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Observational Evidence for Stellar Mass Black Holes J. Astrophys. Astr. (1999) 20, 197–210 Observational Evidence for Stellar Mass Black Holes Tariq Shahbaz, University of Oxford, Department of Astrophysics, Nuclear Physics Building, Keble Road, Oxford, 0X1 3RH, England. e-mail:[email protected] Abstract. I review the evidence for stellar mass black holes in the Galaxy. The unique properties of the soft X-ray transient (SXTs) have provided the first opportunity for detailed studies of the mass-losing star in low-mass X-ray binaries. The large mass functions of these systems imply that the compact object has a mass greater than the maximum mass of a neutron star, strengthening the case that they contain black holes. The results and techniques used are discussed. I also review the recent study of a comparison of the luminosities of black hole and neutron star systems which has yielded compelling evidence for the existence of event horizons. Key words. X-ray sources—black holes. 1. Black holes and the soft X-ray transients It is widely believed that black holes power AGN and some X-ray binaries, and that they inhabit the nuclei of many normal galaxies. The compulsion to believe that black holes are physical objects is driven by our faith in general relativity and the idea that a black hole is a logical end point of stellar and galactic evolution. The proof for the existence of stellar black holes has been a subject of considerable effort for the last three decades, since the first dynamical mass estimate for the compact object in Cyg X-l (Webster & Murdin 1972; Bolton 1972). More recently observational studies have concentrated on the identification of X-ray features unique to black holes and the direct measurement of the dynamical mass of the compact object. After the discovery of the first two black holes (Cyg X-l and LMC X-3) involving High Mass X-ray Binaries (HMXBs), almost all the new black hole systems added to the list (see table 1) belong to the class of low mass X-ray binaries (LMXBs)1. A large fraction of these are part of the subgroup called the soft X-ray transients (SXTs), sometimes also referred to as X-ray novae. These are characterised by massive X-ray outbursts, usually lasting for several months, during which they are discovered by X-ray or γ-ray all-sky monitors such as those on Ginga, Granat, GRO and RXTE (see Fig. 1). The high fraction of black holes among SXTs is a natural consequence of the low accretion rates implied by their evolved secondaries (King, Kolb & Burderi 1996; see section 3.1). 1High and Low mass refer to the mass of the optical donor star. 197 198 Tariq Shahbaz Table 1. Black Hole candidates (adapted from White 1994). * Firm candidates supported by dynamical evidence. The outbursts are due to a thermal instability in the outer regions of the accretion disc (van Paradijs 1996) which occur at mass transfer rates lower than 10–8 M yr–1 ☼ . These outbursts recur on a timescale of decades, but in the interim the SXTs are in a state of quiescence, the most unique feature of this class. It is during this period that it becomes possible to observe the companion star directly and perform both optical and IR photometry and spectroscopy. This is not possible in LMXBs in general, as the light is almost totally dominated by the X-ray irradiated accretion disc. During outburst the X-ray spectra exhibit a hard power-law (with associated flickering) extending to several hundred keV; on which is superposed a softer X-ray component2 with the exception of a few remarkable cases (e.g. GS2023 + 338; see Fig. 2). Thanks to the recent increase in sensitivity of X-ray detectors it has also become possible to study, in detail the X-ray spectra of several SXTs during quiescence. They are found to be very soft with a blackbody temperature of Tbb ~ 0.2 keV (Tanaka & Shibazaki 1996). SXTs are being discovered at a rate of 1–2 per year and, although the number of systems is still small, it has been noted that their galactic distribution (unlike neutron star LMXBs) is consistent with that of Population I objects; they do not cluster towards the galactic center (White 1994), but along the plane of the galaxy. This suggests that the progenitors of the galactic black holes may have been massive Population I OB stars. In the next section I shall review the high energy spectral characteristics of the black hole systems. 2 Hence their name, coined so as to distinguish them from the harder Be X-ray transients, which are a completely different class of objects (van Paradijs & McClintock 1995). [ Evidence for Stellar Mass Black Holes 199 Figure 1. X-ray light curves of four bright soft X-ray transients (from Tanaka & Shibazaki 1996). 2. X-ray observations The X-ray spectra of the black hole SXTs usually show two components (see Fig. 2). One is a power law with a photon index in the range 1.5–2.5 which dominates at high- energies (> 10 keV) and is occasionally detected up to energies of hundreds of keV. The other component is limited to photon energies below 10 keV, and is often called the “ultra-soft” component which is interpreted as the emission from an optically thick, geometrically thin accretion disc (Tanaka & Shibazaki 1996). It is roughly described by a Planck function with a blackbody temperature of kTbb < 1 keV. It 200 Tariq Shahbaz Figure 2. X-ray spectra of several black hole X-ray binaries, showing various combinations of ultra-soft and power-law components (from Gilfanov et al. 1995). should be noted that several important black hole systems, such as GS2023+338 or GRO J0422 + 32, did not exhibit such an ultra-soft component (Tanaka 1989; Sunyaev 1992; see Fig. 2). Also it should be noted that neutron stars can also produce ultra-hard power-law tails when the luminosity drops below a certain limit (Barret & Vedrenne 1994). However, it seems that the high energy tails of the black hole systems might be harder (photon indices α > 2 ) than for neutron star systems (van der Klis 1994). The expected spectrum i.e. the ‘multi-colour’ disc spectrum from the accretion disc has been determined (Mitsuda et al. 1984). Spectral fits with this model have been made to X-ray spectra obtained with GINGA throughout the outbursts of several black hole SXTs (e.g. GS2000+25 and GS1124 – 68). An important feature of these fits was that Rin (the inner edge of the disc) remained constant, while the disc luminosity changed by more than an order of magnitude (Tanaka & Shibazaki 1996). The values of Rin derived from classical multi-colour blackbody fits are consistent with the innermost stable orbit (3 rg) around the compact object. For neutron stars Rin was found to be systematically lower, by a factor of 3–4 (Tanaka 1992) compared to the black holes. However, the above model was simple, (1) it ignored relativistic effects; gravitational redshift effects near the black hole horizon which depend on the inclination angle of the disc and black hole spin and (2) it assumed that the local disc emission was a Planck function; in the hot inner disc where most of the X-ray are emitted, electron scattering may dominate over free-free absorption and so cause the Evidence for Stellar Mass Black Holes 201 colour temperature to be greater than the effective temperature (Ebisawa 1994). A more robust model for the expected luminosity from the accretion disc of black hole SXTs has allowed the actual inner edge of the disc and black hole spin to be determined (Zhang, Cui & Chen 1997). At higher energies, Granat detected the presence of a transient emission feature in GS112468 at ~ 480 keV (see Fig. 2) which has been interpreted as the 511 keV redshifted annihilation line (Sunyaev et al. 1992; Goldwurm et al. 1992). Similar features were also observed in Cyg Xl (Ling & Wheaton 1989) and 1E1740.7 – 2942 (Mandrou 1990), a fact which led some authors to propose this as a new black hole signature. One cannot use any of the above X-ray features as an unambiguous signature of accreting black holes. However, they make a very useful tool in the search for new potential black holes. The most reliable test is still based upon dynamical grounds: the measurement of a mass function exceeding the maximum mass(~ 3M ) beyond ☼ which neutron stars become unstable (Friedman, Ipser & Parker 1986). 3. Optical and infrared observations During the long periods of quiescence when the accretion disc is faint the SXTs are ideal systems to study at optical/IR wavelengths because the low-mass companion star features are strong and observable. This allows firm lower limits to be set on the mass of the compact object by simply obtaining a radial velocity study of the companion star. The dynamical measurement of the mass of the compact object in the black hole SXTs requires both a photometric and spectroscopic study of the companion star. Using Kepler’s law one can derive the binary mass function, which is given by (1) where Ρ is the orbital period, M1 and M2 are the masses of the compact object and companion star respectively, i is the binary inclination and K2 is the radial velocity semi-amplitude of the companion. The efficient discovery of these objects by Ginga, GRO and RXTE means that more than a dozen are now known (see Table 1). The observational details for SXTs which have been most intensively studied are summarised in Table 2.
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