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What are the essential ingredients of ultraluminous X-ray sources? Roberto Soria (CfA & MSSL) Some ULX collaborators: M Cropper, C Copperwheat (MSSL), R Fender (Southampton), Z Kuncic, C Hung (Sydney), D Swartz (MSFC), A Goncalves (Paris-M), M Pakull, F Grise’ (Strasbourg), R. Mushotzky (GSFC) What we’d like to know about ULXs 1) Mass No direct (kinematic) mass determination yet. Two or three candidates perhaps feasible now. 2) How to gain a factor of ~ 50 in apparent Lx with respect to stellar-mass BHs Beamed (microblazars?) Higher BH mass (IMBHs?) Not beamed Super-Eddington luminosity Searching for common features in the ULX population “Soft-excess” in their X-ray spectra? Signature of a cool disk? higher BH mass? Holmberg II X-1 (Lx ~ 2E40 erg/s) “soft excess” kT ~ 0.15 keV Holmberg II X-1 (Lx ~ 2E40 erg/s) Searching for common features in the ULX population “Soft-excess” in their X-ray spectra? Signature of a cool disk? higher BH mass? Most bright ULXs (Lx ~ 1E40 erg/s) have it (Stobbart et al 06) A few do not, pure power-law spectrum (Winter et al 06) Evidence of IMBHs, M ~ 1000 Msun ? “Soft-excess” interpretation is still unclear See also poster by Soria, Goncalves & Kuncic Cool disk emission Smeared absorption lines in fast, ionized outflow Holmberg II X-1 (Lx ~ 2E40 erg/s) Holmberg II X-1 (Lx ~ 2E40 erg/s) Injected spectrum (power-law) Emerging spectra with absorption from ionized, fast-moving outflow (v ~ 0.1 c, nH ~ 3E22) Models by Goncalves et al. References: Gierlinski & Done (2004) Crummy et al (2006) Goncalves & Soria (2006) “Soft-excess” interpretation is still unclear See also poster by Soria, Goncalves & Kuncic Standard disk around IMBH Cool disk emission Non-standard disk Smeared absorption lines in fast, ionized outflow More generally: absorption + re-emission + reflection Essential feature of X-ray spectra: Dominated by non-thermal emission Disk radiates only ~ 10-20% of output accretion power Most power is efficiently transferred from disk to upscattering medium (jet/corona) Disk should be cooler than a standard SS disk for a given BH mass Chilled disk Cooler than standard disk because power is drained from disk into jet+wind+corona see also Z. Kuncic’s talk Chilled disk Cooler than standard disk because power is drained from disk into jet+wind+corona see also Z. Kuncic’s talk (Soria & Kuncic, in prep.) Searching for common features in the ULX population Jets, outflows? Radio cores: not detectable yet (< 0.1 mJy) Resolved jets: not detectable yet Radio lobes: likely detection in a few sources Energy in lobes >~ 1E52 erg Size ~ 50-70 pc Typical fluxes ~ 0.1-0.2 mJy at 5 GHz Radio lobes of a ULX in NGC 5408 (Soria, Fender et al 2006) Subaru B + ATCA 5 GHz CFHT Ha + ATCA 5 GHz Searching for common features in the ULX population Jets, outflows? Radio cores: not detectable yet (< 0.1 mJy) Resolved jets: not detectable yet Radio lobes: likely detection in a few sources Optical nebulae: observed in many bright ULXs sizes ~ 50-400 pc X-ray photoionized or collisionally ionized? HST/ACS Optical nebulae Jet lobes? NGC 1313 X-2 (Pakull, Grise & Motch 2006) ULX 30 pc Star Hot spot? (hot ring?) MF16 “SNR” + ULX, in NGC 6946 (Swartz et al 2006, in prep) = 80 pc Searching for common features in the ULX population Jets, outflows? Likely to be essential ingredient but more evidence needed Searching for common features in the ULX population Young host environment? Not essential for fainter ULXs (Lx <~ 3E39 erg/s) Essential for brighter ULXs (Lx >~ 1E40 erg/s) Only found in spiral & irregular galaxies “Young” = less than 50 Myr Donor = OB star transferring gas on its nuclear timescale Searching for common features in the ULX population Starburst environment? Some ULXs are in starburst galaxies (eg, Cartwheel, Antennae, Mice) Some are in very quiet corners of nuclear starburst or starforming galaxies (eg, NGC 7714, M83, M99) Some are in tidal dwarfs with little star formation (eg, Ho II, Ho IX) NOT AN ESSENTIAL INGREDIENT but some association Searching for common features in the ULX population Super star-clusters? Suggested as site of IMBH formation via O-star coalescence (Portegies Zwart et al; Rasio et al) But inconsistent with ULX observations (except for M82 X-1) Most ULXs found in OB associations or open clusters, with masses <~ a few 1000 Msun NOT AN ESSENTIAL INGREDIENT Searching for common features in the ULX population Colliding or tidally interacting systems? Galaxy-galaxy collisions (eg, ULXs in Antennae, Mice, Cartwheel, NGC 4485/90, NGC 7714/15) Satellite dwarf – galaxy collisions (eg ULX in NGC 4559) HI cloud – disk collisions (eg ULX in M99) Tidal dwarfs and tails (eg ULXs in Ho II, Ho IX) Searching for common features in the ULX population Colliding or tidally interacting systems? Essential or very important ingredient The Antennae NGC 4559 Examples of ULXs formed in colliding events M99 (Soria & Wong 2006) XMM EPIC image (0.2-12 keV) HI contours over R image LX ~ 2 1040 erg/s (see poster by Soria & Wong) High-velocity cloud collision with M99 gas disk Only a coincidence? Searching for common features in the ULX population Low-metallicity environment? Mounting evidence but no systematic study yet (eg, ULXs in Cartwheel, Ho II, NGC 4559, NGC 5408, 1 Zw 18) More massive BH remnants from . expected metal-poor O stars (Mwind ~ Z0.5-0.8) Probably a very important ingredient My (biased) conclusions: I: NATURE OF (MOST) ULXs Simplest model still consistent with the data: BH masses ~ 30 – 100 Msun (upper limit of stellar processes) Age of the accreting systems < 50 Myr (OB donor) II: (SPECULATIVE) FORMATION PROCESS Triggered star formation (eg, ram p from cloud/galaxy collisions) Dynamical collapse of molecular clumps (as opposed to turbulent fragmentation) Fast gas accretion and protostellar mergers in a dense protocluster core (clump mass ~ a few 1000 Msun, much smaller than a super cluster) Massive stellar progenitor, Mstar ~ 200 Msun if metal abundance is low BH with a mass ~ 50-100 Msun Externally-triggered dynamical collapse of a molecular clump in the Milky Way Total mass ~ 1700 Msun Infall rate ~ 10-3 Msun/yr Infall timescale ~ 1.7 105 yr CMM3 has 40 Msun, still accreting & merging 35 Msun 15 Msun 40 Msun Peretto et al. (2006) Very massive stars from clustered star formation exist in the Milky Way & LMC: Pistol star: initial mass ~ 200 Msun (but too metal rich to collapse into a BH) R145 in 30 Dor: M sin3i = (140 +/- 37) Msun III. POWER BUDGET Accretion rate up to ~ 10 times Eddington Luminosity near or a few times Eddington Disk radiates only < 20% of the output power Disks are cooler than standard SS Kin. power available for outflows and jets Can BHs have steady jets when accreting at or above Eddington? ULXs could be test cases for QSO super-Edd accretion and feedback models at high redshift A finis si’. Mersi’ che i l’eve scota’. Black hole masses in ULXs Optical counterparts too faint for direct mass-function determinations X-ray Luminosity function cuts off at ~ 3 x 1040 erg/s Eddington limit suggests M ~ 30 - 200 Msun Higher masses (~ 103 Msun) speculated from X-ray timing and spectral studies BH mass from X-ray spectral models Galactic X-ray binaries generally show: power-law component + thermal disk component Flatter (G ~ 1.5) when LX <~ 0.01 LEdd Steeper (G ~ 2.5) when LX ~ LEdd 4 2 4 2 LX ~ Tin R ~ Tin M -4 Lmax ~ LEdd ~ M ~ Tin X-ray spectrum of NGC4559 X7 (XMM) Power-law (G ~ 2.3) Tbb ~ 0.12 keV X-ray spectrum of NGC4559 X7 (XMM) G ~ 2.0 Disk kTin ~ 0.13 keV Disk kTin ~ 1.9 keV kTphot ~ 0.27 keV LX (erg/s) 1042 1041 1000 Msun Lx = LEdd Hot-disk model 1040 1039 IMBH model 15 Msun GBHs 1038 5 Msun 0.1 0.2 1 Tin 2 (keV) IMBH model Miller, Fabian & Miller (2004) Feng & Kaaret (2005) kTin ~ 0.12 – 0.15 keV M >~ 1000 Msun LX ~ 0.05 – 0.2 LEdd Hot-disk model kTin ~ 1.5 – 2.5 keV M <~ 10 Msun LX ~ 10 LEdd Stobbart, Roberts & Wilms (2006) PROBLEMS: IMBH model kTin ~ 0.12 – 0.15 keV M >~ 1000 Msun LX ~ 0.05 – 0.2 LEdd Hot-disk model kTin ~ 1.5 – 2.5 keV M <~ 10 Msun LX ~ 10 LEdd similar to NLSy1 requires exotic formation processes why do they never reach LEdd? PROBLEMS: IMBH model kTin ~ 0.12 – 0.15 keV M >~ 1000 Msun LX ~ 0.05 – 0.2 LEdd similar to NLSy1 requires exotic formation processes why do they never reach LEdd? Hot-disk model kTin ~ 1.5 – 2.5 keV M <~ 10 Msun LX ~ 10 LEdd ad hoc (esp. ~ 10 keV) standard SS disk should not survive at 10 LEdd ! Alternative model: broad absorption G ~ 2.8 Summary I Unwise to estimate BH masses from X-ray spectra “Soft excess” may be due to absorption New spectral state? (for ULXs and NLSy1?) Steep pl + absorption in fast, dense outflow Very high (steep pl) High/soft (disk) Low/hard (flat pl) . M ULX radio counterparts: proof of IMBHs? “fundamental plane” of BH activity (Merloni, Heinz & DiMatteo 2004; Fender et al 2004) Few ULXs have a radio counterpart M82 (Kording et al 2005) Holmberg II (Miller, Mushotzky & Neff 2005) NGC 5408 (Kaaret et al 2003; Soria, Fender et al 2006) NGC 7424 (Soria, Kuncic et al 2006) NGC 6946 (Swartz et al 2006, in prep) NGC 5408 (zoomed in) Coincidence between: X-ray (~1E40 erg/s) Radio (~ 0.3 mJy at 5 GHz) Ha (~1E36 erg/s) Comparison with X-ray and radio luminosities of Galactic BHs However: Steep radio spectrum (thin synchrotron) Same value in 2000 and 2004 Marginally resolved (radius ~ 30 pc) More likely radio emission from lobes, not core Core = X-rays, flat radio spectrum (if present) Traces the instantaneous accretion state Lobes = steep radio spectrum Trace the integrated jet power over ~ 0.1 Myr Radio lobes or supernova remnant? Not easy to distinguish or disentangle the two (eg, SS433 has SNR + jet lobes) Leptonic jet model in NGC 5408: E ~ 3 1051 erg PJ ~ 7 1038 erg/s over 1.5 105 yr Expansion velocity ~ 80 km/s A SN model (= 99% relativistic protons) would require E ~ 3 1052 erg (A hypernova, perhaps?) NGC 7424 (d ~ 12 Mpc) (Soria, Kuncic, Broderick & Ryder 2006) ULX-2 State transition low/hard high/soft 5 1038 7 1039 erg/s (with thermal plasma) Age = 8 +/- 2 Myr Point-like (< 60 pc) radio source Index ~ - 0.6 (thin synchrotron) LR ~ 3 x Cas A Radio lobe or young SNR? (Soria et al 2006b) Summary II We are starting to find ULX radio counterparts Radio lobes (FR2 microquasars?) or SNR? Many ULX radio lobes may have been misclassified as SNRs if the central X-ray source is off Ratio of ULX radio lobes / “fossil” radio lobes may give us clues on the X-ray duty cycle Radio/ULX associations useful to determine power budget = radiative vs mechanical output (also important for estimating feedback from early quasars) Part III: speculations on ULX formation IMBH formation in a young super-star-cluster? Dynamical friction 106 Msun cluster Mass segregation Runaway core-collapse 1000 Msun BH Stellar collisions/mergers in the core Short-lived, very massive star (~1000 Msun) Hypernova or direct collapse into IMBH Numerical simulations by Portegies Zwart et al and by Gurkan, Rasio et al. Problem: most ULXs are not in super-star-clusters Near OB stars but not inside a bound cluster Have their parent clusters dispersed? Tidal disruption: always too slow (>~ 50 Myr) SN disruption: perhaps….but there are no signs of the dispersed super clusters Were they ejected? Inconsistent with IMBH, would require low BH mass (eg, Zezas et al 2002; Belczynski et al 2005) 106 Msun super star clusters with 1000 Msun BHs Rarely found Probably not needed We only need M ~ 30 -- 200 Msun Suggestion: IMBHs formed in smaller proto-clusters, not super clusters (Soria 2005) Ionized gas protostars Neutral gas t ~ 0.5 Myr protocluster (eg, Kroupa & Boily, 2002-2004; Geyer & Burkert 2001) sh < 10 km/s M ~ 103.5 -- 105 Msun Ideal conditions for forming progenitor “star” with M ~ 100 -- 300 Msun dispersing the protocluster (binding energy <~ 1052 erg) Combination of accretion (large-scale gas inflow) + coalescence in the protocluster core? Dense proto-clusters ideal for coalescence Stellar captures and mergers are favoured by proto-stellar disks / envelopes Collision cross section enhanced at low velocity dispersion (gravitational focussing) Collision rates & maximum BH mass enhanced at high density Merging BHs: most difficult Merging O stars: somewhat easier Merging protostars, molecular cores: easiest Two regimes for coalescence + IMBH formation? M <~ 105 Msun sh < 10 km/s M >~105.5 Msun tcc <~ 0.5 Myr tcc <~ 3 Myr sh >~ 10 km/s IMBH formation in unbound proto-cluster IMBH formation in bound cluster ULX in a sparse OB assoc (size >~ 100 pc) with expanding gas nebula ULX in a cluster (size <~ 3 pc) Additional advantage of the proto-cluster scenario Same physical process that creates massive [O + O] binaries, progenitors of BH HMXBs ULXs = high-luminosity end of HMXBs up to ~ 100-200 Msun Protoclusters near the Cone nebula (Peretto, Andre’ & Belloche 2006) Near-IR contours + 1.2mm continuum Peretto, Andre’ & Belloche (2006) Two necessary ingredients for a massive BH: 1 Supersonic global inflows in protoclusters (as opposed to random turbulent motion) An external trigger may cause compression and dynamical collapse 2 Small mass loss from progenitor star before SN core-collapse Low metal abundance (~ 0.1 solar) reduces mass loss in stellar winds Importance of low metal abundance Heger et al (2003) Galaxy collisions, cloud-disk collisions Triggered star formation Denser protoclusters, dynamical collapse, high-mass stars ULXs, upper end of HMXB distribution (often) starbursts, large number of HMXBs Observational evidence for ULXs in SSCs? ULX in a young super-star-cluster in M82 Lx varying from ~ 1039 to 1041 erg/s Mbh ~ 1000 Msun Mcl ~ 4 105 Msun Portegies Zwart et al, Nature, 2004 Near clusters but not in one ULX in the starburst dwarf NGC 5408 with Lx ~ 1040 erg/s Near B stars but not in a cluster Kaaret et al 2003 Soria et al 2004 Near OB stars but not in a super-star-cluster ULX in the dwarf galaxy NGC 5204 Liu et al 2004 Not in super-star-clusters ULX in the starburst dwarf NGC5408 with Lx ~ 1040 erg/s Two ULXs in NGC4559 with Lx ~ 1 – 4 1040 erg/s NGC4559 X-10: near OB stars, no super cluster A few B stars but no big clusters Cropper et al 2005 Soria et al 2005 Antennae: lots of ULXs, displaced from clusters ULXs are displaced from SSCs by ~ 100 – 300 pc Zezas, Fabbiano et al 2002 Massive proto-stellar mergers Explosive expulsion of gas proto-cluster disruption Binding energy of the gas in a 105 Msun cluster ~ a few 1050 -- 1051 erg Single SN releases ~ 1051 erg Merger of 100 + 100 Msun stars releases ~ 1051 erg (Bally & Zinnecker 2005) Not in clusters 4 ULXs in the colliding galaxies NGC 7714 / 7715 with Lx ~ 2 – 8 1040 erg/s 2 are in clusters, 2 are not Smith et al 2005, AJ, 129, 1350 NGC4559 X-7: near OB stars, no super cluster A few B stars but no SSCs Soria et al 2005