COMPACT OPTICAL COUNTERPARTS OF ULTRALUMINOUS X-RAY SOURCES

Abstract

Using archival Hubble Space Telescope (HST) imaging data, we report the\nmultiband photometric properties of 13 ultraluminous X-ray sources (ULXs) that\nhave a unique compact optical counterpart. Both magnitude and color variation\nare detected at time scales of days to years. The optical color, variability,\nand X-ray to optical flux ratio indicate that the optical emission of most ULXs\nis dominated by X-ray reprocessing on the disk, similar to that of low mass\nX-ray binaries. For most sources, the optical spectrum is a power-law, $F_{\\nu}\n\\propto \\nu^{\\alpha}$ with $\\alpha$ in the range 1.0 to 2.0 and the optically\nemitting region has a size on the order of 1e12 cm. Exceptions are NGC 2403 X-1\nand M83 IXO 82, which show optical spectra consistent with direct emission from\na standard thin disk, M101 ULX-1 and M81 ULS1, which have X-ray to optical flux\nratios more similar to high-mass X-ray binaries, and IC 342 X-1, in which the\noptical light may be dominated by the companion star. Inconsistent extinction\nbetween the optical counterpart of NGC 5204 X-1 and the nearby optical nebulae\nsuggests that they may be unrelated.\n