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) > 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. At the distance to the galaxies 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 Expanding field, cannot help in the study of nearby galaxies. Brief review of what I know and share some of our work and thoughts. 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., 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

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

0.3-7 keV intensity contours Cartwheel galaxy D=122 Mpc Gao et al. (2003) Every source is a ULX A clear link of ULX pop and SF WFPC2 B-band image and 0.3-7 keV intensity contours 0.3-1.5 keV image and 1.5-7 keV contours

Difficult to explain with the IMBH X-ray binary scenario 3x108 yr ~107 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 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. 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)

Cartwheel-X7 L(0.5-10 keV) = 1.3 x 1041 erg/s Might be a composite of multiple sources Timing analysis is needed

Nature of ULXs Background AGNs (~<10%) Very young SNRs 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

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 production of IMBH, this time non-primordial, may be through the mergers of stellar mass BH produced in star clusters or super-star clusters. The cluster will harden through interactions In the second line of argument, the power spectrum exhibits a break at 0.028 mHz. If the relationship between break frequency and compact object mass can be interpolated between stellar and supermassive BH,

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.

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

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

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

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

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

X-ray Spectra of ULXs: Accretion disk structure Log n Log n*Fn Total disk spectrum Annular BB emission It is assumed that the disk is geometrically-thin and optically-thick in the z-direction. Thus each annular element of the disk radiates roughly as a blackbody with a temperature T(R) , where : Sigma x T^4(R) = D(R) Where D(R) is the dissipation rate and sigma x T^4 is the blackbody flux. R_* is the radius of the black hole (or compact object). Dissipation through the disk is independent of the viscosity in the disk – and the dissipation rate is the energy flux through the faces of the thin disk. Thus if the disk is optically-thick in the z-direction, we are justified in assuming that the dissipation rate is equivalent to the blackbody emission.

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 CMCD spectrum Log n*Fn The diagram illustrates a cross-section through a binary star accretion disk. Half of the accretion luminosity is released in the disk - the other half very close to the compact star. The disk is generally assumed to be geometrically-thin although X-ray observations of X-ray binaries indicate that the disk either bulges or flares up at the the outer edges. Such activity is seen in dipping low-mass X-ray binaries (LMXRBs). This may be due to radiation torques warping the disk, to tidal forces exerted by the secondary or due to the impact of the accreting material on the edge of the disk. The disk is optically-thick throughout except at the very inner edges which are probably optically-thin and emit bremsstrahlung radiation. Once the material has fallen low along the disk near the surface of the star, it may heat this surface or fall into the black hole. Log n

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)

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

Model Comparisons LMC X-1 spectrum

Corrected for absorption

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

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!

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

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)

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 keV) 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 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.

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)

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.

ACIS-S contours on optical 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 NGC4565-X4 ACIS-S contours on optical Wang 2004

ULX NGC4565-X4 ACIS-S spectrum Tin = 0.190 (0.191-0.271) keV OVII K NVI K 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

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)?

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

NGC1313-X2 nebula Size ~ 570 x 400 pc V ~ 100 km/s n ~ 0.2 cm-3 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 E W

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

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)

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)

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: Astro-E2: 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