Ultraluminous X-ray sources: a mystery for modern X-ray astronomy

1. Ultraluminous X-ray sources: a mystery for modern X-ray astronomy Tim Roberts Tim Roberts

2. Galaxies in the X-ray regime M83 - “Southern pinwheel galaxy” ESO VLT M83 ESO/VLT image 0.3 – 1 keV ≡ 12 – 40 Å “soft” 1 – 2 keV ≡ 6 – 12 Å 2 – 8 keV ≡ 1.5 – 6 Å “hard” Point sources – X-ray binaries, supernova remnants Hot phase of the interstellar medium Active nucleus M83 Chandra ACIS-S – true X-ray colours Tim Roberts - ULXs

3. X-ray source luminosities 47 GRBs QSOs Galactic populations SNRs AGNs ULXs Log X-ray luminosity (erg s-1) 37 CVs Supermassive black holes Stars NSs & BHs 27 Tim Roberts - ULXs

4. The missing link? Only have evidence for stellar-mass (<20 M) and supermassive (>106 M) black holes Is there a missing link between them? Tim Roberts - ULXs

5. The Eddington limit • Maximum theoretical luminosity for a spherically-accreting object • Balance between gravitational pull inwards and radiative push outwards • Directly proportional to mass of accretor LEdd ~ 1.3 × 1038 (M/M) erg s-1 • ULXs too big for stellar-mass black holes; can’t all be displaced AGN • A new, intermediate-mass class of black holes? Tim Roberts - ULXs

6. How do we test this? • Need the best instruments Chandra (NASA) XMM-Newton (ESA) Workhorse instruments for both - CCD imaging spectrometers Tim Roberts - ULXs

7. Black Hole accretion states Energy spectra from McClintock & Remillard (2006) Photon cm-2 s-1 keV-1 1 10 100 1 10 100 1 10 100 Energy (keV) High (thermal-dominated) ~ 1 – 2 keV disc + PL tail Low/hard Hard PL (Γ ~ 1.5 – 2) dominant, disc absent or truncated, radio jet emission. Least luminous. Very high (steep power-law) Soft PL (Γ > 2.5) plus some hot disc emission. Most luminous. Key point - accreting black hole X-ray spectra can be empirically modelled as the combination of an accretion disc spectrum and a power-law (corona) Tim Roberts - ULXs

8. XMM-Newton evidence for IMBHs • X-ray spectroscopy – cool accretion discs (Miller et al. 2003) NGC 1313 X-1 Tin M-0.25 kTin ~ 0.15 keV c.f. kTin ~ 1 keV for stellar BHs → ~ 1000 M BHs Tim Roberts - ULXs

9. 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 - ULXs

10. ULXs in starburst galaxies • Multiple ULXs (10+) are found in Starburst galaxies • Ongoing star formation  ULXs are short-lived • Requires an infeasibly large underlying population of IMBHs (King 2004) • Alternative: are ULXs in Starbursts “ordinary” high-mass X-ray binaries (HMXBs)? From Gao et al. (2003) Tim Roberts - ULXs

11. In support of stellar-mass BHs • How to exceed Eddington limit: • Relativistic beaming • Radiative anisotropy • Truly super-Eddington discs • Super-Eddington mass transfer rates in HMXBs can fuel ULXs • Blue stellar counterparts – high mass companions? • GRS1915+105 – demonstrates super-Eddington does happen in stellar-mass systems Potential X-ray luminosities for accretion onto a 10 M BH from 2 – 17 M secondaries (Rappaport, Podsiadlowski & Pfahl 2005) Tim Roberts - ULXs

12. Dichotomy • Highest X-ray 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 - ULXs

13. ULX X-ray spectra revisited • Key evidence for IMBHs from “cool disc” in XMM-Newton ULX spectra. 10+ examples • But not all ULXs show this spectral form: several have an “inverted” spectrum • e.g. NGC 55 ULX (Stobbart et al. 2004), NGC 5204 X-1 (Roberts et al. 2005) • Difficult to explain dominant soft power-law physically! NGC 1313 X-1 From Miller et al. (2003) kTin ~ 1.16 keV Γ ~ 2.5 M33 X-8 From Foschini et al. (2004) kTin ~ 0.15 keV “diskbb” – optically-thick accretion disc power-law – hot, optically-thin corona Tim Roberts - ULXs

14. A sample of bright ULXs 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 - ULXs

15. ULX accretion physics • Physical accretion disc plus corona model: cool discs, optically-thick coronae • ULXs operate differently to common Galactic black hole states, except… • “Strong” VHS in XTE J1550-564: energetically-coupled corona/ disc (Done & Kubota 2006). • Key features are a disc that appears cool as its inner regions are obscured by an energetic, optically-thick corona. From Done & Kubota (2006) Tim Roberts - ULXs

16. Holmberg II X-1 Goad, Roberts et al. 2006 • Archetypal luminous ULX (LX > 1040 erg s-1) – top IMBH candidate • Deep XMM-Newton obs (110 ks, though > 60% spoiled by bad space weather) • Best fit spectrum: cool disc (~ 0.2 keV) plus optically-thick ( ~ 6) corona EPIC spectrum of Ho II X-1 Data: pn/MOS1/MOS2 Model: diskpn + comptt Tim Roberts - ULXs

17. X-ray timing measurements – PSDs Frequency regime probed by XMM for bright ULXs Adapted from Vaughan et al. (2005) Courtesy P. Uttley Power Spectral Densities for two AGN and Galactic BH Cygnus X-1 Approximate scaling of break frequencies with mass Tim Roberts - ULXs

18. Ho II X-1: timing • Lack of strong variability • PSD analysis – compare to classic black hole states • Insufficient power for HS • May be in L/H or VHS - energy spectrum says latter • 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 - ULXs

19. How to progress? • X-ray measurements suggest smaller black holes; but no direct mass determination • As with Galactic BHs, ultimate test of the compact object mass in ULXs is dynamical studies • Mass function f(M) requires measurement of orbital period (P) and velocity amplitude of donor star (K*), from line features in optical spectrum: f(M) = MX3sin3i/(M*+MX)2 = PK*3sin3i/2πG • But ULX counterparts have mV ~ 22 – 26 (where identified) in crowded fields – not trivial! Tim Roberts - ULXs

20. 3 steps to the black hole mass • (1) Identify counterpart from HST imaging • Time awarded in cycle 14 to complete local ULXs • (2) Obtain spectra – suitable features? • Gemini spectra awarded for three ULX counterparts • (3) Monitoring campaign Tim Roberts - ULXs

21. New HST imaging ACS WFC F606W F330W F435WF606W mV ~ 24 mV > 27 mV ~ 25 Tim Roberts - ULXs

22. New HST imaging (2) Very blue colours - optical emission from accretion discs? mV ~ 22 mV ~ 25 mV ~ 26 Tim Roberts - ULXs

23. Radial velocity studies Gemini-S GMOS spectrum of NGC 1313 X-2 Possible to derive P, KBH from broad He II 4686Å accretion disc line? Done for Galactic BHs (e.g. Soria et al. 1998). Dynamic mass within reach? Tim Roberts - ULXs

24. State of play • Break present in 2 – 10 keV spectrum of many ULXs – not expected for sub-Eddington IMBHs • Cool disc, optically thick corona - similarities to very high accretion rate Galactic BHs • Ho II timing result – mass limit < 100 M • New theory - possible to produce BHs of up to 100 M in young (low metallicity) stellar populations • Strongly suggests ULXs are larger stellar-mass BHs accreting at around Ledd • Require dynamical mass measurements to prove this; a work in progress Tim Roberts - ULXs