Constraints on M_• and a_• based on a joint fit of the X-ray spectra from Epochs 2 to 6 with the slim-disk model (Table B.4). We calculate the ΔC-stat across the {M_•,  a_•} plane. The best–fit point with the lowest C-stat is marked by the yellow triangle. Areas within 1σ and 2σ confidence levels are in red and blue, respectively. At 1σ for the two–parameter fits, M_• is constrained to be log($M_{•}/M_{\odot })=2.4_{-0.1}^{+0.6}$, while a_• is virtually unconstrained.