Ultra-luminous X-ray sources

Roberto Soria (University College London)M. Cropper, C. Motch, R. Mushotzky, M. Pakull, K. Wu

Outline of this talkOutline of this talk

New results from our XMM-Newton studyNew results from our XMM-Newton studySpectral and timing propertiesOptical counterpartsSimilarities and differences

What have we learnt and what do we do next?What have we learnt and what do we do next?

What are ULXs? What are ULXs? a few open questionsa few open questions

Ultra-luminous X-ray sourcesUltra-luminous X-ray sources

Mass of accreting object max luminosity

log L(erg/s)

30 32 38 39 44 46

CVs,stars

X-ray binaries QSO, AGN

BHNS

Eddington limit: L = 1.3 1038 (M/Msun) erg/s

Flux f L = 4d2 f > 1039 erg/s: ULXULX

ULXULX

Main unsolved issue:Main unsolved issue:Beamed or isotropic emission?Beamed or isotropic emission?

No beaming? No beaming? (ULXs = intermediate-mass BHs)

Relativistic beaming? Relativistic beaming? (ULXs = microblazars)

Mild geometrical beaming? Mild geometrical beaming? (ULXs = HMXBs)

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?)

If so, how are IMBHs formed?

Case study: ULXs in NGC 4559 ULXs in NGC 4559 (Sc, d = 10 Mpc)

Cropper et al 2004, MNRAS; Soria et al 2004, MNRAS

XMM Optical Monitor image(near-UV band)

XMM/EPIC image of NGC 4559

(Cropper et al 2004)

4

3

2

HST/WFPC2 image of NGC4559 (V)

This ULX is in a large star-forming complex at the outer edge of the spiral galaxy

Brightest ULX in NGC 4559

LLx x ~ ~ 3 103 104040 erg/s erg/s

XMM Optical monitor (UV)

CHFT (H)

HST/WFPC2 (Soria et al 2004)

1 arcsec

500 pc

H contours on a WFPC2 V image

H bubble/ring?

B and I contours of the dwarf irregular galaxy near the ULX (HST)

Median Age > 1 Gyrwith a few younger stars (< 30 Myr)

Mini-Cartwheel scenarioMini-Cartwheel scenario

ULX in NGC4559

Satellite dwarf galaxy punching through the gas-rich disk

Density perturbationExpanding density wave

Expanding wave of star formation

ULXs in the Cartwheel galaxy

OB stars, young clusters and ULXs

Clues on the nature of the BHClues 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 triggeredtriggered star formation?

higher stellar masses?

And what about the donor stardonor starand the other stars in the field?

Roche-lobe mass transfer from star to BH can be driven by:

Observed ULX luminosity suggests Roche-lobe mass transfer

Nuclear evolution of the donor star(faster for Supergiants, expand to bigger radii)

Orbital evolution(the binary system expands or shrinks)

Roche-lobe mass transfer from star to BH can be driven by:

Observed ULX luminosity suggests Roche-lobe mass transfer

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

Orbital evolution allows steady mass transferfrom a M ~ 15–20 Msun supergiantonto 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?

MS stars shrink as they lose mass; Sg expandMS stars shrink as they lose mass; Sg expand

No “unusual” optical counterpartsNo “unusual” optical counterparts

Power-law ( ~ 2.3)Tbb ~ 0.12 keV

X-ray spectrum of NGC4559 X7 (XMM)

Thermal component at 0.12 keV = disk emission?

ULX on the tidal bridge between NGC7714/15

NGC 7715

NGC 7714

ULXs and tidal interactions/collisionsULXs and tidal interactions/collisions

More ULXs found in interacting/merging galaxies.interacting/merging galaxies.

Interacting galaxies have higher star-formation rate;number of ULXs ~ SFR (like high-mass XRBs).

Interactions actively favour ULX formation?

M81 group NGC 4449

Why?Why?

ULX ((with radio emissionwith radio emission)) in NGC 5408

Micro-blazar or milli-quasar?

Kaaret et al (2003)Soria et al (2004)

Tbb ~ 0.12 keV

Flaring behaviour especially in hard X-rays(matter ejections? inner-disk oscillations?)

X-ray lightcurves for the ULX in NGC5408

Break in power spectrum suggests a mass M ~ 100 Msun

Power density spectrum for ULX in NGC5408

Work in progressWork 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 progressWork in progress

Relation between ULXs and star formation triggered by collisions

Nature of the donor star, mechanism of mass transfer

Determine radio/X-ray flux ratios, radio spectral index & polarizationDetermine radio/X-ray flux ratios, radio spectral index & polarization

Fradio / Fx

X-ray binariesin low-hard state

microquasarsmicro-blazars

AGN, quasars

Intermediate-mass BHs (“milliquasars”)?

Nature of soft thermal component (disk photon trapping/downscattering?)

Ionised nebulae around ULXs

Work in progressWork in progress

ULXs in molecular clouds? Accreting from molecular clouds?

ULX and CO contours in NGC4559