A Dark Matter Masquerade: Degeneracies in Black Hole and Accretion Inference from X-ray Reflection Measurements and Prospects for Compact Dark Matter Halo Constraints

Abstract

X-ray reflection spectroscopy is an established probe of black hole (BH) spin and accretion geometry, and many studies have examined its systematic uncertainties. In this work, we explore a possible environmental effect associated with compact dark matter (DM) halos. We adopt numerical BH-DM spacetimes and perform general-relativistic ray-tracing calculations to generate broadened Fe\, K$α$ line profiles. For the configurations studied here, compact DM halos shift the lines toward lower observed energies relative to their Kerr counterparts. We fit the simulated profiles with standard Kerr models and find that the inferred spin and inclination can differ from their input values; in some cases, the spin is overestimated. These results suggest that compact DM halos may introduce an additional uncertainty in reflection-based measurements. Conversely, reflection models that include the environment could ultimately help constrain key halo parameters, such as its mass and characteristic scale, complementing gravitational-wave probes.