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
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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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