What are the essential ingredientsWhat are the essential ingredients of ultraluminous X-ray sources?of ultraluminous X-ray sources?

Roberto Soria (CfA & MSSL)Roberto Soria (CfA & MSSL)

Some ULX collaborators: M Cropper, C Copperwheat (MSSL),R Fender (Southampton), Z Kuncic, C Hung (Sydney), D Swartz (MSFC), A Goncalves (Paris-M), M Pakull, F Grise’ (Strasbourg), R. Mushotzky (GSFC)

What we’d like to know about ULXsWhat we’d like to know about ULXs

1)1) MassMass No direct (kinematic) mass determination yet. Two or three candidates perhaps feasible now.

2) How to gain a factor of ~ 50 in apparent 2) How to gain a factor of ~ 50 in apparent LLxx

with respect to stellar-mass BHs

Beamed (microblazars?)

Not beamedHigher BH mass (IMBHs?)

Super-Eddington luminosity

Searching for common features Searching for common features in the ULX populationin the ULX population

““Soft-excess” in their X-ray spectra?Soft-excess” in their X-ray spectra?

Signature of a cool disk? higher BH mass?

Power law ( ~ 2)

“soft excess”

kT ~ 0.15 keV

Holmberg II X-1 (Lx ~ 2E40 erg/s)

Holmberg II X-1 (Lx ~ 2E40 erg/s)

Searching for common features Searching for common features in the ULX populationin the ULX population

““Soft-excess” in their X-ray spectra?Soft-excess” in their X-ray spectra?

Signature of a cool disk? higher BH mass?

Most bright ULXs (Lx ~ 1E40 erg/s) have it (Stobbart et al 06)

A few do not, pure power-law spectrum (Winter et al 06)

Evidence of IMBHs, M ~ 1000 Msun ?

““Soft-excess” interpretation is still unclearSoft-excess” interpretation is still unclear

See also poster by Soria, Goncalves & Kuncic

Cool disk emissionCool disk emission

Smeared absorption linesSmeared absorption linesin fast, ionized outflowin fast, ionized outflow

Holmberg II X-1 (Lx ~ 2E40 erg/s)

Power law ( ~ 2.5)

Holmberg II X-1 (Lx ~ 2E40 erg/s)

Injected spectrum (power-law)Injected spectrum (power-law)

Emerging spectra with absorption Emerging spectra with absorption from ionized, fast-moving outflow from ionized, fast-moving outflow (v ~ 0.1 c, n(v ~ 0.1 c, nHH ~ 3E22) ~ 3E22)

Models by Goncalves et al.Models by Goncalves et al.

References:Gierlinski & Done (2004)Crummy et al (2006)Goncalves & Soria (2006)

““Soft-excess” interpretation is still unclearSoft-excess” interpretation is still unclear

See also poster by Soria, Goncalves & Kuncic

Cool disk emissionCool disk emission

Smeared absorption linesSmeared absorption linesin fast, ionized outflow in fast, ionized outflow More generally: absorption + re-emission + reflectionMore generally: absorption + re-emission + reflection

Standard disk around IMBH

Non-standard disk

Essential feature of X-ray spectra:Essential feature of X-ray spectra:

Dominated by non-thermal emissionDisk radiates only ~ 10-20% of output accretion power

Most power is efficiently transferredfrom disk to upscattering medium (jet/corona)

Disk should be cooler than a standard SS disk for a given BH mass

Chilled diskChilled disk

see also Z. Kuncic’s talk

Cooler than standard disk because power is drained from disk into jet+wind+corona

Chilled diskChilled disk

see also Z. Kuncic’s talk

Cooler than standard disk because power is drained from disk into jet+wind+corona

(Soria & Kuncic, in prep.)

Searching for common features Searching for common features in the ULX populationin the ULX population

Jets, outflows?Jets, outflows?

Radio cores:Radio cores: not detectable yet (< 0.1 mJy)Resolved jets:Resolved jets: not detectable yet Radio lobes:Radio lobes: likely detection in a few sources Energy in lobes >~ 1E52 erg Size ~ 50-70 pc Typical fluxes ~ 0.1-0.2 mJy at 5 GHz

(Soria, Fender et al 2006)

Subaru B + ATCA 5 GHz

CFHT H + ATCA 5 GHz

Radio lobes of a ULXRadio lobes of a ULXin NGC 5408in NGC 5408

Searching for common features Searching for common features in the ULX populationin the ULX population

Jets, outflows?Jets, outflows?

Radio cores:Radio cores: not detectable yet (< 0.1 mJy)Resolved jets:Resolved jets: not detectable yet Radio lobes:Radio lobes: likely detection in a few sourcesOptical nebulae:Optical nebulae: observed in many bright ULXs sizes ~ 50-400 pc X-ray photoionized or collisionally ionized?

ULX

Jet lobes?

Hot spot? (hot ring?)

MF16 “SNR” + ULX, in NGC 6946MF16 “SNR” + ULX, in NGC 6946(Swartz et al 2006, in prep)

Optical nebulaeOptical nebulae

30 pc

HST/ACS

NGC 1313 X-2NGC 1313 X-2(Pakull, Grise & Motch 2006)

= 80 pc

Star

Searching for common features Searching for common features in the ULX populationin the ULX population

Jets, outflows?Jets, outflows?

Likely to be essential ingredient Likely to be essential ingredient but more evidence neededbut more evidence needed

Searching for common features Searching for common features in the ULX populationin the ULX population

Young host environment?Young host environment?

Not essential for fainter ULXs (Lx <~ 3E39 erg/s)

Essential for brighter ULXsEssential for brighter ULXs (Lx >~ 1E40 erg/s) Only found in spiral & irregular galaxies

“Young” = less than 50 MyrDonor = OB star transferring gas on its nuclear timescale

Searching for common features Searching for common features in the ULX populationin the ULX population

Starburst environment?Starburst environment?Some ULXs are in starburst galaxies (eg, Cartwheel, Antennae, Mice)

Some are in very quiet corners of nuclear starburst or starforming galaxies (eg, NGC 7714, M83, M99)

Some are in tidal dwarfs with little star formation (eg, Ho II, Ho IX)

NOT AN ESSENTIAL INGREDIENT NOT AN ESSENTIAL INGREDIENT but some associationbut some association

Searching for common features Searching for common features in the ULX populationin the ULX population

Super star-clusters?Super star-clusters?

Suggested as site of IMBH formation via O-star coalescence (Portegies Zwart et al; Rasio et al)

But inconsistent with ULX observations (except for M82 X-1)

Most ULXs found in OB associations or open clusters, with masses <~ a few 1000 Msun

NOT AN ESSENTIAL INGREDIENTNOT AN ESSENTIAL INGREDIENT

Searching for common features Searching for common features in the ULX populationin the ULX population

Colliding or tidally interacting systems?Colliding or tidally interacting systems?

Galaxy-galaxy collisions (eg, ULXs in Antennae, Mice, Cartwheel, NGC 4485/90, NGC 7714/15)

Satellite dwarf – galaxy collisions (eg ULX in NGC 4559) HI cloud – disk collisions (eg ULX in M99)

Tidal dwarfs and tails (eg ULXs in Ho II, Ho IX)

Searching for common features Searching for common features in the ULX populationin the ULX population

Colliding or tidally interacting systems?Colliding or tidally interacting systems?

Essential or very important ingredientEssential or very important ingredient

The Antennae

NGC 4559Examples of ULXs Examples of ULXs formed in colliding eventsformed in colliding events

M99 (Soria & Wong 2006)

XMM EPIC image (0.2-12 keV)

HI contours over R image

LX ~ 2 1040 erg/s

(see poster by Soria & Wong)

High-velocity cloud collision with M99 gas disk

Only a coincidence?

Searching for common features Searching for common features in the ULX populationin the ULX population

Low-metallicity environment?Low-metallicity environment?

Probably a very important ingredientProbably a very important ingredient

Mounting evidence but no systematic study yet (eg, ULXs in Cartwheel, Ho II, NGC 4559, NGC 5408, 1 Zw 18)

More massive BH remnants expected from metal-poor O stars (Mwind ~ Z0.5-0.8)

.

My (biased) conclusions:My (biased) conclusions:

I: NATURE OF (MOST) ULXsI: NATURE OF (MOST) ULXs

Simplest model still consistent with the data:

BH masses ~ 30 – 100 Msun (upper limit of stellar processes)

Age of the accreting systems < 50 Myr (OB donor)

II: (SPECULATIVE) FORMATION PROCESSII: (SPECULATIVE) FORMATION PROCESS

Triggered star formationTriggered star formation (eg, ram p from cloud/galaxy collisions)

Dynamical collapse of molecular clumpsDynamical collapse of molecular clumps(as opposed to turbulent fragmentation)

Fast gas accretion and protostellar mergersFast gas accretion and protostellar mergersin a dense protocluster corein a dense protocluster core (clump mass ~ a few 1000 Msun, much smaller than a super cluster)

Massive stellar progenitor, MMassive stellar progenitor, Mstarstar ~ 200 M ~ 200 Msunsun

BH with a mass ~ 50-100 MBH with a mass ~ 50-100 Msunsun

if metal abundance is lowif metal abundance is low

Total mass ~ 1700 Msun Infall rate ~ 10-3 Msun/yrInfall timescale ~ 1.7 105 yr

CMM3 has 40 MCMM3 has 40 Msunsun, still accreting & merging , still accreting & merging

Externally-triggered dynamical collapse Externally-triggered dynamical collapse of a molecular clump in the Milky Wayof a molecular clump in the Milky Way

Peretto et al. (2006)

40 M40 Msunsun15 M15 Msunsun

35 M35 Msunsun

Very massive stars from clustered star formation exist in the Milky Way & LMC:

Pistol star: initial mass ~ 200 Msun

(but too metal rich to collapse into a BH)

R145 in 30 Dor: M sin3i = (140 +/- 37) Msun

III. POWER BUDGETIII. POWER BUDGET

Accretion rate up to ~ 10 times Eddington

Luminosity near or a few times Eddington

Disk radiates only < 20% of the output power

Disks are cooler than standard SS

Kin. power available for outflows and jets

Can BHs have steady jets when accreting Can BHs have steady jets when accreting at or above Eddington?at or above Eddington?

ULXs could be test cases for QSO super-Edd ULXs could be test cases for QSO super-Edd accretion and feedback models at high redshiftaccretion and feedback models at high redshift

A finis si’. Mersi’ che i l’eve scota’.

Black hole masses in ULXsBlack hole masses in ULXs

X-ray Luminosity function cuts off at ~ 3 x 10X-ray Luminosity function cuts off at ~ 3 x 104040 erg/s erg/sEddington limitEddington limit suggests suggests MM ~ 30 - 200 ~ 30 - 200 MMsunsun

Optical counterparts too faint for direct Optical counterparts too faint for direct mass-function determinationsmass-function determinations

Higher masses (~ 10~ 1033 MMsunsun) speculated from X-ray timing and spectral studies

BH mass from X-ray spectral modelsBH mass from X-ray spectral models

Galactic X-ray binaries generally show:power-law component + thermal disk component

Flatter ( ~ 1.5) when LX <~ 0.01 LEdd

Steeper ( ~ 2.5) when LX ~ LEdd

LLmaxmax ~ L ~ LEddEdd ~ M ~ T ~ M ~ Tinin

LLXX ~ T ~ Tinin R R22 ~ T ~ Tinin M M

22 44 44

- 4- 4

Power-law ( ~ 2.3)Tbb ~ 0.12 keV

X-ray spectrum of NGC4559 X7 (XMM)

X-ray spectrum of NGC4559 X7 (XMM)

Disk kTin ~ 0.13 keV

~ 2.0

kTphot ~ 0.27 keV

Disk kTin ~ 1.9 keV

Tin

LX

10.20.1

1039

1040

1042

1041

1038

(keV)

(erg/s)

Lx = LEdd

2

5 Msun

15 Msun

1000 Msun

IMBH modelIMBH modelGBHsGBHs

Hot-disk modelHot-disk model

IMBH modelIMBH model

Hot-disk modelHot-disk model

kTkTinin ~ 0.12 – 0.15 keV ~ 0.12 – 0.15 keV

M >~ 1000 MM >~ 1000 Msunsun

LLXX ~ 0.05 – 0.2 L ~ 0.05 – 0.2 LEddEdd

kTkTinin ~ 1.5 – 2.5 keV ~ 1.5 – 2.5 keV

M <~ 10 MM <~ 10 Msunsun

LLXX ~ 10 L ~ 10 LEddEdd

Miller, Fabian & Miller (2004)Feng & Kaaret (2005)

Stobbart, Roberts & Wilms (2006)

IMBH modelIMBH model

Hot-disk modelHot-disk model

kTkTinin ~ 0.12 – 0.15 keV ~ 0.12 – 0.15 keV

M >~ 1000 MM >~ 1000 Msunsun

LLXX ~ 0.05 – 0.2 L ~ 0.05 – 0.2 LEddEdd

kTkTinin ~ 1.5 – 2.5 keV ~ 1.5 – 2.5 keV

M <~ 10 MM <~ 10 Msunsun

LLXX ~ 10 L ~ 10 LEddEdd

similar to NLSy1similar to NLSy1

requires exotic requires exotic formation processesformation processes

why do they why do they never reach Lnever reach LEddEdd??

PROBLEMS:PROBLEMS:

IMBH modelIMBH model

Hot-disk modelHot-disk model

kTkTinin ~ 0.12 – 0.15 keV ~ 0.12 – 0.15 keV

M >~ 1000 MM >~ 1000 Msunsun

LLXX ~ 0.05 – 0.2 L ~ 0.05 – 0.2 LEddEdd

kTkTinin ~ 1.5 – 2.5 keV ~ 1.5 – 2.5 keV

M <~ 10 MM <~ 10 Msunsun

LLXX ~ 10 L ~ 10 LEddEdd

similar to NLSy1similar to NLSy1

requires exotic requires exotic formation processesformation processes

why do they why do they never reach Lnever reach LEddEdd??

PROBLEMS:PROBLEMS:

ad hoc (esp. ~ 10 keV)ad hoc (esp. ~ 10 keV)

standard SS disk shouldstandard SS disk shouldnot survive at 10 Lnot survive at 10 LEddEdd ! !

~ 2.8

Alternative model: broad absorption

Summary ISummary I

Unwise to estimate BH masses from X-ray spectraUnwise to estimate BH masses from X-ray spectra

““Soft excess” may be due to absorptionSoft excess” may be due to absorption

New spectral state? (for ULXs and NLSy1?) New spectral state? (for ULXs and NLSy1?)

Low/hard (flat pl)

High/soft (disk)

Very high (steep pl)

Steep pl + absorption in fast, dense outflow

M.

ULX radio counterparts: proof of IMBHs?ULX radio counterparts: proof of IMBHs?

(Merloni, Heinz & DiMatteo 2004; Fender et al 2004)

““fundamental plane” of BH activityfundamental plane” of BH activity

Few ULXs have a radio counterpartFew ULXs have a radio counterpart

M82 (Kording et al 2005)

Holmberg II (Miller, Mushotzky & Neff 2005)

NGC 5408 (Kaaret et al 2003; Soria, Fender et al 2006)

NGC 7424 (Soria, Kuncic et al 2006)

NGC 6946 (Swartz et al 2006, in prep)

NGC 5408 (zoomed in)

X-ray (~1E40 erg/s)

Radio (~ 0.3 mJy at 5 GHz)

H (~1E36 erg/s)

Coincidence between:Coincidence between:

Comparison with X-ray and radio luminosities of Galactic BHs

Steep radio spectrum (thin synchrotron)

Same value in 2000 and 2004

Marginally resolved (radius ~ 30 pc)

More likely radio emission from lobes, not coreMore likely radio emission from lobes, not core

CoreCore = X-rays, flat radio spectrum (if present) Traces the instantaneous accretion stateLobesLobes = steep radio spectrum Trace the integrated jet power over ~ 0.1 Myr

However:However:

Radio lobes or supernova remnant?Radio lobes or supernova remnant?

Not easy to distinguish or disentangle the two(eg, SS433 has SNR + jet lobes)

Leptonic jet model in NGC 5408:Leptonic jet model in NGC 5408: E ~ 3 10E ~ 3 105151 erg erg PPJJ ~ 7 10 ~ 7 103838 erg/s over 1.5 10 erg/s over 1.5 1055 yr yr

Expansion velocity ~ 80 km/sExpansion velocity ~ 80 km/s

A SN model (= 99% relativistic protons) A SN model (= 99% relativistic protons) would requirewould require E ~ 3 10E ~ 3 105252 erg erg (A hypernova, perhaps?)

NGC 7424 (d ~ 12 Mpc)

(Soria, Kuncic, Broderick & Ryder 2006)

ULX-2

State transitionlow/hard high/soft5 1038 7 1039 erg/s

(with thermal plasma)

Point-like (< 60 pc) radio sourceIndex ~ - 0.6 (thin synchrotron)LR ~ 3 x Cas A

Radio lobe or young SNR?Radio lobe or young SNR?(Soria et al 2006b)

Age = 8 +/- 2 Myr

We are starting to find ULX radio counterparts

Radio lobes (FR2 microquasars?) or SNR?

Many ULX radio lobes may have been misclassified as SNRs if the central X-ray source is off

Ratio of ULX radio lobes / “fossil” radio lobes may give us clues on the X-ray duty cycle

Radio/ULX associations useful to determine power budget = radiative vs mechanical output(also important for estimating feedback from early quasars)

Summary IISummary II

Part III: Part III: speculations on ULX formationspeculations on ULX formation

IMBH formation in a young super-star-cluster?IMBH formation in a young super-star-cluster?

Dynamical friction

Mass segregationRunaway core-collapse

Stellar collisions/mergers in the core

Short-lived, very massive star (~1000 Msun)

Hypernova or direct collapse into IMBH

Numerical simulations by Portegies Zwart et aland by Gurkan, Rasio et al.

101066 M Msunsun cluster cluster

1000 M1000 Msunsun BH BH

Problem: Problem: most ULXs are most ULXs are notnot in super-star-clusters in super-star-clusters

Near OB stars but not inside a bound cluster

Have their parent clusters dispersed?Have their parent clusters dispersed?Tidal disruption: always too slow (>~ 50 Myr)SN disruption: perhaps….but there are no signs no signs of the dispersed super clustersof the dispersed super clusters

Were they ejected?Were they ejected?Inconsistent with IMBH, would require low BH mass(eg, Zezas et al 2002; Belczynski et al 2005)

101066 M Msunsun super star clusters super star clusters

with 1000 Mwith 1000 Msunsun BHs BHs

Rarely foundRarely found

Probably not neededProbably not needed

We only need MM ~ 30 -- 200 ~ 30 -- 200 MMsunsun

SuggestionSuggestion::IMBHs formed in smaller proto-clusters, IMBHs formed in smaller proto-clusters, not super clusters not super clusters (Soria 2005)(Soria 2005)

(eg, Kroupa & Boily, 2002-2004; Geyer & Burkert 2001)

protocluster

cluster

OB assoc

Neutral gasprotostars

Ionized gas

~ 0.5 Myr~ 0.5 Myr

Ideal conditions for

forming progenitor “star” with M ~ 100 -- 300 Msun

dispersing the protocluster (binding energy <~ 1052 erg)

M ~ 103.5 -- 105 Msunh < 10 km/s

Combination of accretion (large-scale gas inflow) Combination of accretion (large-scale gas inflow) + coalescence in the protocluster core?+ coalescence in the protocluster core?

Stellar captures and mergers are favoured by proto-stellar disks / envelopes

Collision cross section enhanced at low velocity dispersion (gravitational focussing)

Collision rates & maximum BH mass enhanced at high density

Dense proto-clusters ideal for coalescenceDense proto-clusters ideal for coalescence

Merging BHs: most difficultmost difficultMerging O stars: somewhat easiersomewhat easierMerging protostars, molecular cores: easiesteasiest

Two regimes for coalescence + IMBH formation?Two regimes for coalescence + IMBH formation?

M <~ 105 Msun

h < 10 km/s

IMBH formation in unbound proto-cluster

tcc <~ 0.5 Myr

M >~105.5 Msun

h >~ 10 km/s

tcc <~ 3 Myr

IMBH formation in bound cluster

ULX in a sparse OB assoc (size >~ 100 pc)with expanding gas nebula

ULX in a cluster (size <~ 3 pc)

Additional advantage of the proto-cluster scenario

Same physical process that creates massive Same physical process that creates massive [O + O] binaries, progenitors of BH HMXBs[O + O] binaries, progenitors of BH HMXBs

ULXs = high-luminosity end of HMXBsULXs = high-luminosity end of HMXBs

up to ~ 100-200 Msun

Protoclusters near the Cone nebula

(Peretto, Andre’ & Belloche 2006)

Near-IR contours + 1.2mm continuum

Peretto, Andre’ & Belloche (2006)

Supersonic global inflows in protoclusters Supersonic global inflows in protoclusters (as opposed to random turbulent motion)(as opposed to random turbulent motion)

Two necessary ingredients for a massive BH:Two necessary ingredients for a massive BH:

An An external triggerexternal trigger may cause may cause compression and dynamical collapsecompression and dynamical collapse

1

2 Small mass loss from progenitor star Small mass loss from progenitor star before SN core-collapsebefore SN core-collapse

Low metal abundance (~ 0.1 solar) reduces mass loss in stellar winds

Importance of low metal abundanceImportance of low metal abundance

Heger et al (2003)

Galaxy collisions, cloud-disk collisionsGalaxy collisions, cloud-disk collisions

Triggered star formation

Denser protoclusters, dynamical collapse,high-mass stars

ULXs, upper end of HMXB distribution

(often) starbursts, large number of HMXBs

Observational evidence for ULXs in SSCs?

ULX in a young super-star-cluster in M82Lx varying from ~ 1039 to 1041 erg/s Mbh ~ 1000 Msun Mcl ~ 4 105 Msun

Portegies Zwart et al, Nature, 2004

Near clusters but not in one

ULX in the starburst dwarf NGC 5408with Lx ~ 1040 erg/s

Near B stars but not in a cluster Kaaret et al 2003Soria et al 2004

Near OB stars but not in a super-star-cluster

ULX in the dwarf galaxy NGC 5204 Liu et al 2004

Not in super-star-clusters

ULX in the starburst dwarf NGC5408with Lx ~ 1040 erg/s

Two ULXs in NGC4559 with Lx ~ 1 – 4 1040 erg/s

NGC4559 X-10: near OB stars, no super cluster

A few B stars but no big clusters

Soria et al 2005Cropper et al 2005

Antennae: lots of ULXs, displaced from clusters

ULXs are displaced from SSCs by ~ 100 – 300 pcZezas, Fabbiano et al 2002

Massive proto-stellar mergersMassive proto-stellar mergers

proto-cluster disruptionproto-cluster disruption

Merger of 100 + 100 Merger of 100 + 100 MMsunsun stars starsreleases ~ 10releases ~ 105151 erg erg (Bally & Zinnecker 2005)

Binding energy of the gas in a 10Binding energy of the gas in a 1055 MMsunsun cluster cluster

~ a few 10~ a few 105050 -- 10 -- 105151 erg erg

Single SN releases ~ 10Single SN releases ~ 105151 erg erg

Explosive expulsion of gas

Not in clusters

4 ULXs in the colliding galaxies NGC 7714 / 7715with Lx ~ 2 – 8 1040 erg/s

Smith et al 2005, AJ, 129, 1350

2 are in clusters, 2 are not

NGC4559 X-7: near OB stars, no super cluster

A few B stars but no SSCs

Soria et al 2005