1. Dynamical constraints on the mass of the black hole in a ULXNo ^
Tim Roberts
Jeanette Gladstone (Alberta)
Andy Goulding, Mark Swinbank, Martin Ward (Durham)
Mike Goad (Leicester)
Andrew Levan (Warwick)

2. Tim Roberts - ULX dynamical masses
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)
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Tim Roberts - ULX dynamical masses

3. Tim Roberts - ULX dynamical masses
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
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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)
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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)
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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)
Weds 26th May 2010
Tim Roberts - ULX dynamical masses

7. Tim Roberts - ULX dynamical masses
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(?)
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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
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Tim Roberts - ULX dynamical masses

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

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

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

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

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

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

19. Tim Roberts - ULX dynamical masses
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?
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Tim Roberts - ULX dynamical masses

20. Mass limits NGC 1313 X-2 Ho IX X-1
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
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Tim Roberts - ULX dynamical masses

21. Tim Roberts - ULX dynamical masses
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?
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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?
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Tim Roberts - ULX dynamical masses

23. Tim Roberts - ULX dynamical masses
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
Weds 26th May 2010
Tim Roberts - ULX dynamical masses