2001ApJ...558..270K

We report deep near-infrared and optical observations of the X-ray point source in the Cassiopeia A supernova remnant, CXO J232327.9+584842. We have identified a J=21.4±0.3 mag and a K_s=20.5±0.3 mag source within the 1 σ error circle, but we believe this source is a foreground Population II star with T_eff=2600-2800 K at a distance of ~2 kpc that could not be the X-ray point source. We do not detect any sources in this direction at the distance of Cas A and therefore place 3 σ limits of R≳25 mag, F675W≳27.3 mag, J≳22.5 mag, and K_s≳21.2 mag (and roughly H≳20 mag) on emission from the X-ray point source, corresponding to M_R≳8.2 mag, M_F675W≳10.7 mag, M_J≳8.5 mag, M_H≳6.5 mag, and M_Ks≳8.0 mag, assuming a distance of 3.4 kpc and an extinction of A_V=5 mag.