369 research outputs found
Optical Counterparts of Ultraluminous X-Ray Sources
Despite much observational and theoretical effort little is presently known
about the nature of the luminous non-nuclear X-ray sources which appear to
largely surpass the Eddington limit of a few solar masses. Here we present
first results of our OHP/ESO/CFHT optical survey of the environments of
variable ultraluminous X-ray sources (ULX) in nearby galaxies. At the position
of several ULX we find emission nebulae of a few hundred parsecs diameter, and
which often show both low and high ionisation emission lines. The gas must
therefore be either photoionized by hard XUV continua, or be shock-ionized in
the expanding bubbles. The nebulae have kinematic ages of some million years
and appear to be directly linked to the highly energetic formation process of
the compact ULX or being inflated by ongoing stellar wind/jet activity. The
discovery of intense HeII lambda4686 nebular recombination radiation together
with comparatively strong [OI] lambda6300 emission around the variable ULX in
dwarf galaxy Holmberg II has allowed us to show that the interstellar medium
acually 'sees' and reprocesses part of the 10^40 erg/s measured at X-ray
wavelengths, if we assume isotropic emission. Strong beaming into our line of
sight which has been advocated to avoid such high luminosities can thus be
excluded, at least for this source.Comment: 8 pages, 8 figures, to appear in the proceedings of the symposium
'New Visions of the X-ray Universe in the XMM-Newton and Chandra Era', 26-30
November 2001, ESTEC, The Netherland
Optical Properties of the Ultraluminous X-ray Source Holmberg IX X-1 and its Stellar Environment
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.Comment: 14 pages, 14 figures, accepted for publication in Ap
The Ultraluminous X-ray Source in Holmberg IX and its Environment
We present optical observations of an ultraluminous X-ray source (ULX) in
Holmberg IX, a dwarf galaxy near M81. The ULX has an average X-ray luminosity
of some 10^{40} erg/s. It is located in a huge (400pc x 300pc) ionized nebula
being much larger than normal supernova remnants. From the observed emission
lines (widths and ratios) we find that the structure is due to collisional
excitation by shocks, rather than by photoionization.
We identify the optical counterpart to be a 22.8 mag blue star (M_V=-5.0)
belonging to a small stellar cluster. From isochrone fitting of our
multi-colour photometry we determine a cluster age of 20 to 50 Myr. We also
discovered strong stellar HeII4686 emission (equivalent width of 10 A) which
proves the identification with the X-ray source, and which suggests the
presence of an X-ray heated accretion disc around the putative black hole.Comment: 2 pages, 2 figures, to appear in the proceedings of the IAU Symposium
230, "Populations of High Energy Sources in Galaxies", Dublin, 15-19 Aug 200
High-excitation nebulae around Magellanic Wolf-Rayet stars
The SMC harbours a class of hot nitrogen-sequence Wolf-Rayet stars (WNE) that
display only relatively weak broad emission lines. This indicates low mass-loss
rates and makes them also hard to detect. However, such stars are possible
emitters of strong He+ Lyman continua that in turn could ionize observable
HeIII regions, i.e. highly excited HII regions emitting nebular HeII4686
emission. We here report the discovery of a rare HeIII region in the SMC which
is located in the OB association NGC249 around the weak-lined WN star SMC WR10.
WR10 is particularly interesting since it is a single star showing the presence
of atmospheric hydrogen. While analysing the spectrum in the framework of two
popular WR atmosphere models, we found for the same input parameters strongly
discrepant predictions (by 1 dex) for the He+ Lyman continuum. A second aspect
of the work reported here concerns the beautiful MCELS images which clearly
reveal a class of strongly [OIII]5007 emitting (blue-coded) nebulae. Not
unexpectedly, most of the 'blue' nebulae are known Wolf-Rayet bubbles, but new
bubbles around a few WRs are also detected. Moreover, we report the existence
of blue nebulae without associated known WRs and discuss the possibility that
they reveal weak-wind WR stars with very faint stellar HeII4686 emission.
Alternatively, such nebulae might hint at the hitherto missing population of
relatively low-mass, hot He stars predicted by massive binary evolution
calculations. Such a binary system is probably responsible for the ionization
of the unique HeII4686-emitting nebula N44C.Comment: Comments: Proceedings of the IAU Symposium 256, The Magellanic
System; J.T. van Loon and J.M. Oliveira (Eds.
Ultraluminous X-ray Sources: Bubbles and Optical Counterparts
Optical studies of ultraluminous X-ray sources (ULX) in nearby galaxies have
turned out to be instrumental in discriminating between various models
including the much advertised intermediate mass black hole hypothesis and
various beaming scenarios. Here we report on ESO VLT and SUBARU observations of
ULX that have revealed the parent stellar clusters with ages of some 60 million
years in two cases. Thus we are able to derive upper limits of about 8 M_sun
for the mass donors in these systems. The optical counterparts are dominated by
X-ray heated accretion disks, and the discovery of the HeII4686 emission line
now allows to derive dynamical masses in these systems. Apparent radial
velocity variations of 300 km/s have been detected in NGC 1313 X-2 which, if
confirmed by further observations, would exclude the presence of IMBH in these
systems.Comment: 4 pages, to appear in the proceedings of IAU Symposium 230,
"Populations of High Energy Sources in Galaxies", Dublin, 15-19 Aug 200
Large Highly-Ionized Nebulae Around Ultra-luminous X-ray Sources
We present the results of deep optical spectroscopic observations using the
LRIS spectrograph on the Keck I 10-m telescope of three ultra-luminous X-ray
sources (ULXs), Ho IX X-1; M81 X-6; and Ho II X-1. Our observations reveal the
existence of large (100 - 200 pc diameter) highly-ionized nebulae, identified
by diffuse He II (4686 Angstrom) emission, surrounding these sources. Our
results are the first to find highly-ionized nebulae of this extent, and the
detection in all three objects indicates this may be a common feature of ULXs.
In addition to the extended emission, Ho IX X-1 has an unresolved central
component containing about one-third of the total He II flux, with a
significant velocity dispersion of ~ 370 km/s, suggestive of the existence of a
photo-ionized accretion disk or an extremely hot early-type stellar
counterpart. Most of the He II emission appears to be surrounded by
significantly more extended Hbeta emission, and the intensity ratios between
the two lines, which range from 0.12 - 0.33, indicate that photo-ionization is
the origin of the He II emission. Sustaining these extended nebulae requires
substantial X-ray emission, in the range ~ 10^{39} - 10^{40} ergs/s, comparable
to the measured X-ray luminosities of the sources. This favors models where the
X-ray emission is isotropic, rather than beamed, which includes the
interpretation that ULXs harbor intermediate-mass black holes.Comment: Accepted for publication in ApJ Letter
Direct Detection of an Ultraluminous Ultraviolet Source
We present Hubble Space Telescope observations in the far UV of the
ultraluminous X-ray source in NGC 6946 associated with the optical nebula MF
16. Both a point-like source coincident with the X-ray source and the
surrounding nebula are detected in the FUV. The point source has a flux of
5E-16 erg s^-1 cm^-2 Ang^-1 and the nebula has a flux of 1.6E-15 erg s^-1 cm^-2
Ang^-1, quoted at 1533 Ang and assuming an extinction of A_V = 1.54. Thus, MF
16 appears to host the first directly detected ultraluminous UV source (ULUV).
The flux of the point-like source is consistent with a blackbody with T ~
30,000 K, possibly from a massive companion star, but this spectrum does not
create sufficient ionizing radiation to produce the nebular HeII flux and a
second, hotter emission component would be required. A multicolor disk
blackbody spectrum truncated with an outer disk temperature of ~16,000 K
provides an adequate fit to the FUV, B, V, I, and HeII fluxes and can produce
the needed ionizing radiation. Additional observations are required to
determine the physical nature of the source.Comment: 4 pages, accepted for ApJ Letter
X-Ray Off States and Optical Variability in CAL 83
CAL 83 was one of the first supersoft X-ray binaries to be discovered and is
considered to be the prototype of its class. In 15 X-ray observations between
1983-1997 it was observed to have nearly constant X-ray luminosity and
temperature, with the exception of one off-state in 1996. We report on a second
X-ray off-state, discovered with a Chandra ACIS-S observation in November 1999.
Comparing the long-term X-ray and MACHO optical light curves, we find that, CAL
83 has exhibited distinct and well-defined low, intermediate, and high optical
states. Transitions between states are not accompanied by color variations.
Both X-ray off states were observed during optical high states and were
followed by optical low states within ~50 days. We discuss possible
explanations for the observed optical and X-ray variations.Comment: 12 pages A&A style with 5 figures; accepted for publication in A&A
Main Journa
The star-forming environment of a ULX in NGC 4559: an optical study
We have studied the candidate optical counterparts and the stellar population
in the star-forming complex around a bright ULX in NGC4559, using HST/WFPC2,
XMM-Newton/OM, and ground-based data. We find that the ULX is located near a
small group of OB stars. The brightest point source in the Chandra error circle
is consistent with a single blue supergiant of mass ~ 20 M_sun and age ~ 10
Myr. A few other stars are resolved inside the error circle: mostly blue and
red supergiants with masses ~ 10-15 M_sun and ages ~ 20 Myr. This is consistent
with the interpretation of this ULX as a black hole (BH) accreting from a
high-mass donor star in its supergiant phase, via Roche-lobe overflow. The
observed optical colors and the blue-to-red supergiant ratio suggest a low
metal abundance: 0.2 <~ Z/Z_sun <~ 0.4 (Padua tracks), or 0.05 <~ Z/Z_sun <~
0.2 (Geneva tracks). The age of the star-forming complex is <~ 30 Myr. H-alpha
images show that this region has a ring-like appearance. We propose that it is
an expanding wave of star formation, triggered by an initial density
perturbation, in a region where the gas was only marginally stable to
gravitational collapse. A possible trigger was the collision with a satellite
dwarf galaxy, visible a few arcsec north-west of the complex, going through the
gas-rich outer disk of NGC4559. The X-ray data favour a BH more massive (M > 50
M_sun) than typical Milky Way BH candidates. The optical data favour a
``young'' BH originating in the recent episode of massive star formation;
however, they also rule out an association with young massive star clusters. We
speculate that other mechanisms may lead to the formation of relatively massive
BHs (~ 50-100 M_sun) from stellar evolution processes in low-metallicity
environments, or when star formation is triggered by galactic collisions.Comment: MNRAS accepted, 19 pages. Contact the first author for
full-resolution picture
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