Test of Kerr-Sen metric with black hole observations

Understand

The Kerr-Sen black hole is a rotating charged black hole solution arising from heterotic string theory.

• In 4-dimensions effective theory the bosonic fields are: a $U(1)$ gauge boson, a Kalb-Ramond 3-form which is equivalent to a pseudoscalar axion in 4-dimensions, the dilaton and the graviton.
• The coupling constants in the theory are $\alpha^{\prime}$ (inverse string tension) and $\kappa$ (inverse reduced Planck mass in 4-dimensions) and the charge of the $U(1)$ field and the axion-photon coupling are related to these two.
• Sen found a black hole solution (the Kerr-Sen black hole) with these fields as the external hair of the black hole.