2020/11/30 by Sebastian H. Völkel, Enrico Barausse, Nicola Franchini +1 · 2 citations
Physics and Astronomy · #gr-qc #astro-ph.HE
paper · pdf · doi:10.1088/1361-6382/ac27ed
published as Class. Quantum Grav. 38 21LT01 (2021) · Published as letter in Classical and Quantum Gravity, 8 pages, 3 figures
arxiv created 2021/10/12 · arxiv updated 2021/10/13
Following up on a recent analysis by Psaltis et al. [Phys. Rev. Lett. 125, 141104 (2020)], we show that the observed shadow size of M87^* can be used to unambiguously and robustly constrain the black hole geometry in the vicinity of the circular photon orbit. Constraints on the post-Newtonian weak-field expansion of the black hole's metric are instead more subtle to obtain and interpret, as they rely on combining the shadow-size measurement with suitable theoretical priors. We provide examples showing that post-Newtonian constraints resulting from shadow-size measurements should be handled with extreme care. We also discuss the similarities and complementarity between the EHT shadow measurements and black-hole gravitational quasi-normal modes.