Ultraluminous Xray sources

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Ultra-luminous X-ray sources
Roberto Soria (University College
London) M. Cropper, C. Motch, R. Mushotzky, M.
Pakull, K. Wu
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Outline of this talk
What are ULXs? a few open questions
New results from our XMM-Newton study Spectral
and timing properties Optical counterparts Similar
ities and differences
What have we learnt and what do we do next?
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Ultra-luminous X-ray sources
CVs, stars
X-ray binaries
QSO, AGN
ULX
BH
NS
log L (erg/s)
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44
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Mass of accreting object max
luminosity
Eddington limit L 1.3 1038 (M/Msun) erg/s
Flux f L 4pd2 f gt 1039 erg/s ULX
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Main unsolved issue Beamed or isotropic
emission?
Mild geometrical beaming? (ULXs HMXBs)
Relativistic beaming? (ULXs microblazars)
No beaming? (ULXs intermediate-mass BHs)
If so, how are IMBHs formed?
Primordial stellar evolution (Pop III stars)?
Normal stellar evolution in peculiar environments?
Mergers of stellar-mass objects in dense clusters?
Other processes? (eg, accretion of satellites
with nuclear BHs?)
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Case study ULXs in NGC 4559 (Sc, d 10 Mpc)
XMM Optical Monitor image (near-UV band)
Cropper et al 2004, MNRAS Soria et al 2004, MNRAS
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XMM/EPIC image of NGC 4559
(Cropper et al 2004)
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2
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HST/WFPC2 image of NGC4559 (V)
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XMM Optical monitor (UV)
This ULX is in a large star-forming complex at
the outer edge of the spiral galaxy
Brightest ULX in NGC 4559
Lx 3 1040 erg/s
CHFT (Ha)
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1 arcsec
500 pc
HST/WFPC2 (Soria et al 2004)
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Ha bubble/ring?
Ha contours on a WFPC2 V image
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B and I contours of the dwarf irregular galaxy
near the ULX (HST)
Median Age gt 1 Gyr with a few younger stars (lt
30 Myr)
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Mini-Cartwheel scenario
Satellite dwarf galaxy punching through the
gas-rich disk
Density perturbation Expanding density wave
ULX in NGC4559
Expanding wave of star formation
OB stars, young clusters and ULXs
ULXs in the Cartwheel galaxy
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Clues on the nature of the BH
Young age (star forming region)
Super-star clusters, young clusters?
Low metal abundance (larger remnant)
Associated with galaxy collision
Different role of pressure and turbulence for
triggered star formation?
higher stellar masses?
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And what about the donor star and the other stars
in the field?
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Observed ULX luminosity suggests Roche-lobe mass
transfer
Roche-lobe mass transfer from star to BH can be
driven by
Nuclear evolution of the donor star (faster for
Supergiants, expand to bigger radii)
Orbital evolution (the binary system expands or
shrinks)
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Observed ULX luminosity suggests Roche-lobe mass
transfer
Roche-lobe mass transfer from star to BH can be
driven by
Nuclear evolution of the donor star (faster for
Supergiants, expand to bigger radii)
Orbital evolution (the binary system expands or
shrinks)
MBH lt Mstar
MBH gt Mstar
The orbit expands
The orbit shrinks
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MS stars shrink as they lose mass Sg expand
Orbital evolution allows steady mass
transfer from a M 1520 Msun supergiant onto a
M 40100 Msun black hole ensuring a luminosity
Lx a few 1040 erg/s for up to 106 yr Are
these at least one class of ULXs?
No unusual optical counterparts
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Power-law (G 2.3)
Tbb 0.12 keV
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Thermal component at 0.12 keV disk emission?
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ULXs and tidal interactions/collisions
NGC 7715
NGC 7714
ULX on the tidal bridge between NGC7714/15
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More ULXs found in interacting/merging galaxies.
M81 group
NGC 4449
Interacting galaxies have higher star-formation
rate number of ULXs SFR (like high-mass XRBs).
Why?
Interactions actively favour ULX formation?
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ULX (with radio emission) in NGC 5408
Kaaret et al (2003) Soria et al (2004)
Micro-blazar or milli-quasar?
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Tbb 0.12 keV
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X-ray lightcurves for the ULX in NGC5408
Flaring behaviour especially in hard
X-rays (matter ejections? inner-disk
oscillations?)
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Power density spectrum for ULX in NGC5408
Break in power spectrum suggests a mass M 100
Msun
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Work in progress
Relation between ULXs and star formation
triggered by collisions
Nature of the donor star, mechanism of mass
transfer
Nature of soft thermal component (disk photon
trapping/downscattering?)
Ionised nebulae around ULXs (images courtesy of
M Pakull)
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Work in progress
Relation between ULXs and star formation
triggered by collisions
Nature of the donor star, mechanism of mass
transfer
Nature of soft thermal component (disk photon
trapping/downscattering?)
Ionised nebulae around ULXs
Determine radio/X-ray flux ratios, radio spectral
index polarization
X-ray binaries in low-hard state
micro- blazars
microquasars
AGN, quasars
Fradio / Fx
Intermediate-mass BHs (milliquasars)?
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Work in progress
ULXs in molecular clouds? Accreting from
molecular clouds?
ULX and CO contours in NGC4559
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A canonical ULX
Lx 2 1040 erg/s
Tbb 120-140 eV
BH mass gt 100 Msun ?
Lbb lt 5 1039 erg/s
Smooth spectrum, no X-ray lines Always on, but
variability by factor 2 Break in
power-density-spectrum? Associated to
star-forming region
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High-velocity-cloud collision with outer disk of
a galaxy?
Initial perturbation
Expanding shocked layer
Collect and collapse
Young clusters OB associations ULXs
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Galaxy interactions/ mergers
High-velocity-cloud collisions
Clustered star formation
ULXs as by-products?
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ULXs and young star clusters
Core collapse of young star clusters
Dynamical friction
Mass segregation Most massive O stars sink to
the cluster centre
Equipartition instability core collapse Runaway
collisions of massive stars

Formation of a 200 - 500 Msun star
(Portegies Zwart et al. Rasio et al.)
Direct collapse into IMBH ? ULX
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ULXs and young star clusters
Many ULXs found in star-forming regions, near but
not exactly in super-star clusters
Formed in SSC then ejected? (SN
kicks?) Constraints on ejection velocity, age,
mass of BH
(eg, Zezas Fabbiano 2002--study of the ULXs in
Antennae)
Is there an alternative explanation?
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Condition for runaway collisions in cluster core
tcc 0.1 trh 107 yr (rh /1pc)3/2 (M /106
Msun)1/2 (1 Msun/ltmgt) lt tnucl 3 106 yr
For typical observed values of masses radii
M lt 104 Msun
We speculate that ULXs formed not in super-star
clusters but in medium-size clusters 103 lt M
lt 104 Msun
(Most super star clusters found near a ULX may
be unrelated)
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NGC7714/15
QSO
Nucl starburst AGN
ULX
XMM/EPIC Soria Motch 2004
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ULX luminosity varies Lx 4-6 1040 erg/s
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ULXs and ionised nebulae
(Pakull Mirioni 02)
NGC1313 X-2
Holmberg IX
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ULXs and ionised nebulae
What drives ionises the expanding gas nebulae?

• many normal supernovae?
• a hypernova?
• jets / outflows from ULX?

Possible observ. test for ULX-from-cluster models
Embedded young clusters with 103 lt M lt 104
Msun release their gas explosively in lt 1
Myr (popping clusters, Kroupa Boily 2002,
Lada et al 1984)

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ULXs and ionised nebulae
Possible observ. test for ULX-from-cluster models
Embedded young clusters with 103 lt M lt 104
Msun release their gas explosively in lt 1
Myr (popping clusters, Kroupa Boily 2002,
Lada et al 1984)

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ULXs and metal abundance
Many ULXs found in metal-poor galaxies Metallicity
effect?
Jeans mass MJ T3/2, Pop III stars more
massive (only relevant at Z lt 10-4 Zsolar)
Why?
Metal-poor stars leave bigger BH remnants
Metallicity effect on the donor star?
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Metallicity and the donor star
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L 1040 erg/s h 0.2
M 10-6 Msun /year
Accretion from Roche-lobe overflow Donor
Roche-lobe-filling O star? (thermal timescale or
nuclear timescale)

• An O star can fill its Roche lobe
• after common envelope phase
• when it expands as a supergiant

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Summary XMMHST study of a bright ULX in
NGC4559 ULXs from smaller clusters, not
super-star clusters? Metallicity effects on
binary stellar evolution (mass radii of BH
progenitor and O donor)
ULXs might be a by-product of clustered star
formation in low-metallicity environments
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Common features of ULX environments?
Interacting galaxies
Star-forming regions / rings / shells
ULXs
Young star clusters
Ionised gas nebulae
Metal-poor environment?
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ULX No 4
Transient X-ray source in the nuclear star cluster
Similar to the nuclei of M33 and other Scd
galaxies
Systematic study of Scd nuclei may be test for
IMBH formation in star clusters
ULX No 3
Located in spiral arm, in star-forming region
near bright young clusters, HII regions
Bright HMXB, Lx 3 1039 erg/s, BH mass 10
-15 Msun
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ULX No 2
Located in interarm region, inner part of the
galaxy No bright young clusters or HII regions
nearby
Lx 1040 erg/s, power-law spectrum
Beamed source? Accreted object?
Radio (MERLIN) and optical (HST/ACS) studies
planned for this source
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MS stars shrink as they lose mass Sg expand
Main sequence
Supergiants with M 1520 Msun