Ultraluminous X-ray Sources

1
Ultraluminous X-ray Sources
Tim Roberts
ULXs in the interacting galaxy pair NGC 4485/4490
(Gladstone Roberts 2008 - also poster B.11)
2
A definition

• ULX an X-ray source in an extra-nuclear region
  of a galaxy with an observed luminosity in excess
  of 1039 erg s-1
• Heterogeneous population - includes some recent
  supernovae - but bulk of sources are black holes
  accreting from a secondary star

The Antennae - Chandra ACIS
3
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?).
• Are we observing a new, 102 105 M?
  intermediate mass class of accreting black hole
  (IMBHs e.g. Colbert Mushotzky 1999)?

4
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
? 1000 M? BHs
c.f. kTin 1 keV for stellar BHs
5
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
6
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
7
The link with massive stars
High mass stars can feed stellar mass black
holes at a sufficient rate to produce the extreme
X-ray luminosity
From Gao et al. (2003)
Potential X-ray luminosities for accretion onto a
10 M? BH from 2 17 M? secondaries (Rappaport,
Podsiadlowski Pfahl 2005)
Populations of ULXs (10) detected in bright
starbursts - ULXs must be short-lived, so cannot
all be IMBHs
8
Physical processes

• Still need to break the Eddington limit
  suggested methods include
• 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)
• Can combine at least two of the above, e.g. King
  (2008) - within Rsph local energy release is kept
  Eddington by driving a bi-conical outflow so
  apparent line-of-sight Bolometric luminosity is

For beaming factor b and super-Eddington rate
M/MEdd
. .
9
Evidence from our own Galaxy

• Super-Eddington luminosities are seen!
• GRS 1915105 has intermittently exceeded LEdd
  over its 15 yr outburst (Done et al. 2004)
• SS433 is super-critically accreting (perhaps
  exceeding M/MEdd by gt103) - if seen face-on it
  would be an ULX (Fabrika Mescheryakov 2001,
  Poutanen et al. 2007)

. .
SS433 cartoon showing jet precession
inclination
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A pause for reflection

• Dichotomy
• X-ray evidence such as extreme 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?

11
X-ray timing PSDs break frequency

• Break frequencies in PSDs related to black hole
  mass and accretion rate (McHardy et al. 2006)
• But most ULXs show little or no variability power
  (Feng Kaaret 2005)
• Break feature in NGC 5408 X-1 PDS _at_ 3 mHz (Soria
  et al. 2004 Strohmayer et al. 2007) implies mass
  of 100 - 1000 M?

Frequency regime probed by XMM for bright ULXs
Adapted from Vaughan et al. (2005)
Scaling of break frequencies with mass, assuming
accretion at mdotEdd
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ULX QPOs

• Two ULXs with known QPOs - both luminous with LX
  gt 1040 erg s-1
• Cannot be beamed
• Scaling arguments from Galactic black holes -
  masses 100 - 1000 M? if in known state (talk by
  Zampieri Casella et al. 2008)

Double QPO in NGC 5408 X-1 (from Strohmayer et
al. 2007)
QPO in M82 X-1 (from Strohmayer et al. 2003)
13
Ho II X-1 timing
Goad et al. 2006

• Ho II X-1 is a good example of a ULX with little
  variability power - can we explain this using
  known accretion states?
• Not disc-dominated
• Insufficient power for high or classic very high
  states
• Energy spectrum not low/hard state
• Similar to ?-class of GRS 1915105 in VHS?
• Band-limited PSD - but dont see variability, so
  must be at high-f ? MBH lt 100 M?.

EPIC-pn light-curve of Ho II X-1 (0.3 6 keV,
100 s binning)
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ULX spectra revisited
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
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ULX spectra vs Galactic black holes

• Physical accretion disc plus corona model cool
  discs (kT 0.1-0.3 keV), optically-thick coronae
  (? 5 - 100)
• ULXs operate differently to common black hole
  states, but
• Strong VHS in XTE J1550-564 (Done Kubota
  2006)
• Disc appears cool as its inner regions are
  obscured by an optically-thick corona.

from Kubota Done (2004)
ultraluminous branch (from Soria 2007)
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A new, ultraluminous accretion state?

• Spectrum defined by apparently cool disc,
  power-law turning over at gt 2 keV. Little or no
  variability power present. Occurs at extreme
  accretion rates

Low hard state in GX339-4 vs a classic ULX, Ho IX
X-1
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The importance of winds

• Hydrodynamical simulations of extreme accretion
  rates (M gtgt MEdd) onto stellar-mass black holes -
  Ohsuga (2006, 2007)
• Extreme wind driven - column 3 ? 1024 cm2 at
  the poles, much higher elsewhere
• Explains coronae, lack of variability power,
  giant nebulaelink to high-Z QSOs, Galactic-scale
  feedback

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Other explanations for spectral break

• Kerr disc models (Makishima et al. 2000)
• 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).
• Fully comptonised VHS with spectrum modified by
  ionised fast outflow (Goncalves Soria 2006).
• Common thread high accretion rate, small black
  holes (MBH lt 100 M?).

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A multi-wavelength perspective

• Optical - counterparts and beambags (cf. Pakull
  Grisé 2008)
• 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)
• Identified ULX counterparts are blue - OB stars
  (e.g. Liu et al. 2004, Kuntz et al. 2005)

Nebula around Ho IX X-1 (Grise Pakull 2006)
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New HST imaging of ULXs
Roberts, Levan Goad (2008) - arXiv0803.4470v1
ACS WFC F606W
Early F supergiant? NB. high extinction
No counterpart, tho very high extinction
Consistent with late O or early B star
F330W F435W F606W
mF606W 23.9
mF606W gt 26
mF606W 24.9
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New HST imaging of ULXs (2)
ACS WFC F606W
Young stellar cluster? (MV 8 - 9)
OB Star (odd colours?) U-B -1.4, B-V
0.1
Real ULX, or related to background galaxy?
F330W F435W F606W
mF606W 22.0
mF606W 24.9
mF606W 25.6
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Are these really secondary stars?

• High LX will affect optical emission
  reprocessing in accretion disc becomes more
  important to optical light as black hole mass
  increases
• Stellar heating stars may be later types than
  initial colour IDs suggest (late B, not late
  O/early B) (Patruno Zampieri 2008 Copperwheat
  et al. 2007) - small black holes
• Alternatively, IMBHs may be favoured (Madhusudhan
  et al. 2008)

Stellar-mass BHs
IMBHs
From Madhusudhan et al. (2008)
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The goal mass functions

• Urgency to finding counterparts race to get
  first ULX mass function
• Best way to resolve mass controversy!

He II 4686Å line from accretion disc of NGC 1313
X-2 300 km s-1 shift (Pakull et al. 2006).
Could be used to constrain RV curve, hence
constrain ULX black hole mass
Radial velocity curve from extragalactic
Wolf-Rayet black hole binary IC 10 X-1. Uses He
II 4686Å line to constrain mass function, find a
black hole mass of 24 - 33 M? (Silverman
Filippenko 2008)
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So, what are ULXs?

• Bulk of evidence - few keV X-ray spectral breaks,
  star formation link etc - argues most ULXs are
  extreme accretion rate, small (lt 100 M?) black
  holes
• ULX is an accretion state, not a source class
• Cannot rule out some larger IMBHs - NGC 5408 X-1,
  M82 X-1 and HLXs (with LX gt 1041 erg s-1) are the
  best candidates?
• Mass functions are within reach - will resolve
  the controversy for at least some ULXs

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(Re-)fueling problem

• Best way of providing fuel supply companion
  star.
• Alternative molecular cloud disruption (Krolik
  2004).
• New modelling very difficult to form stable
  IMBH-ULXs, underpredict ULXs by 10-100
  (Madhusudhan et al 2006).
• Though plausible in dense stellar clusters
  (Baumgardt et al. 2006).

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A note on BH states
Energy spectra from McClintock Remillard (2003)
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 (G 1.5 2) dominant, disc
absent or truncated, radio jet emission. Least
luminous.
Very high (steep power-law) Soft PL (G gt 2.5)
plus some hot disc emission. Most luminous.
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M82 X-1 Best IMBH candidate?

• Extreme ULX in M82 (LX,peak 1041 erg s-1).
• Central density of MGG 11 sufficient for creation
  of IMBH via stellar mergers/collapse.
• Detection of X-ray QPOs flux is relatively
  isotropic.
• BUT could be nucleus of accreted galaxy etc.

From Portegies-Zwart et al. (2004) See also
Kaaret et al. (2001), Strohmayer Mushotzky
(2003), King Dehnen (2005), Mucciarelli et al.
(2006)