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

TL;DR

The paper shows that a hidden compact companion near Sgr A* can dynamically scour the inner DM spike, dramatically suppressing the Galactic Center $J$-factor and hence the expected annihilation flux. It develops a scouring-radius framework that links the companion’s energy deposition to modifications of the density profile, and validates this with full line-of-sight integrations across a broad parameter space in $m_2$, $r_2$, $t_{\rm BH}$, and $\gamma_{\rm sp}$. The key finding is a sharp dichotomy: steep Gondolo–Silk spikes ($\gamma_{\rm sp}>2$) resist suppression, while shallower or heated spikes ($\gamma_{\rm sp}\lesssim 2$, especially $\gamma_{\rm sp}\lesssim 1.8$) can see $J$-factor reductions by one to two orders of magnitude for ${m_2}\sim 10^4$–$10^5\,M_\odot$ and $r_2$ of order a few hundred AU. This introduces significant astrophysical degeneracies in indirect-detection constraints and motivates multimessenger approaches to detect or constrain a hidden companion and to reinterpret GC observations in light of dynamical spike evolution.

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. For canonical Gondolo-Silk spikes with $γ_{\rm sp} \gtrsim 2$, we find that the distribution is remarkably resilient and that the companion produces at most mild reductions of the $J$-factor. In contrast, for pre-heated or otherwise shallow spikes with $γ_{\rm sp} \lesssim 1.8$, even a modest $\sim 10^{4}\,M_\odot$ companion on a O(100) AU orbit and O(Gyr) age can suppress the annihilation flux by one to two orders of magnitude. 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. Our findings demonstrate that neglecting a dark companion can lead to substantial overestimates of the Galactic Center $J$-factor, with direct consequences for interpreting gamma-ray, neutrino, and antimatter searches for annihilating dark matter.

Figures (6)

• Figure 1: Observational constraints on a hypothetical dark companion to Sgr A*. Shaded regions indicate exclusions from: (red) S-star orbital stability requiring $<1$% perturbations to the gravitational potential Naoz:2019; (blue) proper motion limits from VLBA/EHT astrometry Reid:2004EHT:2022 constraining reflex motion $<1 km$$s^{-1}$; (gray) gravitational-wave merger timescale $<10$ Myr, incompatible with long-lived scouring Naoz:2019. The unshaded central region corresponds to the viable parameter space explored in this work.
• Figure 2: Suppression of the annihilation $J$ factor as a function of the companion mass $m_2$ for several orbital separations $r_2$. Solid, dashed, and dotted lines correspond to $\gamma_{\rm sp}=2.3,\,2.0,\,1.8$, respectively. The shaded band indicates the observationally allowed range of $m_2$ (see Fig. \ref{['fig:constraints']}).
• Figure 3: Dependence of the $J$-factor suppression $J_{\rm mod}/J_{\rm sp}$ on the spike slope $\gamma_{\rm sp}$, for representative values of $m_2$ and $r_2$. The vertical dashed line marks the transition between the Gondolo--Silk regime ($\gamma_{\rm sp}>2$) and the dynamically heated regime ($\gamma_{\rm sp}<2$).
• Figure 4: Validation of the analytical approximation (Eq. \ref{['eq:Jratio_approx']}, solid lines) against numerical line-of-sight integrations (points) across multiple spike slopes. Agreement is fair for $\gamma_{sp} \lesssim$ 2, and adequate for steeper spikes, confirming that the simple scaling captures the essential physics of J-factor suppression.
• Figure 5: Parameter-space maps of the $J$-factor suppression ratio $J_{\rm mod}/J_{\rm sp}$. Left: dependence on companion mass $m_2$ and orbital separation $r_2$ for $\gamma_{\rm sp}=2.0$ and $t_{\rm BH}=5$ Gyr. Right: dependence on spike slope $\gamma_{\rm sp}$ and system age $t_{\rm BH}$ for $m_2=10^4\,M_\odot$ and $r_2=200$ AU. Contours mark suppression levels of $J_{\rm mod}/J_{\rm sp}=0.9,\,0.5,\,0.1$. The gray shaded region in the left panel represents the observationally excluded region from Figure \ref{['fig:constraints']}.