Response to Comment on "A noninteracting low-mass black hole-giant star binary system".

Van den Heuvel and Tauris argue that if the red giant star in the system 2MASS J05215658+4359220 has a mass of 1 solar mass ( ), then its unseen companion could be a binary composed of two 0.9 stars, making a triple system. We contend that the existing data are most consistent with a giant of mass [Formula: see text] , implying a black hole companion of [Formula: see text] .

A noninteracting low-mass black hole-giant star binary system.

Black hole binary systems with companion stars are typically found via their x-ray emission, generated by interaction and accretion. Noninteracting binaries are expected to be plentiful in the Galaxy but must be observed using other methods. We combine radial velocity and photometric variability data to show that the bright, rapidly rotating giant star 2MASS J05215658+4359220 is in a binary system with a massive unseen companion.