Ultraluminous X-ray Sources

Andrew King, University of Leicester

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

² do ULXs contain intermediate—mass black holes, M » 102 – 104 M¯ (IMBH) ?

Penn State 22.5.04

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?or3. 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

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’

high—mass X—ray binaries:

low-mass donor black hole with unstableaccretion disc (cool edges)

(2) bright, long-lived soft X-ray transient outbursts (SXTs)

² present in both ellipticals and spirals² long outbursts like GRS 1915+105 (on since 1992)

² LEdd = 4.4 £ 1039 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

How does an X—ray binary appear so luminous?

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 » M2c2 » 1052 erg

» hypernova energy

² ULX nebulae larger than SNR

² supermassive BH analogue M- 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 HLXs (L > 1041 erg s-1 ) may be captured satellite galaxy nuclei

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

² ULX – star formation and HLX – galaxy formation links