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					New insights into ultraluminous X-ray
  sources from XMM-Newton/EPIC
            observations


                                 Tim Roberts
                                 Ann-Marie Stobbart, Bob Warwick,
                                 Mike Goad, Leigh Jenkins (Leicester)
                                 Martin Ward (Leicester/Durham)
                                 Jörn Wilms (Warwick)

Luminous X-ray sources in M101
                                 Phil Uttley, James Reeves (GSFC)
   (from Jenkins et al. 2004)
    ULXs and IMBHs
       ULXs – discrete X-ray
        sources with LX > 1039 erg s-1.                             NGC 1313 X-1
       But at these luminosities LX >                                    Power-law
        LEdd for a ~ 10 M black hole                                       + diskbb
        – a new class of ~ 100 – 104
        M intermediate-mass black
        holes (IMBHs) required?                              kTin ~ 0.15 keV
       Supporting evidence from
        “soft excess” in XMM-Newton
        ULX spectra (e.g. Miller et al.                    T  M-0.25
        2003). Now 10+ examples.
                                                  cf. kTin ~ 1 – 2 keV for stellar BHs
2                  Tim Roberts - New insights into ULXs          Tuesday 12th April 2005
    But……
       Multiple ULXs (10+) are found in
        Starburst       galaxies      –   e.g.
        Cartwheel galaxy (Gao et al.
        2003). Ongoing star formation 
        ULXs are intrinsically short-lived.
       Requires an infeasibly large
        underlying population of IMBHs
        (King 2004).
       Alternative: are ULXs in
        Starbursts high-mass X-ray
        binaries (HMXBs)?                                     From Gao et al. (2003)
       NB – no comparable population
        in ellipticals (Irwin et al. 2004).

3                      Tim Roberts - New insights into ULXs                Tuesday 12th April 2005
    Stellar-mass BHs as ULXs
       Possible mechanisms for breaking Eddington limit:
        –   Beaming by relativistic jets (e.g. Körding et al. 2002).
        –   Anisotropic radiation patterns (King et al. 2001).
        –   True super-Eddington discs (Begelman 2002, Ebisawa et al. 2003).
       Podsiadlowski et al. (2003), Rappaport et al. (2005) – super-
        Eddington mass transfer rates in HMXBs – account for most ULXs.
       Blue stellar counterparts to several ULXs.
       At least three stellar mass BHs (albeit LMXBs) in our galaxy have
        been seen to reach super-Eddington luminosities – GRS1915+105
        does so frequently (McClintock & Remillard 2003).
       Some ULXs do have stellar-mass disc temperatures (~1 – 2 keV).
       But not much recent observational evidence from ULX X-ray
        diagnostics…….
4                     Tim Roberts - New insights into ULXs   Tuesday 12th April 2005
    The NGC 55 ULX

      Combined XMM-Newton                  DSS image with EPIC contour
          EPIC image




      See Stobbart, Roberts & Warwick, 2004, MNRAS, 351, 1063.
5              Tim Roberts - New insights into ULXs       Tuesday 12th April 2005
                                               kTin ~ 0.9
                                                     keV
              Γ~4
    XMMU J001528.9-391319




                                                                                1039 erg s-1
    • Source exhibits temporal variability including dipping.
    • Dips most prominent at high energies.
6                 Tim Roberts - New insights into ULXs      Tuesday 12th April 2005
        From Zhang et al. (2000)
                                                                                  NGC 55 ULX


    A problematic spectrum
                                    kT ~ 100 keV
                                            VHS of GX 339-4

        Dominance of power-law continuum at soft energies not seen
         before in Galactic systems – though e.g. GRS 1915+105 high
         state extrapolates below 2 keV to this spectrum.
        Disc parameters extreme (high kTin, low Rin) but plausible for
         slim disc accretion onto a stellar-mass (or slightly bigger) BH.
        Problem is dominant soft ~ 1 – 1.5 keV, – cannot be disc-
        kT ~ 0.2 – 0.5 keV           kT power-law τ
         Comptonisation (too few photons below peak in disc
                                            ~ soft
         emissivity). Unlikely to be jet – too10 (Γ ~ 3 – 4 vs ~ 1.5 – 2
         for jet). Power-law form is not consistent with thermal emission
         from outflow/wind. Insurmountable problem?
        Possible explanation: greater spectral complexity. “3-layer”
         model of Zhang et al. (2000) – based on the Solar atmosphere -
         cold inner disc, warm & optically-thick accretion disc
         atmosphere, much hotter optically-thin corona.

7                                  Tim Roberts - New insights into ULXs   Tuesday 12th April 2005
    Other examples of “new” spectrum
                       kTin ~ 1.16 keV
                                                  From Roberts et
                                                     al. (2005)
                           Γ ~ 2.5




           M33 X-8                                                  NGC 5204 X-1

       This spectrum is seen in second LX ~ 1039 erg s-1 ULX – M33 X-8
        (Foschini et al. 2004).
       More luminous (LX ~ 5 × 1039 erg s-1) NGC 5204 X-1 data well fit
        by both “IMBH model”, i.e. cool accretion disc (kTin ~ 0.2 keV) +
        hard power-law continuum (Γ ~ 2), and “non-standard”
        description (Γ ~ 3.3, kTin ~ 2.2 - 2.8 keV).
8                    Tim Roberts - New insights into ULXs              Tuesday 12th April 2005
    A sample of bright ULXs
       How prevalent is the “new” spectrum in ULXs?
        Particularly in comparison to an IMBH spectrum?
       Select 13 (predominantly archival) ULXs observed by
        XMM-Newton/EPIC with (a) ~20 ks or more EPIC
        exposure, and (b) > 10 ct/ks in ROSAT HRI. Expect ~
        few thousand counts per source.
       Full range of LX covered (1039 – few × 1040 erg s-1).
       Uniform reduction to produce clean spectra for
        comparison of empirical models and state-of-the-art
        physical models.
       Analysis ongoing…

9                 Tim Roberts - New insights into ULXs   Tuesday 12th April 2005
     Empirical models
        Absorbed multi-colour disc blackbody spectrum
         (diskbb in XSPEC) rejected at high significance for all
         data.
        Absorbed power-law continuum not rejected at 95%
         confidence in only 4 datasets (including 3 lowest
         quality).
        IMBH model produces “good” (χ2ν ~ 1) fits in 7 sources
         (“better” in 2 more). Find kTin ~ 0.1 – 0.25 keV, Γ ~ 1.6
         – 2.5. Masses circa. 1000 M for IMBH.
        Problem: Γ too small? Theory and observations show
         Γ > 2.5 for high-state black hole accretion discs.
10                  Tim Roberts - New insights into ULXs   Tuesday 12th April 2005
     2-10 keV curvature

        Other four datasets much prefer “new” spectrum.
                     IMBH model             From Roberts et al. (2005)

        But this description not rejected in 6/7 IMBH
                                                          “New” 4.
         candidates, and as equally plausible as IMBH in model
        Key discriminator: curvature in 2-10 keV regime.
        Broken power-law versus power-law fits over 2-10 keV:
         5 significant improvements (> 3σ via F-test), 2
         marginal.    Correspond with preference for “new”
         spectrum.



11                   Tim Roberts - New insights into ULXs   Tuesday 12th April 2005
     Physical models (1)

        Slim disc model (e.g. Watarai et al. 2001; Ebisawa et
         al. 2004); XSPEC parameterisation courtesy of K.
         Ebisawa.
        At ~LEdd expect advection-dominated optically-thick
         discs – differences to “standard accretion disc, e.g. Rin
         decreases below ISCO as Mdot increases.
        Provides poor fits in most cases; problems with
         degeneracy between α and MBH, Mdot.
        MBH typically 10 – 50 M and < 100 M in all but one
         case. Gives Mdot in 0.1 – 10 in Eddington units.
12                  Tim Roberts - New insights into ULXs   Tuesday 12th April 2005
     Physical models (2)
                 kT ~ 100 keV


        Physically       self-consistent    accretion      disc +
         comptonisation model: diskpn+eqpair in XSPEC.
        Fits well to 5 datasets; a further 5 have good fits, or
         only moderately worse than, other1(empirical) models.
                kT ~ 0.2 – 0.5 keV        kT ~ – 1.5 keV, τ
        But only two fits look like IMBHs: cool disc, low optical
                                               ~ 10
         depth (kT ~ 0.3 keV, τ ~ 1).
        Other fits have cool discs (kT ~ 0.2 keV) but are
         optically thick (τ ~ 6 – 10). INCONSISTENT WITH
         IMBHs!
        cf. Zhang’s 3-layer model…

13                  Tim Roberts - New insights into ULXs   Tuesday 12th April 2005
     Holmberg II X-1
        Archetypal luminous ULX (LX > 1040 erg s-1).
        Deep XMM-Newton observation (80 ks, though >
         50% spoiled by bad space weather).
        X-ray spectrum: not well-fit by any model. Best
         empirical description: IMBH model (kTin ~ 0.2 keV, Γ
         ~ 2.6). But diskpn+eqpair provides best overall fit
         with kT ~ 0.2 keV, τ ~ 6.6.
        Also – lack of variability puzzling.
        PDS – red noise, only seen above poisson noise
         level at < 10 mHz. Not consistent with ~1000 solar
         mass BH in high state.
14                 Tim Roberts - New insights into ULXs   Tuesday 12th April 2005
     Conclusions
        Detailed spectroscopy – some ULXs just don’t look like
         IMBHs with “standard” accretion disc + corona spectra
         extrapolated from Galactic BHs (2 – 10 keV
         curvature/flat Γ).
        Highly compton-thick layer may be key evidence –
         ionised surface of bloated accretion disc fed by super-
         Eddington inflow of material from high-mass
         secondary. BH mass few 10s of M.
        Lack of short-term variability supports Compton-thick
         layer.
        However, only conclusive means of ending this debate
         is to derive a dynamical mass limit on the BH from
         orbital dynamics…and that’s another story!
15                 Tim Roberts - New insights into ULXs   Tuesday 12th April 2005

				
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posted:1/15/2013
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