2006ApJ...636..848F

We set new near-infrared and optical magnitude limits for the central X-ray point source (XPS) in the Cassiopeia A supernova remnant based on HST images. Near-infrared images of the center of Cas A taken with the NICMOS 2 camera in combination with the F110W and F160W filters (~J and H bands) have magnitude limits ≥26.2 and ≥24.6, respectively. These images reveal no sources within a 1".2 radius (corresponding to a 99% confidence limit) of the Chandra XPS position. The NICMOS data, taken together with broadband optical magnitude limits (R∼28 mag) obtained from a deep STIS CCD exposure taken with a clear filter (50CCD), indicate that the XPS luminosities are very low in the optical/NIR bands (e.g., L_H<3x10²⁹ ergs/s) with no optical, J-, or H-band counterpart to the XPS easily detectable by HST. The closest detected object lies 1".8 from the XPS's nominal coordinates, with magnitudes R=25.7, m_F110W=21.9, and m_F160W=20.6, and is a foreground, late-type star as suggested by Kaplan, Kulkarni, and Murray. We discuss the nature of the Cas A central compact object on the basis of these near-infrared and optical flux limits.