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

1. 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) Phil Uttley, James Reeves (GSFC) Luminous X-ray sources in M101 (from Jenkins et al. 2004)

2. ULXs and IMBHs • ULXs – discrete X-ray sources with LX> 1039 erg s-1. • But at these luminosities LX> LEdd for a ~ 10 M black hole – a new class of ~ 100 – 104Mintermediate-mass black holes (IMBHs) required? • Supporting evidence from “soft excess” in XMM-Newton ULX spectra (e.g. Miller et al. 2003). Now 10+ examples. NGC 1313 X-1 Power-law + diskbb kTin ~ 0.15 keV T  M-0.25 cf. kTin ~ 1 – 2 keV for stellar BHs Tim Roberts - New insights into ULXs

3. 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)? • NB – no comparable population in ellipticals (Irwin et al. 2004). From Gao et al. (2003) Tim Roberts - New insights into ULXs

4. 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……. Tim Roberts - New insights into ULXs

5. The NGC 55 ULX Combined XMM-Newton EPIC image DSS image with EPIC contour See Stobbart, Roberts & Warwick, 2004, MNRAS, 351, 1063. Tim Roberts - New insights into ULXs

6. kTin~ 0.9 keV XMMU J001528.9-391319 Γ ~ 4 1039 erg s-1 • Source exhibits temporal variability including dipping. • Dips most prominent at high energies. Tim Roberts - New insights into ULXs

7. A problematic spectrum From Zhang et al. (2000) NGC 55 ULX 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 power-law – cannot be disc-Comptonisation (too few photons below peak in disc emissivity). Unlikely to be jet – too soft (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. kT ~ 0.2 – 0.5 keV kT ~ 1 – 1.5 keV, τ ~ 10 Tim Roberts - New insights into ULXs

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

9. 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… Tim Roberts - New insights into ULXs

10. 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. Tim Roberts - New insights into ULXs

11. 2-10 keV curvature • Other four datasets much prefer “new” spectrum. • But this description not rejected in 6/7 IMBH candidates, and as equally plausible as IMBH in 4. • 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. IMBH model From Roberts et al. (2005) “New” model Tim Roberts - New insights into ULXs

12. 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 αand MBH, Mdot. • MBH typically 10 – 50 M and < 100 M in all but one case. Gives Mdot in 0.1 – 10 in Eddington units. Tim Roberts - New insights into ULXs

13. 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, other (empirical) models. • But only two fits look like IMBHs: cool disc, low optical 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… kT ~ 0.2 – 0.5 keV kT ~ 1 – 1.5 keV, τ ~ 10 Tim Roberts - New insights into ULXs

14. 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. Tim Roberts - New insights into ULXs

15. 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! Tim Roberts - New insights into ULXs