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									        Ultraluminous X-ray Sources

  Andrew King, University of Leicester
           Penn State 22.5.04

² Lx(apparent) > 1039 erg s-1 = LEdd(10 M¯)


² do ULXs contain intermediate—mass
  black holes, M » 102 – 104 M¯ (IMBH) ?
major constraint: ULX – star formation
connection, e.g. Antennae
Using IMBH to make ULXs in star-forming
             galaxies

1. If IMBH are primordial (Pop III),
  new star clusters must `light up’ accretion:
  -- unclear how a primordial IMBH
  acquires a companion star
IMBH formation in dense star clusters?
either
2. merge stars, tmerge << tMS and build up
   large M (Gurkan et al. 2003; Portegies
   Zwart et al., 2004)
   problem: mass loss in merger?
or
3. merge black holes  IMBH (Miller &
   Hamilton, 2002)
   problem: GR reaction:
   merged BH lost from cluster with low M
in all 3 cases, any ULX is formed in a cluster

² most ULXs are observed near but outside
  clusters -- must eject (with companion
  star?)

² make at most 1 ULX per cluster, i.e.
  > 105 M¯ needed to make each ULX
could ULXs instead be an unusual
phase of X-ray binary evolution?


        (King et al., 2001)
     (Grimm, Gilfanov & Sunyaev, 2003)
no break at 1039 erg s-1: most ULXs are HMXBs
        likely candidates: 2 types

(1) high—mass X—ray binaries

 thermal—timescale mass transfer rate
 Mdot(tr) » Mdonor/tKH » 10-4 - 10-3 M¯ yr-1

 nuclear-timescale mass transfer rates
 comparable: black hole mass can grow
 significantly
star formation

MS evolution of massive stars, < 108 yr

high-mass X-ray binary
(wind-fed) » 104, 5 yr


star fills Roche lobe,
very high Mdot,
ULX phase, » 103, 4 yr
    ULX phase reached in < 108 yr after SF
     high—mass X—ray binaries:


² present in star-forming regions

² found near but outside clusters – SNe kicks

² thermal—timescale phase is like SS433 viewed
   `from the side’
(2) bright, long-lived soft X-ray transient outbursts
                        (SXTs)




   low-mass donor       black hole with unstable
                        accretion disc (cool edges)
² present in both ellipticals and spirals
² long outbursts like GRS 1915+105 (on since 1992)
How does an X—ray binary appear so luminous?

                 39
² LEdd = 4.4 £ 10 erg s-1

(20 M¯ BH, hydrogen-depleted accretion)

 two ways of increasing this: (1)

² GRS 1915+105 has L > 6 £ 1039 erg s-1
   with BH mass 14M¯, i.e. > 3 LEdd

 ² with mild anisotropy apparent luminosity
   can reach » 4 £1040 erg s-1
               luminosity (2)

² extremely high mass transfer rates
  Mdot (tr) » 103 – 104 Mdot(Edd)

² outer disc `unaware’ of this until
   radius REdd where

      GMMdot(tr) /REdd » Ledd_
² then total disc luminosity is

  Ldisc = Ledd[1 + ln(Mdot(tr)/Mdot(Edd)]

             » 10LEdd
² thus expect L » 1 – 4 £ 1040 erg s-1 for
   20M¯ BH with hyper-Eddington accretion

² characteristic blackbody radius R » 109 cm

² cf ultrasoft components in ULXs e.g.
  NGC 1313 (Miller et al 2003: – if instead
  R is assumed to relate to BH size, get
  M » 103 M¯)
     Outflows from ULXs

² Mdot >> Mdot(Edd), so most mass expelled

² optically thick outflow with
   Mdot(out)v » LEdd/c

² outflow momentum sweeps up ISM

 nebula
  Eout » h M2c2 » 1052 erg

» hypernova energy

² ULX nebulae larger than SNR

² supermassive BH analogue 
   M-s relation for galaxies:
Gao et al., 2003
      star formation ring began expanding
      t* = 3 £ 108 yr ago, but takes < 107 yr to pass
      any radius

ULXs live tlife < 107 yr, so number of `dead’
ones inside ring is

      N > (n/bd)(t*/tlife) > 300/bd

where b is anisotropy and d is duty cycle
(both <1) (King, 2004)
² mass transfer lifetime ~ M2/L of ULX
               < 107 yr

² companion star’s MS lifetime < 107 yr,
  otherwise ULXs form after ring has passed

² consistent with 3000 super—Eddington
   HMXBs with M2 > 15M¯

² but IMBH binaries transient (small disc) so
  duty cycle d << 1

² requires > 3£ 104 IMBH, and thus > 1010M¯ in
  clusters, most mass not accreted
² population properties of ULXs in
  star-forming galaxies similar to HMXBs,
  but incompatible with IMBH

² luminosities suggest HMXBs in
   super-Eddington phase

² outflows  nebulae

 most ULXs are HMXBs or SXTs
² exception? M82 ULX : L > 1041 erg s-1
    too high for stellar-mass BH ?

 other sources possible too, but may be
 superpositions (check variability)
² number of such `hyperluminous X—ray sources’
  (HLXs) is very small – at most one per few
  galaxies

² Occam’s razor: try existing BH models –
  stellar—mass binaries or galactic nuclei

² not stellar—mass: galactic nuclei?
   (King & Dehnen, 2004)
² hierarchical merging
  every large galaxy has 10 – 100 satellites




² most orbits miss host, but occasional collisions
² if colliding satellite retains central
   BH and star cluster, tides trigger accretion,
   just like AGN

² satellite can have BH mass > 104 M¯

² accretion time << orbital timescale: HLX
  activity only close to galaxy plane


² passage of satellite stimulates star formation:
  HLX accompanied by stellar—mass ULXs
                    Summary

² most ULXs are stellar—mass XRBs rather than
  IMBHs (L < 1041 erg s-1 )

² high L from large accretion rate, super—Eddington
  accretion or anisotropic emission

² HLXs (L > 1041 erg s-1 ) may be captured satellite
  galaxy nuclei
² ULX – star formation and HLX – galaxy formation
  links

								
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