Dynamical constraints on the mass of the black hole in a ULX

1. No Dynamical constraints on the mass of the black hole in a ULX ^ Tim Roberts Jeanette Gladstone (Alberta) Andy Goulding, Mark Swinbank, Martin Ward (Durham) Mike Goad (Leicester) Andrew Levan (Warwick)

2. ULX mass estimates • Wide variety of methods used to make indirect measurements of BH mass (e.g. Zampieri & Roberts 2009) • X-ray luminosity & spectra • Cool discs as IMBHs (e.g. Miller et al. 2003) • Disc-dominated spectral models giving MBH ~ 10 – 80 M (e.g. Vierdayanti et al. 2006) • Super-Eddington accretion models implying MBH < 100 M (e.g. Gladstone et al. 2009) Tim Roberts - ULX dynamical masses

3. ULX mass estimates (2) • X-ray timing measurements • Scaling QPO, break frequencies gives IMBHs (e.g. Strohmayer & Mushotzky 2009) • But lack of variability in some ULXs argues for small BHs (e.g Heil et al. 2009) • Optical constraints • Counterpart magnitudes & colours (e.g. Patruno & Zampieri 2010) • All degenerate, reliant on assumptions Tim Roberts - ULX dynamical masses

4. Dynamical masses: a heritage • To ‘weigh’ a BH – measure binary period and radial velocity shifts P From companion star: K* From compact object: Early Cyg X-1 data (Bolton 1975) from stellar absorption lines Radial velocity curve of GRO J1655 from He II line (Soria et al. 1998) Tim Roberts - ULX dynamical masses

5. Extragalactic dynamical masses • Recently: two X-ray binaries outside our Galaxy with BH dynamical masses • M33 X-7 (Orosz et al. 2007) – eclipsing system • IC 10 X-1 (Prestwich et al. 2007) – WR-BH binary with MBH ~ 24 – 33 M IC 10 X-1 radial velocity curve (Silverman & Filippenko 2009) Tim Roberts - ULX dynamical masses

6. Problems with doing this for ULXs… • Going from ~1 Mpc to several Mpc… • Faint counterparts (mv ~ 20.5 at best; typically > 24, Roberts et al. 2008) • Confused fields (stars & nebulae) • Observational challenge! HST data for Ho II X-1 (Kaaret et al. 2004) Tim Roberts - ULX dynamical masses

7. Previous attempts • Kaaret & Corbel (2009) – NGC 5408 X-1 • 6 VLT/FORS observations in 3 days • No absorption lines from stellar companion; broad He II (face-on system?) • Pakull, Grise et al. – NGC 1313 X-2 • Early observations show Δv ~ 380 km s-1 in He II line between epochs (Pakull et al. 2006) • Follow-up in 2008/9 – no radial velocity variations(?) Tim Roberts - ULX dynamical masses

8. Towards a ULX dynamical mass • We have been developing a programme to attempt to obtain ULX dynamical masses • 3-step process: • Identify counterparts from Chandra astrometry and HST imaging (Jeanette Gladstone thesis; Gladstone et al. in prep.) • Obtain pilot optical spectroscopy observations to identify useful spectral features • Undertake the RV measurements campaign Tim Roberts - ULX dynamical masses

9. Best targets Luminous ULXs (> 5 ×1039 erg s-1) NGC 1313 X-2 m555 = 23.3 NGC 5204 X-1 m606 = 22.3 Ho IX X-1 m555 = 22.5 Tim Roberts - ULX dynamical masses

10. Pilot spectroscopy results • Gemini G-MOS spectra obtained • B600 grating • Gemini-S • NGC 1313 X-2, 3 hr • Gemini-N • NGC 5204 X-1, 0.8 hr • Ho IX X-1, 1.5 hr Tim Roberts - ULX dynamical masses

11. New programme on Gemini • GN-2009B-Q-21: Ho IX X-1 (18.6 hours) • 1.5 hours per observation • GS-2009B-Q-20: NGC 1313 X-2 (29.2 hours) • 2.5 hours per observation • Strategy: request 10 observations performed on nightly intervals (when possible) • Liu et al. (2009): ~ 6 day period in NGC 1313 X-2 • Madhusudhan et al. (2008): 1 – 10 day periods expected for small BHs in ULXs Tim Roberts - ULX dynamical masses

12. Data obtained • NGC 1313 X-2: 1 obs. Oct, 9 in 13 nights Dec 2009 • Ho IX X-1: 5 obs. late Dec 2009, 5 obs. Feb 2010. Portion of 2-D spectrum from 26th Dec 2009 observation of Ho IX X-1 Tim Roberts - ULX dynamical masses

13. He II 4686Å shifts relative to nebula Data from Ho IX X-1 Tim Roberts - ULX dynamical masses

14. Periodicity? Best fit: P = 1.73 days, K = 77 km s-1 Reduced χ2 ~ 5 NOT PERIODIC Tim Roberts - ULX dynamical masses

15. Any better luck with NGC 1313 X-2? Tim Roberts - ULX dynamical masses

16. No. Best fit P = 3.01 days, K = 61 km s-1 Reduced χ2 ~ 4 Tim Roberts - ULX dynamical masses

17. Is all lost? Ho IX X-1 data No – He II line profile is broad and variable on timescales of days Tim Roberts - ULX dynamical masses

18. Similarly for NGC 1313 X-2 He II line profile broad and highly variable on timescales of days! Tim Roberts - ULX dynamical masses

19. Implications • He II emission from very close to ULX • Solution: accretion disc reprocessing? • Broadening implies not completely face-on • Line widths consistent with location very far out in disc… • …or a small velocity component in line-of-sight • If this is from disc, can use limit from scatter in velocity shift to put lower limit on BH mass • But other possibility: emission in wind? Tim Roberts - ULX dynamical masses

20. Mass limits 2σ upper limits: K < 97 km s-1 (Ho IX X-1); K < 182 km s-1 (NGC 1313 X-2) NGC 1313 X-2 6-day photometric period Ho IX X-1 Tim Roberts - ULX dynamical masses

21. One more thing… • Stack all 15 hrs observations to get high quality optical spectrum • Ho IX X-1 looks featureless, very blue - dominated by accretion disc light? Tim Roberts - ULX dynamical masses

22. NGC 1313 X-2: more interesting • Continuum less strong in 25 hrs on NGC 1313 X-2 • Possible stellar features appearing at short wavelengths? Tim Roberts - ULX dynamical masses

23. Conclusions • Ambitious programme to constrain the dynamical mass of two ULXs attempted • He II line centroid varies; but not periodically • Places no strong limit on BH mass • He II line is however broad and highly variable on timescales of days • Could be from the accretion disc: implying source close to face-on? Or from a wind? • Stacked spectra: very different counterparts Tim Roberts - ULX dynamical masses