We compute the quasinormal frequencies of $d$-dimensional spherically symmetric black holes with leading string $\alpha'$ corrections in the eikonal limit for tensorial gravitational perturbations and scalar test fields.
We find that, differently than in Einstein gravity, the real parts of the frequency are no longer equal for these two cases.
The corresponding imaginary parts remain equal to the principal Lyapunov exponent corresponding to circular null geodesics, to first order in $\alpha'$.
We also compute the radius of the shadow cast by these black holes.
Eikonal quasinormal modes and shadow of string-corrected $d$-dimensional black holes · Around
C. G. Callan, R. C. Myers and M. J. Perry, Black Holes in String Theory
1989
Earlier work this paper cites.
V. Cardoso and J. P. S. Lemos, Black hole collision with a scalar particle in four-dimensional, five-dimensional and seven-dimensional anti-de Sitter space-times: Ringing and radiation
B. Toshmatov, Z. Stuchlík, J. Schee and B. Ahmedov, Electromagnetic perturbations of black holes in general relativity coupled to nonlinear electrodynamics , Phys. Rev. D97