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Populations of X-ray sources in star-forming galaxies. Roberto Soria ( MSSL) K Wu, A Kong, M Pakull, R Kilgard , D Swartz. Contents. introduction why studying X-ray sources in other galaxies. luminosity and colour distributions

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Populations of X-ray sources

in star-forming galaxies

Roberto Soria (MSSL)

K Wu, A Kong, M Pakull, R Kilgard, D Swartz


  • introduction

    why studying X-ray sources in other galaxies

  • luminosity and colour distributions

    and what they tells us about the host galaxy

  • different physical classes of X-ray sources

  • case studies: M83, NGC300, M74, NGC 4449

  • multiwavelength comparisons

  • ”ultra-luminous sources”

what they are and how to test the models

Why studying X-ray sources in galaxies

  • use discrete X-ray sources and diffuse emission

    as a tool to understand galactic activity and evolution

  • do statistical studies of X-ray source populations

  • spatial distribution

  • luminosity & color distribution

  • different classes of compact objects

  • understand the properties of individual X-ray sources

spectra, lightcurves, state transitions,...

External triggers

Internal triggers?

Cold gas


Stellar evolution

PNe, SNe II, Ib/c

Hot gas (shocks)

Diffuse soft X-rays

Compact remnants


X-rays from accretion

Basic steps:


  • luminosity (count rate) distribution

  • spatial distribution, multi-band comparisons/identification

  • colour distributions for different classes of sources

Distinguish different physical types of X-ray binaries, SNR, SNe

Use X-ray sources as probes of galaxy structure and evolution

Cumulative luminosity distribution

of the discrete X-ray sources in a galaxy






Starburst/star-forming regions

“normal” spiral population



Breaks in the luminosity distribution

Luminosity functions in M83

Luminosity functions in M81

outside disk

outside disk

starburst nucleus

Breaks/features in the luminosity function may depend on:

  • Eddington limit for the neutron starsdistance indicator

  • ageing of the X-ray binary population(Wu 2001)

galactic history indicator

galactic nucleus

X-ray binary

Super-soft source


Wind / XRB?


X-ray pulsar

T ~ 0.6 keV

Starburst nucleus

High abund of Ne, Mg, Si,S

Low Fe/O, Fe/C

High C/O

T ~ 0.4 keV

ISM may be enriched by:

winds from WR stars,

core-collapse SNe

Spiral arms

Identification of the X-ray sources:

multiwavelength comparisons



HST/WFPC2 greyscale, Chandra contours

Ha greyscale (SSO), Chandra contours (0.3--8 keV)

Colour-colour plot

for bright M83 sources

X-ray binaries (BH, NS)

Soft sources (SNR +)

Supersoft sources

Candidate X-ray SNR are

associated to brighter HII regions


Ha greyscale (SSO), Chandra point sources

6 cm radio greyscale (VLA), Chandra point sources

...almost none in M31

Courtesy of S Trudolyubov et al, 2003 submitted

NGC 300

XMM OM image

(UV filters)

NGC 300

XMM OM image

(UV filters)

Optical SNRs

Radio SNRs

Comparing samples of SNR

  • radio-identified SNR: dense HII regions

    core-collapse SNe (young population)

  • optically-identified SNR: low-density regions

mostly Type Ia (old population)

  • X-ray SNR: both, but brighter when associated to radio SNR

Radio + X-ray (+ optical)



X-ray + optical


Type Ia

(Young) core-collapse X-ray SNe

SN in NGC 4449

  • thermal spectrum: emission from hot, shocked gas

  • non-thermal spectrum: dominated by synchrotron radiation (power-law spectrum)

SN 1978k in NGC 1313

Colour distribution for M74

SN2002ap in M74 seen by XMM

Thermal X-ray emission from SNe

Type II

  • High mass loss rate, low velocity wind,

SN 1993J

low velocity ejecta (< 30,000 km/s)


Hard X-rays first

Soft X-rays later





Optically thick

cool shell

L > ~ 1038erg/s

Thermal X-ray emission from SNe

Type Ib/c

  • Low mass loss rate, high velocity wind,

SN 2002ap

low velocity ejecta (< 30,000 km/s)


Hard X-rays negligible

Soft X-rays always visible




Optically thin

cool shell


Thermal X-ray emission from SNe

Type Ib/c

  • Relativistic ejecta? (> 100,000 km/s?)

SN 1998bw

Hard X-rays, g-rays


H, g




Thermal X-ray emission from SNe

Type Ib/c

  • Relativistic ejecta? (> 100,000 km/s?)

SN 1998bw

Hard X-rays, g-rays


H, g


“Ultra-luminous” sources

Emitted luminosity > Eddington limit for M = 7 Msun

2 x1038



Neutron stars

Black holes


Black holes






1 arcsec

X-1: diskbb (Tcol~ 0.6keV)

+ pow(G~ 2.6)

X-7:pow(G~ 2.1)

Where are they?

Found in 20% of spiral galaxies

40% of ellipticals (7 in Fornax A, 6 in NGC 1553)

most tidally-disrupted starburst (10 in Antennae)

Very old populations

Very young populations


All are persistent

Most variable by a factor of a few (over hours/yrs)

Long duty cycle? If so, how many quiescent sources?

State transitions?

Some similarities with Galactic BH (Roberts et al 2000)

X-ray spectrum?

Most are fitted by diskbb with scattering

T Tcol = f Teff where f ~1.5 – 3; Tcol~ 1keV

X-6 in M 81 (Swartz et al 2002)

Tcol= 1.1keV, Lx = 2.7 x 1039 erg/s

Others are fitted by a simple power-law

Some are “super-soft”, T ~ 0.07keV, Lbol~ 1039 erg/s

A few can be identified as SNR

Three possibilities for accreting ULX

M > 10 Msun , L < Ledd


M ~ 10 Msun , L >~ Ledd


M <~ 10 Msun , L <~ Ledd


Problem not settled yet, need better observations


Intermediate-mass BH: how to form them?

  • primordial (see eg Rees)

    feeding -- from a molecular cloud? (Grindlay)

    -- by capturing a companion?

  • in globular clusters from SN explosion of very massive stars?

    (from merging of many smaller BH?)

NO. Ineffective because of slingshot effect

  • in super star clusters ( = young globular clusters?)

    from merging of many stars  star of 500 Msun

     sinks to the cluster centre  SN  IMBH?

(eg, Ebisuzaki et al 2001)

...and how to observe them?

Optical counterparts, lightcurves, accretion disk lines

obtain mass function


“Super-Eddington” sources (not really)

Frad (L=Ledd) = Fgrav

Ledd = (4pcG) M / k = 1.3 1038 (M/Msun ) (0.40/k)

Thomson scattering opacity

Effective opacity for clumpy medium < for homogeneous medium

Shaviv 1998

Witt & Gordon 1996

Isichenko 1992 (“percolation theory”)

Where to observe this?

Look out for winds

Accretion disks around BH (Begelman 2002)

Classical novae (Shaviv 2001)

Wolf-Rayet, supergiant stars, h Car (Shaviv 2000)

Dust scattering in clumpy ISM (WG96)

Super-soft sources? AGN? Starburst galaxies?


Non-isotropic emission:

how to beam it?

(King et al 2001)

(Fabrika et al 2000)

Thermal-timescale mass transfer phase?

high mass transfer rate beaming?

Analogy with Galactic microquasars/microblazars

...and how to verify if it is beamed?

Optical (narrow-band) observations of X-ray ionized nebulae around ULX

can tell us whether X-ray source is beamed

(Pakull & Mirioni 2002)

New pieces of the ULX puzzle

Colours/spectra, time variability, spatial distribution

consistent with normal X-ray binaries


New pieces of the ULX puzzle


Most ULXs are in interacting/merging galaxies

(Swartz et al 2003)

Tidal interactions  higher star formation


ULXs in star-forming galaxies are young objects

Optical counterparts are O stars, OB associations


Many ULXs are in low-metallicity environments

(see Pakull’s work)

Weaker stellar wind  higher mass of the BH remnant

Most likely explanation for ULXs?

a 30-50 MsunBH accreting from an O star

via Roche-lobe overflow

Lx <~ Ledd

Lx = h M c2

Work in progress by Podsiadlowski, Heger, Langer, etc

Most likely explanation for ULXs?

a 30-50 MsunBH accreting from an O star

via Roche-lobe overflow

Three classes of X-ray binaries?

  • NS or BH accreting from a low-mass star via Roche-lobe overflow (LMXB)

  • NS or BH accreting from a high-mass star via stellar wind (HMXB)

  • NS or BH accreting from a high-mass star via Roche-lobe overflow (ULX + LMC X-4)

Statistical studies

of X-ray populations


Groups of galaxies


in nearby galaxies

Studies of individual sources

X-ray studies of

high-redshift galaxies