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

Cosimo Bambi, Katherine Freese, Sunny Vagnozzi, and Luca Visinelli

Phys. Rev. D 100, 044057 – Published 29 August 2019

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 center 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 $0.95 ≲ a_{*} \leq 1$ 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.

Received 1 May 2019

DOI:https://doi.org/10.1103/PhysRevD.100.044057

Physics Subject Headings (PhySH)

Alternative gravity theories Classical black holes General relativity equations & solutions

Gravitation, Cosmology & Astrophysics

Authors & Affiliations

Cosimo Bambi^1,*, Katherine Freese^2,3,4,†, Sunny Vagnozzi^2,3,5,‡, and Luca Visinelli ^3,6,7,§

¹Center for Field Theory and Particle Physics and Department of Physics, Fudan University, 200438 Shanghai, China
²The Oskar Klein Centre for Cosmoparticle Physics, Stockholm University, Roslagstullsbacken 21A, SE-106 91 Stockholm, Sweden
³The Nordic Institute for Theoretical Physics (NORDITA), Roslagstullsbacken 23, SE-106 91 Stockholm, Sweden
⁴Leinweber Center for Theoretical Physics, Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA
⁵Kavli Institute for Cosmology (KICC) and Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, United Kingdom
⁶Department of Physics and Astronomy, Uppsala University, Lägerhyddsvägen 1, SE-75120 Uppsala, Sweden
⁷Gravitation Astroparticle Physics Amsterdam (GRAPPA), Institute for Theoretical Physics Amsterdam and Delta Institute for Theoretical Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands

^*bambi@fudan.edu.cn
^†ktfreese@umich.edu
^‡sunny.vagnozzi@fysik.su.se
^§l.visinelli@uva.nl

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 100, Iss. 4 — 15 August 2019

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Kerr black hole: deviation from circularity $Δ C$ defined in Eq. (5) as a function of the Kerr BH dimensionless spin parameter $a_{*} = a / M$ and observation angle $θ_{obs}$ . The region above the black line is excluded by the measured circularity of M87* reported by the Event Horizon Telescope collaboration in [29].
Reuse & Permissions
Figure 2
Superspinar: The green hatched region shows the allowed region of parameter space for superspinars, in agreement with the circularity and size of the EHT observation, at fixed observation angle $θ_{obs} = 17 °$ . The parameters shown are the superspinar dimensionless spin parameter $a_{*}$ and radius-to-mass ratio $R_{ss} / M$ . The color coding illustrates the deviation from circularity $Δ C$ defined in Eq. (5). The regions to the left of the black curve on the left side of the figure, to the right of the black curve on the right side of the figure, and above the black curve on the top of the figure, are excluded by the circularity of M87* inferred by the Event Horizon Telescope collaboration in [29]. In the two regions within the green lines, the size of the superspinar shadow matches the size reported by the EHT collaboration [34]. The intersection between the region allowed by the circularity limits and the region allowed by the size limits is given by the region hatched in green.
Reuse & Permissions
Figure 3
Superspinar: The green hatched region shows the allowed region of parameter space for superspinars, in agreement with the circularity and size of the EHT observation, at fixed dimensionless spin parameter $a_{*} = 1.1$ . Here the parameters explored are the superspinar observation angle $θ_{obs}$ and radius-to-mass ratio $R_{ss} / M$ . The color coding illustrates the deviation from circularity $Δ C$ defined in Eq. (5). The region to the right of the black line is excluded by the measured circularity of M87* reported by the Event Horizon Telescope collaboration in [29]. The white region to the right of the figure features extreme deviations from circularity ( $Δ C ≫ 100 %$ ) and was not explored for practical reasons. The two green lines bound the region in which the size of the superspinar lies within the range given by observations, Eq. (6). The intersection between the region allowed by the circularity limits and the region allowed by the size limits is given by the region hatched in green. One can see that values of $5 ≳ R_{ss} / M ≳ 1$ are allowed, depending on the observation angle.
Reuse & Permissions
Figure 4
Top: shadows of a Kerr BH for $a_{*} = 0.999$ and for an observation angle $θ_{obs} = 90 °$ (left) and $θ_{obs} = 17 °$ (right). Bottom: shadows of a superspinar for $a_{*} = 1.1$ , an observation angle $θ_{obs} = 17 °$ , and for $R_{ss} = 0.5 M$ (left) and $R_{ss} = 2.5 M$ (right). The unit of length on the $x$ axis is $M$ . The shadows on the left-hand side are excluded by the EHT image: the deviation from circularity for the Kerr BH is too large, and the superspinar is too small and oblate. The shadows on the right-hand side are instead allowed by the EHT image. Note that from Eq. (6) the radius of the EHT image is $\approx 5.5$ in units of $M$ , which is consistent with the two shadows on the right-hand side.
Reuse & Permissions

Physical Review D

covering particles, fields, gravitation, and cosmology