The Nature of Ultraluminous X-Ray Sources in NGC 4565

Abstract

We report the optical identifications of two X-ray-luminous point sources in the spiral galaxy NGC 4565 based on archive data of Chandra and the Hubble Space Telescope. The central X-ray point source, RX J1236.3+2559, is found to be the nucleus of NGC 4565 with an X-ray luminosity of L_X~4.3×10³⁹ ergs s^-1. We show that its multiband properties are consistent with its being a low-luminosity active galactic nucleus. A faint optical counterpart with B~25.1 and I~24.0 was identified for the off-nucleus X-ray point source, RX J1236.2+2558. Its extinction-corrected B magnitude is estimated to be 24.5. The X-ray-to-optical flux ratio (f_X/f_B) is about 540. From the optical and X-ray properties, we argue that RX J1236.2+2558 is an ultraluminous X-ray compact source with L_X~6.5×10³⁹ ergs s^-1. The source is probably located in a faint globular cluster at the outer edge of NGC 4565's bulge.