Ultra-luminous X-ray sources Roberto Soria (University College London) M. Cropper, C. Motch, R. Mushotzky, M. Pakull, K. Wu

Outline of this talk 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? What are ULXs? a few open questions

Ultra-luminous X-ray sources Mass of accreting object max luminosity log L (erg/s) CVs, stars X-ray binariesQSO, AGN BHNS Eddington limit: L = (M/M sun ) erg/s ULX Flux f L = 4  d 2 f > erg/s: ULX ULX

Main unsolved issue: Main unsolved issue: 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?

ULXs in NGC 4559 Case study: 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 L x ~ 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 Gyr with a few younger stars (< 30 Myr)

Mini-Cartwheel scenario ULX in NGC4559 Satellite dwarf galaxy punching through the gas-rich disk Density perturbation Expanding density wave Expanding wave of star formation ULXs in the Cartwheel galaxy OB stars, young clusters and ULXs

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 triggered for triggered star formation? higher stellar masses?

donor star And what about the donor star and 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) M BH < M star The orbit shrinks M BH > M star The orbit expands

Orbital evolution allows steady mass transfer from a M ~ 15–20 M sun supergiant onto a M ~ 40–100 M sun black hole ensuring a luminosity L x ~ a few erg/s for up to ~ 10 6 yr Are these at least one class of ULXs? MS stars shrink as they lose mass; Sg expand No “unusual” optical counterparts

Power-law (  ~ 2.3) T bb ~ 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/collisions

interacting/merging galaxies. More ULXs found in 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?

(with radio emission) ULX (with radio emission) in NGC 5408 Micro-blazar or milli-quasar? Kaaret et al (2003) Soria et al (2004)

T bb ~ 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 M sun Power density spectrum for ULX in NGC5408

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 Determine radio/X-ray flux ratios, radio spectral index & polarization F radio / F x X-ray binaries in low-hard state microquasars micro- blazars AGN, quasars Intermediate-mass BHs (“milliquasars”)? Nature of soft thermal component (disk photon trapping/downscattering?) Ionised nebulae around ULXs

Work in progress ULXs in molecular clouds? Accreting from molecular clouds? ULX and CO contours in NGC4559