Testing the rotational nature of the supermassive object M87* from the circularity and size of its first image (1904.12983v2)

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.

• The paper examines M87*'s rotation by analyzing the shadow's circularity and size to constrain viewing angles and spin parameters.
• It employs a comparison of EHT observations with Kerr black hole predictions, excluding high-spin scenarios for angles above 45°.
• The study explores the superspinar hypothesis, offering insights into alternative quantum gravity effects beyond traditional general relativity.

Insights into M87*'s Rotational Nature through EHT Observations

The paper "Testing the rotational nature of the supermassive object M87* from the circularity and size of its first image" investigates the nature of the supermassive object M87* at the center of the galaxy M87, utilizing the first direct imaging from the Event Horizon Telescope (EHT). The focus of the paper is on deriving constraints on M87* by analyzing the circularity and size of its shadow, thus providing insights into the object's rotational characteristics.

The authors begin by considering M87* as a Kerr black hole, characterized by two primary parameters: the viewing angle $\theta_{\rm obs}$ and the dimensionless spin parameter $a_*$, or $a/m$. Their analysis shows that the EHT data excludes Kerr black holes for observation angles $\theta_{\rm obs} \gtrsim 45^\circ$ when $0.95 \lesssim a_* \leq 1$, whereas for spins $a_* \lesssim 0.9$, there are no such constraints. This inference is based on matching the observed circularity of the shadow to theoretical predictions, thus ruling out certain angles and spins that would result in non-circular shadows.

The paper also explores the intriguing possibility that M87* could be a superspinar—an object exceeding the Kerr bound ($|a_*| > 1$). Superspinars are hypothetical entities proposed within string theory that could evade the issues of singularities due to quantum gravity effects, leading to significant deviations in shadow structure. The analysis of the EHT image shows that under some parameter configurations, such as $R_{\rm ss}/M$ being about 1 to 5, and $a_* (> 1)$, the shadows could align with observations for specific viewing angles, notably around $\theta_{\rm obs} \lesssim 10^\circ$. This suggests that superspinars remain a viable explanation for M87*'s nature within certain parameter space regions, challenging the notion that the EHT observations only support traditional Kerr black holes.

The implications of these findings are substantial. They provide a framework for future testing of alternative theories beyond general relativity, especially those tied to quantum gravity. The potential discovery of superspinars could offer indirect evidence supporting string theory and related high-energy physics. Moreover, these observations guide the parameter space exploration for astrophysical black holes, fostering a deeper understanding of their physical properties and contributing to the broader effort of unifying gravity with quantum mechanics.

Future observational advancements, potentially through space-based very-long-baseline interferometry or higher frequency regimes, may further refine these constraints and illuminate the nature of supermassive black holes and other compact objects. As technologies develop, opportunities to probe these extreme environments will enhance our understanding of gravitational physics and inform theoretical developments in cosmology and quantum gravity.