Near-Infrared and Optical Limits for the Central X-Ray Point Source in the Cassiopeia A Supernova Remnant

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., LH < 3 × 1029 ergs s-1) 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, mF110W = 21.9, and mF160W = 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.