@article{9084ecb754f14ee29a6e3dbe52015cc4,
title = "The optical counterpart of M101 ULX-1",
abstract = "We have identified the optical counterpart of the ultraluminous X-ray source M101 ULX-1 (CXOKM101 J140332.37+542102), by comparing Hubble Space Telescope Advanced Camera for Surveys images with Chandra ACIS-S images. The optical counterpart has V = 23.75 and colors consistent with those for a mid-B supergiant. Archival Wide Field Planetary Camera 2 observations show that the source brightness is constant to within ∼0.1 mag. The physical association of this source with the ULX is confirmed by Gemini Multi-Object Spectrograph observations that show spatially unresolved He II λ4686 and He I λ5876 emission. These results suggest that M101 ULX-1 is a high-mass X-ray binary, but deep spectroscopic monitoring observations are needed to determine the detailed properties of this system.",
keywords = "X-rays: binaries, X-rays: individual (M101 ULX-1)",
author = "Kuntz, {K. D.} and Gruendl, {Robert A.} and Chu, {You Hua} and Chen, {C. H.Rosie} and Martin Still and Koji Mukai and Mushotzky, {Richard F.}",
note = "Funding Information: This work is based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (US), the Particle Physics and Astronomy Research Council (UK), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina). We thank M. Mountain and J.-R. Roy for granting Gemini Director{\textquoteright}s Discretionary time and P. Gomez for executing the GMOS observations, I. Jorgensen, M. Bergman, and R. C. Smith for advising us on the calibration and reduction of GMOS data, and N. Walborn for useful discussions regarding spectral classification. This program is supported by the Chandra grant SAO GO-5600587.",
year = "2005",
month = feb,
day = "10",
doi = "10.1086/428571",
language = "English (US)",
volume = "620",
pages = "L31--L34",
journal = "Astrophysical Journal",
issn = "0004-637X",
publisher = "American Astronomical Society",
number = "1 II",
}