Double instability of Schwarzschild black holes in Einstein-Weyl-scalar theory

Abstract: We study the stability of Schwarzschild black hole in Einstein-Weyl-scalar (EWS) theory with a quadratic scalar coupling to the Weyl term. Its linearized theory admits the Lichnerowicz equation for Ricci tensor as well as scalar equation. The linearized Ricci-tensor carries with a regular mass term ($m^2_2$), whereas the linearized scalar has a tachyonic mass term ($-1/m^2_2$). It turns out that the double instability of Schwarzschild black hole in EWS theory is given by Gregory-Laflamme and tachyonic instabilities. In the small mass regime of $m_2<0.876$, the Schwarzschild black hole becomes unstable against Ricci-tensor perturbations, while tachyonic instability is achieved for $m_2<1.174$. The former would provide a single branch of scalarized black holes, whereas the latter would induce infinite branches of scalarized black holes.