Transcript Document

Ultraluminous X-ray Sources
Tim Roberts
ULXs in the interacting galaxy pair
NGC 4485/4490 (Gladstone &
Roberts 2008 - also poster B.11)
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
Wednesday 28th May 2008
The Antennae - Chandra ACIS
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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)?
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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
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LX – kTin relationship
 IMBH candidates
occupy separate
part of parameter
space to stellarmass BHs.
 Strong evidence
for IMBHs as new
class underlying
luminous ULXs.
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From
Miller et al.
(2004)
LX  T4
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Vanishing IMBHs problem
From Grimm, Gilfanov
& Sunyaev (2003)
Break at ~ 2 ×
1040 erg s-1
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 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.
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The link with massive stars
From Gao et al. (2003)
High mass stars can feed
stellar mass black holes at a
sufficient rate to produce the
extreme X-ray luminosity
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
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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
Ý  For beaming factor b and super-Eddington
LEdd 
M
L
. .
1 ln Ý 
rate
M/M
b 
M Edd 
Edd
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Evidence from our own Galaxy
 Super-Eddington luminosities are
seen!
 GRS 1915+105 has intermittently
exceeded LEdd over its ~15 yr
outburst (Done et al. 2004)
 SS433 is super-critically
accreting
(perhaps exceeding
. .
M/MEdd by >103) - if seen face-on
it would be an ULX (Fabrika &
Mescheryakov 2001, Poutanen
et al. 2007)
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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?
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X-ray timing – PSDs & break frequency
 Break frequencies in PSDs related
to black hole mass and accretion
rate (McHardy et al. 2006)
Frequency
regime
probed by
XMM for
bright
ULXs
0.98
Ý
Tbreak  M1.12
m
BH
Edd
 But most ULXs show little or no
variability power (Feng & Kaaret
2005)

 Break feature in NGC 5408 X-1
PDS @ ~3 mHz (Soria et al. 2004;
Strohmayer et al. 2007) implies
mass of 100 - 1000 M
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Adapted from Vaughan
et al. (2005)
Scaling of break frequencies with mass,
assuming accretion at mdotEdd
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ULX QPOs
Double QPO in
NGC 5408 X-1
(from
Strohmayer et
al. 2007)
QPO in M82
X-1 (from
Strohmayer et
al. 2003)
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 Two ULXs with known
QPOs - both luminous
with LX > 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)
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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 “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.
Wednesday 28th May 2008
EPIC-pn light-curve of Ho II X-1
(0.3 – 6 keV, 100 s binning)
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ULX spectra revisited
 Look at best archival
XMM-Newton data
 Demonstrate that
2-10 keV spectrum fit
by a broken powerlaw in all of the
highest quality data
Stobbart, Roberts & Wilms 2006
Disc
Power-law
 Invalidates IMBH
model - hard
component is not a
simple 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)
from Kubota
& Done
(2004)
 ULXs operate differently to
common black hole states,
but…
 “Strong” VHS in XTE J1550564 (Done & Kubota 2006)
“ultraluminous
branch” (from
Soria 2007)
 Disc appears cool as its inner
regions are obscured by an
optically-thick corona.
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A new, ultraluminous accretion state?
 Spectrum defined by
apparently cool disc,
power-law turning over at
> 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
• ) onto stellar-mass black
M
Edd
holes - Ohsuga (2006, 2007)
 Extreme wind driven - column
~ 3  1024 cm2 at the poles,
much higher elsewhere
 Explains coronae, lack of
variability power, giant
nebulae…link 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 < 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)
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Nebula around Ho IX X-1
(Grise & Pakull 2006)
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F330W F435W F606W ACS WFC F606W
New HST imaging of ULXs
Early F supergiant?
NB. high extinction
m
F606W
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2008
= 23.9
No counterpart, tho’
very high extinction
m
> 26
Roberts, Levan & Goad (2008)
- arXiv:0803.4470v1
Consistent with late O
or early B star
F606W
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X-ray Sources
mF606W = 24.9
20
F330W F435W F606W ACS WFC F606W
New HST imaging of ULXs (2)
Young stellar cluster?
(MV ~ 8 - 9)
m
F606W
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2008
= 22.0
OB Star (odd colours?):
U-B ~ -1.4, B-V ~ 0.1
m
= 24.9
Real ULX, or related to
background galaxy?
F606W
Tim Roberts - Ultraluminous
X-ray Sources
mF606W = 25.621
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)
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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!
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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 (< 100 M) black holes
 ULX is an accretion state, not a source class
 Cannot rule out some larger IMBHs - NGC 5408 X1, M82 X-1 and HLXs (with LX > 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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