what are ultraluminous x-ray sources? tim roberts

What are ultraluminous X-ray sources?

Tim Roberts

Wednesday 5th September 2007 Tim Roberts - What are ULXs? 2

A definition, and an opportunity

ULX: an X-ray source in an extra-nuclear region of a galaxy with an observed luminosity in excess of 1039 erg s-1

Now accessible Chandra has resolved

populations in starburst galaxies

XMM-Newton: detailed timing and spectroscopy

The Antennae - Chandra ACIS


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 106 M); fall to Galactic centre in a Hubble time due to effects of dynamical friction.

Too massive for stellar remnants (3M MBH 18M). NB. Evolution of a single star very unlikely to form black hole

more massive than ~ 20 M (Fryer & Kalogera 2001). Are we observing a new, 102 – 105 M “intermediate mass”

class of accreting black hole (IMBHs; e.g. Colbert & Mushotzky 1999)?


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

c.f. kTin ~ 1 keV for stellar BHs

→ ~ 1000 M BHs


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


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 LEdd like this.

From Grimm, Gilfanov & Sunyaev (2003)

Break at ~ 2 × 1040 erg s-1


ULXs in starburst galaxies Multiple ULXs (10+) are

found in starburst galaxies. Ongoing star formation

ULXs are intrinsically short-lived.

Requires an infeasibly large underlying population of IMBHs (King 2004).

Alternative: are ULXs in starbursts high-mass X-ray binaries (HMXBs)?

From Gao et al. (2003)


In support of stellar-mass BHs

How to exceed Eddington limit: 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)

Super-Eddington mass transfer rates in HMXBs can fuel ULXs.

Blue stellar counterparts – high mass companions?

Potential X-ray luminosities for accretion onto a 10 M BH from 2 – 17 M secondaries (Rappaport, Podsiadlowski & Pfahl 2005)


Evidence from our own Galaxy GRS 1915+105 has intermittently

exceeded the Eddington limit over its ~15 year outburst (Done et al. 2004)

SS433 is super-critically accreting (perhaps exceeding mdotEdd by >103) - if seen face-on it would be an ULX (Begelman et al. 2006, Poutanen et al. 2007) Precessing jet/outflow - link to ULX

nebulae? SS433: cartoon showing jet precession &

inclination


The situation in early 2005 …

Dichotomy X-ray evidence such as extreme luminosities,

cool accretion discs and M82 X-1 QPOs point to IMBHs, but…

Other evidence stacking up in favour of smaller black holes.

Which one is the correct interpretation?


Key evidence for IMBHs from “soft excess” in XMM-Newton ULX spectra. 10+ examples.

But mass estimate only valid from disc-dominated HS spectrum; ULXs patently aren’t in HS!

Furthermore, 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-1From Miller et al. (2003)

kTin ~ 0.15 keV

“diskbb” – optically-thick accretion discpower-law – hot, optically-thin corona

ULX X-ray spectra revisited

kTin ~ 1.16 keV

Γ ~ 2.5

M33 X-8From Foschini et al. (2004)


A sample of bright ULXs

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

Stobbart, Roberts & Wilms 2006

Disc

Power-law


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)


Other explanations for spectral break “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). Physical similarities to optically-thick corona?

Fully comptonised VHS with spectrum modified by ionised fast outflow (Goncalves & Soria 2006).

Common thread: high accretion rate, small black holes.


X-ray timing – PSDs & QPOs Break frequencies in PSDs

related to black hole mass and accretion rate by McHardy et al. (2006)

QPOs are detected; but only low-frequency limited diagnostic value? Suggest 100 - 1000 M

IMBHs in M82 X-1, NGC 5408 X-1 (Strohmayer et al. 2003, 2007)

Adapted from Vaughan et al. (2005)

Frequency regime

probed by XMM for

bright ULXs

Scaling of break frequencies with mass, assuming accretion at mdotEdd

98.012.1 EddBHbreak mMT


Ho II X-1: timing PSD analysis – compare to

classic BH states. 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.

Lack of variability predicted for hyper-accretion (Ohsuga 2007)

EPIC-pn light-curve of Ho II X-1

(0.3 – 6 keV, 100 s binning)

Goad et al. 2006


Monitoring NGC 5204 X-1

First XMM-Newton or Chandra programme looking at ULX variability on time scales days - weeks.

Similar amplitude of variability to GRS 1915+105 (tho’ no limit-cycle variations).

Roberts et al. 2006


Spectral variability in NGC 5204 X-1 As flux increases,

gain hard counts. Spectral modelling -

heating/cooling of optically thick corona.

Behaviour the same as XTE J1550-564 – strong VHS!

Could also describe disc heating

+ 50-ks

+ 5-ks


NGC 4490 revisited Population of ULXs in NGC

4485/90 pair - spectral variability follows harder-as-brighter trend

Seen in many ULXs now Recent examples - flaring

event in NGC 1365 X1 (Soria et al. 2007), cool disc component of NGC 1313 X-2 not following LX T4 (Feng & Kaaret 2007)

Gladstone & Roberts in prep.


A sky full of ULXs

ULXs are now showing up as a minority population in deep surveys (Hornschemeier et al. 2004, Watson et al. 2005)

Cross-correlation of 2XMMp with RC3 - 297 ULXs (contamination @ ~15% level) This is with many ULXs in starbursts excluded! ~ 1 in 5 with LX > 1040 erg s-1

3 candidate hyper-luminous X-ray sources (HLXs) Some indication of spectral changes with luminosity/

energy - but data quality mixed


A multi-wavelength perspective Optical - counterparts and

bubbles 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)

Optical/IR modelling of counterparts - small black holes (~100 M or less; Copperwheat et al. 2007)

HST/ACS images of NGC 2403 X-1 - Roberts

et al. (in prep)

F330W

F435W

F606W


So, what are ULXs? Bulk of evidence - few keV X-ray spectral breaks,

spectral variability, star formation link etc - argues most ULXs are extreme accretion rate, smallish (<100 M) black holes

ULX is an accretion state, not a source class Formation of black holes up to 100 M possible in

massive binary stars (Belczynski et al. 2006) or very massive, low metallicity stars (Fryer & Kalogera 2001)

Cannot rule out individual cases of larger IMBHs - HLXs are the best candidates?

what are ultraluminous x-ray sources? tim roberts

Documents

tim roberts slide

luminous ulxs

bhs slide

thick coronae ulxs

sample of bright ulxs

roberts wilms

disc powerlaw slide

ongoing star formation