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Ultraluminous X-ray Sources in Nearby Galaxies

Ultraluminous X-ray Sources in Nearby Galaxies. Q. Daniel Wang (Univ. of Massachusetts, Amherst) In collaboration with Yangsen Yao, David Smith, Yu Gao, etc. M51: X-ray sources & H  image ( Terashima & Wilson 2003): Large, medium, and small circles: L(0.5-8 keV) > 10 39 ,

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Ultraluminous X-ray Sources in Nearby Galaxies

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  1. Ultraluminous X-ray Sources in Nearby Galaxies Q. Daniel Wang (Univ. of Massachusetts, Amherst) In collaboration with Yangsen Yao, David Smith, Yu Gao, etc.

  2. M51: X-ray sources & H image (Terashima & Wilson 2003): Large, medium, and small circles: L(0.5-8 keV) > 1039, (5-10) × 1038, and (1-5) × 1038 erg/s Ultraluminous X-ray sources (ULXs) are extra-nuclear persistent point sources, each with isotropic Lx > (1-3) x 1039 erg/s, or > the Eddington luminosity of a ~10 Msun object. Not seen in Local Group galaxies (probably except for GRS 1915+105, Lx~1039 erg/s; MBH ~ 14 Msun; Grener et al. 2001).

  3. Why are ULXs interesting? • The brightest X-ray sources in galaxies (except for AGNs) • Potentially intermediate-mass black holes (IMBHs) • a link between stellar and supermassive BHs • probably with a cosmic mass density > that of supermassive BHs • Remnants of Pop III stars and/or formed in star cluster? • Impacts on the ISM • associated with very energetic structures • Acceleration of cosmic rays?

  4. Outline • Brief history • Where to find ULXs? • Nature of ULXs: stellar mass BHs or IMBHs? • X-ray Properties • Temporal • Spectral: Comptonized multi-color disk (CMCD) modeling • Evidence for IMBHs • How to form IMBHs? • ULXs and their environs • Summary and Future • Recent Review papers: • Miller & Colbert (2003) • Van der Marel (2003)

  5. Brief History of ULX study • Discovered with Einstein X-ray Observatory (Long et al. 1983; Fabbiano 1998) • A few were characterized with ROSAT and ASCA (e.g., Colbert & Mushotzky 1999; Makishima et al. 2000) • Chandra  accurate positioning for IDs • XMM-Newton  good S/N for spectral and timing analysis • Recent extensive multi-wavelength observations and theoretical studies

  6. Where to find ULXs? • The ULX rate (Bregman & Liu 2004): • 0.29±0.08 ULXs per 1010 Lo,sun for spirals • 0.02±0.05 ULXs per 1010 Lo,sun for ellipticals • Tend to be associated with SF regions • Brighter ULXs tend to be found in outskirts of galaxies: • e.g., M81 X-9 (Wang 2002), Cartwheel galaxy (Gao et al. 2003), and NGC4559 X-7 (Soria et al 2003). • low metallicity effect? • Lower mass-loss rate  more massive BHs • Longer Roche-Lobe filling phase

  7. The Antennae 18 ULXs! Fabbiano et al. (2003)

  8. Cartwheel galaxy Gao et al. (2003) D=122 Mpc WFPC2 B-band image and 0.3-7 keV intensity contours 0.3-1.5 keV image and 1.5-7 keV contours

  9. At least, 10 ULXs in the ring ULXs are close to, but typically not right on, optical peaks (too much extinction?) Lifetime of the ULX phase is < 107 yr Total number of dead ULXs ~ 300/bd b – beaming factor d – duty cycle Assuming one IMBH formed from a ~3x105 Msun cluster, a total > 108 Msun/ cluster mass is need - efficiency to form a ULX, e.g., capturing a companion. 3x108 yr ~107 yr • Alternatives are probably fine: • IMBHs are from Pop III stars • IMBHs powered by the SN fallback (Wang 2002; Li 2003) • X-ray binaries with Stellar-mass BHs and with strong beaming • Very young SNRs Difficult to explain with the IMBH X-ray binary scenario King (2004)

  10. Cartwheel-X7 • L(0.5-10 keV) = 1.3 x 1041 erg/s • Might be a composite of multiple sources

  11. Nature of ULXs • Background AGNs (~<10%) • Normally optical, IR, and/or radio bright (e.g., Foschini et al. 2002) • Very young SNRs • With Lx up to ~1041 erg/s (SN1988Z; Fabian & Terlevich 1996), easily IDed in optical and radio • However, some may contain bright X-ray compact sources, e.g., NGC 6946 MF16: • Bright radio and optical nebula • age ~ 3.5 x 103 yr • Variable in X-ray on both short and long scales (Roberts & Colbert 2003) • Hard X-ray spectrum similar to most other ULXs • Most of ULXs appear to be accreting BHs

  12. Stellar-mass or intermediate-mass? • Truly super-Eddington • E.g., accretion disks with radiation-driven inhomogeneity (Begelman 2002). But the limit is probably less than a factor of 10 higher. • Beamed or jetted toward us (King 2002; Markoff et al. 2001) • Similar to Galactic microquasars • Strong temporal variability expected • Several ULXs do show such variability • But most ULXs remain steady • Perpendicular to the disks, thus no eclipsing • A couple of ULXs do show possible orbital periods

  13. X-ray temporal variability • Mostly persistent (within a factor of < 2). • Strong aperiodic variability in a few ULXs, e.g., M101-P098 (Mukai et al. 2003). • A few with apparent periodic variability. • PDS of some ULXs show a low frequency break: • E.g., 0.028 mHz for NGC4559-X7 (Cropper et al. 2004)  103 Msun, interpolated from the break frequency and mass relationship between stellar and supermassive BHs.

  14. ULX M101-P098 (Mukai et al. 2003) beamed emission or changing photo-sphere?

  15. QPO of ULX M82-X41.4+60 • QPO – mostly a disk phenomenon • o = 54 mHz consistent with the IMBH, compared to o ~ 1 Hz for stellar mass BH • Narrow QPO peak (fwhm=10 mHz) and large amplitude, ruling out multiple scattering Strohmayer, & Mushotzky (2003) XMM-Newton/EPIC > 2 keV data

  16. Circinus galaxy X-1 • Lx ~ 4 x 1039 erg/s • Apparent period ~ 7.5 hr • An eclipsing binary? Bauer et al. (2001)

  17. Terashima & Wilson (2003) ACIS M51-TW#69 • Apparent 2.1 hr period • Very broad dips • Drastic spectral steepening with decreasing flux. Eclipsing? PN+MOS Smith & Wang 2004

  18. M51-TW#69: PN+MOS spectrum of • L(0.5-8)=1.3x1039 erg/s • Power law with a photon index = 1.8 • Consistent with being completely Comptonized

  19. Total disk spectrum Log n*Fn Annular BB emission Log n X-ray Spectra of ULXs:Accretion disk structure

  20. CMCD spectrum Comptonization of MCD Problems with MCD+PW model: • Nonphysical extension of PW to low energies • No radiation transfer • Little insight to the properties of the corona and its relation to the disk (e.g., incl. angle) MCD spectrum Log n*Fn Log n

  21. Implementation of a CMCD model, based on Monte-Carlo simulations • Spherically symmetric corona with a thermal electron energy distribution • Parameters: Te, , Rc, , plus Tin and normalization  (Rin/D)2. • Assuming that Rin (after various corrections) is the last stable circular orbit radius, the BH mass M=c2Rin/G. Yao et al. (2004) Wang et al. (2004)

  22. Test examples: LMC X-1 and X-2 • Independently estimates of , MBH, and NH • Data from PeppoSAX • Broad-band coverage • No pile-up • Spectral change LMC X-1 spectrum

  23. Model Comparisons LMC X-1 spectrum

  24. Corrected for absorption

  25. Comparisons of key measurements LMC X-1 Incl. angle (deg) M (Msun) NH (1020 cm-2) Tin (keV) Indep. Est. 24 <  < 64 4 < M < 12.5 -- CMCD 23 (< 43) 6.7 (?-?) 50(49 – 51) 0.93 MCD+PW 79(74 – 84)0.93 LMC X-3 Indep. Est.  < 70 deg > 7 3.8(3.1 – 4.6)a CMCD 59 (< 69) 6.9 (?-?) 4.5(4.2 - 4.7) 0.98 MCD+PW 7.6(6.7 – 8.5)1.02 a from X-ray absorption edge study

  26. Spectral evolution of LMC X-1 early part Tin=0.91 keV  = 0.5 late part Tin=0.99 keV  = 2 No Rin changes is needed!

  27. ULX Spectral Fits M81-X9 Notice the effect of the incl. angle Wang et al. 2004

  28. XMM-Newton Observations of Six ULXs in nearby galaxies Source Galaxy type D(Mpc) • NGC1313 X-1/X-2 SB(s)d 3.7 • IC342 X-1 Scd 3.3 • M81 X-9 Im 3.6 • NGC5408 X-1 IB(s)m 4.8 • NGC3628 X-1 Sbc 10.0 Wang et al. (2004)

  29. ULX spectral analysis PN+MOS spectra fitted with the CMCD model

  30. ULX Spectral Fit Results • Satisfactory fits to the spectra. • Tin (~0.05-0.3 keV) values consistent with the IMBH interpretation. • Constraints on accretion disk properties such as incl. angle, etc.

  31. Inferred Parameters from Spectral Fits • BH mass on the order of ~ 103 Msun each. • Accretion at a fraction of their Eddington rates. Wang et al. (2004)

  32. Evidence for IMBHs • No unambiguous detections of individual IMBHs yet, only observational hints (van der Marel 2002): • ULXs • High X-ray luminosities • Low frequency QPO or PDS breaks • A few possible eclipsing binaries, thus no jet boosting • Spectra consistent with MCDs of low Tin (~0.2 keV) plus Comptonization • Some show hard/low-soft/high transitions, typical of BH candidate binaries. • microlensing events • Optical kinematics of centers of nearby galaxies and globular clusters.

  33. How to form IMBHs? • Remnants of Pop III stars (Madau & Rees 2001) • A couple of 102 Msun each is predicted. • Grow by capturing stars in star clusters. • Induce SF in GMCs around them? • Young star clusters • Formed in a runaway core collapse and merger of MS stars (Portegies Zwart & McMillan 2002; Miller & Hamilton 2002) • Fed by Roche lobe overflow from a tidally captured stellar companion (circularized without being destroyed by tidal heating; Hopman et al. 2004). • Accreting IMBHs may outlive the host clusters. • Globular clusters (Taniguchi et al. 2000)

  34. Multi-wavelength counterparts • Rarely radio-bright • Only known candidates: • NGC5408-2E1400 (0.26 mJy at 4.8 GHz; Kaaret et al. 2003) • M81-X6 (0.095 mJy at 8.3 GHz; Swartz et al. 2003) • But consistent with Galactic micro-quasar radio luminosities. • Optical/UV counterpart • Few ULXs have relatively firm IDs • E.g., NGC 5204 ULX –B0 Ib supergiant plus NV emission line (Liu et al. 2004), predicting ~ an orbit period of 10 days.

  35. NGC4565-X4 NGC 4565 • Edge-on Sb galaxy • Low SF rate • The ULX is on the side with little disk absorption. • The Galactic foreground NH ~ 1.2x1020 cm-2.  Measurement of the intrinsic absorption in the ULX ACIS-S contours on optical Wang 2004

  36. NVI K OVII K ULX NGC4565-X4 • Tin = 0.190 (0.191-0.271) keV • L(0.5-10 keV) = 7 x 1039 erg/s • M ~ 103 Msun • Incl. angle = 18 (17-41) deg • NH = 2.5 (1.9 – 2.7 ) x 1021 cm-2 • In contrast to the Galactic value of 1.3 x 1020 cm-2 • A warm absorber? Similar to the IMBH (M ~ 104 - 105 Msun) AGN of NGC4395 (Shih et al. 2003) ACIS-S spectrum

  37. ULX NGC4565-X4 • The optical counterpart as a globular cluster (Wu et al. 2002) • An IMBH formed in a globular cluster (Taniguchi et al. 2000)?

  38. Impacts of ULXs on Environments M81-X9 Nebula Size ~ 260x350 pc Shock-heating Wang 2002 Wang (2002)

  39. E W Pakull & Mirioni 2002 NGC1313-X2 nebula • Size ~ 570 x 400 pc • V ~ 100 km/s • n ~ 0.2 cm-3 • E ~ 1.0 x 1053 erg, assuming an 1-D wind bubble

  40. HoII X-1: an X-ray-ionized nebulae • Abnormally high [OIII]/H ratio (Remillard, Rappaport & Macri 1995) • Strong He++ recombination line 4686 • Requiring He+ Lyman continuum (~ 54 -200 eV) ~0.3-1.3 1040 erg/s • Agreeing with the observed Lx. • Excluding significant non-isotropic X-ray beaming Pakull & Mirioni 2002

  41. Nature of the ULX and energetic shell associations • Superbubble? • Timescale mismatch: • Dynamic time of such a shell (~ R/v) is too short (~< 106 yr). • Ionization of the shells is primarily due to shock heating  age of the OB association ~> 107 yrs. • Too much energy is required: • Typically 1052 – 1053 erg, or 1039 – 1040 erg/s (or 1 SN per 104 -105 yr), energetically similar to 30 Doradus. • Hypernova remnant? • Shell - interstellar remnant • ULX – stellar remnant, accreting from • Fallback of the ejecta • Accreting binary with an original or captured companion (Is the timescale too short?) Wang (2002)

  42. Shell powered by an X-ray binary? Available binding energy (~GMBHMc/rBH ~ 1054 Mc erg; rBH MBH) Required mechanical energy output ~ radiation luminosity Consistent with other accreting systems (microquasars or AGNs). Wind probably at a speed of ~ c. Disk winds are observed in X-ray spectra of binaries and AGNs. UV/soft X-ray ionization of nebulae High electron temperature (H/H~105 K for M81-X9) Diffuse boundaries (due to long X-ray absorption path-length)

  43. Summary and Conclusions • ULXs represent a heterogeneous population • Very young SNRs • Stellar mass BHs with beamed and/or mildly super-Eddington X-ray emission • IMBHs accreting from HN/SN fallbacks or companions, though no conclusive evidence yet • A self-consistent Comptonized MCD spectral model has been developed and tested • Satisfactory fits to several best-observed IMBHs  estimates of BH masses, plus constraints on disk incl. angle, etc. • ULXs are often associated with highly-ionized and/or very energetic nebulae. • Clues to their origins • Constraints on outflows from accreting systems

  44. Future • Longer exposures with Chandra/XMM-Newton: • Variability: power spectrum break, QPO, and orbital period • High S/N spectra for more sources  diversity and spectral state changes. • Astro-E2: • high resolution spectrometer for study both emission and absorption lines • Sensitivity to higher energy photons  better constraints on Comptonization • Multi-wavelength follow-up: • IR/Optical/UV ID  nature of source, dynamic mass, etc. • Nebulosity  beam effect, energy output, and origin

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