Seeing Unseen Gravitational Wave Inspirals at the Galactic Center

Abstract

The Galactic Center is expected to host a population of extremely large mass-ratio inspirals (X-MRIs) migrating toward the supermassive black hole. We propose an observational approach utilizing the secular gravitational torque exerted by X-MRIs on the highly eccentric S-star S301. This approach is intrinsically limited by the stochastic nature of the surrounding stellar-mass black hole cusp. As S301 propagates through this granular medium, it experiences discrete gravitational encounters. Standard orbital fitting procedures inherently assume a smooth background potential; applying them to stochastic trajectories averages these discrete velocity discontinuities into a continuous secular drift. This introduces a systematic bias that mimics the X-MRI precession signal, invalidating traditional mass estimation techniques. To resolve this degeneracy, we implement a methodology based on the Wavelet Transform Modulus Maxima method to characterize the local Hölder regularity of the orbital motion. This model isolates the singular discontinuities induced by stellar-mass black hole encounters from the continuous torque of the X-MRI distribution, allowing for detection even when instantaneous accelerations are below the instrumental noise threshold. We demonstrate analytically that S301 is highly sensitive to the interior X-MRI population while remaining dynamically decoupled from the secular influence of the exterior cusp. Combining precision astrometry and radial velocity projections, we establish that a minimal population of two X-MRIs is detectable within a five to nine-year timeframe, allowing us to constrain the presence of an unseen orbiter population prior to the launch of space-based gravitational wave detectors.