Transcript Document

What are ultraluminous X-ray sources?
Tim Roberts
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
The Antennae - Chandra ACIS
 Chandra has resolved
populations in starburst
galaxies
 XMM-Newton: detailed
timing and spectroscopy
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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).
 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)?
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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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ULXs in starburst galaxies
 Multiple ULXs (10+) are
found in starburst galaxies.
 Ongoing star formation 
ULXs are intrinsically shortlived.
 Requires an infeasibly large
underlying population of
IMBHs (King 2004).
 Alternative: are ULXs in
starbursts high-mass Xray binaries (HMXBs)?
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From Gao et al. (2003)
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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?
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Potential X-ray luminosities for
accretion onto a 10 M BH from 2
– 17 M secondaries (Rappaport,
Podsiadlowski & Pfahl 2005)
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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 faceon it would be an ULX
(Begelman et al. 2006,
Poutanen et al. 2007)
 Precessing jet/outflow - link to ULX SS433: cartoon showing jet
nebulae?
precession & inclination
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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?
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ULX X-ray spectra revisited
 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!
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NGC 1313 X-1
kTFrom
1.16 keV
in ~ Miller
et al.
Γ ~ 2.5(2003)
M33 X-8
From Foschini
et al. (2004)
kTin ~
0.15 keV
“diskbb” – optically-thick
accretion disc
power-law – hot, optically-thin
corona
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A sample of bright ULXs
 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 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, opticallythick corona.
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From Done & Kubota (2006)
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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.
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X-ray timing – PSDs & QPOs
 Break frequencies in PSDs
related to black hole mass
and accretion rate by
McHardy et al. (2006)
T break  M
1 . 12
BH
 0 . 98
m Edd
 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)
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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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Ho II X-1: timing
Goad et al. 2006
 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)
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EPIC-pn light-curve of Ho II X-1
(0.3 – 6 keV, 100 s binning)
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Monitoring NGC 5204 X-1
Roberts et al. 2006
 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).
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Spectral variability in NGC 5204 X-1
+ 50-ks
+ 5-ks
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 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
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NGC 4490 revisited
Gladstone & Roberts in prep.
 Population of ULXs in NGC
4485/90 pair - spectral
variability follows harder-asbrighter 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)
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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
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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)
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HST/ACS images of
NGC 2403 X-1 - Roberts
et al. (in prep)
F330W
F435W
F606W
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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?
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