Ultraluminous Xray Sources in Nearby Galaxies Q Daniel
Ultraluminous X-ray Sources in Nearby Galaxies Q. Daniel Wang (Univ. of Massachusetts, Amherst) In collaboration with Yangsen Yao, David Smith, Yu Gao, etc.
M 51: X-ray sources & H image (Terashima & Wilson 2003): Large, medium, and small circles: L(0. 5 -8 ke. V) > 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).
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?
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)
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
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. , M 81 X-9 (Wang 2002), Cartwheel galaxy (Gao et al. 2003), and NGC 4559 X-7 (Soria et al 2003). – low metallicity effect? • Lower mass-loss rate more massive BHs • Longer Roche-Lobe filling phase
The Antennae 18 ULXs! Fabbiano et al. (2003)
D=122 Mpc Cartwheel galaxy Gao et al. (2003) WFPC 2 B-band image and 0. 3 -1. 5 ke. V image and 0. 3 -7 ke. V intensity contours 1. 5 -7 ke. V contours
3 x 108 yr • 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 ~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 Stellarmass BHs and with strong beaming • Very young SNRs • Assuming one IMBH formed from a ~3 x 105 Msun cluster, a total > 108 Msun/ cluster mass is need - efficiency to form a ULX, e. g. , capturing a companion. Difficult to explain with the IMBH X-ray binary scenario King (2004)
Cartwheel-X 7 • L(0. 5 -10 ke. V) = 1. 3 x 1041 erg/s • Might be a composite of multiple sources
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 (SN 1988 Z; Fabian & Terlevich 1996), easily IDed in optical and radio – However, some may contain bright X-ray compact sources, e. g. , NGC 6946 MF 16: • 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
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
X-ray temporal variability • Mostly persistent (within a factor of < 2). • Strong aperiodic variability in a few ULXs, e. g. , M 101 -P 098 (Mukai et al. 2003). • A few with apparent periodic variability. • PDS of some ULXs show a low frequency break: – E. g. , 0. 028 m. Hz for NGC 4559 -X 7 (Cropper et al. 2004) 103 Msun, interpolated from the break frequency and mass relationship between stellar and supermassive BHs.
ULX M 101 -P 098 (Mukai et al. 2003) beamed emission or changing photosphere?
QPO of ULX M 82 -X 41. 4+60 QPO – mostly a disk phenomenon • o = 54 m. Hz consistent with the IMBH, compared to o ~ 1 Hz for stellar mass BH • Narrow QPO peak ( fwhm=10 m. Hz) and large amplitude, ruling out multiple scattering XMM-Newton/EPIC > 2 ke. V data Strohmayer, & Mushotzky (2003)
Circinus galaxy X-1 • Lx ~ 4 x 1039 erg/s • Apparent period ~ 7. 5 hr • An eclipsing binary? Bauer et al. (2001)
M 51 -TW#69 Terashima & Wilson (2003) ACIS • Apparent 2. 1 hr period • Very broad dips • Drastic spectral steepening with decreasing flux. Eclipsing? PN+MOS Smith & Wang 2004
M 51 -TW#69: PN+MOS spectrum of • L(0. 5 -8)=1. 3 x 1039 erg/s • Power law with a photon index = 1. 8 • Consistent with being completely Comptonized
X-ray Spectra of ULXs: Accretion disk structure Log n*Fn Total disk spectrum Annular BB emission Log n
Comptonization of MCD Problems with MCD+PW model: Log n*Fn MCD spectrum CMCD spectrum Log n • 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)
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=c 2 Rin/G. Yao et al. (2004) Wang et al. (2004)
Test examples: LMC X-1 and X-2 • Independently estimates of , MBH, and NH • Data from Peppo. SAX – Broad-band coverage – No pile-up – Spectral change LMC X-1 spectrum
Model Comparisons LMC X-1 spectrum
Corrected for absorption
Comparisons of key measurements LMC X-1 Incl. angle (deg) Indep. Est. 24 < < 64 CMCD 23 (< 43) MCD+PW M (Msun) 4 < M < 12. 5 6. 7 (? -? ) NH (1020 cm-2) -50(49 – 51) 79(74 – 84) Tin (ke. V) 0. 93 LMC X-3 Indep. Est. CMCD MCD+PW a < 70 deg 59 (< 69) >7 6. 9 (? -? ) from X-ray absorption edge study 3. 8(3. 1 – 4. 6)a 4. 5(4. 2 - 4. 7) 0. 98 7. 6(6. 7 – 8. 5) 1. 02
Spectral evolution of LMC X-1 early part Tin=0. 91 ke. V = 0. 5 No Rin changes is needed! late part Tin=0. 99 ke. V =2
ULX Spectral Fits M 81 -X 9 Notice the effect of the incl. angle Wang et al. 2004
XMM-Newton Observations of Six ULXs in nearby galaxies • • • Source NGC 1313 X-1/X-2 IC 342 X-1 M 81 X-9 NGC 5408 X-1 NGC 3628 X-1 Galaxy type SB(s)d Scd Im IB(s)m Sbc D(Mpc) 3. 7 3. 3 3. 6 4. 8 10. 0 Wang et al. (2004)
ULX spectral analysis PN+MOS spectra fitted with the CMCD model
ULX Spectral Fit Results • Satisfactory fits to the spectra. • Tin (~0. 05 -0. 3 ke. V) values consistent with the IMBH interpretation. • Constraints on accretion disk properties such as incl. angle, etc.
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)
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 ke. V) 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.
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 & Mc. Millan 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)
Multi-wavelength counterparts • Rarely radio-bright – Only known candidates: • NGC 5408 -2 E 1400 (0. 26 m. Jy at 4. 8 GHz; Kaaret et al. 2003) • M 81 -X 6 (0. 095 m. Jy 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 –B 0 Ib supergiant plus NV emission line (Liu et al. 2004), predicting ~ an orbit period of 10 days.
NGC 4565 • Edge-on Sb galaxy • Low SF rate • The ULX is on the side with little disk absorption. • The Galactic foreground NH ~ NGC 4565 -X 4 1. 2 x 1020 cm-2. Measurement of the intrinsic absorption in the ULX ACIS-S contours on optical Wang 2004
ULX NGC 4565 -X 4 NVI K OVII K • • • Tin = 0. 190 (0. 191 -0. 271) ke. V L(0. 5 -10 ke. V) = 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 NGC 4395 (Shih et al. 2003) ACIS-S spectrum
ULX NGC 4565 -X 4 • The optical counterpart as a globular cluster (Wu et al. 2002) • An IMBH formed in a globular cluster (Taniguchi et al. 2000)?
Impacts of ULXs on Environments M 81 -X 9 Nebula Size ~ 260 x 350 pc Shock-heating Wang 2002 Wang (2002)
Pakull & Mirioni 2002 NGC 1313 -X 2 nebula • • E W 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
Ho. II X-1: an X-rayionized nebulae • Abnormally high [OIII]/H ratio (Remillard, Rappaport & Macri 1995) • Strong He++ recombination line 4686 • Requiring He+ Lyman continuum (~ 54 -200 e. V) ~0. 3 -1. 3 1040 erg/s • Agreeing with the observed L x. • Excluding significant nonisotropic X-ray beaming Pakull & Mirioni 2002
Nature of the ULX and energetic shell associations • Superbubble? – Timescale mismatch: • Dynamic time of such a shell (~ R/v) is too short (~< 10 6 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)
• Shell powered by an X-ray binary? – Available binding energy (~GMBHMc/r. BH ~ 1054 Mc erg; r. BH 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 M 81 -X 9) – Diffuse boundaries (due to long X-ray absorption path-length)
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
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-E 2: – 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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