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

TL;DR

This study investigates the optical properties and stellar environment of the ULX Holmberg IX X-1, revealing insights into its stellar companion, accretion disk, and variability through multi-wavelength observations.

Contribution

It provides the first detailed analysis of the optical counterpart and environment of Holmberg IX X-1, constraining the companion star's mass and the system's accretion processes.

Findings

Optical counterpart is part of a young stellar cluster with age < 20 Myr.

The companion star's mass is constrained to be > 10 M_sun, possibly > 25 M_sun.

The system shows photometric variability likely due to ellipsoidal effects or disk reprocessing.

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.