Shadows of Einstein-dilaton-Gauss-Bonnet black holes

TL;DR

This paper investigates the shadows of Einstein-dilaton-Gauss-Bonnet black holes, finding they are generally smaller than Kerr black holes and that shadow observations alone cannot easily distinguish these exotic objects.

Contribution

The study provides the first detailed analysis of EdGB black hole shadows, comparing static and rotating cases, and highlights the subtle differences from Kerr black holes.

Findings

EdGB black hole shadows are smaller than Kerr counterparts.

Differences in shadow radii are less than one percent.

Shadow features do not reveal exotic properties of EdGB black holes.

Abstract

We study the shadows of the fully non-linear, asymptotically flat Einstein-dilaton-Gauss-Bonnet (EdGB) black holes (BHs), for both static and rotating solutions. We find that, in all cases, these shadows are smaller than for comparable Kerr BHs, i.e. with the same total mass and angular momentum under similar observation conditions. In order to compare both cases we provide quantitative shadow parameters, observing in particular that the differences in the shadows mean radii are never larger than the percent level. Therefore, generically, EdGB BHs cannot be excluded by (near future) shadow observations alone. On the theoretical side, we find no clear signature of some exotic features of EdGB BHs on the corresponding shadows, such as the regions of negative (Komar, say) energy density outside the horizon. We speculate that this is due to the fact that the Komar energy interior to the light rings (or more precisely, the surfaces of constant radial coordinate that intersect the light rings in the equatorial plane) is always smaller than the ADM mass, and consequently the corresponding shadows are smaller than those of comparable Kerr BHs. The analysis herein provides a clear example that it is the light ring impact parameter, rather than its "size", that determines a BH shadow.