Ultraluminous X-ray Research Articles

Constraints on Accretion in Ultraluminous X-Ray Sources fromSpitzerIRS Observations of NGC 4485/4490: Infrared Diagnostic Diagrams

Constraining the astrophysical nature of ultraluminous X-ray (ULX) sources, which have X-ray luminosities exceeding 1039 ergs s-1, has been elusive due to the optical faintness of any counterparts. With high spectral resolution observations in the ~10-30 μm wavelength range we have conducted an experiment to study six ULX sources in the NGC 4485/4490 galaxy pair. We have found that five of the six ULXs, based on mid-infrared spectral diagnostics, show the characteristic higher ionization features that are found in AGNs. The sixth source, ULX-1, is consistent with being a supernova remnant. The chief infrared spectral diagnostics used are the ratios of [S III]/[Si II] versus [Ne III]/[Ne II]. In two instances fits to the continuum and polycyclic aromatic hydrocarbon (PAH) features also indicate higher dust temperatures, which are characteristic of accreting sources. Overall, however, we find that the continuum is dominated by stellar processes, and the best diagnostic features are the emission lines. High spectral resolution studies in the mid-infrared thus appear to show great promise for determining the astrophysical nature of ULXs.

XMM‐NewtonArchival Study of the Ultraluminous X‐Ray Population in Nearby Galaxies

We present the results of an archival XMM-Newton study of the bright X-ray point sources (LX > 1038 ergs s-1) in 32 nearby galaxies. From our list of approximately 100 point sources, we attempt to determine if there is a low-state counterpart to the ultraluminous X-ray (ULX) population, searching for a soft-hard state dichotomy similar to that known for Galactic X-ray binaries and testing the specific predictions of the intermediate-mass black hole (IMBH) hypothesis. To this end, we searched for low-state objects, which we defined as objects within our sample that had a spectrum well fitted by a simple absorbed power law, and high-state objects, which we defined as objects better fitted by a combined blackbody and a power law. Assuming that low-state objects accrete at approximately 10% of the Eddington luminosity (as found by Done & Gierlinski) and that high-state objects accrete near the Eddington luminosity, we further divided our sample of sources into low- and high-state ULX sources. We classify 16 sources as low-state ULXs and 26 objects as high-state ULXs. As in Galactic BH systems, the spectral indices, Γ, of the low-state objects, as well as the luminosities, tend to be lower than those of the high-state objects. The observed range of blackbody temperatures for the high state is 0.1-1 keV, with the most luminous systems tending toward the lowest temperatures. We therefore divide our high-state ULXs into candidate IMBHs (with blackbody temperatures of approximately 0.1 keV) and candidate stellar mass BHs (with blackbody temperatures of approximately 1.0 keV). A subset of the candidate stellar mass BHs have spectra that are well fitted by a Comptonization model, a property similar to Galactic BHs radiating in the very high state near the Eddington limit.

Optical Counterparts of Ultraluminous X-Ray Sources

We present the optical identification and characterization of counterparts of four objects previously cataloged as ultraluminous X-ray (ULX) sources. The objects were selected from the E. Colbert & A. Ptak catalog. The optical counterparts are identified as pointlike objects with magnitudes in the range of ~17-19. The optical spectra of three of the sources (IXO 32, 37, and 40) show the presence of emission lines typical of quasars. The position of these lines allows a precise estimation of their redshifts (2.769, 0.567, and 0.789 for IXO 32, 37, and 40, respectively). The fourth X-ray source, IXO 35, is associated with a red object that has a spectrum typical of an M star in our Galaxy. These identifications are useful for building clean samples of ULX sources, selecting suitable targets for future observations, and performing statistical studies on the different populations of X-ray sources.

Black Holes in Galactic Nuclei, X-Ray Binaries and Ultraluminous X-Ray Sources

AbstractThis review summarizes the astrophysical evidence for the existence of black holes provided by their gravitational influence on nearby matter. Two classes of accreting black holes have now been observationally verified: supermassive black holes (SMBHs) in galactic nuclei, and stellar-mass black holes in X-ray binaries (XRBs). With the recent re-discovery of ultra-luminous X-ray (ULX) sources, fresh evidence has also emerged for the existence of a third class of accreting black holes: intermediate-mass black holes (IMBHs). The properties of the three classes of accreting black holes are briefly discussed.

The Ultraluminous X‐Ray Source Population from the Chandra Archive of Galaxies

One hundred fifty-four discrete non-nuclear Ultra-Luminous X-ray (ULX) sources, with spectroscopically-determined intrinsic X-ray luminosities greater than 1 e39 ergs/s, are identified in 82 galaxies observed with Chandra's Advanced CCD Imaging Spectrometer. Source positions, X-ray luminosities, and spectral and timing characteristics are tabulated. Statistical comparisons between these X-ray properties and those of the weaker discrete sources in the same fields (mainly neutron star and stellar-mass black hole binaries) are made. Sources above approximately le38 ergs per second display similar spatial, spectral, color, and variability distributions. In particular, there is no compelling evidence in the sample for a new and distinct class of X-ray object such as the intermediate-mass black holes. 83% of ULX candidates have spectra that can be described as absorbed power laws with index = 1.74 and column density = 2.24e21 l per square centimeter, or approximately 5 times the average Galactic column. About 20% of the ULX's have much steeper indices indicative of a soft, and likely thermal, spectrum. The locations of ULXs in their host galaxies are strongly peaked towards their galaxy centers. The deprojected radial distribution of the ULX candidates is somewhat steeper than an exponential disk, indistinguishable from that of the weaker sources. About 5--15% of ULX candidates are variable during the Chandra observations (which average 39.5 ks). Comparison of the cumulative X-ray luminosity functions of the ULXs to Chandra Deep Field results suggests approximately 25% of the sources may be background objects including 14% of the ULX candidates in the sample of spiral galaxies and 44% of those in elliptical galaxies implying the elliptical galaxy ULX population is severely compromised by background active galactic nuclei. Correlations with host galaxy properties confirm the number and total X-ray luminosity of the ULXs are associated with recent star formation and with galaxy merging and interactions. The preponderance of ULXs in star-forming galaxies as well as their similarities to less-luminous sources suggest they originate in a young but short-lived population such as the high-mass X-ray binaries with a smaller contribution (based on spectral slope) from recent supernovae. The number of ULXs in elliptical galaxies scales with host galaxy mass and can be explained most simply as the high-luminosity end of the low-mass X-ray binary population.

A Transition to a Low/Soft State in the Ultraluminous Compact X-Ray Source Holmberg II X-1

We present three XMM-Newton observations of the ultraluminous compact X-ray source Holmberg II X-1 in its historical brightest and faintest states. The source was in its brightest state in 2002 April with an isotropic X-ray luminosity of ~2 × 1040 ergs s-1 but changed to a peculiar low/soft state in 2002 September in which the X-ray flux dropped by a factor of ~4 and the spectrum softened. In all cases, a soft excess component, which can be described by a simple or multicolor disk blackbody (kT ~ 120-170 eV), is statistically required in addition to a power-law continuum (Γ ~ 2.4-2.9). Both spectral components became weaker and softer in the low/soft state; however, the dramatic variability is seen in the power-law component. This spectral transition is opposite to the "canonical" high/soft-low/hard transitions seen in many Galactic black hole binaries. There is a possible contribution from an optically thin thermal plasma. When this component is taken into account, the spectral transition appears to be normal—a drop of the power-law flux and a slightly softer blackbody component in the low state.

Ultraluminous X-Ray Sources as Intermediate-Mass Black Holes Fed by Tidally Captured Stars

The nature of ultraluminous X-ray (ULX) sources is presently unknown. A possible explanation is that they are accreting intermediate-mass black holes (IBHs) that are fed by Roche lobe overflow from a tidally captured stellar companion. We show that a star can circularize around an IBH without being destroyed by tidal heating (in contrast to the case of massive black holes in galactic centers, where survival is unlikely). 6 M 1 10 M BH , We find that the capture and circularization rate is ∼ yr 1 , almost independently of the cluster’s relaxation 8 5 # 10 time. We follow the luminosity evolution of the binary system during the main-sequence Roche lobe overflow phase and show it can maintain ULX source–like luminosities for greater than 10 7 yr. In particular, we show that the ULX source in the young cluster MGG-11 in starburst galaxy M82, which possibly harbors an IBH, is well explained by this mechanism, and we predict that 10% of similar clusters with IBHs have a tidally captured circularized star. The cluster can evaporate on a timescale shorter than the lifetime of the binary. This raises the possibility of a ULX source that outlives its host cluster, or even lights up only after the cluster has evaporated, in agreement with observations of hostless ULX sources. Subject headings: black hole physics — galaxies: star clusters — stellar dynamics — X-rays: binaries

An Observational Diagnostic for Ultraluminous X-Ray Sources

We consider observational tests for the nature of ultraluminous X-ray (ULX) sources. These tests must distinguish between thermal-timescale mass transfer onto stellar-mass black holes, leading to anisotropic X-ray emission, and accretion onto intermediate-mass black holes. We suggest that long-term transient behavior via the thermal-viscous disk instability could discriminate between these two possibilities for ULX sources in regions of young stellar populations. Thermal-timescale mass transfer generally produces stable disks and persistent X-ray emission. In contrast, mass transfer from massive stars to black holes produces unstable disks and thus transient behavior, provided that the black hole mass exceeds some minimum value MBH, min. This minimum mass depends primarily on the donor mass and evolutionary state. We show that MBH, min 50 M☉ for a large fraction (90%) of the mass transfer lifetime for the most likely donors in young clusters. Thus, if long-term monitoring reveals a large transient fraction among ULX sources in a young stellar population, these systems would be good candidates for intermediate-mass black holes in a statistical sense; information about the donor star is needed to make this identification secure in any individual case. A transient ULX population would imply a much larger population of quiescent systems of the same type.

The Optical Counterpart of an Ultraluminous X‐Ray Source in NGC 5204

Ultraluminous X-ray (ULX) sources are extranuclear point sources in external galaxies with LX = 1039-1041 ergs s-1 and are among the most poorly understood X-ray sources. To help understand their nature, we are trying to identify their optical counterparts by combining images from the Hubble Space Telescope (HST) and the Chandra X-Ray Observatory. Here we report on the optical counterpart for the ULX in NGC 5204, which has average X-ray luminosity of ~3 × 1039 ergs s-1 and has varied by a factor of 50% over the last 10 years. A unique optical counterpart to this ULX is found by carefully comparing the Chandra ACIS images and HST WFPC2 and ACS/HRC images. The spectral energy distribution and the HST/STIS far-ultraviolet (FUV) spectrum of this object show that it is a B0 Ib supergiant star with peculiarities, including the N V λ1240 emission line, which is uncommon in B stellar spectra but has been predicted for X-ray-illuminated accretion disks and seen in some X-ray binaries. Study of its FUV spectrum leads to a binary model for this ULX in which the B0 Ib supergiant is overflowing its Roche lobe and accreting onto the compact primary, probably a black hole. This picture predicts an orbital period of ~10 days for different black hole masses, which can be tested by future observations.

Spectral Evolution of an Ultraluminous Compact X-Ray Source in NGC 253

We report an observational result for a ULX in NGC253. We find that the spectral evolution is well consistent with the standard behavior. That is, the source satisfies the relation L d i s k ∞ T i n 4 as established for black hole binaries. By comparing with black hole binaries, we estimate black hole mass of this ULX as 7M.

ChandraObservations of the Interacting NGC 4410 Galaxy Group

We present high resolution X-ray imaging data from the ACIS-S instrument on the Chandra telescope of the nearby interacting galaxy group NGC 4410. Four galaxies in the inner portion of this group are clearly detected by Chandra, including the peculiar low luminosity radio galaxy NGC 4410A. In addition to a nuclear point source, NGC 4410A contains diffuse X-ray emission, including an X-ray ridge extending out to about 12 (6 kpc) to the northwest of the nucleus. This ridge is coincident with an arc of optical emission-line gas, which has previously been shown to have optical line ratios consistent with shock ionization. This structure may be due to an expanding superbubble of hot gas caused by supernovae and stellar winds or by the active nucleus. The Chandra observations also show four or five possible compact ultra-luminous X-ray (ULX) sources (L(x) >= 10^39 erg/s) associated with NGC 4410A. At least one of these candidate ULXs appears to have a radio counterpart, suggesting that it may be due to an X-ray binary with a stellar-mass black hole, rather than an intermediate mass black hole. In addition, a faint diffuse intragroup X-ray component has been detected between the galaxies (L(x) ~ 10^41 erg/s). This supports the hypothesis that the NGC 4410 group is in the process of evolving via mergers from a spiral-dominated group (which typically have no X-ray-emitting intragroup gas) to an elliptical-dominated group (which often have a substantial intragroup medium).

Chandra-detected X-Ray Sources in the Nearby Spiral Scd Galaxy NGC 2403

We present an analysis of a 35.6 ks observation made with the Chandra X-Ray Observatory of the nearby spiral Scd galaxy NGC 2403. Forty-one discrete sources are detected at a significance of 3 ? or greater within the portion of the galaxy sampled by the Chandra detectors. The luminosities of the discrete sources range from ~1036 through ~1039 ergs s-1 over the energy range of 0.5?10 keV. We have also analyzed Einstein, ROSAT, and ASCA observations of this galaxy to search for time variability from the detected sources. One luminous X-ray source seen in an Einstein observation of this galaxy is not detected by Chandra or in a separate observation made with ASCA, while a luminous source observed by Chandra was not detected by Einstein or ASCA. For the four most luminous X-ray sources detected by the Chandra observation?including one previously identified as an ultraluminous X-ray source?spectral fits to these sources provide the best broadband CCD spectra to date. We have searched for counterparts at optical and radio wavelengths. Only one of the 35 optically identified supernova remnants (SNRs) in this galaxy, MFBL 31, is clearly associated with an X-ray source in our sample. Three other optically identified SNRs?MFBL 15, MFBL 17, and MFBL 29?may also have X-ray counterparts, but the associations are far less robust. A similar search for X-ray counterparts to known discrete radio sources seen toward NGC 2403 yields a single coincidence between an X-ray source and a candidate radio SNR, TH-2. None of the detected X-ray sources are coincident with either candidate globular clusters or massive star clusters associated with NGC 2403. The only historical supernova in NGC 2403, SN 1954J, is not detected; neither is an X-ray counterpart to the galaxy's optical nucleus detected. Finally, we confirm the diffuse emission results of Fraternali et al.

X-Ray Spectroscopic Evidence for Intermediate-Mass Black Holes: Cool Accretion Disks in Two Ultraluminous X-Ray Sources

We have analyzed an XMM-Newton observation of the nearby spiral galaxy NGC 1313, which contains two "ultraluminous" X-ray (ULX) sources. We measure isotropic luminosities of LX = 2.0 × 1040 ergs s-1 and LX = 6.6 × 1039 ergs s-1 for NGC 1313 X-1 and X-2 (0.2-10.0 keV, assuming a distance of 3.7 Mpc). The spectra statistically require soft and hard spectral components to describe the continuum emission; some prior studies of ULX sources have claimed cool soft components with lower statistics. The improvement over several single-component models exceeds the 8 σ level of confidence for X-1; the improvement for X-2 is significant at the 3 σ level. The soft components in these ULX spectra are well fitted by multicolor disk blackbody models with color temperatures of kT ≃ 150 eV. This temperature differs markedly from those commonly measured in the spectra of stellar mass (10 M☉) black holes in their brightest states (kT ≃ 1 keV). It is expected that the temperature of an accretion disk orbiting a black hole should decrease with increasing black hole mass. If the soft components we measure are due to emission from the inner region of an accretion disk, and disks extend close to the innermost stable circular orbit at the accretion rates being probed, the low color temperatures may be interpreted as spectroscopic evidence of black holes with intermediate masses: MBH ≃ 103 M☉. Simple Eddington scaling arguments suggest a minimum mass of MBH ~ 102 M☉. NGC 1313 X-1 and X-2 are found in optical nebulae, which may indicate that anisotropic emission geometries are unlikely to account for the fluxes observed.

BL Lac identification for the ultraluminous X-ray source observed in the direction of NGC 4698

We report the identification of the optical and radio counterparts of the ultraluminous X-ray (ULX) source XMMU J124825.9+083020 (NGC 4698-ULX1). The optical spectrum taken with the VLT yields a redshift of , which implies that the ULX is not associated with the nearby galaxy NGC 4698. The spectral energy distribution calculated from the available data indicates that the source is likely to be a BL Lac object. The possible synchrotron peak at X-ray energies suggests that this source may be a γ-ray emitter.

A Possible X-Ray Periodicity at Several Tens of Hours of an Ultraluminous Compact X-Ray Source in IC 342

A long (155 hr) ASCA observation was performed of two ultraluminous compact X-ray sources, source 1 and source 2, in the spiral galaxy IC 342. While source 1, which was in a hard spectral state, varied little, source 2, which was in its soft spectral state, varied significantly on a timescale of about 1 day. The rms variation amplitude amounts to 5% in the 2-10 keV band but is less than 4% in 0.7-2 keV. The variation involves statistically significant changes in the parameters describing multicolor disk blackbody emission from this source. The variation is possibly periodic, with a period of either 31 ± 2 or 41 ± 3 hr. Both are consistent with the orbital period of a semidetached binary formed by a black hole and a main-sequence star of several tens of solar masses. These results reinforce the interpretation of these X-ray objects in terms of accreting massive stellar black holes.

Discovery of Spectral Transitions from Two Ultraluminous Compact X-Ray Sources in IC 342

Two ASCA observations were made of two ultraluminous compact X-ray sources (ULXs) in the spiral galaxy IC 342. In the 1993 observation, source 2 showed a 0.5-10 keV luminosity of 6 × 1039 ergs s-1 (assuming a distance of 4.0 Mpc) and a hard power-law spectrum of photon index ~1.4. As already reported, source 1 was ~3 times brighter on that occasion and exhibited a soft spectrum represented by a multicolor disk model with an inner-disk temperature of ~1.8 keV. The second observation, made in 2000 February, revealed that source 1 had made a transition into a hard spectral state, while source 2 made a transition into a soft spectral state. The ULXs are therefore inferred to exhibit two distinct spectral states, and they sometimes make transitions between them. These results significantly reinforce the scenario that describes ULXs as mass-accreting black holes.

Accretion disk spectra of super-luminal jet sources and ultra-luminous compact X-ray sources in nearby spiral galaxies

The Ultra-luminous Compact X-ray Sources (ULXs)in nearby spiral galaxies and the Galactic super-luminaljet sources sharethe common spectral characteristic that they haveextremely high disk temperatures which cannot be explainedin the framework of the standard accretion disk modelin the Schwarzschild metric. We have calculated an extreme Kerr disk model to examine if the Kerr disk model can instead explain the observed `too hot' accretion disk spectra.We found that the Kerr disk spectrum becomes significantly hardercompared to the Schwarzschild disk only when the disk is highlyinclined.For super-luminal jet sources, which are known to beinclined systems, the Kerr disk model may thuswork if we choose proper values for the black hole angular momentum. For the ULXs, however, the Kerr disk interpretation will be problematic,as is is highly unlikely that their accretion disks are preferentiallyinclined.