The subcritical accelerating Schwarzschild-Bertotti-Robinson family

We identify the general finite-$\mathrm{\ell }$ neutral Alekseev–Astorino Schwarzschild–Bertotti–Robinson family with the subcritical branch of the independently obtained accelerating Schwarzschild–Bertotti–Robinson solution. The exact scale-free dictionary is \begin{equation*} B^2M^2=\frac{m^2}{b^2+\ell^2},\qquad AM=\frac{m\ell}{b^2+\ell^2},\qquad \frac{A}{B}=\frac{\ell}{\sqrt{b^2+\ell^2}}, \end{equation*} up to the north–south orientation convention. Thus finite real $\mathrm{\ell }$ covers precisely the acceleration interval $\mathrm{\mid }A\mathrm{\mid }<\mathrm{\mid }B\mathrm{\mid }$: for $\mathrm{\ell }\mathrm{\ne }0$ the corresponding OP acceleration parameter is nonzero and the two axial openings are unequal, although no acceleration horizon is present. We establish the identification by canonical Weyl-rod matching and horizon-shape comparison, an independent conicity check, and an exact off-horizon reconstruction of the metric and Maxwell field. The centered limit $\mathrm{\ell }=0$ is the nonaccelerating Schwarzschild–BR solution, whereas $\mathrm{\mid }\mathrm{\ell }\mathrm{\mid }\to \mathrm{\infty }$ approaches the critical boundary $\mathrm{\mid }A\mathrm{/}B\mathrm{\mid }=1$ from below. The Astorino parameter $\mathrm{\ell }$ therefore parametrizes subcritical acceleration rather than an additional independent neutral black-hole degree of freedom.