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The Event Horizon Telescope (EHT) collaboration has recently released thefirst image of a black hole (BH), opening a new window onto tests of generalrelativity in the strong field regime. In this paper, we derive constraints onthe nature of M87* (the supermassive object at the centre of the galaxy M87),exploiting the fact that its shadow appears to be highly circular, and usingmeasurements of its angular size. We first consider the simple case where M87*is assumed to be a Kerr BH. We find that the inferred circularity of M87*excludes Kerr BHs with observation angle $\theta_{\rm obs} \gtrsim 45^{\circ}$for dimensionless rotational parameter $0.95 \lesssim a_* \leq 1$ whereas theobservation angle is unbounded for $a_* \lesssim 0.9$. We then consider thepossibility that M87* might be a superspinar, i.e. an object described by theKerr solution and spinning so fast that it violates the Kerr bound by having$|a_*| > 1$. We find that, within certain regions of parameter space, theinferred circularity and size of the shadow of M87* do not exclude thepossibility that this object might be a superspinar.

Testing the rotational nature of the supermassive object M87* from the circularity and size of its first image