Viscous damping in Manko–Novikov horizonless objects: A phenomenological GW190814 study

Abstract

The gravitational-wave event GW190814 contains a secondary component of mass ∼2.6M⊙ in the “mass gap” between neutronstars and stellar-mass black holes. We explore whether a horizonless ultra-compact object described by a Manko–Novikov (MN) exteriorcan be made compatible with ergoregion-instability constraints by adding an absorptive surface. Because perturbations of the MN spacetimeare not known to separate and no matched interior solution is specified, the calculation is deliberately phenomenological. The round-tripgain is modeled as G(ω) = (1−κ)ZSR(ω), where κ is related to an effective surface shear viscosity η. For the illustrative point a/M = 0.6,q = −0.1, a calibrated amplification input ZSR = 1.150 ± 0.020 gives κcrit ≈ 0.130 ± 0.018. The associated viscosity scale is of order1019 Pas with factor-of-few uncertainty from surface physics and the unknown MN perturbation spectrum. Echo times and frequencies arereported only as exploratory estimates within an exotic-compact-object analogy; detector relevance is not assessed.