Null integrability and photon phenomenology in the accelerated Schwarzschild black hole (2509.23432v1)

Abstract: In this study we analyse null geodesics in an accelerated Schwarzschild black hole sustained by a cosmic string/strut (C-metric) in the sub-extremal regime. Using dimensionless variables and a Mino-type parameter we cast the equations into a complete first order form. For photons the conformal Hamilton-Jacobi equation separates and the radial and polar motion reduce to Bierman-Weierstrass form, yielding new closed analytic solutions. This provides a clean classification of photon trajectories, identifies a fixed photon cone that replaces equatorial symmetry when the acceleration is nonzero, and locates a single spherical photon surface shared by all latitudes. On the observational side, we prove that the shadow seen by any static observer is an exact circle: its screen radius depends on the acceleration and observer location but not on the conicity, so the local shadow cannot bound the string tension. We provide compact expressions for the photon ring orbital frequency and Lyapunov exponent and use them to obtain eikonal quasinormal estimates; we also show how a single shadow measurement at known position uniquely infers the dimensionless acceleration. As diagnostics of the background, we compute the (generally unequal) surface gravities of the black hole and acceleration horizons and derive an exact redshift law between static worldlines.