$) static metric by Simpson and Visser.