Ultraluminous Compact X-ray Sources Research Articles

Detailed Spectral Study of an Ultra-Luminous Compact X-Ray Source, M81 X-9, in the Disk-Dominated State

Abstract We report on the results of detailed spectral studies of an ultra-luminous X-ray source (ULX), M81 X-9 (Holmberg IX X-1), conducted with XMM-Newton on 2001 April 22 and with ASCA on 1999 April 6. On both occasions, the source showed an unabsorbed 0.5–10 keV luminosity of $\sim 2 \times 10^{40} \,\mathrm{erg} \,\mathrm{s}^{-1}$ (assuming a distance of 3.4 Mpc) and a soft spectrum, apparently represented by a multi-color disk model with an innermost disk temperature of 1.3–1.5 keV. Adding a power-law model further improved the fit. However, as previously reported, the high luminosity cannot be reconciled with the high disk temperature within the framework of the standard accretion disk radiating at a sub-Eddington luminosity. Therefore, we modified the multi-color disk model, and allowed the local disk temperature to scale as $\propto r^{-p}$ on the distance $r$ from the black hole, with $p$ being a free parameter. We then found that the XMM-Newton and the ASCA spectra can be both successfully reproduced with $p \sim 0.6$ and the innermost disk temperature of 1.4–1.8 keV. These flatter temperature profiles suggest a deviation from the standard Shakura-Sunyaev disk, and are consistent with predictions of a slim-disk model.

Observational evidence for intermediate-mass black holes: ultra-luminous X-ray sources

AbstractIncorporating early data from the Suzaku satellite launched in July 2005, properties of Ultra-Luminous compact X-ray sources (ULXs) were studied in close comparison with those of Galactic and Magellanic black-hole binaries. Based on an analogy between these two types of X-ray sources, ULXs showing power-law type spectra are considered to host Comptonized accretion disks, while those with multicolor-disk type spectra are interpreted to harbor “slim” disks. The analogy also suggests that ULXs are radiating near their Eddington limits, and hence their central black holes are significantly more massive than the ordinary stellar-mass black holes contained in Galactic and Magellanic black-hole binaries. In this sense, ULXs can be regarded as intermediate-mass black holes.

Radiatively Inefficient Accretion Flow in the Nucleus of NGC 1097

We present a model for the accretion flow around the supermassive black hole in the LINER nucleus of NGC 1097 that fits the optical to X-ray spectral energy distribution (SED). The X-ray segment of the SED is based on observations with the Chandra X-Ray Observatory, which are reported here for the first time. The inner part of the flow is modeled as a radiatively inefficient accretion flow (RIAF), and the outer part as a standard thin disk. The value of the transition radius (r(tr) approximate to 225R(S), where R-S = 2GM/c(2)) between the RIAF and the outer thin disk was obtained from our previous fitting of the double-peaked Balmer emission line profile, which originates in the thin disk. The black hole mass was inferred from measurements of the stellar velocity dispersion in the host galaxy. When these parameters are used in the accretion flow model, the SED can be successfully reproduced, which shows that the line profile model and the accretion flow model are consistent with each other. A small remaining excess in the near-UV is accounted for by the contribution of an obscured starburst located within 9 pc from the nucleus, as we reported in an earlier paper. The radio flux is consistent with synchrotron emission of a relativistic jet modeled by means of the internal shock scenario. In an appendix we also analyze the Chandra X-ray observations of the similar to 1 kpc circumnuclear star-forming ring and of an ultraluminous compact X-ray source located outside the ring.

Slim Disks with Transition Regions and Applications to Microquasars and Narrow‐Line Seyfert 1 Galaxies

Slim disks have been received much attention because of the increasing evidence for the super-critical accreting objects. In this paper, we make an attempt to construct a unified model, in which the viscosity and the dimensionless accretion rate can span rather wide ranges. We replace blackbody radiation under diffusion approximation with a bridged formula, which accounts for both blackbody radiation and thermal bremsstrahlung in optically-thick and -thin cases, respectively. Thus this allows us to investigate the structures of and the emergent spectra from slim disks in a wider parameter space, covering transition regions from optically thick to optically thin. We show that there is a maximum transition radius, roughly $R_{\rm tr, max}/R_g\sim 50$ when $\dot{M}/\dot{M}_{\rm C}\sim 15$. The emergent spectra from the unified model of the accretion disk have been calculated. A simple model of hot corona above the slim disk is taken into account for the hard X-ray spectrum in this paper based on Wang & Netzer (2003). We have applied the present model to the microquasar GRS 1915+105, narrow line Seyfert 1 galaxies RE J1034+396 and Akn 564. Our model can explain well the broadband X-ray spectra of narrow line Seyfert 1 galaxies, microquasars and possible ultra-luminous compact X-ray sources. The present model can be widely applied to the candidates of super-critical accreting objects.

Chandra Observation of Luminous Sources in the Nearby Irregular Galaxy NGC 4449

Abstract Using archival Chandra ACIS-S data, $0.5 \hbox{--} 8.0 \,\mathrm{keV}$ X-ray spectra of two luminous X-ray sources in the nearby dwarf irregular galaxy NGC 4449 were studied. One, with an extremely high luminosity of $1.3 \times 10^{39} \;\mathrm{erg} \,\mathrm{s}^{-1}$ in the $0.5 \hbox{--} 8.0 \;\mathrm{keV}$ band, shows a spectrum that is well described with a power-law model of photon index $\sim 2$. Its properties are consistent with those of ultraluminous compact X-ray sources observed in nearby galaxies. The spectrum of the other, with a luminosity of $2.7 \times 10^{38} \,\mathrm{erg} \,\mathrm{s}^{-1}$ in the same band, is successfully represented with a so-called multi-color disk blackbody emission model with an inner-most disk temperature of $\sim 0.59 \,\mathrm{keV}$. Its spectral parameters are typical of ordinary black hole binaries observed in our Galaxy and the Large Magellanic Cloud. These young population objects, together with a bright supernova remnant and diffuse hot gas already reported, suggest that the X-ray emission from irregular galaxies is generally enhanced by their recent star-forming activities.

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.

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.

Accretion Disk Spectra of the Ultra-Luminous X-Ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources

Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and Galactic superluminal jet sources share the common spectral characteristic that they have unusually high disk temperatures which cannot be explained in the framework of the standard optically thick accretion disk in the Schwarzschild metric. On the other hand, the standard accretion disk around the Kerr black hole might explain the observed high disk temperature, as the inner radius of the Kerr disk gets smaller and the disk temperature can be consequently higher. However, we point out that the observable Kerr disk spectra becomes significantly harder than Schwarzschild disk spectra only when the disk is highly inclined. This is because the emission from the innermost part of the accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen for a face-on disk. The Galactic superluminal jet sources are known to be highly inclined systems, thus their energy spectra may be explained with the standard Kerr disk with known black hole masses. For ULXs, on the other hand, the standard Kerr disk model seems implausible, since it is highly unlikely that their accretion disks are preferentially inclined, and, if edge-on Kerr disk model is applied, the black hole mass becomes unreasonably large (greater than or approximately equal to 300 Solar Mass). Instead, the slim disk (advection dominated optically thick disk) model is likely to explain the observed super- Eddington luminosities, hard energy spectra, and spectral variations of ULXs. We suggest that ULXs are accreting black holes with a few tens of solar mass, which is not unexpected from the standard stellar evolution scenario, and their X-ray emission is from the slim disk shining at super-Eddington luminosities.

Accretion Disk Spectra of the Ultra-luminous X-ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources

Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and Galactic superluminal jet sources share the common spectral characteristic that they have unusually high disk temperatures which cannot be explained in the framework of the standard optically thick accretion disk in the Schwarzschild metric. On the other hand, the standard accretion disk around the Kerr black hole might explain the observed high disk temperature, as the inner radius of the Kerr disk gets smaller and the disk temperature can be consequently higher. However, we point out that the observable Kerr disk spectra becomes significantly harder than Schwarzschild disk spectra only when the disk is highly inclined. This is because the emission from the innermost part of the accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen for a face-on disk. The Galactic superluminal jet sources are known to be highly inclined systems, thus their energy spectra may be explained with the standard Kerr disk with known black hole masses. For ULXs, on the other hand, the standard Kerr disk model seems implausible, since it is highly unlikely that their accretion disks are preferentially inclined, and, if edge-on Kerr disk model is applied, the black hole mass becomes unreasonably large (≥ 300M☉). Instead, the slim disk (advection dominated optically thick disk) model is likely to explain the observed super-Eddington luminosities, hard energy spectra, and spectral variations of ULXs. We suggest that ULXs are accreting black holes with a few tens of solar mass, which is not unexpected from the standard stellar evolution scenario, and that their X-ray emission is from the slim disk shining at super-Eddington luminosities.

Accretion Disk Spectra of Ultraluminous X‐Ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources

Ultraluminous compact X-ray sources (ULXs) in nearby spiral galaxies and Galactic superluminal jet sources share the common spectral characteristic that they have unusually high disk temperatures that cannot be explained in the framework of the standard optically thick accretion disk in the Schwarzschild metric. On the other hand, the standard accretion disk around the Kerr black hole might explain the observed high disk temperature, since the inner radius of the Kerr disk becomes smaller and the disk temperature can consequently be higher. However, we point out that the observable Kerr disk spectra become significantly harder than Schwarzschild disk spectra only when the disk is highly inclined. This is because the emission from the innermost part of the accretion disk is Doppler boosted for an edge-on Kerr disk while hardly seen for a face-on disk. The Galactic superluminal jet sources are known to be highly inclined systems; thus, their energy spectra may be explained with the standard Kerr disk with known black hole masses. For ULXs, on the other hand, the standard Kerr disk model seems implausible, since it is highly unlikely that their accretion disks are preferentially inclined, and if the edge-on Kerr disk model is applied, the black hole mass becomes unreasonably large (300 M☉). Instead, the slim-disk (advection-dominated optically thick disk) model is likely to explain the observed super-Eddington luminosities, hard energy spectra, and spectral variations of ULXs. We suggest that ULXs are accreting black holes with a few tens of solar masses, which is not unexpected from the standard stellar evolution scenario, and that their X-ray emission is from the slim disk shining at super-Eddington luminosities.

Comptonization in Super‐Eddington Accretion Flow and Growth Timescale of Supermassive Black Holes

Super-Eddington accretion onto black holes (BHs) may occur at ultraluminous compact X-ray sources in nearby galaxies, Galactic microquasars and narrow-line Seyfert 1 galaxies (NLS1s). Effects of electron scattering (opacity and Comptonization) and the relativistic correction (gravitational redshift and transverse Doppler effect) on the emergent spectra from super-Eddington accretion flows onto non-rotating BHs are examined for 10^{1.5} and 10^{6.5} M_sun BH masses (M_BH). With mdot [= Mdot / (L_Edd / c^2), where Mdot is the accretion rate] > 100, the spectral hardening factor via electron scattering is \lesssim 2.3 - 6.5. Due to the mdot-sensitive hardening factor, the color temperature of the innermost radiation is not proportional to L^{0.25}, differing from the simplest standard accretion disk. The model is applied to optical--soft X-ray emission from NLS1s. We pick up one NLS1, namely PG 1448+273 with an inferred M_BH of 10^{6.4} M_sun, among the highest mdot candidates. The broadband spectral distribution is successfully reproduced by the model with an extremely high mdot (= 1000) and the viscosity parameter alpha of 0.01. This implies that this object, as well as some other highest mdot systems, is really young: the inferred age, M_BH / Mdot, is about 10^6 years. We also briefly discuss the distribution of mdot for transient and highly variable NLS1s, finding that those are located at 3 \lesssim mdot \lesssim 300. Such a moderately high accretion rate is indicative of thermal instability. Furthermore, mdot for a possible type-2 counterpart of NLS1s, NGC 1068, is found to be similar to mdot for NLS1s.

The nature of the ultraluminous X-ray sources inside galaxies and their relation to local QSOs

Received Abstract. It is suggested that many of the ultraluminous compact x-ray sources now being found in the main bodies of galaxies, particularly those that are active, like M82, NGC 3628 and others, arelocal QSOs, or BL Lac objects, with high intrinsic redshifts in the process of being ejected from those galaxies. Evidence in support of this hypothesis is summarized.

Another interpretation of the power-law-type spectrum of an ultraluminous compact X-ray source in IC 342

The ultraluminous compact X-ray sources (ULXs) generally show a curving spectrum in the 0.7–10 keV ASCA bandpass, which looks like a high-temperature analogue of the disc-dominated high/soft-state spectra seen in Galactic black hole binaries (BHBs) at high mass accretion rates. Several ULXs have been seen to vary, and to make a transition at their lowest luminosity to a spectrum which looks more like a power law. These have been previously interpreted as the analogue of the power-law-dominated low/hard state in Galactic BHBs. However, the ULX luminosity at which the transition occurs must be at least 10–50 per cent of the Eddington limit assuming that their highest luminosity phase corresponds to the Eddington limit, while for the Galactic BHBs the high/soft–low/hard transition occurs at a few per cent of the Eddington limit. Here we show that the apparently power-law spectrum in a ULX in IC 342 can be equally well fitted over the ASCA bandpass by a strongly Comptonized optically thick accretion disc with a maximum temperature of ∼1 keV. Recent work on the Galactic BHBs has increasingly shown that such components are common at high mass accretion rates, and that this often characterizes the very high (or anomalous) state. Thus we propose that the power-law-type ULX spectra are not to be identified with the low/hard state, but rather represent the Comptonization-dominated very high/anomalous state in the Galactic BHBs.

External Compton emission from relativistic jets in Galactic black hole candidates and ultraluminous X-ray sources

Galactic binary systems that contain a black hole candidate emit hard X-rays in their low luminosity mode. We show that this emission can be understood as due to the Compton scattering of photons from the companion star and/or the accretion disk by relativistic electrons in a jet. The same electrons are also responsible for the radio emission. Two sources -- XTE J1118+480 and Cygnus X-1 -- are modelled as representatives of black holes with low and high luminosity companion stars respectively. We further show that the ultraluminous compact X-ray sources observed in nearby galaxies have the properties expected of stellar mass black holes with high luminosity companions in which the jet is oriented close to our line of sight.

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.

Ultraluminous X-Ray Sources in External Galaxies

We investigate models for the class of ultraluminous nonnuclear X-ray sources (i.e., ultraluminous compact X-ray sources [ULXs]) seen in a number of galaxies and probably associated with star-forming regions. Models in which the X-ray emission is assumed to be isotropic run into several difficulties. In particular, the formation of sufficient numbers of the required ultramassive black hole X-ray binaries is problematic, and the likely transient behavior of the resulting systems is not in good accord with observation. The assumption of mild X-ray beaming suggests instead that ULXs may represent a short-lived but extremely common stage in the evolution of a wide class of X-ray binaries. The best candidate for this is the phase of thermal-timescale mass transfer that is inevitable in many intermediate- and high-mass X-ray binaries. This in turn suggests a link with the Galactic microquasars. The short lifetimes of high-mass X-ray binaries would explain the association of ULXs with episodes of star formation. These considerations still allow the possibility that individual ULXs may contain extremely massive 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.

The Nature of Ultraluminous Compact X‐Ray Sources in Nearby Spiral Galaxies

Studies were made of ASCA spectra of seven ultra-luminous compact X-ray sources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. 1994), M81 X-6 (Fabbiano 1988b; Kohmura et al. 1994; Uno 1997), IC 342 Source 1 (Okada et al. 1998), Dwingeloo 1 X-1 (Reynolds et al. 1997), NGC 1313 Source B (Fabbiano & Trinchieri 1987; Petre et al. 1994), and two sources in NGC 4565 (Mizuno et al. 1999). With the 0.5--10 keV luminosities in the range 10^{39-40} ergs/s, they are thought to represent a class of enigmatic X-ray sources often found in spiral galaxies. For some of them, the ASCA data are newly processed, or the published spectra are reanalyzed. For others, the published results are quoted. The ASCA spectra of all these seven sources have been described successfully with so called multi-color disk blackbody (MCD) emission arising from optically-thick standard accretion disks around black holes. Except the case of M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not to exceed the Eddington limits, the black holes are inferred to have rather high masses, up to ~100 solar masses. However, the observed innermost disk temperatures of these objects, Tin = 1.1--1.8 keV, are too high to be compatible with the required high black-hole masses, as long as the standard accretion disks around Schwarzschild black holes are assumed. Similarly high disk temperatures are also observed from two Galactic transients with superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high disk temperature may be explained by the black hole rotation, which makes the disk get closer to the black hole, and hence hotter.