Ultraluminous X-ray Sources (ULXs) and Intermediate Mass Black Holes (IMBHs) (For Journal Club)

1. Ultraluminous X-ray Sources (ULXs) andIntermediate Mass Black Holes (IMBHs) (For Journal Club) J. F. Wu Tsinghua Center for Astrophysics Department of Physics, Tsinghua University Jianfeng Wu, THCA

2. Main References • Miller M. C., & Colbert E. J. M., 2004, International Journal of Modern Physics D, 13, 1, (astro-ph/0308402, final accepted version) • Makishima K. et al., 2000, Astrophys. J.,535, 632 Jianfeng Wu, THCA

3. Contents I. Introduction II. Ultraluminous X-ray Sources III. Black Holes in Globular Clusters and as MACHOs IV. Formation Mechnisms for IMBHs V. Alternative Explanations of ULXs VI. Implications of IMBHs VII. Outlook Jianfeng Wu, THCA

4. Introduction Description of Black Holes: Kerr-Newman Metric: “No hair” theorem: three parameters to describe the black hole completely —— Mass M —— Angular Momentum a = J/M —— Charge Q Only the first two are significant for real black holes: Schwarzschild black hole (non-spinning) and Kerr black hole (spinning). Jianfeng Wu, THCA

5. Introduction • Stellar-mass Black Holes in X-ray Binaries: mass estimated from the careful radial velocity measurement of the companion • Supermassive Black Holes in the Center of Galaxies: tremendous mass in very small region e.g.: M87 Two Distinct Populations of Black Holes: (Greiner et al., 2001) (Orosz 2002) (Tegmark2002) Jianfeng Wu, THCA

6. Introduction • Intermediate Mass Black Holes (IMBHs): Suspicion: —— IMBHs may form in the center of dense clusters Observations: —— Ultraluminous X-ray Sources ( ULXs ) —— an excess of dark mass in the cores of globulars Jianfeng Wu, THCA

7. Ultraluminous X-ray Sources • Preparation: • ISCO ( Innermost Stable Circular Orbit ): Black hole with an accretion disk is usually an X-ray source. The inner edge of the accretion disk is near to the innermost stable circular orbit, which is 3 Rs for the Schwarzschild black holes while 0.5 ~ 4.5 Rs for the Kerr black holes. Jianfeng Wu, THCA

8. Ultraluminous X-ray Sources • Preparation: • Eddington luminosity: Isotropic assumption for radiation: Radiation force no greater than the gravity: Jianfeng Wu, THCA

9. Ultraluminous X-ray Sources • Definitions: Eddington luminosity limits the bolometric energy output of the object. The X-ray luminosity in 2 – 10 keV band will be a factor of a few – 10 times smaller. So: The lower limit to the X-ray luminosity for a ULX is 1039.0 ergs s-1. The upper limit is not specified but for usually objects Lx < 1040.5 ergs s-1. “Ultraluminous” is with respect to normal X-ray binaries. Another name for ULXs is “Intermediate-luminosity X-ray Objects” (IXOs), indicating their luminosities are intermediate between those of normal stellar-mass black hole X-ray binaries and AGNs. Jianfeng Wu, THCA

10. Ultraluminous X-ray Sources • Historical observations: Einstein and ROSAT • Einstein:detected central X-ray Sources with luminosity > 1039 ergs s-1, but cannot tell whether sources are single or multiple, or whether really coincide with the nuclei because of the bad angular resolution (1' ). • ROSAT: • not coincident with the nucleus • presented in every five nearby galaxies on average • brightest ULXs Lx ~ 9 ×1040ergs s-1, implying mass > 700 M☉ • locations in galaxies: —— spiral: near but distinct from the dynamical center —— elliptical: almost exclusively in the halos Jianfeng Wu, THCA

11. Ultraluminous X-ray Sources • X-ray Energy Spectra of ULXs • Multicolor Disk (MCD) Blackbody Model: each annulus of the accretion disk is assumed to radiate as a blackbody with a radius-dependent temperature the inferred temperature Tin of the innermost portion of the disk is related to the mass of the black hole: Jianfeng Wu, THCA

12. Ultraluminous X-ray Sources • X-ray Energy Spectra of ULXs • ASCA Spectral Modeling of ULXs: Makishima et al., 2000, Astrophys. J., 535, 632 usually with a ( soft ) MCD component for the disk emission, plus a (hard) power-law component for the presumedly Comptonized disk emission High Temperature Problem: fitting kTin≈ 1.1 – 1.8 keV, while Galactic stellar mass BHXBs typically kTin≈ 0.4 – 1 keV Solutions: —— Stellar mass but beaming —— Modify thin disk model: change 2, not effective —— Use slim disk model: not effective —— Kerr black holes: demand high inclination angle KEY: ULXs are not well represented by a simple MCD disk model after all. e.g.: CMCD for XMM-Newton Spectra: 0.05 – 0.3 keV ( Wang et al. 2004) Jianfeng Wu, THCA

13. Ultraluminous X-ray Sources • X-ray Energy Spectra of ULXs • XMM-Newton and Chandra Spectral Modeling of ULXs: Often fit with a single model ( either MCD or power-law) —— Power-law with  ~ 5: super-soft —— Power-law with  ~ 2: may actually be background nuclei —— MCD model: for spectra with MCD component: no high temperature problem (~ 0.1 keV) —— Fe K lines (6.4 -7.0 keV, M82): not favor of beaming mechanism but indirect evidence for IMBH Note: nearly all the spectra of ULXs now are in spiral galaxies, for those in ellipticals may have harder spectra. CMCD Fitting for NGC 1313 X-1, Tin = 0.199 keV ( Wang et al., 2004) Jianfeng Wu, THCA

14. Ultraluminous X-ray Sources • ULXs and Host Galaxy Type: • Spiral & Starburst Galaxies: Antennae and Cartwheel —— ULXs are directly related to young star population —— ULXs may be a special type of HMXB with beamed X-ray emission • Elliptical Galaxies: —— elliptical galaxies with ULXs have a larger number per galaxy than do the spiral galaxies with ULXs —— HMXB scenario not for all ULXs Chandra Spectra show that ULXs in ellipticals are distinct from those in spirals. Jianfeng Wu, THCA

15. Ultraluminous X-ray Sources • X-ray Variability of ULXs: • Long-term ( months or longer ) variability of ULXs in many nearby spiral galaxies: —— reject the system or group scenarios • Variability with time scale less than a few week: —— tempting to interpret as orbital periods • Variability with short timescales ( second to minutes): ——the same level of fractional rms amplitude as variability on longer timescales, variability at the few percent level would be detectable out to the Nyquist frequency of observations of the brightest ULXs Jianfeng Wu, THCA

16. Ultraluminous X-ray Sources • X-ray Variability of ULXs: • QPOs: one case of 54 mHz ( Strohmayer & Mushotzky 2003) —— disfavor of beaming scenario: “if the source is really a beamed stellar-mass black hole, the variability in the disk emission (which is nearly isotropic) would have to be of enormous amplitude to account for the observations” Combined spectral and temporal analysis —— Powerful tool in diagnosing ULX emission processes normal BHXB: soft spectra in high state, hard spectra in low state anomalous : soft spectra in low state, hard spectra in high state ( Antennae) —— microquasars ? Jianfeng Wu, THCA

17. Ultraluminous X-ray Sources • Multiwavelength associations: study the environment of ULXs ( companion, accretion disk and jets) • Optical counterparts: strong link between ULXs and star clusters: —— spirals: star-forming region, young cluster with O giants/supergiants: HMXB scenario —— ellipticals: globular clusters • Radio counterparts: just beginning Kaaretet al. (2003), NGC 5408 X-1: relativistically beamed jet emission —— microblazar? Jianfeng Wu, THCA

18. Black Holes in Globular Clusters and as MACHOs • Observational Evidences for IMBHs in centers of globular clusters: promising but not compelling • Detailed modeling of properties of individual objects: M/L • Core rotation detecting: —— IMBH in a binary system with a stellar mass black hole —— a massive black hole binary in the core • X-ray Observations: Bondi-Hoyle accretion onto the central black hole can produce emission in various bands, the most prominent perhaps being X-rays and radio. • Microlensing detection: Jianfeng Wu, THCA

19. Formation Mechanisms for IMBHs • Stellar mass limit —— not from core collapse recently • Black holes in very early universe: prior to Big Bang nucleosynthesis, lock matter in non-baryonic form —— horizon mass increase: transition at uncomfortably low energies —— perturbation spectrum strongly peaked and finely tunned • Population III Stars: • Conditions: above 250 M☉directly collapse —— large Jeans mass: T3/2 —— zero metallicity star: little loss mass (insignificant winds, weak pulsations) • Problems: lack of observational constraints on Population III star —— Cooling: mass cannot reach several hundred solar masses —— number of zero metallicity stars: Jianfeng Wu, THCA

20. Formation Mechanisms for IMBHs • Growing in Dense Stellar Cluster: • Dynamics in stellar cluster: —— more massive stars & binaries tend to sink towards the core —— three body interactions: one single and one tightened binaries (collision and merge) —— IMBHs captured and sink towards the core of young stellar cluster ( X-ray source) • In globular clusters: —— Merging of binaries: kick outside the cluster —— Capture of a stellar mass black hole around the IMBHs: —— Tightening if a BH/BH binary by a Kozai resonance • In young clusters: —— Multiple collision to a given ( central ) object: associations between ULXs and star formation region —— Questions and simulations: promising Jianfeng Wu, THCA

21. Alternate Explanations for ULXs • Beaming: the flux along the axis of symmetry can be enhanced by a factor of tens compared to the isotropic Eddington flux in spirals —— beamed sources involving HMXBs in ellipticals —— beamed sources involving LMXBs • Advantages: —— based on known source —— explain the associations of ULXs and star forming regions in spirals • Challenges: —— ULXs with no such rapid variability as stellar mass BHXBs ( e.g. Cyg X-1, 100 Hz ) —— cannot explain the QPOs —— theoretical basis of relativistic outflow are not well established Jianfeng Wu, THCA

22. Alternate Explanations for ULXs • Super Eddington Emission: • Magnetic field: B > 1013G, suppress the Thomason Scattering —— neither black hole or accretion disk have no required magnetic field • Supernovae: enormous accretion rate for principal neutrino emission —— unimportant for normal accretion in X-ray Binaries • Anistropy: radiation causes accretion matter to clump which linked by weak magetic field ( simulation ), radiation moves from low density medium —— total luminosity < 10 times Eddington limit, still need high mass Jianfeng Wu, THCA

23. Alternate Explanations for ULXs • Motivation for stellar mass models: • High temperature problem: —— unwarranted, need careful examination on ULX spectra • Luminosity function shows no evidence for a new component: not compelling —— number of ULXs so small with huge error bar —— any change in source population would change the slope • IMBH can’t evolve in a binary: orbit period excess a year —— capture companion in star cluster • IMBH can’t grow in clusters: binary-single interaction prevent —— direct collision & binary-binary interactions • IMBH can’t separate from clusters: supernova kicks can’t —— three body interaction kicks Jianfeng Wu, THCA

24. Alternate Explanations for ULXs • Motivation for stellar mass models: —— No definitive observations exist for any single ULX, let alone the class of ULXs, that rule out intermediate-mass black holes, or beaming, or super-Eddington emission. —— the current disagreements exist because the crucial parameter — the mass — has not been measured observationally. —— one cannot make definitive statements about the entire class of ULXs, so it may be that some ULXs conform to each of the models proposed. Jianfeng Wu, THCA

25. Implications of IMBHs • Formation of SMBHs: • IMBHs sink to the center of galaxies —— seeds for SMBHs by gas accretion • Coalescence of IMBHs —— angular momentum —— accretion • Gravitational Radiation Sources • Coalescence in stellar clusters • Inspiral of stellar mass objects into IMBHs: LISA • Inspiral of IMBHs into SMBHs: LISA —— Measuring spacetime near the rotating black holes Jianfeng Wu, THCA

26. Outlook • Radial velocity measurement: mass —— Optical/UV/IR companion • X-ray observations of ULXs energy spectra: • X-ray timing observations: tens of milliseconds • Multiwavelength observations of ULXs: optical counterparts, broadband spectra, overall luminosity • Kinematics of globular clusters: detection of IMBH in center • Tasks for three kinds of models: more solid theoretical bases, accounting for observations • Gravitational waves: “Debate on the nature of IMBHs will undoubtedly continue until rigorous measurements of the masses of IMBHs are possible.” ( Radial velocity measurement or gravitational wave detection) Jianfeng Wu, THCA

27. References • References: • Greiner J., Cuby J. G. & McCaughrean M. J., 2001, Nature, 414, 522 • Kaaret P., et al., 2003, Science, 299, 365 • Makishima K. et al., 2000, Astrophys. J.,535, 632 • Miller M. C., & Colbert E. J. M., 2004, International Journal of Modern Physics D, 13, 1, (astro-ph/0308402, final accepted version) • Orosz J. A., 2002, astro-ph/0209041 • Strohmayer T. E., & Mushotzky R. F., 2003, Astrophys. J.,586, L61 • Tegmark M., 2002, Science, 296, 1427 • Wang Q. D., et al., astro-ph/0403413 Jianfeng Wu, THCA

28. The End Thanks for your attention! Tsinghua Center for Astrophysics & Physics Department, Tsinghua University 100084 Beijing, P. R. China jfwu03@mails.tsinghua.edu.cn ftp://166.111.16.2/incoming/study/journal_club/Wujf_ULXs_IMBHs/ Jianfeng Wu, THCA