Ultraluminous Xray sources Roberto Soria 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 30 X-ray binaries ULX NS BH 32 38 39 Mass of accreting object QSO, AGN 44 46 max luminosity Eddington limit: L = 1. 3 1038 (M/Msun) erg/s Flux f L = 4 pd 2 f > 1039 erg/s: ULX log L (erg/s)

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)

3 2 4 HST/WFPC 2 image of NGC 4559 (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/WFPC 2 (Soria et al 2004)

Ha bubble/ring? Ha contours on a WFPC 2 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 NGC 4559 Expanding wave of star formation OB stars, young clusters and 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 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 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 NGC 4559 X 7 (XMM) Power-law (G ~ 2. 3) Tbb ~ 0. 12 ke. V

Thermal component at 0. 12 ke. V = disk emission?

ULXs and tidal interactions/collisions NGC 7715 NGC 7714 ULX on the tidal bridge between NGC 7714/15

More ULXs found in interacting/merging galaxies. M 81 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 ke. V

X-ray lightcurves for the ULX in NGC 5408 Flaring behaviour especially in hard X-rays (matter ejections? inner-disk oscillations? )

Power density spectrum for ULX in NGC 5408 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 NGC 4559