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

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 θ obs 45 • for dimensionless rotational parameter 0.95 a * ≤ 1 whereas the observation angle is unbounded for a * 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.

Upper bound on the GUP parameter using the black hole shadow

“…As I pointed out in Introduction, the value θ o = 17 • is conceived of as the angle between the jet and the observer, assuming that the jet direction is orthogonal to the M87* equatorial plane. The range for the rotation parameter presented here is in agreement with studies like [3], where the Kerr metric is adopted in order to describe the M87* shadow. As we can see in Table I, λ < 10 45 is the worst value compared to other upper bounds provided by different approaches, like the Lamb shift, Landau levels and scanning tunneling microscope in Ref.…”

“…According to Ref. [9], at the observer position,γ is described by the tetrad e µ a , i.e., γ = ζ (−e 0 + sin α cos βe 1 + sin α sin βe 2 + cos αe 3…”

“…Following Refs. [3,[50][51][52], let us construct two observables for the black hole shadow. The first one indicates the difference between the x and y axes.…”

“…According to the Event Horizon Telescope collaboration [1], a measure for the shadow deformation is provided by the deviation from circularity, which is conceived of as deviation from the root-mean-square of the radius l(r p ). Following Bambi et al [3] (but here another parameterization is adopted 6 ), the shadow radius l(r p ) is 1. 10 45 1.…”

“…Even with the good agreement with the Kerr metric, options are not totally ruled out. Due to the uncertainty on both the rotation parameter and observation angle, options to Kerr metric are still candidates for M87* shadow [3]. This article explores that avenue and applies the M87* parameters to Kerr-like objects, like rotating regular black holes.…”