dynamical constraints on the mass of the black hole in a ulx

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

No

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)

Weds 26th May 2010 Tim Roberts - ULX dynamical masses 2

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


Dynamical masses: a heritage

To ‘weigh’ a BH – measure binary period and radial velocity shifts


Early Cyg X-1 data (Bolton 1975) from stellar absorption lines

€

f (M) =MX

3 sin3 i

M* + MX( )2 =

PK*3

2πG

K*

P

€

f (M) =M*

3 sin3 i

M* +MX( )2 =

PKX3

2πG

From companion star:

From compact object:

Radial velocity curve of GRO J1655 from He II line (Soria et al. 1998)

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)

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)

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(?)


Towards a ULX dynamical mass

We have been developing a programme to attempt to obtain ULX dynamical masses

3-step process:I. Identify counterparts from Chandra astrometry

and HST imaging (Jeanette Gladstone thesis; Gladstone et al. in prep.)

II. Obtain pilot optical spectroscopy observations to identify useful spectral features

III. Undertake the RV measurements campaign


Best targets


NGC 1313 X-2m555 = 23.3

NGC 5204 X-1m606 = 22.3

Ho IX X-1m555 = 22.5

Luminous ULXs (> 5 ×1039 erg s-1)

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


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 ULXsWeds 26th May 2010 Tim Roberts - ULX dynamical masses 11

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

He II 4686Å shifts relative to nebula


Data from Ho IX X-1

Periodicity?


Best fit: P = 1.73 days, K = 77 km s-1

Reduced χ2 ~ 5

NOT PERIODIC

Any better luck with NGC 1313 X-2?


No.


Best fit P = 3.01 days, K = 61 km s-1

Reduced χ2 ~ 4

Is all lost?


No – He II line profile is broad and variable on

timescales of days

Ho IX X-1 data

Similarly for NGC 1313 X-2


He II line profile broad and highly variable on timescales of days!

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?Weds 26th May 2010 Tim Roberts - ULX dynamical masses 19

Mass limits


NG

C 1

313

X-2

Ho

IX X

-12σ upper limits: K < 97 km s-1 (Ho IX X-1);

K < 182 km s-1 (NGC 1313 X-2)

6-day photometric

period

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?


NGC 1313 X-2: more interesting

Continuum less strong in 25 hrs on NGC 1313 X-2

Possible stellar features appearing at short wavelengths?


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 counterpartsWeds 26th May 2010 Tim Roberts - ULX dynamical masses 23

dynamical constraints on the mass of the black hole in a ulx

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