Optical Properties of the Ultraluminous X-ray Source Holmberg IX X-1 and its Stellar Environment

Abstract

Holmberg IX X-1 is an archetypal ultraluminous X-ray source (ULX). Here we study the properties of the optical counterpart and of its stellar environment using optical data from SUBARU/Faint Object Camera and Spectrograph,GEMINI/GMOS-N and Hubble Space Telescope (HST)/Advanced Camera for Surveys, as well as simultaneous Chandra X-ray data. The V ~ 22.6 spectroscopically identified optical counterpart is part of a loose cluster with an age <~ 20 Myr. Consequently, the mass upper limit on individual stars in the association is about 20 M_sun. The counterpart is more luminous than the other stars of the association, suggesting a non-negligible optical contribution from the accretion disk. An observed UV excess also points to non-stellar light similar to X-ray active low-mass X-ray binaries. A broad HeII4686 emission line identified in the optical spectrum of the ULX further suggests optical light from X-ray reprocessing in the accretion disk. Using stellar evolutionary tracks, we have constrained the mass of the counterpart to be >~ 10 M_sun, even if the accretion disk contributes significantly to the optical luminosity. Comparison of the photometric properties of the counterpart with binary models show that the donor may be more massive, >~ 25 M_sun, with the ULX system likely undergoing case AB mass transfer. Finally, the counterpart exhibits photometric variability of 0.14 mag between two HST observations separated by 50 days which could be due to ellipsoidal variations and/or disk reprocessing of variable X-ray emission.

14 pages, 14 figures, accepted for publication in ApJ

References in corpus (14)

• The XMM-Newton Serendipitous Survey. V. The Second XMM-Newton Serendipitous Source Catalogue
• The ACS Nearby Galaxy Survey Treasury IX. Constraining asymptotic giant branch evolution with old metal-poor galaxies
• X-ray observations of ultraluminous X-ray sources
• A 62 Day X-Ray Periodicity and an X-Ray Flare from the Ultraluminous X-Ray Source in M82
• The ultraluminous X-ray source NGC 1313 X-2 - Its optical counterpart and environment
• Spectroscopy of Optical Counterparts of Ultraluminous X-ray Sources
• Star-formation in the HI bridge between M81 and M82
• Optical emission from massive donors in ULX binary systems
• Irradiation models for ULXs and fits to optical data
• Models for the Observable System Parameters of Ultraluminous X-ray Sources
• Holmberg Ix: The Nearest Young Galaxy
• The Identification of New Stellar Groupings in the M81 Debris Field
• Ultraluminous X-ray Sources: Beambags and Optical Counterparts
• The ULX NGC 1313 X-2 : an optical study revealing an interesting behavior