Hints of Dark Matter Spikes in Low-mass X-ray Binaries: a critical assessment

Three black-hole low-mass X-ray binaries (LMXBs) in the Milky Way show rates of period decay which cannot be easily explained by standard mechanisms. Recently, it has been claimed that the anomalous period decays in two of these systems may be explained by dynamical friction due to very high dark matter (DM) densities around the black holes. We critically assess these claims by performing $N$-body simulations of binaries embedded in dense DM ``spikes". We simulate the previously-studied systems XTE J1118+480 and A0620--00, as well as studying the third binary Nova Muscae 1991 for the first time in this context. These simulations show that feedback on the DM distribution plays a crucial role and we rule out previously-claimed shallow DM spikes. We set lower limits on the steepness $γ$ of DM density profiles required to explain the period decay in these LMXBs, requiring $γ\gtrsim 2.15-2.20$ in XTE J1118+480 and A0620--00 and $γ\gtrsim 2.3$ in Nova Muscae 1991. Improved modeling of the long-term evolution of binaries embedded in DM spikes may allow us to exclude even larger densities in future.