Ultraluminous X-ray Sources

1. Ultraluminous X-ray Sources Tim Roberts ULXs in the interacting galaxy pair NGC 4485/4490 (Gladstone & Roberts 2008 - also poster B.11)

2. A definition • ULX: an X-ray source in an extra-nuclear region of a galaxy with an observed luminosity in excess of 1039 erg s-1 • Heterogeneous population - includes some recent supernovae - but bulk of sources are black holes accreting from a secondary star The Antennae - Chandra ACIS Tim Roberts - Ultraluminous X-ray Sources

3. A new class of black hole? • But Eddington limit for spherical accretion: LEdd ~ 1.3 × 1038 (M/M) erg s-1 hence ULXs contain  10 M compact objects – larger still if accretion sub-Eddington – massive black holes. • Not super-massive BHs (MBH  106M); fall to Galactic centre in a Hubble time due to effects of dynamical friction. • Too massive for stellar remnants (3M  MBH  18M). • Are we observing a new, 102 – 105M “intermediate mass” class of accreting black hole (IMBHs; e.g. Colbert & Mushotzky 1999)? Tim Roberts - Ultraluminous X-ray Sources

4. X-ray evidence for IMBHs • X-ray spectroscopic evidence – cool accretion discs (Miller et al. 2003). NGC 1313 X-1 T  M-0.25 kTin ~ 0.15 keV → ~ 1000 M BHs c.f. kTin ~ 1 keV for stellar BHs Tim Roberts - Ultraluminous X-ray Sources

5. LX – kTin relationship • IMBH candidates occupy separate part of parameter space to stellar-mass BHs. • Strong evidence for IMBHs as new class underlying luminous ULXs. From Miller et al. (2004) LX T4 Tim Roberts - Ultraluminous X-ray Sources

6. Vanishing IMBHs problem • But some problems with IMBHs, most notably… • X-ray luminosity function (XLF), normalised to star formation rate, unbroken over 5 decades. • XLF break at ~ 0.1 LEdd for 1000-M IMBHs. • No other source population switches off at 0.1 LEddlike this. From Grimm, Gilfanov & Sunyaev (2003) Break at ~ 2 × 1040 erg s-1 Tim Roberts - Ultraluminous X-ray Sources

7. The link with massive stars High mass stars can feed stellar mass black holes at a sufficient rate to produce the extreme X-ray luminosity From Gao et al. (2003) Potential X-ray luminosities for accretion onto a 10 M BH from 2 – 17 M secondaries (Rappaport, Podsiadlowski & Pfahl 2005) Populations of ULXs (10+) detected in bright starbursts - ULXs must be short-lived, so cannot all be IMBHs Tim Roberts - Ultraluminous X-ray Sources

8. Physical processes • Still need to break the Eddington limit; suggested methods include: • Relativistic beaming (e.g. Körding et al. 2002) • Radiative anisotropy (e.g. King et al. 2001) • Truly super-Eddington discs (e.g. Begelman 2002; Heinzeller & Duschl 2007) • Can combine at least two of the above, e.g. King (2008) - within Rsph local energy release is kept ~ Eddington by driving a bi-conical outflow; so apparent line-of-sight Bolometric luminosity is For beaming factor b and super-Eddington rate M/MEdd . . Tim Roberts - Ultraluminous X-ray Sources

9. Evidence from our own Galaxy • Super-Eddington luminosities are seen! • GRS 1915+105 has intermittently exceeded LEdd over its ~15 yr outburst (Done et al. 2004) • SS433 is super-critically accreting (perhaps exceeding M/MEdd by >103) - if seen face-on it would be an ULX (Fabrika & Mescheryakov 2001, Poutanen et al. 2007) . . SS433: cartoon showing jet precession & inclination Tim Roberts - Ultraluminous X-ray Sources

10. A pause for reflection • Dichotomy • X-ray evidence such as extreme luminosities and cool accretion discs point to IMBHs, but… • Other evidence stacking up in favour of smaller black holes. Which one is the correct interpretation? Tim Roberts - Ultraluminous X-ray Sources

11. X-ray timing – PSDs & break frequency • Break frequencies in PSDs related to black hole mass and accretion rate (McHardy et al. 2006) • But most ULXs show little or no variability power (Feng & Kaaret 2005) • Break feature in NGC 5408 X-1 PDS @ ~3 mHz (Soria et al. 2004; Strohmayer et al. 2007) implies mass of 100 - 1000 M Frequency regime probed by XMM for bright ULXs Adapted from Vaughan et al. (2005) Scaling of break frequencies with mass, assuming accretion at mdotEdd Tim Roberts - Ultraluminous X-ray Sources

12. ULX QPOs • Two ULXs with known QPOs - both luminous with LX > 1040 erg s-1 • Cannot be beamed • Scaling arguments from Galactic black holes - masses ~100 - 1000 M if in known state (talk by Zampieri; Casella et al. 2008) Double QPO in NGC 5408 X-1 (from Strohmayer et al. 2007) QPO in M82 X-1 (from Strohmayer et al. 2003) Tim Roberts - Ultraluminous X-ray Sources

13. Ho II X-1: timing Goad et al. 2006 • Ho II X-1 is a good example of a ULX with little variability power - can we explain this using known accretion states? • Not disc-dominated • Insufficient power for high or classic very high states • Energy spectrum not low/hard state • Similar to “χ”-class of GRS 1915+105 in VHS? • Band-limited PSD - but don’t see variability, so must be at high-f MBH < 100 M. EPIC-pn light-curve of Ho II X-1 (0.3 – 6 keV, 100 s binning) Tim Roberts - Ultraluminous X-ray Sources

14. ULX spectra revisited Stobbart, Roberts & Wilms 2006 • Look at best archival XMM-Newton data • Demonstrate that 2-10 keV spectrum fit by a broken power-law in all of the highest quality data • Invalidates IMBH model - hard component is not a simple power-law Disc Power-law Tim Roberts - Ultraluminous X-ray Sources

15. ULX spectra vs Galactic black holes from Kubota & Done (2004) • Physical accretion disc plus corona model: cool discs (kT ~ 0.1-0.3 keV), optically-thick coronae ( ~ 5 - 100) • ULXs operate differently to commonblack hole states, but… • “Strong” VHS in XTE J1550-564 (Done & Kubota 2006) • Disc appears cool as its inner regions are obscured by an optically-thick corona. “ultraluminous branch” (from Soria 2007) Tim Roberts - Ultraluminous X-ray Sources

16. A new, ultraluminous accretion state? • Spectrum defined by apparently cool disc, power-law turning over at > 2 keV. Little or no variability power present. Occurs at extreme accretion rates Low hard state in GX339-4 vs a classic ULX, Ho IX X-1 Tim Roberts - Ultraluminous X-ray Sources

17. The importance of winds • Hydrodynamical simulations of extreme accretion rates (M >> MEdd) onto stellar-mass black holes - Ohsuga (2006, 2007) • Extreme wind driven - column ~ 3  1024 cm2 at the poles, much higher elsewhere • Explains coronae, lack of variability power, giant nebulae…link to high-Z QSOs, Galactic-scale feedback • • Tim Roberts - Ultraluminous X-ray Sources

18. Other explanations for spectral break • Kerr disc models (Makishima et al. 2000) • “Slim” accretion discs (e.g. Watarai et al. 2000) • Accretion disc structure changes at highest accretion rates (close to the Eddington limit). • Model disc profile T(r)  r-p; standard disc has p = 0.75, slim disc p = 0.5. • Recent work finds p ~ 0.6 for ULXs (e.g. Tsuneda et al. 2006, Vierdayanti et al. 2006, Mizuno et al. 2007). • Fully comptonised VHS with spectrum modified by ionised fast outflow (Goncalves & Soria 2006). • Common thread: high accretion rate, small black holes (MBH < 100 M). Tim Roberts - Ultraluminous X-ray Sources

19. A multi-wavelength perspective • Optical - counterparts and “beambags” (cf. Pakull & Grisé 2008) • Bubbles also seen in radio (e.g. Lang et al. 2007) • Spitzer observations of NGC 4490 - AGN-like emission lines from ULXs (Vazquez et al. 2007) • Identified ULX counterparts are blue - OB stars (e.g. Liu et al. 2004, Kuntz et al. 2005) Nebula around Ho IX X-1 (Grise & Pakull 2006) Tim Roberts - Ultraluminous X-ray Sources

20. New HST imaging of ULXs Roberts, Levan & Goad (2008) - arXiv:0803.4470v1 ACS WFC F606W Early F supergiant? NB. high extinction No counterpart, tho’ very high extinction Consistent with late O or early B star F330W F435WF606W mF606W = 23.9 mF606W > 26 mF606W = 24.9 Tim Roberts - Ultraluminous X-ray Sources

21. New HST imaging of ULXs (2) ACS WFC F606W Young stellar cluster? (MV ~ 8 - 9) OB Star (odd colours?): U-B ~ -1.4, B-V ~ 0.1 Real ULX, or related to background galaxy? F330W F435WF606W mF606W = 22.0 mF606W = 24.9 mF606W = 25.6 Tim Roberts - Ultraluminous X-ray Sources

22. Are these really secondary stars? • High LX will affect optical emission; reprocessing in accretion disc becomes more important to optical light as black hole mass increases • Stellar heating: stars may be later types than initial colour IDs suggest (late B, not late O/early B) (Patruno & Zampieri 2008; Copperwheat et al. 2007) - small black holes • Alternatively, IMBHs may be favoured (Madhusudhan et al. 2008) Stellar-mass BHs IMBHs From Madhusudhan et al. (2008) Tim Roberts - Ultraluminous X-ray Sources

23. The goal: mass functions • Urgency to finding counterparts: race to get first ULX mass function • Best way to resolve mass controversy! He II 4686Å line from accretion disc of NGC 1313 X-2; 300 km s-1 shift (Pakull et al. 2006). Could be used to constrain RV curve, hence constrain ULX black hole mass Radial velocity curve from extragalactic Wolf-Rayet black hole binary IC 10 X-1. Uses He II 4686Å line to constrain mass function, find a black hole mass of ~ 24 - 33 M (Silverman & Filippenko 2008) Tim Roberts - Ultraluminous X-ray Sources

24. So, what are ULXs? • Bulk of evidence - few keV X-ray spectral breaks, star formation link etc - argues most ULXs are extreme accretion rate, small (< 100 M) black holes • ULX is an accretion state, not a source class • Cannot rule out some larger IMBHs - NGC 5408 X-1, M82 X-1 and HLXs (with LX > 1041 erg s-1) are the best candidates? • Mass functions are within reach - will resolve the controversy for at least some ULXs Tim Roberts - Ultraluminous X-ray Sources