Thermal Accretion Disk Spectra Based Tests of General Relativity ^†

The continuous X-ray flux of stellar-mass black holes provides an excellent source of data to learn about the astrophysics of accretion disks and about the spacetime itself. The extraction of information, however, depends heavily on our ability to correctly model the astrophysics and the theory of gravity, and the quality of the data. By combining a relativistic ray-tracing and Markov-Chain Monte-Carlo sampling technique, I show that the incorporation of the spin parameter through a slowly-rotating approximation, is not able to break the complex degeneracies of the model and therefore, when introducing modifications beyond general relativity it is very challenging to perform tests of general relativity with this type of observations. As a particular case, I show that it not possible to distinguish the small-coupling, slow-rotation black hole solution of dynamical Chern–Simons gravity from the Kerr solution with current instruments.