Photon Rings and Shadow Size for General Integrable Spacetimes
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Date
2023-08-08
Authors
Salehi, Kiana
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
According to the no-hair theorem, the unique characteristics of astrophysical black holes
are their masses and spins. However, recent observations from the Event Horizon Telescope
(EHT) images of M87 and Sgr A* have allowed us to place constraints on possible
deviations from this theory. To interpret these observations and compare them to other
near horizon scale observations, we introduce a model-agnostic framework that explores
deviations while maintaining generality. We start by considering a general spherically symmetric
metric, which effectively applies for a polar observer in the slow rotation limit and
then follow by relaxing these constraints to axi-symmetric and stationary spacetimes. We
propose a nonperturbative, nonparametric spacetime-domain characterization of shadow
size and related measurements that makes explicit the nature and power (or lack thereof) of
shadow-size-based constraints, and facilitates comparisons among observations and targets.
Furthermore, we demonstrate that relying solely on shadow size measurements does not
impose a direct limitations on the value of the gtt component of the metric. However, in the
case of spherically symmetric spacetime, it can impose a constraint on the radial derivative
of gtt, while a more intricate constraint arises for the axi-symmetric spacetime. Moreover,
the measurement of shadows and potential future observations of multiple photon rings
do not provide any valuable information concerning the ergo-region and frame-dragging in
axi-symmetric spacetime.
Description
Keywords
Gravity