Bibcode
Grisé, F.; Kaaret, P.; Pakull, M. W.; Motch, C.
Referencia bibliográfica
The Astrophysical Journal, Volume 734, Issue 1, article id. 23 (2011).
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6
2011
Revista
Número de citas
47
Número de citas referidas
46
Descripción
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 He II λ4686
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.