The optical counterpart of M101 ULX-1

K. D. Kuntz, Robert A. Gruendl, You Hua Chu, C. H.Rosie Chen, Martin Still, Koji Mukai, Richard F. Mushotzky

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish (US)
Pages (from-to)L31-L34
JournalAstrophysical Journal
Issue number1 II
StatePublished - Feb 10 2005


  • X-rays: binaries
  • X-rays: individual (M101 ULX-1)

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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