Observational Characteristics of X-ray Binaries

Transcription

1 Observational Characteristics of X-ray Binaries 1. Introduction More than 300 galactic X-ray binaries, with L x ~ ergs -1. Concentrated towards the galactic center and the galactic plane, some in globular clusters. The all-sky map generated by Uhuru 1

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4 Cbservational characteristics of x-ray binaries This model was confirmed by the discovery of the pulsating X-ray binary source Cen X-3 in Regular pulsations with a period of 4.84 s ( neutron star s rotational period). Both the X-ray eclipses and the pulse period have a same modulation with the period of days ( orbital period). 4

5 The mass function M sin i f ( M) = = K P (1 e ) 15.6M ( M ) 3 3 opt /2 2 X orb X + Mopt implies a lower limit of 15.6 M for the mass of the companion star of Cen X-3. Thus the X-ray pulsar is moving in a very close orbit around a massive companion star. 5

6 Masses of Compact Objects in XRBs 6

7 Classification of X-ray binaries XRBs are conventionally divided into two classes: high-mass X-ray binaries (HMXBs) and low-mass X-ray binaries (LMXBs) 7

8 Characteristics HMXBs X-ray spectra Relatively hard X-ray spectra with kt 15 kev in exponential fits or a power law energy index of ~0 1 LMXBs Soft X-ray spectra with kt ~ 5-10 kev in exponential fits Time variability Often regular pulsations; no X-ray bursts; often X-ray eclipses. Often X-ray bursts and quasi periodic oscillations; regular pulsations in a few cases 8

9 Optical counterparts Space distribution Massive (>10 M ) and early type (O and early B) stars; L opt /L x >1 Concentrated towards the galactic plane; young stellar population, age < 10 7 yr Faint blue optical counterparts (late type or degenerate stars) ; L opt /L x <0.1( low mass, <1 M ) Concentrated towards the galactic center; fairly wide spread around the galactic plane; old stellar population, age (5-15) 10 9 yr 9

10 2. Neutron star high-mass X-ray binaries Supergiant/X-ray binaries Be/X-ray binaries 10

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13 Characteristics SG/X-ray binaries Be/X-ray binaries Optical counterparts Evolved (giant, Of or blue super giant) stars, with R~10-30R, L opt > 10 5 L and M > 20M Un-evolved stars of spectral type O9Ve to B2Ve characterized by emission lines (predominantly Balmer lines), with R<5-10 R, L opt < L the Orbits Circular orbits with P orb ~ days and M ~8-20 M Eccentric orbits with P orb ~10 days to several years 13

14 Time variability X-ray eclipse and periodic ellipsoidal light variations in many cases Mass and X-ray emission transfer The optical stars fill, or nearly fill their Roche lobes; Persistent X-ray sources; Mass transfer is due to incipient Roche lobe overflow and/or capture of spherical wind Rare X-ray eclipse and periodic ellipsoidal light variations The Be stars underfill their Roche lobes; Often transient X-ray sources i ; The outbursts are due to mass accretion from the equatorial wind 14

15 X-ray luminosities Group of HMXBs Average log L x (ergs -1 ) SG/X 36.7±0.9 (0.3) Be/X in outburst 36.6±0.8 (0.3) Be/X in quiescence 33.3±1.2 (0.3) Magellanic sources 38.4±0.5 (0.2) 15

16 Pulse period distribution The pulse periods are distributed between s and 10 4 s. So far about 40 systems both reliable orbital and spin periods are known. In the P s -P orb diagram (Corbet diagram) LMXBs, supergiant HMXBs and Be/X-ray binaries distribute in different regions. For Be/X-ray binaries there is a strong correlation between P s and P orb. 16

17 The Corbet diagram 17

18 INTEGRAL X-ray Binaries (astro-ph/ , ) The INTEGRAL observatory has performed a detailed survey of the Galactic plane. The most important result of INTEGRAL is the discovery of many new high-energy sources concentrated in the Galactic plane, and in the Norma arm. Many of them are HMXBs hosting a neutron star orbiting around an O/B companion, in most cases a supergiant star. 18

19 They divide into two classes: (1) Obscured HMXBs exhibiting a huge intrinsic and local extinction; 19

20 (2) Supergiant Fast X-ray Transients (SFXTs) exhibiting fast and transient outbursts They sporadically undergo bright flares up to peak luminosities of erg s 1, with duration of a few hours for each single flare. In quiescence the luminosity ca be as low as erg s 1. (astro-ph ) 20

21 Pulse period changes Long term monitoring of the pulse periods of X-ray pulsars have revealed three types of behavior: For disk-fed sources, a secular decrease with time (spin up) with erratic variations around the trend. 21

22 Observations with CGRO show that the long-term spin-up trend is in fact the result of alternating ( ) day - (10-20) yr intervals of steady spin-up and spin-down with a magnitude several times larger than the long-term spin-up trend! This has challenged the traditional model for neutron star accretion disk interaction. 22

23 For wind-fed sources, random walk or secular spin-down. Probably they are also undergoing rapid switching behavior. 23

24 For transient sources, rapid spin-up during (giant) outbursts and spin-down during quiescence.. 24

25 Pulse profiles There are dramatic differences in pulse shape and amplitude between one pulsar and another. In some cases, the pulse profile shows a dependence on energy and luminosity. 25

26 The pulse profiles of many pulsars can be reproduced simply by a beam pattern. If the angles of the magnetic axis and the light of sight with respect to the rotation axis are α and β, respectively, then depending on whether α+β <π/2, or >π/2, one or both magnetic poles are visible. This will lead to a single pulse or double pulses. ( 26

27 If the beam has a maximum either along or perpendicular to the magnetic axis, the beam can be described as a pencil beam or a fan beam respectively. In some cases an offset of the magnetic axis and/or two polar cap emission regions with different sizes are required to give an asymmetric pulse profile. 27

28 Pulsar spectra Continuum: The energy spectrum of an X-ray pulsar is characterized by α N0E E Ec N( E) = α N0E exp[ ( E Ec) / E f ] E > Ec where N is the photon number, the power-law energy index α = Typical values for the cutoff energy E c and E f both lie in the range kev. 28

29 Other spectrum components: Soft X-ray absorption by circumstellar matter hydrogen column density N H distance. Fluorescent iron lines at ~ kev, due to the fluorescent emission from less ionized iron in the cool, circumstellar matter. 29

30 Cyclotron lines detected at energies ~ kev (E 0 =11.6B 12 /(1+z) kev), corresponding to neutron star surface magnetic field strengths of (1-10) G. The spectrum of Her X-1 with a cyclotron absorption line feature at around ~38 kev. 30

31 The cyclotron line energy vs. the cutoff energy 31

32 3. Neutron Star Low-mass X-ray binaries (LMXBs) HMXBs LMXBs 32

33 Space distribution The LMXBs contain the globular cluster X-ray sources, X-ray bursters, soft X-ray transients, and the bright galactic bulge X-ray sources. There are tens of luminous (>10 35 ergs -1 ) X-ray sources located in globular clusters, which are two orders of magnitude more than expected from the total mass in globular clusters relative to that in the Galaxy (Katz 1975). Tidal capture or exchange encounter in cluster core may favor the production of X-ray binaries (Fabian, Pringle, and Rees 1975). 33

34 Orbital period distribution The orbital periods range from ~12 min to 17 days. Comparison of the distribution of orbital periods of LMXBs and cataclysmic variables (CVs) shows that, there are very few CVs with P orb < 1 hr, and within the period gap between 2 and 3 hrs; in the case of the LMXBs the period gap may extend down to < 1 hr (why?). 34

35 X-ray orbital light curves Eclipses in LMXBs are fewer than expected if the mass transfer occurs via a thin accretion disk. Thick accretion disks block the X-ray sources in edge-on systems. The accretion disk is thick because the incoming gas stream creates turbulence at the outer edge of the disk. 35

36 Dips in light curves are ascribed to material that is projected up above the disk plane by a splash point where the gas stream hits the accretion disk. 36

37 For some systems that are viewed almost edge on, the compact X-ray source is hidden by the disk, but X-rays are still seen because they are scattered in a photo-ionized corona above the disk (accretion disk corona, ADC). This makes the source appear extended and results in partial eclipse. 37

38 The observed properties of an LMXB depend on the viewing angle Viewing angle Low inclination (<70 ) Intermediate inclination (70-80 ) High inclination (>80 ) Properties No X-ray dips or eclipses, optical modulation (from the heated companion). Periodic dips, sometimes brief eclipses. Partial eclipses of ADC. 38

39 Spectral emission NS LMXBs are divided into Z and atoll sources, according to their patterns in the X-ray colour-colour diagrams (CDs). Soft color: log of count ratio ( )/( ) kev Hard color: log of count ratio ( )/( kev 39

40 Z sources display the horizontal branch (HB), the normal branch (NB) and the flaring branch (FB); atoll sources follow a curved branch, with a banana at the right and one of more islands at the left. The motion of an LMXB through the CD patterns is one-dimensional: a source always moves smoothly following the pattern rather jumps through the diagram. 40

41 It is believed that the position and sequence of the pattern is reflected by the mass accretion rate and its change (?). The optical and UV emission suggest that for atoll sources, mass accretion rate increases from the island to the left of the banana branch and then from left to right along the banana, and mass accretion rate increases in the sense HB NB FB for Z sources (?). 41

42 (1) High luminosity/z-source systems The continuum spectra can be represented by a two component model: isothermal blackbody from a boundary layer, and the sum of blackbodies from a multi-color disk, or a Comptonized disk spectrum. 42

43 (2) Low luminosity/atoll systems The spectra are harder than in Z sources, and are modeled as simple power-law spectra with energy index ~1 and an exponential high-energy cutoff with a temperature of 5-20 kev. (3) Line emission. Iron K line emission at 6.7 kev was observed in LMXBs, most likely arising in a photo-ionized ADC. 43

44 (4) Optical emission Typical L x /L opt ratio ~ , except for the ADC sources, which have ratio of L x /L opt ~20 (because the central X-ray source is hidden). Most of the optical emission of LMXBs originates in an accretion disk as in CVs. Contrary to CVs, whose disks radiate internally generated energy, the main source of optical emission of LMXBs is the absorption of X-rays by the accretion disk and the subsequent re-radiation of this energy as low energy photons (reprocessing of X-rays). 44

45 X-ray bursts (1) Type I bursts Burst profiles Rising times <1 s to 10 s, decaying times ~ 10 s to minutes. X-ray spectra The time-dependent spectra are well described by a blackbody spectrum with an approximately constant radius and a decreasing temperature during burst decay. 45

46 The blackbody temperature can be used to determine the apparent blackbody radius of the burst-emitting region through 4 1/ 2 R = d( F / σt bb bol ) In some bursts the Eddington critical luminosity is exceeded and atmospheric layers are lifted off the star s surface, leading to a photospheric expansion, followed by a gradual recontraction while the luminosity remains close to the Eddington limit. 46

47 Burst intervals The longer the waiting time, the higher the burst fluence. 47

48 Interpretation Thermonuclear explosions of H and/or He on NS surface. Type I bursts are a distinct feature of neutron stars (Narayan & Heyl 2003, ApJ, 599, 419). 48

49 Burst energetics The total amount of energy emitted is ~ ergs. The observed ratio α of the luminosity in persistent emission between bursts and in X-ray bursts range from ~10 to Theoretically, 2 Lp ηgmc & α = = ~ (25 100)( M / M )( R/10 km) 2 Θ Lb ηn Mc & The lower and upper values correspond to hydrogen and helium burning respectively. 49

50 Mass-radius relation of neutron stars For a uniform spherical emitter of radius R, at a distance d, we have L b = 4π R σt = 4πd Fb Since the energy of each photon and also the rate at which photons arrive undergo gravitational redshift, 2 1/2 R = R(1 + z) = R[1 2 GM /( Rc )] Thus the measurement leads to information about the radius R and the mass M of the neutron star, and thereby the EOS of neutron star matter. 50

51 (2) Type II X-ray bursts Type II X-ray bursts have been observed in the Rapid Burster (MXB ) and possibly in GRO J For the RB both type I and II bursts have been observed. Burst duration from ~2 s to ~ 11 min, interval from ~1 s to ~1 hr. The fluence in a type II burst is approximately proportional to the interval to the next type II burst. 51

52 Spectra The burst peak luminosities range from ~ to ~ ergs -1. The spectra can be approximated by that of a blackbody with constant temperature ~1.8 kev (no spectral softening during burst decay). Interpretation Accretion disk instability (?) 52

53 Quasi-periodic oscillations (QPOs) QPOs are intensity fluctuations with a preferred frequency. Approximately symmetric peaks in the power spectrum whose ratio of full width at half maximum (FWHM) and centroid frequency does not exceed 0.5 (broader features are termed noise). 53

54 There are several kinds of QPOs in NS LMXBs. 54

55 (1) Horizontal-branch QPOs (HBOs): QPO frequency ranges from 5 to 60 Hz. There is a positive correlation between the HBO frequency and the X-ray intensity. (2) Normal- and flaring-branch QPOs (NBOs and FBOs). NBOs frequencies ~ 4.5 and 7 Hz. FBOs occur on a small part (~10% of the total extent) of the FB nearest the NB. Their frequencies increase from ~ 6 Hz near the NB-FB junction to ~20 Hz up the FB. 55

56 (3) Millisecond oscillations (i) Millisecond pulsations (astro-ph/ ) 56

57 (ii) Burst oscillations In the initial phase of type I bursts when the burning front is spreading, the energy generation is inherently very anisotropic (also due to magnetic fields and patchy burning), leading to periodic or quasi-periodic phenomena. 57

58 It is widely accepted that the burst oscillations arise due to a hot spot or spots in an atmospheric layer of the neutron star rotating slightly slower than the star itself because it expanded by 5-50 m in the X-ray burst but conserved its angular momentum. So the oscillation frequency (or its sub-harmonics) represents the spin frequency of the neutron star. 58

59 (iii) Kilo-Hertz QPOs More than 20 sources have shown khz QPOs. There are two simultaneous QPO peaks ( twin peaks ) in the Hz range. Sco X-1 4U

60 The frequency of both peaks usually increases with X-ray flux, with the peak separation Δν remaining roughly close to (but not equal to) ν s or ν s /2. 60

61 On timescales of hours to a day, khz OPO frequency usually increases with X-ray flux. But the same positive relation does not apply to another source or the same source with longer timescale. 61

62 Saturation of khz QPO frequency (?) 62

63 There are good correlations between khz QPOs and low-frequency phenomena in WDs, NSs and BHCs, implying a common mechanism for the origin of the QPOs in these systems. 63

64 Theoretical models (a) The beat-frequency model (Miller et al. 1998, ApJ, 508, 791) The frequency ν 2 of the upper peak is the Keplerian orbital frequency ν orb of accreting matter at some preferred radius (e.g. the magnetospheric radius or the sonic radius) in the accretion disk. The lower peak frequency ν 1 is the beat frequency between ν 2 and the spin frequency ν s, so ν 1 = ν orb ν s, but this relation is inconsistent with observations (Δν changes with ν 1 or ν 2 ). 64

65 65

66 (b) The relativistic precession disk model (Stella & Vietri 1998, ApJ, 492, L59; 503, 350) Inclined eccentric free-particle orbits around a spinning neutron star show both nodal precession (a wobble of the orbital plane) due to relativistic frame dragging and relativistic periastron precession. The frequency ν 2 of the upper peak is the orbital frequency ν orb of accreting matter at some radius. The lower peak frequency ν 1 is the periastron precession of the orbit. 66

67 67

68 (c) The two-oscillator model (Titarchuk & Osherovich, 1999, ApJ, 518, L95; Osherovich & Titarchuk 1999, ApJ, 523, L73) The lower peak frequency ν 1 is the Keplerian frequency at the outer edge of a viscous transition layer between the disk and neutron star surface. Blob being thrown out of this layer into the magnetosphere oscillate both radially and perpendicular to the disk, producing two harmonics of another low-frequency QPO as well as the upper khz peaks: 68

69 69

70 (d) The MHD oscillation model (Zhang 2004; Li & Zhang 2005; Shi & Li 2009) The upper and lower khz QPO frequencies are identified to be the rotational frequencies and the MHD (e.g. standing kink modes of) loop oscillations at the inner edge of the accretion disk, respectively. 70

71 4. Black Hole binaries Discovery of the black hole (BH) nature of Cyg X-1 in

72 Casares (2006) 72

73 (Remillard & McClintock 2006 ARA&A) 73

74 Soft X-ray transients A large fraction of the BHC are X-ray transients, called soft X-ray transients or X-ray novae. The X-ray flux increases by more than two orders of magnitude within several days. The flux declines on time scales of several tens of days to more than one hundred days. 74

75 Many (possibly all) transients are recurrent. The intervals between outbursts vary from less than one month to tens of years or more, the duty cycle There are similarities between the outbursts of LMXBs and those of dwarf novae (DNe). Thermal disk instability model has been proposed for transient outbursts. 75

76 bservational characteristics of x-ray binaries BH diagnostics (1) Lack of pulses and Type I X-ray bursts. (2) Spectra Ultrasoft spectra NS LMXBs can have very soft spectra (e.g. Cir X-1) though such soft spectra in NS systems are much less common. High energy power law tail above 20 kev. High-soft and low-hard states (transition around ~10 37 ergs -1 ). 76

77 (3) For a given orbital period, quiescent BH binaries are ~100 times dimmer than quiescent NS binaries (from astro-ph/ ) 77

78 State transitions Spectra of XTE J (from astro-ph/ ) See, however, Miller et al (ApJ, 652, L113; 653, 525) for possible presence of a cool disk component in low/hard state. 78

79 In a BH transient outburst sources tend to follow a harder trajectory from low to high luminosity, and make the main hard-soft (LS HS) transition at much higher luminosity than the soft-hard (HS LS) one, behavior that is often called hysteresis. 79

80 Variability Large amplitude (rms variation ~30%) flickering, QPOs. Low-frequency QPOs (LFQPOs; roughly Hz) have been detected on one or more occasions for 14 BHBs They are seen in the Steep Power-law (SPL) state, the hard:spl intermediate state, and in some hard states, particularly when the X-ray luminosity is high. This behavior clearly ties LFQPOs to the non-thermal component of the X-ray spectrum. 80

81 High-frequency QPOs (HFQPOs; Hz) have been detected in seven sources. They do not shift freely in frequency in response to sizable luminosity changes. All of the strong detections of HFQPOs above 100 Hz occur in the SPL state. 81

82 Name BH Mass (Msun) HF QPO frequencies (Hz) GRO J ~6 300, 450 XTE J ~10 184, 276, 188, 249~276 GRS ~14 41, 67, 113, 165, 328 H ~ 160, 240, 166, 242 XTE J ~9 150~200 4U ~ 184, 100~300 XTE J ~ 110~270 82

83 Microquasars 83

84 Measuring Black Hole Spin (1) Polarimetry - a gradual change of the plane of polarization with energy is a purely relativistic effect, and the magnitude can give a direct measure of a (2) Continuum Fitting Continuum radiation R in R ISCO a 84

85 (3) Fe K line profile 85

86 (4) High Frequency Quasi-Periodic Oscillations 86

87 5. Luminous X-ray sources in galaxies X-ray source population in galaxies It was a surprise to find with Einstein observations that many normal spiral galaxies also had central X-ray sources. 87

88 The subsequent X-ray observatories ROSAT and ASCA expanded our knowledge of the X-ray properties of galaxies, but did not produce the revolutionary leap originated by the first Einstein observations. Only with Chandra s sub-arcsecond angular resolution, populations of individual X-ray sources can be separated from the diffuse emission of hot interstellar gases, both spatially and spectrally at the distance of the Virgo Cluster and beyond. 88

89 NGC 1313 Observed with Einstein, ROSAT, Chandra 89

90 Two typical observations of galaxies with Chandra: the spiral M83 and the elliptical NGC

91 X-ray luminosity Functions (XLFs) XLFs have been used to characterize different XRB populations in the Milky Way, but these studies have always suffered the distance uncertainty. External galaxies, instead, provide clean source samples. Moreover, the detection of X-ray source populations in a wide range of different galaxies allows us to explore global population differences connected with the age and/or metallicity. 91

92 LMXBs in Early-type galaxies LMXBs can account for a large fraction of the X-ray emission of some early type galaxies. 92

93 Two breaks have been reported in the XLFs of E and S0 galaxies: the first is a break at ~2-5 x ergs -1, near the Eddington limit of an accreting neutron star, which may be related to the transition in the XLF between neutron star and black hole binaries. 93

94 Association of LMXBs with globular clusters (GCs) Sarazin et al. (2003) point out that the fraction of LMXBs associated with GCs increases from spiral bulges (MW, M31 ~10-20%), to S0s ~20%, E ~50%, and cd~70%, suggesting that most of LMXBs may originate from GCs. (Pooley et al ApJ 591, L131) 94

95 Other authors conclude that the relationship between the fraction of LMXBs found in GCs and the GC specific frequency is consistent with the simple relationship expected if field LMXB originate in the field while GC LMXB originate in GCs (Juett 2005; Irwin 2005). 95

96 LMXBs preferentially are found in luminous GCs, and in red, younger and/or metal rich, clusters (V-I >1.1), rather than in blue, older and/or metal poor, ones. (Kundu, Maccarone & Zepf 2002, 2003). (Age or metallicity effect?) 96

97 Young XRB populations Luminous HMXBs are expected to dominate the emission of star forming galaxies. These sources, resulting from the evolution of a massive binary system where the more massive star has undergone a supernova event, are short-lived (~ yr), and constitute a marker of recent star formation: their number is likely to be related to the galaxy star formation rate (SFR). 97

98 The HMXB XLF is overall flatter than that of LMXBs, with a cumulative power-law slope of 0.6 to 0.4; in other words, young HMXBs populations contain on average a larger fraction of very luminous sources than the old LMXB populations. 98

99 Grimm, Gilfanov & Sunyaev (2003) suggest that there is a universal XLF of star-forming populations with a simple power-law with cumulative slope

100 Both the number and total luminosity of HMXBs in a galaxy are directly related to the SFR and can be used as an independent SFR indicator. for for 100

101 The differences of the XLF in different regions of a galaxy, and in galaxies with different SFR 101

102 These differences may be related to the aging of the X-ray source population, which will be gradually depleted of luminous young (and short-lived) sources associated with more massive, faster-evolving, donor stars, and also to metallicity effects. 102

103 bservational characteristics of x-ray binaries Ultraluminous X-ray sources (ULXs) ULXs are the most luminous point-like extra-nuclear X-ray sources found in nearby galaxies. Observed (isotropic) X-ray luminosities in excess of ergs -1, the Eddington luminosity for a 10 M BH L = 4πGMm / σ ( M / M ) ergs E p T Θ 103

104 Location in galaxies ULXs are associated with both star-forming regions in spiral and irregular galaxies, and the old stellar population in elliptical galaxies. In spirals, ULXs are often near, but distinct from the dynamical centers of the galaxies. In ellipticals, ULXs are almost exclusively in the halos of the galaxies. 104

105 The brightest ULXs are in the brightest FIR galaxies. The mean X-ray luminosity of ULXs in elliptical galaxies is less than in spiral galaxies. Swartz et al

106 Association of ULXs with active star-forming stellar populations 106

107 Spectra and spectral variability The ASCA X-ray spectra generally consist of a power law (Γ~1.2) and a disk-blackbody component with T in = kev. They also show spectral transitions between soft/high and hard/low states, similar to the state changes observed in Galactic black hole binaries. 107

108 XMM-Newton spectra of two ULXs in NGC 1313 (X-1 and X-2) led to highly significant detections of soft accretion disk components, with temperatures of kt~150 ev, consistent with accretion disks of IMBHs (Miller et al. 2003, 2004). KT m& m in 1/2 1/4 1/4 α 108

109 Intermediate Mass Black Holes or XRBs? (1) Stellar-mass BHs with real super-eddington X-ray emission (Begelman 2002) or anisotropic emission (King et al. 2001) or relativistically beamed emission (Kordng, Falcke & Markoff 2002) L X =bl sph =10 40 bl 40, b=ω/4π, or b= [γ(1-βcosθ)] -p (2) Intermediate mass ( M ) BHs with sub-eddington luminosities (IMBHs, Colbert & Mushotzky 1999; Miller & Hamilton 2002). 109

110 Problems: Modeling ULX spectra Eddington limit practical? Alternative models (1) Stellar-mass black holes accreting from the supernova fallback disk (Li 2003) (2) IMBHs accreting from molecular clouds (Krolik 2004) (3) Young pulsars (Perna and Stella 2004) 110

111 Evidence for the Beaming Model Many ULXs are associated with star forming regions. Similar characteristics with the Galactic microquasar SS433 and GRS Possible massive optical counterpart observed. Jet from a ULX discovered in the dwarf irregular galaxy NGC (Karret et al. 2003) 111

112 LMXBs as luminous X-ray sources (1) Transient (long orbital period) LMXBs Trudolyubov & Priedhosky (2004) report only one recurrent transient in their study of GC sources in M31, although 80% of these sources show some variability; however, they also find six persistent sources in the ergs -1 luminosity range. (2) Ultra-compact binaries (3) Persistent LMXBs with a CB disk 113

113 A black hole in a globular cluster? (astro-ph/ ) In a globular cluster in the Virgo Cluster giant elliptical galaxy NGC X-ray luminosity of about ergs -1, varying by a factor of 7 in a few hours. 114

114 Difficulties for the Beaming Model (1) Discovery of 54 mhz QPO and broad Fe K line from a ULX in the starburst galaxy M

115 (2) The ~7 hr periodic dipping/eclipsing modulation seen in a putative ULX in the Circinus galaxy (Bauer et al. 2001) 116

116 (3) Emission nebulae of a few hundred pc diameter are found to be present at or around several ULXs 117

117 The problem(s) with IMBHs (1) The XLF has an unbroken power-law form for 5 decades up to a luminosity of~ ergs -1, this break occurs at ~10% of the Eddington luminosity for the~1000m black holes. (2) Association of ULXs with star formation. 118

118 Formation Scenarios for IMBHs (1) Merging of stars in a young dense stellar cluster followed by direct collapse into an IMBH (Portegies Zwart et al. 1999). (2) Merging of binaries that have a black hole with initial mass of ~50 M in a globular cluster (Miller & Hamilton 2002). (3) Evolution of primordial population III stars (Madau & Rees 2001). 119

119 Most ULXs could be explained by stellar-mass black holes that are either super-eddington, or subject to some sort of beaming. However, it is still diffcult to reconcile the most extreme ULXs - those above ergs -1 with simple stellar-mass black hole systems. 120

120 121

121 Are IMBHs more likely to be transients? (Kalogera et al. 2004) 122 (Li 2004)

122 Summary There may exist several types of ULXs with different nature. Most of the ULXs are likely to be XRBs, but the most luminous X-ray sources may be IMBHs. Future investigations may focus on: radial velocity measurements; X-ray observations of the energy spectra; X-ray timing observations; multiwavelength observations; theories of BH formation and evolution. 123

123 References 1. Bhattacharya, D. and van den Heuvel, E. P. J. 1991, Phys. Rep., 203, 1 2. Lewin, W. H. G., van Paradijs, J., and van den Heuvel, E. P. J. 1995, X-ray binaries (chapters 1-4, 6) 3. Nagase, F. 1989, PASJ, 41, 1 4. Bildsten, L. et al. 1997, ApJS, 113, Van der Klis, M. 2000, ARA&A, 38, 717 (astro-ph/ ) 6. Tanaka, Y. and Shibazaki, N. 1996, ARA&A, 34, Remillard, R. D. and McClintock, J. E. 2006, ARA&A, (astro-ph/ ) 8. Casares, J. 2007, astro-ph/ Miller, J. M. 2007, astro-ph/

124 10. Fabbiano, G. 2006, astro-ph/ Fabbiano, G. and White, N. E. 2006, astro-ph/ Miller, M. C. and Colbert, E. J. M. 2003, astro-ph/ Roberts, T. P. 2007, astro-ph/ i Be/X-ray binaries are so called hard transients. They may be completed unobservable for periods ranging from months to years, and then suddenly turn on as a strong X-ray source for a duration of weeks to months. There are two classes of outbursts in Be/X-ray binaries. Type I outbursts are qusi-periodic, due to the neutron star s motion in an eccentric orbit. Type II outbursts are irregular, which may be caused by mass ejection from the Be star. But there are a few sources like X Per are persistent with low and stable X-ray luminosity. 125