Breaking Parameter Degeneracies in a Magnetically Charged Black Hole Embedded in a Hernquist Dark-Matter Halo: A Multi-Observable Analysis

Abstract

We study the degeneracy of intrinsic and environmental parameters for BH observables in a static spacetime sourced by a nonlinear magnetic monopole immersed in a Hernquist dark-matter halo. We explore four complementary probes; the shadow radius $R_{sh}$, eikonal quasinormal-mode frequencies $M\omega_R$, weak gravitational lensing $\hat{\theta}_\infty$, and neutrino-antineutrino annihilation $\dot{Q}/\dot{Q}_{Newt}$, and map their degeneracy contours in the $(g/\mathcal{M},\alpha/\mathcal{M})$ plane at fixed $\beta/\mathcal{M}$. Different parameter combinations yield signatures nearly indistinguishable from a Schwarzschild black hole single-observable diagnostics cannot uniquely constrain the magnetic charge and halo amplitude. The degeneracy contours are, however, mutually non-parallel: the slopes $d\alpha/dg$ along constant-$R_{sh}$ and constant-$M\omega_R$ contours differ by a factor $\sim 5$, so their combination breaks the remaining degeneracy and constrains both parameters simultaneously. We compute the QNMs spectra using a high-order WKB method with Pad\'e resummation. The magnetic charge raises the real oscillation frequency while the halo lowers it; the cancellation is observable-dependent and does not persist across all four channels. An expansion around an asymptotically renormalized Schwarzschild background of mass $\mathcal{M}=M+\alpha$ shows that at fixed $\mathcal{M}$ both sectors reduce $R_{sh}$ at first perturbative order. For weak lensing, $\mathcal{M}$ alone determines the leading deflection, first subleading correction depends on $\mathcal{Q}=g^2+4\alpha\beta$, separating total halo mass from halo concentration. For neutrino-pair annihilation, the magnetic charge suppresses the deposition rate by raising the lapse, while the halo enhances it through the reverse mechanism.