Kasner critical scaling in Einstein--Maxwell--scalar black holes with nonlinear collapse and scalarization

A critical scaling exponent of $1\mathrm{/}2$ was numerically observed for the Kasner parameter of asymptotically flat scalarized black holes and conjectured to be universal [Phys.\ Rev.\ Lett.\ $\mathbf{\text{136}}$, 251402 (2026)]. In the same static, spherically symmetric Einstein–Maxwell–scalar setting, we show that this exponent is not universal. We develop a rigorous mathematical framework and construct couplings realizing exponents $1\mathrm{/}(2m)$ for any integer $m\ge 1$ by tuning higher-order terms at fixed quadratic coupling parameter $\mu$. Nonlinear interior collapse produces a spacelike Kasner singularity with ${\beta }_{\mathrm{K}}\propto {\epsilon }^{-1}$, where $\epsilon$ is the value of scalar field at the event horizon. An exterior branch satisfying $q\mathrm{/}{q}_{\star }-1\propto {\epsilon }^{2m}$, therefore, yields ${\beta }_{\mathrm{K}}\propto \mathrm{\mid }q\mathrm{/}{q}_{\star }-1{\mathrm{\mid }}^{-1\mathrm{/}(2m)}$, where $q$ is the charge-to-mass ratio and $q_{\star }$ its critical value. We also determine the prefactor explicitly. This recovers the $1\mathrm{/}2$ law and establishes a hierarchy including $1\mathrm{/}4$ and $1\mathrm{/}6$.