New Optical Constraints on the Presence of a Compact Central Object in Cassiopeia A

Abstract

Deep optical imaging at the location of the pointlike X-ray source at the center of the ≃300 yr old Cassiopeia A supernova remnant discovered in the Chandra first-light observation is presented. R-band images obtained with the Steward Observatory 2.3 m telescope and direct CCD on four nights in 1999 September are used to set a new brightness limit of R ≳ 26.3 mag (1 σ) on the presence of any optical object associated with the X-ray source. This new limit implies MR ≳ 10.3 mag and LX(0.2-10 keV)/Lopt ≳ 800. Accretion models are the most constrained by the optical results. Binary accretion models and scenarios involving very low accretion rates from a disk onto a neutron star or a black hole similar to models for quiescent transient low-mass X-ray binaries are effectively ruled out. Fallback accretion models may be ruled out in some circumstances, but the emitted optical and infrared flux is sensitive to the location of the transition radius, the geometry of the disk, and its inclination to the line of sight.