super-Eddington Luminosities Research Articles

Discovery of a Bright Transient Ultraluminous X-Ray Source, Suzaku J1305–4931 in NGC4945

Abstract An X-ray source, Suzaku J1305$-$4931, was discovered in the south-west arm of a nearby Seyfert II galaxy, NGC4945, at 0.5-10keV flux of $2.2 \times 10^{-12}$ergcm$^{-2}$s$^{-1}$ during a Suzaku observation conducted on 2006 January 15-17. It was undetectable in a shorter observation on 2005 August 22-23, with an upper limit of $1.7 \times 10^{-14}$ergcm$^{-2}$s$^{-1}$. At a distance of 3.7Mpc, the bolometric luminosity of the source became $L_{\rm bol} = 4.4 \times 10^{39} \alpha$ergs$^{-1}$, where $\alpha = ({\rm\cos} 60^\circ \!/ \cos i)$ and $i$ is the disk inclination. The time-averaged X-ray spectrum of the source is described by a multi-color disk model, with an innermost disk temperature of $T_{\rm in} = 1.69_{-0.05}^{+0.06}$keV. It varied by a factor of $2$ in intensity, following a clear correlation of $L_{\rm bol} \propto T_{\rm in}^4$. The innermost disk radius is inferred to stay constant at $R_{\rm in} = 79_{-3.9}^{+4.0} \alpha^{1/2}$ km, suggesting the presence of a standard accretion disk. Relating $R_{\rm in}$ with the last stable orbit around a non-rotating black hole yields a rather low black-hole mass, $\sim 9 \alpha^{1/2}$ solar masses, which would imply that the source is shining at a considerable super-Eddington luminosity. These results can be better interpreted by invoking sub-Eddington emission from a rapidly spinning black hole with 20-130 solar masses.

Populations of Bright X-ray Sources in the Starburst Galaxies NGC 4038/4039

Assuming a naive star formation history, we construct synthetic X-ray source populations, using a population synthesis code, for comparison with the observed X-ray luminosity function (XLF) of the interacting galaxies NGC 4038/4039. We have included high- and intermediate-mass X-ray binaries, young rotation-powered pulsars and fallback disk-fed black holes in modeling the bright X-ray sources detected. We find that the majority of the X-ray sources are likely to be intermediate-mass X-ray binaries, but for typical binary evolution parameters, the predicted XLF seems to be steeper than observed. We note that the shape of the XLFs depends critically on the existence of XLF break for young populations, and suggest super-Eddington accretion luminosities or the existence of intermediate-mass black holes to account for the high luminosity end and the slope of the XLF in NGC 4038/4039.

Modeling of the Super‐Eddington Phase for Classical Novae: FiveIUENovae

We present a light-curve model for the super-Eddington luminosity phase of five classical novae observed with IUE. Optical and UV light curves are calculated based on the optically thick wind theory with a reduced effective opacity for a porous atmosphere. Fitting a model light curve with the UV 1455 A light curve, we determine the white dwarf mass and distance to be 1.3 M☉, 4.4 kpc for V693 CrA; 1.05 M☉, 1.8 kpc for V1974 Cyg; 0.95 M☉, 4.1 kpc for V1668 Cyg; 1.0 M☉, 2.1 kpc for V351 Pup; and 1.0 M☉, 4.3 kpc for OS And.

A Stellar-Mass Black Hole in the Ultraluminous X-Ray Source M82 X-1?

We have analyzed the archival XMM-Newton data of the archetypal Ultra-Luminous X-ray Source (ULX) M82 X-1 with an LO5 ksec exposure when the source was in the steady state. Thanks to the high photon statistics from the large effective area and long exposure, we were able to discriminate different X-ray continuum spectral models. Neither the standard accretion disk model (where the radial dependency of the disk effective temperature is T(r) proportional to r(sup -3/4)) nor a power-law model gives a satisfactory fit. In fact, observed curvature of the M82 X-1 spectrum was just between those of the two models. When the exponent of the radial dependence (p in T(r) proportional to r(sup -P)) of the disk temperature is allowed to be free, we obtained p = 0.61 (sup +0.03)(sub -0.02). Such a reduction of p from the standard value 3/4 under extremely high mass accretion rates is predicted from the accretion disk theory as a consequence of the radial energy advection. Thus, the accretion disk in M82 X-1 is considered to be in the Slim disk state, where an optically thick Advection Dominant Accretion Flow (ADAF) is taking place. We have applied a theoretical slim disk spectral model to M82 X-1, and estimated the black hole mass approximately equal to 19 - 32 solar mass. We conclude that M82 X-1 is a stellar black hole which has been produced through evolution of an extremely massive star, shining at a several times the super-Eddington luminosity.

The planets capture model of V838 Monocerotis: conclusions for the penetration depth of the planet(s)

V838 Mon is the prototype of a new class of objects. Understanding the nature of its multistage outburst and similar systems is challenging. So far, several scenarios have been invoked to explain this group of stars. In this work, the planets-swallowing model for V838 Mon is further investigated, taking into account the findings that the progenitor is most likely a massive B-type star. We find that the super-Eddington luminosity during the eruption can explain the fast rising times of the three peaks in the optical light curve. We used two different methods to estimate the location where the planets were consumed. There is a nice agreement between the values obtained from the luminosities of the peaks and from their rising time-scale. We estimate that the planets were stopped at a typical distance of one solar radius from the centre of the host giant star. The planets-devouring model seems to give a satisfying explanation to the differences in the luminosities and rising times of the three peaks in the optical light curve of V838 Mon. The peaks may be explained by the consumption of three planets or alternatively by three steps in the terminal falling process of a single planet. We argue that only the binary merger and the planets-swallowing models are consistent with the observations of the new type of stars defined by V838 Mon.

A Close Look at the Population of Supersoft and Quasi‐soft X‐Ray Sources Observed in M31 withXMM‐Newton

The deepest X-ray images of M31, obtained with XMM-Newton, are examined to derive spectral and statistical properties of the population of the very soft X-ray sources. When classifying supersoft X-ray sources (SSSs) with criteria based on the same hardness ratios defined for recent Chandra observations, one-quarter of the selected SSSs turn out to be supernova remnants (SNRs). Another quarter of the SSSs are spatially coincident with recent classical novae, although they are less than 10% of the nova population observed in the last 25 yr. Only 3 out of the 15 non-SNR SSSs show clear variability, with X-ray flux variation of more than 1 order of magnitude within a few months. Two of these sources display additional, smaller amplitude variability on timescales of several minutes. Their broadband spectra and those of the novae are approximately fitted with a blackbody or white dwarf atmospheric model at near-Eddington luminosity for the distance of M31. Two SSSs appear to reach very large, perhaps super-Eddington luminosities for part of the time and probably eject material in a wind until the luminosity decreases again after a few months. Most quasi-soft sources (QSSs) are repeatedly detected. I discuss the possibility that most QSSs in M31 may be SNRs or foreground neutron stars. Two X-ray sources with both a soft and hard component are in the positions of a recurrent nova and a possible symbiotic nova, but they are probably black hole transients.

A Modeling of the Super-Eddington Luminosity in Nova Outbursts: V1974 Cygni

We have modeled nova light curves exceeding the Eddington luminosity. It has been suggested that a porous structure develops in nova envelopes during the super Eddington phase and the effective opacity is much reduced for such a porous atmosphere. Based on this reduced opacity model, we have calculated envelope structures and light curves of novae. The optically thick wind model is used to simulate nova winds. We find that the photospheric luminosity and the wind mass-loss rate increase inversely proportional to the reducing factor of opacities, but the wind velocity hardly changes. We also reproduce the optical light curve of V1974 Cygni (Nova Cygni 1992) in the super-Eddington phase, which lasts 13 days from the optical peak 1.7 mag above the Eddington luminosity.

Supernovae in helium star-compact object binaries: a possibleγ-ray burst mechanism

Helium star-compact object binaries, and helium star-neutron star binaries in particular, are widely believed to be the progenitors of the observed double-neutron-star systems. In these, the second neutron star is presumed to be the compact remnant of the helium star supernova. In this paper, the observational implications of such a supernova are discussed, and in particular are explored as a candidate γ-ray burst mechanism. In this scenario, the supernova results in a transient period of rapid accretion on to the compact object, extracting via magnetic torques its rotational energy at highly super-Eddington luminosities in the form of a narrowly beamed, strongly electromagnetically dominated jet. Compton scattering of supernova photons advected within the ejecta, and photons originating at shocks driven into the ejecta by the jet, will cool the jet and can produce the observed prompt emission characteristics, including the peak-inferred isotropic energy relation, X-ray flash characteristics, subpulse light curves, energy-dependent time lags and subpulse broadening, and late time spectral softening. The duration of the burst is limited by the rate of Compton cooling of the jet, eventually creating an optically thick, moderately relativistically expanding fireball that can produce the afterglow emission. If the black hole or neutron star stays bound to a compact remnant, late term light curve variability may be observed as in SN 2003dh.

The mass of the black hole in 3C 273

In this paper we apply the reverberation method to determine the mass of the black hole in 3C273 from the Ly a and C iv emission lines using archival IUE observations. Following the standard assumptions of the method, we find a maximum-likelihood estimate of the mass of 6.59 10^9 Mo, with a 1 sigma confidence interval 5.69-8.27 10^9 Mo. This estimate is more than one order of magnitude larger than that obtained in a previous study using Balmer lines. We reanalyze the optical data and show that the method applied to the Ha, Hb, and Hg Balmer lines produce mass estimates lower by a factor 2.5, but already much larger than the previous estimate derived from the same lines. The finding of such a high mass in a face-on object is a strong indication that the gas motion is not confined to the accretion disk. The new mass estimate makes 3C273 accreting with an accretion rate about six times lower than the Eddington rate. We discuss the implications of our result for the broad-line-region size and black-hole mass vs luminosity relationships for the set of objects for which reverberation black-hole masses have been obtained. We find that, while objects with super-Eddington luminosities might theoretically be possible, their existence is not necessarily implied by this sample.

An Extended Grid of Nova Models. II. The Parameter Space of Nova Outbursts

This paper is a sequel to an earlier paper devoted to multiple, multicycle nova evolution models (Prialnik & Kovetz 1995, first paper of the series), which showed that the different characteristics of nova outbursts can be reproduced by varying the values of three basic and independent parameters: the white dwarf mass-M_{WD}, the temperature of its isothermal core-T_{WD} and the mass transfer rate-Mdot. Apart from being the largest computational classical novae parameter-space survey, we show here that the parameter space is constrained by several analytical considerations and find its limiting surfaces. Consequently, we extend the grid of multicycle nova evolution models presented in Paper I almost to its limits, adding multicycle nova outburst calculations for a considerable number of new parameter combinations. In particular, the extended parameter pace that produces nova eruptions includes low mass transfer rates down to 5e-13 Msun/yr, and more models for low T_{WD}. Resulting characteristics of these runs are added to the former parameter combination results, to provide a full grid spanning the entire parameter space for Carbon-Oxygen white dwarfs. The full grid covers the entire range of observed nova characteristics, even those of peculiar objects, which have not been numerically reproduced until now. Most remarkably, runs for very low Mdot lead to very high values for some characteristics, such as outburst amplitude A>~20, high super-Eddington luminosities at maximum, heavy element abundance of the ejecta Z_{ej}~0.63 and high ejected masses m_{ej}~7e-4 Msun.

Black hole accretion discs and jets at super-Eddington luminosity

Super-Eddington accretion discs with and around black holes with mass 10 M⊙ are examined by two-dimensional radiation hydrodynamical calculations extending from the inner disc edge to 5 × 104rg and lasting up to ∼106rg/c. The dominant radiation pressure force in the inner region of the disc accelerates the gas vertically to the disc plane, and jets with 0.2–0.4c are formed along the rotational axis. In the case of the lower accretion rate, the initially anisotropic high-velocity jet expands outward and becomes gradually isotropic flow in the distant region. The mass-outflow rate from the outer boundary is as large as ∼1019–1023 g s−1, but it is variable and intermittent with time; that is, the outflow switches occasionally to inflow in the distant region. The luminosity also varies as ∼1040–1042 erg s−1 on a long time-scale. On the other hand, the jet in the case of the higher accretion rate maintains its initial anisotropic shape even after it goes far away. The mass-outflow rate and the luminosity attain steady values of 3 × 1019 g s−1 and 1.3 × 1040 erg s−1, respectively. In accordance with the local analysis of the slim accretion disc model, the disc is thermally unstable in the case of but stable in the case of . The super-Eddington model with promises to explain the small collimation degree of the jet and the large mass-outflow rate observed in the X-ray source SS 433.

DISTINGUISHING BARE QUARK STARS FROM NEUTRON STARS

Observations to date cannot distinguish neutron stars from self-bound bare quark stars on the basis of their gross physical properties such as their masses and radii alone. However, their surface luminosity and spectral characteristics can be significantly different. Unlike a normal neutron star, a bare quark star can emit photons from its surface at super-Eddington luminosities for an extended period of time. We present a calculation of the photon bremsstrahlung rate from the bare quark star's surface, and indicate improvements that are required for a complete characterization of the spectrum. The observation of this distinctive photon spectrum would constitute an unmistakable signature of a strange quark star and shed light on color superconductivity at stellar densities.

Young Crab‐like Pulsars and Luminous X‐Ray Sources in Starbursts and Optically Dull Galaxies

Recent Chandra observations of nearby galaxies have revealed a number of ultraluminous X-ray sources (ULXs) with super-Eddington luminosities, away from the central regions of nonactive galaxies. The nature of these sources is still debated. We argue that a fraction of them could be young, Crab-like pulsars, the X-ray luminosity of which is powered by rotation. We use the pulsar birth parameters estimated from radio pulsar data to compute the steady state pulsar X-ray luminosity distribution as a function of the star formation rate (SFR) in the galaxy. We find that ~10% of optically dull galaxies are expected to have a source with LX 1039 ergs s-1, while starburst galaxies should each have several of these sources. We estimate that the X-ray luminosity of a few percent of galaxies is dominated by a single bright pulsar with LX 1039 ergs s-1, roughly independently of its SFR. We discuss observational diagnostics that can help distinguish the young pulsar population in ULXs.

Constraints on QSO Models from a Relation between the QSO Luminosity Function and the Local Black Hole Mass Function

QSOs are believed to be powered by accretion onto massive black holes (BHs). In this paper, assuming that each central BH in nearby galaxies has experienced the QSO phase and ignoring BH mergers, we establish a relation between the QSO luminosity function (LF) and the local BH mass function (MF). The QSOLF is jointly controlled by the luminosity evolution of individual QSOs and the triggering history of the accretion onto seed BHs. By comparing the time integral of the QSOLF with that inferred from local BHs, we separate the effect of the luminosity evolution of individual QSOs from the effect of the triggering history. Assuming that the nuclear luminosity evolution includes two phases (first increasing at the Eddington luminosity with growth of BHs and then declining), we find that observations are generally consistent with the expected relation between the QSOLF and the local BHMF and obtain the following constraints on QSO models and BH growth: (i) The QSO mass-to-energy efficiency should be ≳0.1. (ii) The lifetime (defined directly through the luminosity evolution of individual QSOs here) should be ≳4 × 107 yr. The characteristic declining timescale in the second phase should be significantly shorter than the Salpeter timescale τSp, and BH growth should not be dominated by the second phase. (iii) The ratio of obscured QSOs/AGNs to optically bright QSOs should be not larger than 7 at MB ~ -23 and 3 at MB ~ -26 if = 0.31, and not larger than 1 at MB ~ -23 and negligible at MB ~ -26 if = 0.1. (iv) It is unlikely that most QSOs are accreting at super-Eddington luminosities. We point out that it is hard to accurately estimate the value of the QSO lifetime from the QSOLF and/or the local BHMF, if it is longer than a certain value (e.g., ~4τSp in this study). We discuss the importance of accurate measurements of the intrinsic scatter in the BH mass and velocity dispersion relation of local galaxies and the scatter in the bolometric correction of QSOs. We also discuss some possible applications of the work in this paper, such as to the study of the demography of QSOs and the demography of normal galaxies at intermediate redshift.

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.

ULXs, Accretion Disk Fragmentation, and Super-Eddington Luminosities

We suggest a new instability in disk/corona systems which leads to fragmentation of accretion disks in strong magnetic fields under radiation pressure and allows Super-Eddington radiation from the fragmented accretion disk.

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.

GRS 1915+105: the brightest Galactic black hole

We compare the evolution of spectral shape with luminosity in GRS 1915+105 with that of ‘normal’ black holes. The pathological variability of GRS 1915+105, which probably indicates a disc instability, does not require that GRS 1915+105 belongs in a different class to all the other objects. At comparable fractions of Eddington luminosity its spectra and (more importantly) apparent disc stability are both similar to that seen in the ‘normal’ black holes. Its unique limit-cycle variability only appears when it radiates at uniquely high (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.

Radiation from hot, bare, strange stars

We present the results of numerical simulations of stationary, spherically outflowing, pair winds, with total luminosities of L=10^{35}- 10^{42} ergs/s. These results have direct relevance to the emission from hot, bare, strange stars, which are thought to be powerful sources of electron-positron pairs created by the Coulomb barrier at the quark surface. The spectra of emergent photons and pairs are calculated. For L > 2x10^{35} erg/s, photons dominate the emerging emission. As L increases from 10^{35} to 10^{42} ergs/s, the mean photon energy decreases from ~ 400-500 keV to 40 keV, while the spectrum changes in shape from a wide annihilation line to being nearly blackbody with a high energy (> 100 keV) tail. Such a correlation of the photon spectrum with the luminosity, together with the fact that super-Eddington luminosities can be achieved, might be a good observational signature of hot, bare, strange stars.