High Black Hole Mass Research Articles

The eclipsing massive X-ray binary M 33 X-7: New X-ray observations and optical identification

The eclipsing X-ray binary M 33 X 7 was in the field of view during several observations of our XMM-Newton M 33 survey and in the archival Chandra observation 1730 which cover a large part of the 3.45 d orbital period. We detect emission of M 33 X 7 during eclipse and a soft X-ray spectrum of the source out of eclipse that can best be described by bremsstrahlung or disk blackbody models. No significant regular pulsations of the source in the range 0.25-1000 s were found. The average source luminosity out of eclipse is 5 10 37 erg s 1 (0.5-4.5 keV). In a special analysis of DIRECT observations we identify as optical counterpart a B0I to O7I star of 18.89 mag in V which shows the ellipsoidal heating light curve of a high mass X-ray binary with the M 33 X 7 binary period. The location of the X-ray eclipse and the optical minima allow us to determine an improved binary period and ephemeris of mid-eclipse as HJD (2 451 760:61 0:09) N (3:45376 0:00021). The mass of the compact object derived from orbital parameters and the optical companion mass, the lack of pulsations, and the X-ray spectrum of M 33 X 7 may indicate that the compact object in the system is a black hole. M 33 X 7 would be the first detected eclipsing high mass black hole X-ray binary.

How Low Can a Black Hole Go?

Black holes have been detected in many systems, and the range of their estimated masses is thought to reflect different mechanisms of formation. Stellar mass black holes (those with masses of 3 to 15 solar masses) are the remnants of supernovae, and any star with a mass greater than about 3 solar masses will collapse and form a black hole; on the other hand, lower mass stars will form neutron stars. Supernova models suggest that lower mass black holes should be more abundant than higher mass black holes and that there should be a continuum from neutron stars to stellar mass black holes. Observations, however, reveal a gap at 3 to 5 solar masses, where no black holes have been identified.

INTEGRALdiscovery of a bright highly obscured galactic X-ray binary source IGR J16318-4848

INTEGRALregularly scans the Galactic plane to search for new objects and in particular for absorbed sources with the bulk of their emission above 10− 20 keV. The first new INTEGRALsource was discovered on 2003 January 29, 0.5 ◦ from the Galactic plane and was further observed in the X-rays with XMM-Newton. This source, IGR J16318−4848, is intrinsically strongly absorbed by cold matter and displays exceptionally strong fluorescence emission lines. The likely infrared /optical counterpart indicates that IGR J16318−4848 is probably a High Mass X-Ray Binary neutron star or black hole enshrouded in a Compton thick environment. Strongly absorbed sources, not detected in previous surveys, could contribute significant ly to the Galactic hard X-ray background between 10 and 200 keV.

On the central black hole mass in Mkn 501

We analyse the apparent disagreement between the mass estimates of the central black hole(s) in Mkn~501 based on (i) the observations of the host galaxy, (ii) the high energy (HE) emission mechanism, and (iii) the modulation of the beamed radiation by a black hole (BH) binary system. While method (i) seems to imply a central mass > 5\times 10^8 M_{sun}, method (ii) suggests a BH mass less than 6 \times 10^7 M_{sun}. We critically discuss the estimates inferred from (i) showing that current uncertainties may permit a central mass as low as ~(2-3)\times 10^8 M_{sun}. We demonstrate that in this case the estimates (i) and (ii) might be brought into agreement by assuming a binary BH system where the jet dominating the HE emission originates from the less massive (secondary) BH as suggested by method (iii). On the other hand, if Mkn~501 has in fact a high central BH mass of order 10^9 M_{sun}, a change of fundamental assumptions seems to be required in the context of several HE emission models. We show, that in this case a binary scenario following (iii) may be still possible if the jet which dominates the emission emerges from the more massive (primary) BH and if the binary evolution passes through phases of super-Eddington accretion and/or decreased conversion efficiency.

The Independence of Active Galactic Nucleus Black Hole Mass and Radio Loudness

We revisit the issue of whether radio loudness in active galactic nuclei (AGNs) is associated with central black hole mass, as has been suggested in the literature. We present new estimates of black hole mass for 295 AGNs (mostly radio quiet), calculating their radio loudnesses from published radio and optical fluxes, and combine with our previously published values, for a sample of 452 AGNs for which both black hole mass and radio loudness (or upper limits thereto) are known. Among the radio-quiet AGNs, there are now many black holes with mass larger than 109 M?, extending to the same high black hole masses as radio-loud AGNs of the same redshifts. Over the full sample, the black hole masses of radio-loud and radio-quiet AGNs span the same large range, 106-1010 M?. We conclude that radio loudness in AGNs does not depend strongly on central black hole mass.

Physical parameters of the high-mass X-ray binary 4U1700-37

We present the results of a detailed non-LTE analysis of the ultraviolet and optical spectrum of the O6.5 Iaf + star HD 153919 - the mass donor in the high-mass X-ray binary 4U1700-37. We find that the star has a luminosity log(L=L)= 5:82 0:07, Te = 35 000 1000 K, radius R = 21:9 +1:3 0:5 R, mass-loss rate ˙ M = 9:5 10 6 M yr 1 , and a significant overabundance of nitrogen (and possibly carbon) relative to solar values. Given the eclipsing nature of the system these results allow us to determine the most likely masses of both components of the binary via Monte Carlo simulations. These suggest a mass for HD 153919 of M= 58 11 M - implying the initial mass of the companion was rather high (>60 M). The most likely mass for the compact companion is found to be Mx = 2:44 0:27 M, with only 3.5 per cent of the trials resulting in a mass less than 2.0 M and none less than 1.65 M. Such a value is significantly in excess of the upper observational limit to the masses of neutron stars of 1.45 M found by Thorsett & Chakrabarthy (1999), although a mass of 1.86 M has recently been reported for the Vela X-1 pulsar (Barziv et al. 2001). Our observational data is inconsistent with the canonical neutron star mass and the lowest black hole mass observed (>4.4 M; Nova Vel). Significantly changing observational parameters can force the compact object mass into either of these regimes but, given the strong proportionality between M and Mx, the O-star mass changes by factors of greater than 2, well beyond the limits determined from its evolutionary state and surface gravity. The low mass of the compact object implies that it is dicult to form high mass black holes through both the Case A & B mass transfer channels and, if the compact object is a neutron star, would significantly constrain the high density nuclear equation of state.

Formation of high mass X-ray black hole binaries

The discrepancy in the past years of many more black-hole soft X-ray transients (SXTs), of which a dozen have now been identified, had challenged accepted wisdom in black hole evolution. Reconstruction in the literature of high-mass X-ray binaries has required stars of up to ∼40 M ⊙ to evolve into low-mass compact objects, setting this mass as the limit often used for black hole formation in population syntheses. On the other hand, the sheer number of inferred SXTs requires that many, if not most, stars of ZAMS masses 20–35 M ⊙ end up as black holes ( Portegies Zwart et al., 1997; Ergma and van den Heuvel, 1998). In this paper we show that this can be understood by challenging the accepted wisdom that the result of helium core burning in a massive star is independent of whether the core is covered by a hydrogen envelope, or ‘naked’ while it burns. The latter case occurs in binaries when the envelope of the more massive star is transferred to the companion by Roche Lobe overflow while in either main sequence or red giant stage. For solar metallicity, whereas the helium cores which burn while naked essentially never go into high-mass black holes, those that burn while clothed do so, beginning at ZAMS mass ∼20 M ⊙ , the precise mass depending on the 12C( α, γ) 16O rate as we outline. In this way the SXTs can be evolved, provided that the H envelope of the massive star is removed only following the He core burning. Whereas this scenario was already outlined in 1998 by Brown et al. [NewA 4 (1999) 313], their work was based on evolutionary calculations of Woosley et al. [ApJ 448 (1995) 315] which employed wind loss rates which were too high. In this article we collect results for lower, more correct wind loss rates, finding that these change the results only little. We go into the details of carbon burning in order to reconstruct why the low Fe core masses from naked He stars are relatively insensitive to wind loss rate. The main reason is that without the helium produced by burning the hydrogen envelope, which is convected to the carbon in a clothed star, a central 12C abundance of ∼1/3 remains unburned in a naked star following He core burning. The later convective burning through 12C+ 12C reactions occurs at a temperature T∼80 keV. Finally, we show that in order to evolve a black hole of mass ≳10 M ⊙ such as observed in Cyg X-1, even employing extremely massive progenitors of ZAMS mass ≳60 M ⊙ for the black hole, the core must be covered by hydrogen during a substantial fraction of the core burning. In other words, the progenitor must be a WNL star. We evolve Cyg X-1 in an analogous way to which the SXTs are evolved, the difference being that the companion in Cyg X-1 is more massive than those in the SXTs, so that Cyg X-1 shines continuously.

Slim-Disk Model for Ultraluminous X-Ray Sources

The Ultra Luminous X-ray Sources (ULXs) are unique in exhibiting moderately bright X-ray luminosities, $L_{\rm x} \sim 10^{38-40} {\rm erg~s^{-1}}$, and relatively high blackbody temperatures, $\Tin \sim 1.0-2.0 {\rm keV}$. From the constraint that $L_{\rm x}$ cannot exceed the Eddington luminosity, $L_{\rm E}$, we require relatively high black-hole masses, $M\sim 10-100 M_\odot$, however, for such large masses the standard disk theory predicts lower blackbody temperatures, $\Tin < 1.0$ keV. To understand a cause of this puzzling fact, we carefully calculate the accretion flow structure shining at $\sim L_{\rm E}$, fully taking into account the advective energy transport in the optically thick regime and the transonic nature of the flow. Our calculations show that at high accretion rate ($\dot M \ga 30~L_{\rm E}/c^2$) an apparently compact region with a size of $\Rin \simeq (1-3)\rg$ (with $\rg$ being Schwarzschild radius) is shining with a blackbody temperature of $\Tin \simeq 1.8 (M/10M_\odot)^{-1/4}$ keV even for the case of a non-rotating black hole. Further, $\Rin$ decreases as $\dot M$ increases, on the contrary to the canonical belief that the inner edge of the disk is fixed at the radius of the marginally stable last circular orbit. Accordingly, the loci of a constant black-hole mass on the H-R diagram (representing the relation between $L_{\rm x}$ and $\Tin$ both on the logarithmic scales) are not straight but bent towards the lower $M$ direction in the frame of the standard-disk relation.

Evolution of black holes in the galaxy

In this article we consider the formation and evolution of black holes, especially those in binary stars where radiation from the matter falling on them can be seen. We consider a number of effects introduced by some of us, which are not traditionally included in binary evolution of massive stars. These are (i) hypercritical accretion, which allows neutron stars to accrete enough matter to collapse to a black hole during their spiral-in into another star. (ii) The strong mass loss of helium stars, which causes their evolution to differ from that of the helium core of a massive star. (iii) The direct formation of low-mass black holes ( M∼2 M ⊙) from single stars, a consequence of a significant strange-matter content of the nuclear-matter equation of state at high density. We discuss these processes here, and then review how they affect various populations of binaries with black holes and neutron stars. We have found that hypercritical accretion changes the standard scenario for the evolution of binary neutron stars: it now usually gives a black-hole, neutron-star (BH-NS) binary, because the first-born neutron star collapses to a low-mass black hole in the course of the evolution. A less probable double helium star scenario has to be introduced in order to form neutron-star binaries. The result is that low-mass black-hole, neutron star (LBH-NS) binaries dominate the rate of detectable gravity-wave events, say, by LIGO, by a factor ∼20 over the binary neutron stars. The formation of high-mass black holes is suppressed somewhat due to the influence of mass loss on the cores of massive stars, raising the minimum mass for a star to form a massive BH to perhaps 80 M ⊙. Still, inclusion of high-mass black-hole, neutron-star (HBH-NS) binaries increases the predicted LIGO detection rate by another ∼30%; lowering of the mass loss rates of Wolf–Rayet stars may lower the HBH mass limit, and thereby further increase the merger rate. We predict that ∼33 mergers per year will be observed with LIGO once the advanced detectors planned to begin in 2004 are in place. Black holes are also considered as progenitors for gamma ray bursters (GRB). Due to their rapid spin, potentially high magnetic fields, and relatively clean environment, mergers of black-hole, neutron-star binaries may be especially suitable. Combined with their 10 times greater formation rate than binary neutron stars this makes them attractive candidates for GRB progenitors, although the strong concentration of GRBs towards host galaxies may favor massive star progenitors or helium-star, black-hole mergers. We also consider binaries with a low-mass companion, and study the evolution of the very large number of black-hole transients, consisting of a black hole of mass ∼7 M ⊙ accompanied by a K or M main-sequence star (except for two cases with a somewhat more massive subgiant donor). We show that common envelope evolution must take place in the supergiant stage of the massive progenitor of the black hole, giving an explanation of why the donor masses are so small. We predict that there are about 22 times more binaries than observed, in which the main-sequence star, somewhat more massive than a K- or M-star, sits quietly inside its Roche Lobe, and will only become an X-ray source when the companion evolves off the main sequence. We briefly discuss the evolution of low-mass X-ray binaries into millisecond pulsars. We point out that in the usual scenario for forming millisecond pulsars with He white-dwarf companions, the long period of stable mass transfer will usually lead to the collapse of the neutron star into a black hole. We then discuss Van den Heuvel's “Hercules X-1 scenario” for forming low-mass X-ray binaries, commenting on the differences in accretion onto the compact object by radiative or semiconvective donors, rather than the deeply convective donors used in the earlier part of our review. In Appendix A we describe the evolution of Cyg X-3, finding the compact object to be a black hole of ∼3 M ⊙, together with an ∼10 M ⊙ He star. In Appendix B we do the accounting for gravitational mergers and in Appendix C we show low-mass black-hole, neutron-star binaries to be good progenitors for gamma ray bursters.

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.

ASCA Observations of Two Ultra-Luminous Compact X-Ray Sources in the Edge-On Spiral Galaxy NGC 4565

The edge-on spiral galaxy NGC 4565 was observed for ∼ 35 ks with ASCA in the 0.5–10 keV energy band. The X-ray emission was dominated by two bright sources, which could be identified with two point-like X-ray sources seen in the ROSAT HRI image. The observed 0.5–10 keV fluxes of these sources, 1.7 × 10−12 erg s−1 cm−2 and 0.7 × 10−12 erg s−1 cm−2, imply bolometric luminosities of 1.2 × 1040 erg s−1 and 4.6 × 1039 erg s−1, respectively. They exhibit similar spectra, which can be explained by emission from optically thick accretion disks with inner-disk temperature of 1.4–1.6 keV. One of them, coincident in position with the nucleus, shows too low absorption to be the active nucleus seen through the galaxy disk. Their spectra and high luminosities suggest that they are both mass-accreting black-hole binaries. However, the black-hole mass required by the Eddington limit is rather high (≥ 50 M⊙), and the observed disk temperature is too high to be compatible with the high black-hole mass. Several attempts have been made to solve these problems.

The formation of high-mass black holes in low-mass X-ray binaries

We suggest that high-mass black holes; i.e., black holes of several solar masses, can be formed in binaries with low-mass main-sequence companions, provided that the hydrogen envelope of the massive star is removed in common envelope evolution which begins only after the massive star has finished He core burning.

Contribution of High‐Mass Black Holes to Mergers of Compact Binaries

We consider the merging of high-mass black-hole binaries of the Cyg X-1 type, in which the O-star companion becomes a neutron star following a supernova explosion. Our estimated rate of mergers, $\sim 2\times 10^{-5} yr^{-1}$ for the Galaxy, is relatively low because of the paucity of high-mass black holes. None the less, because of their high masses, these black-hole, neutron-star binaries could contribute importantly to the merging sought by LIGO. From stellar evolutionary calculations including mass loss, we estimate that a ZAMS mass of $\sim 80 \msun$ is necessary before a high-mass black hole can result in a massive binary. We estimate the detection rate for LIGO of high-mass black hole neutron-star mergers to be a factor $\sim 40$ greater than that for binary neutron stars. We suggest how our high cut-off mass can be reconciled with the requirements of nucleosynthesis, and show that a bimodal distribution with masses of black holes can account, at least qualitatively, for the many transient sources which contain high-mass black holes.

On the Formation of Low-Mass Black Holes in Massive Binary Stars

Recently (Brown \& Bethe 1994) it was suggested that most stars with main sequence mass in the range of about $18 - 30 M_{\odot}$ explode, returning matter to the Galaxy, and then go into low-mass ($\geq 1.5 M_{\odot}$) black holes. Even more massive main-sequence stars would, presumably, chiefly g o into high-mass ($\sim 10 M_{\odot}$) black holes. The Brown-Bethe estimates gave approximately $5 \times {10}^{8}$ low-mass black holes in the Galaxy. A pressing question, which we attempt to answer here, is why, with the possible exception of the compact objects in SN1987A and 4U\,1700--37, none of these have been seen. We address this question in three parts. Firstly, black holes are generally ``seen'' only in binaries, by the accretion of matter from a companion star. High mass black holes are capable of accreting more matter than low-mass black holes, so there is a selection effect favoring them. This, in itself, would not be sufficient to show why low-mass black holes have not been seen, since neutron stars (of nearly the same mass) are seen in abundance. Secondly, and this is our main point, the primary star in a binary ---the first star to evolve--- loses its hydrogen envelope by transfer of matter to the secondary and loss into space, and the resulting ``naked'' helium star evolves differently than a helium core, which is at least initially covered by the hydrogen envelope in a massive main-sequence star. We show that primary stars in binaries can end up as neutron stars even if their initial mass substantially exceeds the mass limit for neutron star formation from single stars ($\sim 18 M_{\odot}$). An example is 4U\,1223--62, in which we suggest that the initial primary mass exceeded $35 M_{\odot}$, yet X-ray pulsations

Observational Evidence for Supermassive Black Holes

A growing body of evidence from stellar dynamics in the nuclei of galaxies indicates that supermassive black holes of 107–109 M0 are common. The two best cases are M31 and M32, for which dark, central mass concentrations are the only straightforward interpretation. M87 continues to be a possible location of an even more massive black hole, but new observations and models by the author and D. Richstone effectively rule out the high black hole mass ∼5 × 109 M0 claimed by Sargent, Young, and collaborators. New data are available for several other nearby galaxies which also show kinematic signatures that could also be due to supermassive black holes. The Hubble Space Telescope will play the key role in strengthening these cases and eliminating, for the best examples, alternative models which do not require supermassive black holes.