# Presentation: 293988

## Ultra- Luminous X-ray Sources in Nearby GalaxiesRichard Mushotzky & Susan Neff GSFC  

- Why should these objects be of interest to this meeting?
- They are a numerous (seen in  ~1/4 of all galaxies) population of possible intermediate mass (20-5,000M) black holes.
- At the present time their true nature is not well understood and in fact they maybe several types of objects in this category
- For this talk we shall use the term ULX to refer to objects whose bolometric luminosity is greater than the Eddington limit for a 20 M black hole (2.8x1039 ergs/sec) - there can be a large correction from x-ray luminosity in a given band  to bolometric luminosity
- LEdd =4cGM/k= 1.3 x 1038 M erg s

- Chandra image of the rapidly star forming galaxy NGC4038- the “Antenna”

## IXO – Defined by Observed Properties  

- IXO = Intermediate Luminosity X-ray Object
-        ULX = Ultra-Luminous X-ray Source
-         SES = Super-Eddington Source
-         SLS = Super-Luminous Source
- Unresolved (< 0.6” with *Chandra*)
- Lx  > 2x1039 ergs/sec, 0.5-10kev
- Not at galaxy nucleus

-   XRB: LX  =  1036 - 1038 ergs/sec
-   AGN: LX  >  1042 ergs/sec
-   If LX = 5 x 1039 = LEdd , Macc > 25Msun
-   If LX < LEdd,   Macc 100 -1000 Msun
-   If LX ~ 10-8 LEdd (LLAGN),  Macc~ 106Msun
-   MBH < 10Msun by “normal” stellar evolution
-                             (even from very massive stars)

## IXOs – Model Classes

- Supernovae in dense
-       environments (have been detected Lx~1038-1041 ergs/sec SN 1995N)
- Blast-driven SNR
- Pulsar wind nebula
- Non-isotropic X-ray binaries
- "Normal" high-mass x-ray binaries (HMXB)
- Micro-blazars (beamed emission, relativistic jets)
- Accretion onto very massive objects:
- Intermediate-mass Black Holes (IMBH)
- “Lost” LLAGN (Low-luminosity AGN)

## Ultra- Luminous X-ray Sources in Nearby Galaxies 

- What sort of data do we have?
- A census of these objects from archival Rosat (Colbert and Ptak 2002), Chandra and XMM data
- X-ray spectra from ASCA,Chandra and XMM
- X-ray time variability on long (years) to short (seconds) time scales
- Counterparts in other wavelength bands (optical, radio)
- Luminosity functions
- Correlations with galaxy properties

## Ultra- Luminous X-ray Sources in Nearby Galaxies 

- What are the arguments against their being M>20M objects?
- They are primarily astronomical (see King 2003)
- Difficulties in forming and feeding them
- if M>20 they cannot form from stellar evolution of single “normal” massive stars
- binary stellar evolutionary scenarios indicate that the companion (which provides the “fuel”) should also be massive and therefore cannot be long lived.
- To “acquire” a stellar companion is rather difficult

- Their statistical properties
- they tend to be associated with recent star formation
- a small number have optical associations with bright stars
- some of they show transitions similar to that seen in galactic black holes
- the overall luminosity function of the sources does not have a “feature” associated with the ULXs

- In order to accommodate the observed high luminosities with “low” masses one requires beaming

## Ultra- Luminous X-ray Sources in Nearby Galaxies 

- What are the arguments against their being “normal” M<20M objects?
- DATA
- Their E> 2 keV x-ray spectra are much harder than normal galactic black holes
- The state transitions they undergo are often in the opposite sense from galactic objects
-  Their luminosities can reach 1000 Ledd for a 1 solar mass object
- There is evidence against beaming (QPOs, broad Fe Lines, eclipses)
- There is a lack of optical Ids (massive stars would be seen)
- they lie near, but not in star forming regions

- Theory
- there is no known mechanisms for providing the required beaming other than relativistic effects
- the observed luminosity function is not consistent with beaming
- they are not “ultraluminous” in other wavelength bands- like AGN are

- There are a few ULXs with highly luminous photo-ionized nebulae around them- require high luminosity to photoionize them
- some of them have x-ray spectra which are indicative of high masses.

## X Ray Binaries as IXO’s     

- Non-isotropic emission due to thick accretion disk ?
- Luminosity (mass) therefore lower
- Many more IXO’s are not aimed at us
- Don’t expect to see periodic (eclipse) behavior or
- Fe K line
- QPO
- Most IXO’s probably HMXBs ?
- Prefer star-forming regions
- Lifetimes consistent with starburst ages
- Possibly really are super-Eddington
- "leaky“ thin disks (Begelman)
- rapidly spinning BH's (Terashima et al. 2001)
- short-lived thermal timescales mass transfer (King et al. 2001)
-     (requires lots more XRB than we think are there)
-  XRB’s are likely to be ejected from clusters
- result of Sne which form accreting BH / NS
- three-body interactions with other cluster members + binary hardening

## Reminders:  X Ray Binaries (XRB)

- High- / Low-mass x-ray binaries (HMXB/LMXB)
- High-mass / low-mass refers to *   donor* star
- Must form binaries and keep together through formation of Black Hole or Neutron Star

- LMXB
-  White dwarf
-   donor star
-  Accretor was probably
-   lower mass, slower
-   evolution to NS / BH
-  Prefer bulges and
-   globular clusters
-  P ~ 10hrs
-  Ages:   Old  > 109 yrs

- HMXB
-  Young, massive
-   donor star
-  Accretor was massive,
-   short-lived star
-  Prefer star-forming
-   regions, spiral arms
-  P ~ days
-  Ages:  Young  ~107-8 yrs

##   IMBH’s (10 - 1000Msun)

-  How are they formed?
- Direct evolution of Population III stars
- Typical stellar mass ~100Msun in early Universe
- No metals  no radiative mass loss, no pulsational instability
- Pop III < 140 Msun evolve like Pop I and II,
-     but form more massive remnants
- 140Msun < Pop III star < 260 Msun, no remnant
- Above 260Msun, collapse directly to BH
- Grow in globular clusters
- Grow through stellar and BH mergers (iff cluster core collapse)
- Tend to be ejected from cluster in binary interactions
- How are they fed?
- Possibly accreting directly from ISM (dense clouds)
- Any donor star must have been captured

## Low luminosity AGN (LLAGN)

- May be in all “normal” galaxies
- Known to occur in >40% of local galaxies
- 103 – 106 times less luminous than QSO’s
- Not just “scaled down’’ AGN
- Low accretion power, sub-Eddington, radiatively inefficient
- Different SED’s, low ionization (2/3)
- Radio loud
- Different X-ray spectra
*Probably* ~105 Msun BH’s
- Currently not eating much
- Possible to sustain activity on stellar winds alone
**Only definitive intermediate mass object (NGC4395 with M~10****4****-10****5****M)**

- LLAGN model: inner low radiative-efficiency accretion flow (LRAF) irradiates outer thin disk

## The data

- If they are 20-1000 M BHs where do they come from?
- The early universe?- detailed caluculations of the first stars to form (e.g. Abel et al) show that M~200-1000M objects should be created. They are fairly numerous and should lie in regions that will later become galaxies
- Created in dense stellar regions (e.g. globular clusters Miller et al 2002, dense star clusters Portegies Zwart, et al 2002)

- NGC5408-radio-optical x-ray location (Kaaret et al 2002)-notice large region of star formation to the west, and lack of obvious optical counterpart on HST image

## What are they related to??

- Are the ULXs intermediate mass  black holes ?
- Should have properties that scale from that of AGN at high mass and galactic black holes at low mass
- Time variability
- Broad band spectra
- Detailed spectra in x-ray/radio/optical

- Are they something else?
- Beamed lower mass objects
- A black hole accreting/radiating in a new mode?

- Many of the observed properties should scale with mass
- x-ray spectral form
- Characteristic x-ray time scale
- These scalings have been observed !!!

## What can we learn from optical associations 

- If one can uniquely “identify” the optical counterpart one could estimate the mass of the ULX, estimate its evolutionary history and perhaps discriminate between models.
- If the nebulae that are seen are associated with the ULX then one can use them as calorimeters to derive the true isotropic luminosity of the object

- IC342- Association of ULX with a unusual supernova remnant (Roberts et al 2003)

## IXO’s – Counterparts(15 April 2003)

- 1 inside X-ray ionized nebula
- strong HeII 4686 and [OI] 6300
**requires 3-13x10****39**** ergs/sec** to produce
-           observed optical emission lines
- 4+ in bubble-like nebulae, 200-400 pc
- 2+ with probable O-star counterpart
- 4+ in other nebulae
- diffuse H alpha centered on X-ray source,
- 1 with possible stellar counterpart
- 11 within/near larger HII regions
- 3 with possible OB stellar counterpart
- 3+ in massive young star cluster (SSC)
- Several associated with globular clusters
- mostly elliptical galaxies
- 4 with radio counterparts (+7 probable IDs)
- Combination of “old” (glob cluster) and “young” (star forming regions) locations and low mass (dwarfs) and high mass (elliptical galaxy) locations

- Field is changing very fast!

## What can we learn from optical associations 

- The size of some of these nebulae are very big ~200-600pc and very energetic with kinetic energies of ~1052-1053 ergs/sec much more than SNR
- Detailed optical spectra of these nebulae can distinguish between shock and photoionization and whether these are excited by the central x-ray source, are “hypernova” remnants or something else

- Set of  Association of ULX with a highly ionized nebula (Pakull and Mirioni 2003)

- NGC1313- far from star forming regions

## Optical countparts of ULXs 

- In a small nuber of cases there is a optical counterpart to the ULX- however the sensitivity of HST is such that **only the most luminous stars can be recognized****; m-M=5logD-5 the most luminous stars have M=-8 and the HST limit is typically m=25; this gives D=40Mpc **
- Even with the Chandra error circles there is often not a unique counterpart.

- NGC 5204- Chandra and HST images- source breaks up into 3 objects- the brightest source could be a F supergiant
-  Mv=-8.1 (**the brightest normal stars ever get)** Roberts et al

- Chandra counterpart in M81- optical counterpart a O8V star

## Optical/radio countparts of ULXs 

- It is not clear at present whether the ULX and the nebula are physically related- and if so how
- Is the nebulae ionized by the ULX (Pakull et al)
- Is the nebula a supernova remnant related to the creation of the ULX
- Is the nebula simply a sign of recent star formation
- The nebulae **are not HII regions**
- The x-ray sources are often near, but not in HII regions (star forming regions)

- M81 x-11 (Liu e al 2003)- the x-ray luminosity dominates the bolometric luminosity** **

## High Quality Chandra Data and an optical counterpart

- One of the brightest of the ULXs in a dwarf companion of M81 -shows an optical nebulae with ground based data, not a supergiant star- x-ray to optical ratio is > 100
- L(H)~1038 ergs/sec (Wang 2003) and ~300pc in size-much bigger than young SNR
- This source has the x-ray spectrum well fit by a disk black body.
- L(x)~1.6x1040ergs/sec. L(BOL)~1.3X1041 M> 125M

## How Much energy do we expect in other wavebands ?

- The x-ray flux(f(x) of a L(x)=1040 ergs/cm2 ULX
- 3.5x10-12(D/5mpc)2; a 22-25th mag optical counterpart has f(opt) =0.1- 1.3x10-15 erg/cm2/sec
- Thus f(x)/f(opt)~150->2500
**For a “typical’ active galaxy (quasar) f(x)/f(opt)~1**
- For x-ray detected binaries the optical light comes from either the companion star (high mass x-ray binaries) or the accretion disk (low mass)
- For systems with the light dominated by the disk **f(x)/f(opt) ~100-10****4****;**** **
- The present optical data are consistent with the visual light coming from an accretion disk  scaling from x-ray binaries in the Milkyway
- However, it is not yet ruled out that much of the optical light comes from a massive companions

- X-ray/optical relation for x-ray selected AGN

- f(x)/f(opt)=1

## Nature of the Host galaxy 

- ULXs can occur in any galaxy- while they are most frequent in rapidly star forming galaxies they also occur in dwarfs and in elliptical galaxies which do not have any present day star formation- however in ellipticals the maximal luminosity is 1040 ergs/sec
- In NGC720 a nearby giant elliptical with no star formation the number of ULXs is comparable to that of activ star forming galaxies (Jeltema et al 2003)

- X-ray luminosity function in different galaxy types

- Luminosity of ULX vs IR luminosity and galaxy type (Swartz et al 2003)

## Relation to statistical properties of galaxies 

- In spiral galaxies the number of ULX’s is related to the star formation rate
- Combing the sources from several galaxies scaled by the star formation rate results in a smooth luminosity function (Grimm et al 2003)

- SFR M/yr

- # of ULX

## Nature of the Host galaxy 

- ULXs can occur in any galaxy- while they are most frequent in rapidly star forming galaxies they also occur in dwarfs and in elliptical galaxies which do not have any present day star formation- however in ellipticals the maximal luminosity is 1040 ergs/sec
- In NGC720 a nearby giant elliptical with no star formation the number of ULXs is comparable to that of activ star forming galaxies (Jeltema et al 2003)

- ULX in dwarf galaxy- optical image and x-ray contours

- NGC720- Chandra image- optical contour

## IXO’s – luminosity functions

- Galaxies with higher star-formation rates (higher LFIR) have
- flatter compact-source luminosity functions
- brighter IXOs
- more IXO’s
-  IXO production scales with star formation rate

- 0.1        LX (1039)         10

- 1      LX (1039)      5        10        20

- N(>L)

- N(>L)

- Swartz et al.
- 2003

## IXO Flux Variations

- Variability frequently observed
- Usually between observations (months-years)
- Sometimes intra-observation (hours)
- Some IXOs may be periodic
- IC 342;  31 or 41 hrs (HMXB)
- Cir X-1; 7.5 hrs (>50 Msun BH)
- M51 X-1; 2.1 hr (LMXB?)

- Cir X-2;   P = 7.5 hrs
- Consistent with >50 Msun
- BH in eclipsing binary
- Bauer et al. 2001

- M51 X-1
- P ~ 2.1 hr
- Liu et al. 2002

- IXO’s in NGC 4485/4490
- Roberts et al. 2002

## X-ray Time variability

- Detection of periodicities can help determine the mass of the objects
- For a mass ratio of q = M1=M2 < 0:8, the Roche Lobe radius is
-  Rcr = 0:46( M1/M1+M2)1/3
- in which  is the separation between the donor and the accretor, and M2 is the mass of the accretor.
- Combined with Kepler's 3rd law, Porb =8.9(R)3/2(M_x000c_)1/2 hours.
- For a late-type low mass star, the mass-radius relation is  R=M (solar units) and periods of 2-8 hours translates to **mass of the donor** of 0.2-0.4 M

- The mass of the compact object (the accretor) cannot be determined from the period alone- if eclipses are detected then other constraints are possible.
- the fraction of the period spent in eclipse is related to the size of the Roche lobe of the binary companion and hence to the companion to compact object mass ratio

- Periodic “Dips”- Material in the accretion stream?

## X-ray Time Variability 

- Detection of .06Hz QPOs in the x-ray flux from the ULX in M82- the x-ray brightest ULX (Strohmeyer and Mushotzky 2003)

- Rg=GM/c2

- It has been known for many years that galactic black holes exhibit “quasi-periodic oscillations” (QPOs)
- While these are not perfectly understood they are clearly associated with the accretion disk and represent characteristic length scales with narrow widths, close to the black hole
- If the  QPO frequency is associated with the Kepler frequency at the innermost circular orbit around a Schwarzschild black hole,.
- Thus a frequency of .06 Hz translates to an upper limit on the mass of 1.9x104 M , consistent with the observed luminosity and a efficiency of ~0.1
- Assuming the QPO  originates at R~6Rg the innermost stable radius of a Schwarschild black hole

## Power Density Spectra

- The fourier transform of the x-ray time series, the power density spectra shows for many galactic black holes a simple form, flat at low frequencies and steep at high frequencies,
- The PDS for AGN shows a similar form, but the break frequency scaling as the mass of the objects
- Preliminary analysis of high signal to noise XMM data for several ULXs show low overall power and a flat power law PDS- it is not clear how this yet constrains models ; but **at first glance the ULX do not possess the “characteristic BH” power spectra  **

## Nature of the X-ray Spectrum 

- There is a large literature on the x-ray spectra of Milkyway black holes
- Generically the spectra fall into 2 broad classes
- Powerlaw spectra (low state)
- Disk Black body +power law (high state)

- When the objects are more luminous (high state) their spectra are more “thermal” in nature- this is expected because the accretion flow becomes optically thick
- The x-ray spectra of the ULXs **can be  different**
- Many of the most luminous objects are well fit by **a very hot** disk black body model” without a power law,

## X-ray Spectra

- If the spectra were due to the sum of black body radiation in an accretion disk (the disk black body model) there is a simple relation between temperature, luminosity and mass (Ebisawa et al 2002)

- Since the implied masses of the ULXs> 20M, Tcol< 1 keV -but many sources have Tcol > 2 keV-
- Detailed calculations indicate that this “color temperature” problem is not generally solved in a Kerr metric

- M82

- The ULXs are too “hot” for their inferred “Eddington limited” mass - either the spectral model, masses or interpretation is wrong

## Reminder:  Galactic XRB Spectra

- Low/hard - high/soft: - *most* galactic XRBs:  innermost spherical hot flow (and/or jet)   is most prominent during low luminosity states, accretion disk dominates in high luminosity states.
- High/hard  low/soft :                    *a few* galactic XRBs:                hot inner halo is more responsive to accretion changes than large disk

## Accretion Disk Spectra

- The broad band spectra of a optically thick accretion disk can be calculated- if the optical/IR luminosity can be observed it can be directly compared to the theoretical prediction, normalized to the x-ray.
- Recently (Miller et al 2003) several IXOs have beenfound which have a 2 component x-ray spectrum- the temperature of the soft component is low T~0.15keV - high total mass

## X-ray Spectral Features

- In many AGN and galactic black holes there is a spectral feature associated with reprocessing of the central x-ray radiation by the accretion disk a broad Fe K line
- This line is broadened by dynamics in the disk and shows that the disk “directly sees” the radiation from the central source
- “Beamed” AGN (e.g. Bl Lac objects) do not show this feature
- The ULX in M82 shows a broad Fe K line-so far only object with enough S/N to test
**Existence of broad Fe K line shows that continuum is not beamed**

## IXO’s as XRB’s or Microquasars

**Pro:**
- LX (beamed) okay from normal accretion-*by construction*
- HMXB lifetimes well-matched to starburst
- Soft/high – hard/low spectral changes
- Distances from clusters ok for mass
- No “knee” in luminosity functions
- Correlation with rapid star formation

**Con:**
- No radio jets observed
- Few radio counterparts
- Good accretion disk fits don’t support beaming from jets
- Hard/high – soft/low spectral changes
- Tin derived from MCD fits is too hot
- QPOs/eclipses reject beaming
- Presence of “soft” components in some objects suggest high masses

## IXO’s as IMBHor “lost” LLAGN

- Pro:
- Several IXO associated with GC’s
- Proximity to clusters  stars to capture
- LX / LRadio consistent for LLAGN
- Two ULX with very soft components;  cool disk  MBH ~ 103 Msun
- At least one “real” case

- Con:
- Tin too high for MCD (T~ M-1/4)
- Found in young starforming regions – not enough time to grow to 105 Msun in ~108 yrs
- Not usually near galaxy centers (where IMBX / SMBX should sink)
- Luminosity functions (usually) have constant slope across LX boundary
- When in dwarf galaxy, would be significant fraction of total mass

## Conclusion

- There is no direct evidence for beaming- and in 3 sources evidence against beaming (1 QPO and 2 eclipsing sources)
- There is possible evidence for high intrinsic luminosity in several objects
- The x-ray spectra do no resemble theoretical predictions
- The x-ray power spectra may be different from expectations
- The evolutionary history and origin of these objects is not certain
- The correlations with star formation and galaxy type are violated in several objects
- The ULXs do not “look like” scaled up GBHs or scaled down AGN nor like beamed versions of either one

- The sum of the results do not “hang together”
- Either we are dealing with 3 or more “new” types of objects or we have to re-think what a black hole should “look like”
- We clearly have found some “unknown” AGN

## Microblazars as IXOs

- Accreting BH or NS with thick inner disk
- If looking down jet, will see inside of funnel / thick  disk as additional luminosity,  object will appear brighter
- Not much beaming of X-rays

- Luminosity functions  similar to those expected from XRBs for L<1039.
- Possible “knee” in luminosity functions at L~1039?

- Elliptical

- X-ray point sources: Luminosity Functions

- Spirals

- Starbursts

- M83

- NGC ???

- M82

## IXO’s – luminosity functions

- Galaxies with higher star-formation rates (higher LFIR) have
- flatter compact-source luminosity functions
- brighter IXOs
- more IXO’s
-  IXO production scales with star formation rate

- 0.1        LX (1039)         10

- 1      LX (1039)      5        10        20

- N(>L)

- N(>L)

- Swartz et al.
- 2003

## IXO’s – X-ray Spectra

- Spectral Variability
- Majority (so far) change low/hard to high/soft (like XRB’s)
- Significant minority change high/hard to low/soft
- Many fit by multi-color disk (MCD) models
- BUT, derived Tin often too high for assumed BH mass
- Many others fit by hard power law + soft component
- ?Soft from disk, hard from photons upscattered by hot electrons (jet)?
- ?Hard power-law from expanding SNR?

## NGC 3256

- D ~ 56 Mpc
- Very luminous IR + Xray
- Highest LIR in local Universe
- Near top of X-ray luminous starbursts (LX ~1042 ergs/sec)
- Just past merging
- 200 kpc tidal tails
- Single galaxy body
- Double nucleus (radio and NIR)
- Northern nucleus starburst
- Southern nucleus - ??hidden AGN??
- Major starburst  superwind
- Population of ~40 compact radio sources, mostly SNR

- HST, true-color, Zepf et al. 1998

##  IXOs in NGC 3256 

*Chandra *finds 14
-     discrete sources,
- All IXOs
- 20% LX in IXOs
- IXO Locations
- Mostly in starburst
- Two at “nuclei”
- Several IXO near high metallicity
-       starburst knots (IXOs 7,10,11,13,9,6)
- Sizes < 140pc
- Sizes + LX’s  10-30 "normal" HMXB
-      in each of 14 regions 1/2 size of 30 Dor.

- Diffuse emission                                Compact sources          Lira et al. 2002

- N3256

- N4038

- Full-resolution

- Binned

- X-ray contours,  H greyscale

- NGC 3256

## NGC 3256 – IXO Radio Counterparts         

- 3 IXO’s have radio counterparts
- 2 compact , one resolved
- Other IXO’s near but not coincident with radio emission
- Both radio “nuclei” are  IXO’s
- Sizes < 50pc
- Points embedded in diffuse emission
- Steep radio spectra
- Radio + X-ray  two LLAGN
- Radio too bright for XRB’s
- Requires 600-1000 HMXB’s
- Radio and X-ray too bright for SNR
- Requires ~1000 CasA’s
- No GRB observed in N3256
- Properties consistent with LLAGN
- Lrad / Lx consistent with LLAGN
- SED is right shape

- 2cm

- 3.6cm

- 900pc

- 0.3-10keV

## NGC 3256 – IXO environment

- H, WFPC2 ramp

- 3000A, WFPC2

- HST images              H, red    3000A, blue

- N nucleus, SSC
- S nucleus, obscured
- Lots of H, young stars

## NGC 3256 – HST / STIS Observations 

- H, WFPC2 ramp

-     (from HST archive)
- Centered on northern “nucleus”
- 0.1x52 & 0.2x52 slits
- G750M and G430L gratings
- 6 slightly offset pointings

## NGC 3256 – Northern Nucleus 

- STIS spectra show:
- Strong H, [NII], and SII lines
- Weak [OIII], weak continuum
- H and NII lines are broad, ~450km/sec
- Velocity shear indicative of disk
-      ~250km/sec over ~80pc
- M~108Msun
- Strongly suggestive of SMBH

- [NII]    H        [NII]

- 0.1”N

- Nuc.

- 0.1”S

-  [NII]    H       [NII]

- STIS, 0.2x52slit