  Einstein's theory of General Relativity predicts that the spacetime metric
around astrophysical black holes is described by the Kerr solution. In this
work, we employ state-of-the-art in relativistic reflection modeling to analyze
a selected set of NuSTAR spectra of Galactic black holes to obtain the most
robust and precise constraints on the Kerr black hole hypothesis possible
today. Our constraints are much more stringent than those from other
electromagnetic techniques, and with some sources we get stronger constraints
than those currently available from gravitational waves.
