Transcript What are ULXs? - University College London

Ultra-luminous X-ray sources
Roberto Soria (University College London)
M. Cropper, C. Motch, R. Mushotzky, M. Pakull, K. Wu
Outline of this talk
What are ULXs? a few open questions
New results from our XMM-Newton study
Spectral and timing properties
Optical counterparts
Similarities and differences
What have we learnt and what do we do next?
Ultra-luminous X-ray sources
CVs,
stars
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X-ray binaries
ULX
NS BH
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QSO, AGN
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log L
(erg/s)
Mass of accreting object
max luminosity
Eddington limit: L = 1.3 1038 (M/Msun) erg/s
Flux f
L = 4pd2 f > 1039 erg/s: ULX
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?)
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
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)
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)
1 arcsec
500 pc
HST/WFPC2 (Soria et al 2004)
Ha bubble/ring?
Ha contours on a WFPC2 V image
B and I contours of the
dwarf irregular galaxy
near the ULX (HST)
Median Age > 1 Gyr
with a few younger stars
(< 30 Myr)
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
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?
And what about the donor star
and the other stars in the field?
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)
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 < Mstar
The orbit shrinks
MBH > Mstar
The orbit expands
MS stars shrink as they lose mass; Sg expand
Orbital evolution allows steady mass transfer
from a M ~ 15–20 Msun supergiant
onto a M ~ 40–100 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
X-ray spectrum of NGC4559 X7 (XMM)
Power-law (G ~ 2.3)
Tbb ~ 0.12 keV
Thermal component at 0.12 keV = disk emission?
ULXs and tidal interactions/collisions
NGC 7715
NGC 7714
ULX on the tidal bridge between NGC7714/15
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?
ULX (with radio emission) in NGC 5408
Kaaret et al (2003)
Soria et al (2004)
Micro-blazar or milli-quasar?
Tbb ~ 0.12 keV
X-ray lightcurves for the ULX in NGC5408
Flaring behaviour especially in hard X-rays
(matter ejections? inner-disk oscillations?)
Power density spectrum for ULX in NGC5408
Break in power spectrum suggests a mass M ~ 100 Msun
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)
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
microquasars
microblazars
AGN, quasars
Fradio / Fx
Intermediate-mass
BHs (“milliquasars”)?
Work in progress
ULXs in molecular clouds? Accreting from molecular clouds?
ULX and CO contours in NGC4559