Optical Sources near the Bright X-Ray Source in NGC 1073

Abstract

New HST observations show that the bright X-ray source in the face-on spiral galaxy NGC 1073 is located near a ring of recent star formation with an age of 8-16 Myr. This strengthens the association of X-ray sources in spiral galaxies emitting near or above the Eddington limit for a 20 M_solar black hole with recent star formation events. Two candidate optical counterparts of the X-ray source are found. The X-ray-to-optical flux ratios of both are consistent with those of low-mass X-ray binaries and higher than most high-mass X-ray binaries, suggesting that reprocessing of X-rays contributes to the optical light. The optical magnitude and color of one candidate is consistent with that predicted for an X-ray binary with an initial donor mass of 6-8 M_solar. However, the same X-ray binary evolution model underestimates the X-ray luminosity. An X-ray source list for the field is presented, which includes detections of the nucleus of NGC 1073, three quasars, and an M3e brown dwarf star with high proper motion.