Inclination-dependent spectral and timing properties in transient black hole X-ray binaries

We use a simple one-dimensional parametrization of timing properties to show that hard and hard-intermediate state transient black hole X-ray binaries with the same power spectral shape have systematically harder X-ray power-law emission in higher inclination systems. We also show that the power spectral shape and amplitude of the broad-band noise (with low-frequency quasi-periodic oscillations, QPOs, removed) are independent of inclination, confirming that it is well correlated with the intrinsic structure of the emitting regions and that the ‘type C’ QPO, which is inclination dependent, has a different origin to the noise, probably geometric. Our findings suggest that the power-law emission originates in a corona which is flattened in the plane of the disc, and not in a jet-like structure which would lead to softer spectra at higher inclinations. However, there is tentative evidence that the inclination dependence of spectral shape breaks down deeper into the hard state. This suggests either a change in the coronal geometry and possible evidence for contribution from jet emission, or alternatively an even more optically thin flow in these states.