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

## TL;DR

This paper uses the circularity and size of M87*'s shadow from EHT data to test whether it is a Kerr black hole or a superspinar, providing constraints on its spin and nature.

## Contribution

It introduces a method to constrain the spin and nature of M87* using shadow circularity and size, considering both Kerr black holes and superspinar models.

## Key findings

- Circularity excludes high-spin Kerr BHs at certain angles.
- Shadow size constrains the spin parameter of M87*.
- Superspinar possibility remains viable within some parameter space.

## Abstract

The Event Horizon Telescope (EHT) collaboration has recently released the first image of a black hole (BH), opening a new window onto tests of general relativity in the strong field regime. In this paper, we derive constraints on the 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 using measurements 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 the observation angle is unbounded for $a_* \lesssim 0.9$. We then consider the possibility that M87* might be a superspinar, i.e. an object described by the Kerr solution and spinning so fast that it violates the Kerr bound by having $|a_*| > 1$. We find that, within certain regions of parameter space, the inferred circularity and size of the shadow of M87* do not exclude the possibility that this object might be a superspinar.