Indirect Evidence for Dark Matter Density Spikes around Stellar-Mass Black Holes

Abstract: It has been suggested for a long time that dark matter would form a density spike around a black hole. However, no promising evidence has been observed so far to verify this theoretical suggestion. Here, we report the existence of a dark matter density spike around each of the two nearby stellar-mass black holes (A0620-00 and XTE J1118+480). The dynamical friction between dark matter and the companion stars can satisfactorily explain the abnormally fast orbital decays in the two binaries. The calculated spike index for A0620-00 and XTE J1118+480 are $\gamma=1.71^{+0.01}_{-0.02}$ and $\gamma=1.85^{+0.04}_{-0.04}$ respectively, which are close to the lower regime predicted by the stellar heating model. It may provide a possible indirect evidence for the existence of dark matter density spikes around stellar-mass black holes. We anticipate that analyzing observational data of nearby black hole X-ray binaries would be a new way to reveal the nature of dark matter.