The Scoured Spike: Suppression of Indirect Dark Matter Signals by a Hidden Companion

Abstract

A massive ``dark companion'' -- such as an intermediate-mass black hole or other compact dark object -- orbiting the supermassive black hole at the Galactic Center can dynamically reshape the surrounding dark-matter spike. Through gravitational heating and angular-momentum exchange, the companion excavates a ``scoured'' region that lowers the inner density and suppresses the expected annihilation signal. We quantify this effect by computing the suppression of the dark-matter annihilation $J$-factor induced by such a companion, combining an analytic scouring-radius model with full numerical integrations of the modified density profile. We scan the parameter space of companion mass, orbital separation, system age, and spike slope, explicitly including the interplay with the annihilation plateau. \textcolor{black}{For canonical Gondolo--Silk spikes with $\gamma_{\rm sp} \gtrsim 2$, the scouring radius grows only weakly with injected energy because the binding energy is concentrated at small radii; however, once scouring occurs, the $J$-factor suppression can be substantial, for sufficiently massive or long-lived companions. In the shallower-spike regime with $\gamma_{\rm sp} \lesssim 2$, the same companion drives $r_{\rm scour}$ to much larger multiples of $R_{\rm core}$, and even a modest ${\sim}10^{4}\,M_\odot$ companion on a $\mathcal{O}(100)\,{\rm AU}$ orbit and $\mathcal{O}({\rm Gyr})$ age can suppress the annihilation flux by one to two orders of magnitude. Whether a steep spike is significantly suppressed therefore depends on the companion's mass and lifetime, not merely on the sign of $(2-\gamma_{\rm sp})$.} The numerical results are accurately captured (typically at the $\lesssim 10\%$ level) by a simple fitting formula in terms of a dimensionless scouring parameter that measures the ratio between the scoured region and the annihilation core. [...]