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Low metallicity and UltraLuminous X-ray Sources

Low metallicity and UltraLuminous X-ray Sources. Luca Zampieri INAF-Astronomical Observatory of Padova M. Mapelli ( Universita` di Milano Bicocca ) E. Ripamonti ( Universita` di Milano Bicocca ) S. Bressan ( INAF-Astronomical Observatory of Padova )

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Low metallicity and UltraLuminous X-ray Sources

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  1. Low metallicity and UltraLuminous X-ray Sources Luca Zampieri INAF-Astronomical Observatory of Padova M. Mapelli (Universita` di Milano Bicocca) E. Ripamonti (Universita` di Milano Bicocca) S. Bressan (INAF-Astronomical Observatory of Padova) M. Colpi (Universita` di Milano Bicocca) T. Roberts (Durham University) Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  2. Outline • Possible evidences of preferential location of ULXs in low metallicity environments • Metallicity of the nebula around NGC 1313 X-2 • Results of the statistical analysis of a sample of 66 galaxies observed in X-rays with SFR and metallicities measurements • A fraction of ULXs may originate from massive stars in low metallicity environments • Conclusions Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  3. X-ray spectra: similar to BHC spectra X-ray flux variability, Lx-spectral variability High L end of the XLF of XRBs ULXs X-ray binaries with massive donors X-ray and optical orbital modulations Often in young stellar environ. QPOs in the power spectrum Detection of stellar optical counterparts ULXs ULXs are pointlike, off-nuclear X-ray sources in nearby galaxies with L >> Ledd for 1 Msun (L>1.0e39 erg/s) Most compelling evidence of binary nature provided by detection of a modulation in the X-ray light curve of two ULXs, interpreted as the orbital period: M82 X-1  P~62 days (Kaaret et al. 2006a,b) NGC5408X-1  P~115 days (Strohmayer 2010; but see poster by Foster et al.)

  4. ULXs: some recent advancements • HLX1 in ESO243−49(Farrell et al. 09), with an inferred isotropic L~1.0e42 erg/s, interpreted as first unambiguous identification of an IMBH (Godet et al. 09; Webb et al. 10; Wiersema et al. 10). But interpretation of the X-ray spectra not unique (Soria et al. 2010) • Jet-inflated bubble around a powerful microquasar in the galaxy NGC 7793, of size and energy content comparable to those around ULXs (Pakull, Soria & Motch 10; Soria et al. 10) Impiombato et al. (2010) • Optical modulation of 6 days in NGC 1313 X-2 (Liu et al. 09) * Significance is low (~3 sigma; Impiobato et al. 10) * System consistent only with a 12-15 Msun main sequence donor dumping matter on a 50-100 Msun BH (Patruno & Zampieri 10) Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  5. ULXs: somerecent advancements • X-ray quasi-periodic oscillations (QPOs) at frequencies of 3-4 mHz from a transient ULX in M82 (Feng & Kaaret 10) • Curvature in high S/N X-ray spectra  low temperature (T~5-10 keV) optically thick (tau~3-5) coronae (Gladstone et al. 09) • Critical review of available Mbh estimates  Mbh >> 100 Msun not required for the majority of bright and persistent ULXs (Zampieri & Roberts 09) • Emerging picture: * Most ULXs are accreting X-ray binaries with massive companions in a rather peculiar state ('ultraluminous state') * Some may host IMBHs, but most of them do not require BHs this massive  However, not necessarily all ULXs contain stellar-mass (<10-20 Msun) BHs Definitive answer from measurement of dynamical mass function of ULX counterparts (Roberts et al. 10; Motch et al. 10) Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  6. Preferential location of ULXs in low metallicity environments? • Subsample of galaxies with X-ray pointed observations (ROSAT, Chandra, XMM) from the Catalog of Neighboring Galaxies (Karachentsev et al. 04) • Specific ULX frequency decreases with increasing host galaxy mass indicating that smaller, lower metallicity systems have more ULXs per unit mass (Swartz et al. 2008; Walton et al., in prep.) Swartz et al. (2008) Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  7. Preferential location of ULXs in low metallicity environments? • (+) Measurement of the metallicity of the stellar environment also attempted (NGC 4559 X-7, Cropper et al. 2004; NGC 1313 X-2, Grise’ et al. 08)  subsolar abundance • (+) Optical spectrum of the nebula of Ho II X-1  Z~0.1 Zsun (Pakull & Mirioni 02), but XMM-Newton RGS spectrum  Z~0.6 Zsun (Goad et al. 06) • (-) Analysis of high signal-to-noise ratio XMM spectra of a sample of 14 ULXs, trying to determine the oxygen abundance from the detection of K-shell photoionization edges  solar abundance (Winter et al. 2007) Simulated EPIC-pn spectra fitted with same input model with zero oxygen+edge systematically overestimate the abundance (Pintore & Zampieri 10) Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  8. Metallicity of the nebula around NGC 1313 X-2 Comparison with a grid of photoionization models computed with Cloudy (Ferland 96) • Agreement with metallicity of HII regions in NGC 1313 (Ryder 93; Walsh & Roy 97; Hadfield & Crowther 07) Z=0.15-0.5 Zsun Ripamonti et al. (2010) Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  9. Galaxies observed in X-rays with measurements of SFR and metallicity • Statistical analysis of a sample of galaxies hosting ULXs plus a number of galaxies of the Local Group (excluded ellipticals) • 66 galaxies observed in X-rays with SFR and Z measurements • SFR  average of available estimates • Z from intensity of emission lines of HII regions (calibration of Pilyugin & Thuan 05) Mean Z = 0.22+/-0.11 Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  10. Galaxies observed in X-rays with measurements of SFR and metallicity • Strong correlation between Nulx and SFR (e.g. Grimm et al. 2003; Mineo et al. 2010)  Nulx ~ A*SFR A=1.20+/-0.2 • There is marginal (~2 sigma) evidence of different normalizations between high-red (Z>0.2 Zsun) and low-black (Z<0.2 Zsun) metallicity galaxies NULX Mapelli et al. (2010) Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  11. Galaxies observed in X-rays with measurements of SFR and metallicity • Is there a residual dependence on Z after subtracting off the dominant effect of SFR? • Fit of Nulx/SFR vs Z using a power-law wrt a fit with a constant: improvement significant at the 96% confidence level Mapelli et al. (2010) Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  12. ULXs forming in low metallicity environments • Besides SFR, metallicity may be a further ingredient playing a role in forming and/or triggering the ULX phenomenon: How to explain this connection? • Try to explan it within the framework of BH formation from the direct collapse of massive stars in low-metallicity environments: If a massive star retains an envelope more massive than ~30-40 Msun at the time of explosion, it may collapse directly to form a BH (Zampieri 02; Heger et al. 03) The BH mass is comparable to the final star mass: Mbh>30 Msun (e.g. Fryer 99; Fryer & Kalogera 01; Belczynski et al. 10)  massive (stellar-remnant) BH Lower Z stars tend to have more massive envelopes because mass loss is less efficient  Low Z environment may produce more and bigger BHs Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  13. ULXs forming inlow metallicity environments Zampieri & Roberts (2009) Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  14. ULXs forming in low metallicity environments Bressan et al. (2010) • 60 Msun He core (WR star) from 120 Msun star (Bressan, Marigo, Girardi et al. 10) • A possibly independent piece of evidence: Dynamical mass of IC 10 X-1  23-33 Msun (Prestwich et al. 07; Silverman & Filippenko 08) Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  15. ULXs forming in low metallicity environments mmax=120 Msun  maximum stellar mass mprog(Z)  minimum MS mass to form a BH through direct collapse from Portinari et al. (98) and Belczynski et al. (10) mprog(0.3 Zsun)=80 Msun; mprog(0.01 Zsun)=60 Msun Distribution of BHs  IMF (Kroupa 2001) above mprog mmin=0.08 Msun  minimum stellar mass tco=10 Myr  characteristic lifetime of a binary companion (typical of a 15 Msun donor) Nbh  number of massive BHs that have an active phase Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  16. ULXs forming in low metallicity environments • Cartwheel: Nulx explained with a reasonable production efficiency and fraction of star-forming mass ending up in BHs (Mapelli et al. 09) • Linear fit is statistically consistent with the observed correlation Mapelli et al. (2010) Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

  17. Conclusions • Marginal statistical evidence of Z dependence in ULX formation: More, and more accurate measurements needed • A fraction of ULXs (most of the the bright tail) may be originating from the direct collapse of massive stars formed in low-metallicity environments • Binary evolution at low Z? See Vicky’s talk Z dependence possibly connected to: * Evolution of massive stars in low Z environments * Relative strengths of the ULX-HMXB formation pathways (Linden et al. 10) • Hydrodynamics of direct BH formation * How much energy/mass is ejected? * How symmetric is it? What is the typical natal kick? Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ

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