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

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

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

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

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

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

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

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

Radio lobes and X-ray hot spots in the microquasar S26

We have studied the structure and energetics of the powerful microquasar/shock-ionized nebula S26 in NGC 7793, with particular focus on its radio and X-ray properties. Using the Australia Telescope Compact Array, we have resolved for the first time the radio lobe structure and mapped the spectral index of the radio cocoon. The steep spectral index of the radio lobes is consistent with optically-thin synchrotron emission; outside the lobes, the spectral index is flatter, suggesting an additional contribution from free-free emission, and perhaps ongoing ejections near the core. The radio core is not detected, while the X-ray core has a 0.3-8 keV luminosity ~6 x 10^{36} erg/s. The size of the radio cocoon matches that seen in the optical emission lines and diffuse soft X-ray emission. The total 5.5-GHz flux of cocoon and lobes is ~2.1 mJy, which at the assumed distance of 3.9 Mpc corresponds to about 3 times the luminosity of Cas A. The total 9.0-GHz flux is ~1.6 mJy. The X-ray hot spots (combined 0.3-8 keV luminosity ~2 x 10^{37} erg/s) are located ~20 pc outwards of the radio hot spots (ie, downstream along the jet direction), consistent with a different physical origin of X-ray and radio emission (thermal-plasma and synchrotron, respectively). The total particle energy in the bubble is ~10^{53} erg: from the observed radio flux, we estimate that only about a few 10^{50} erg are stored in the relativistic electrons; the rest is in protons, nuclei and non-relativistic electrons. The X-ray-emitting component of the gas in the hot spots contains ~10^{51} erg, and ~10^{52} erg over the whole cocoon. We suggest that S26 provides a clue to understand how the ambient medium is heated by the mechanical power of a black hole near its Eddington accretion rate.Comment: Accepted by MNRAS on 2010 July 12. Twelve pages, 8 figures, size = 1.3 MB. Contact the authors for higher-res figure