Accretion Disk Spectra of the Ultraluminous Xray Sources in Nearby Spiral Galaxies and Galactic supe

1
Accretion Disk Spectra of the Ultra-luminous
X-ray Sources in Nearby Spiral Galaxies and
Galactic superluminal jet sources

• Ken Ebisawa
• ISDC, NASA/GSFC, USRA
• Piotr Zycki
• Copernicus Center
• Aya Kubota
• ISAS

2
Ultra-luminous X-ray Sources (ULX)

• Discovered with Einstein in nearby spiral
  Galaxies (e.g., Fabbiano 1988)
• LX(0.5-10 keV)1039-1040 erg s-1
• Too bright for X-ray binaries, too dim for AGN
• Most sources are located off-center of the Galaxy
  (Colbert and Mushotzky 1999)
• gt100 M? not to exceed the Eddington limits?

3
Characteristics of ULX

• Significant time variation (Source1 in IC342
  Okada et al. 1998)
• Compact object in nature

4
Characteristics of ULX

• High-low transition? (Source1 and 2 in IC342
  Kubota et al. 2001)
• Orbital modulation (?) from Source 2 (Sugiho et
  al. 2001) , from a ULX in Circinus galaxy (Bauer
  et al. 2001)
• Similar to Galactic black hole candidates

5
Characteristics of ULX

• Thermal spectrum, like standard optically thick
  accretion disk, in the bright state
• Disk temperature too high for given luminosity
  and mass, assuming Schwarzschild black hole
  (Okada et al. 1998 Makishima et al. 2000)
• Similar to Galactic superluminal jet sources
  GRS1915105 and GRO J1655-40 (Zhang, Cui and Chen
  1997)

6

• Optically thick accretion disk around a
  Schwarzschild black hole
• Innermost radius 6 Rg 6 GM/c2
• mass, mass accretion rate (luminosity) ? spectral
  shape determined

Maximum disk color temperature for Schwarzschild
black hole T(max)col 1.2 keV ((Tcol/Teff)/1.7)
(M/MEdd)1/4 (M/7M?)-1/4 This is directly measured
from observed spectral shape Kerr disk can be
much hotter, as innermost radius ?1.24 Rg
.
.
7
M1.8 M? L 0.4 LEdd
M9.4 M? L 11 LEdd

• Too hot accretion disks in ULX and superluminal
  jet sources
• To explain the observation, you need either too
  large mass accretion rate or too small mass, as
  long as standard disk around Schwarzschild black
  hole is assumed

8
Mconst, Ldisk Tin4
Super-Eddington

• Makishima et al. (2000)

9
How to explain the too-hot accretion disk?

• Standard accretion disk around Kerr black hole
  can explain the hard disk spectra? (Zhang, Cui
  and Chen 1997 Makishima et al. 2000)
• Apply Kerr disk spectra to ULX and superluminal
  jet sources

10
Inclined Kerr disk is brighter in high energies
Kerr disk
Scwarzschild disk
Laor, Netzer and Piran (1990) Transfer function
for a0.998 available with xspec
11

• Hard emission from very inner parts (1.26 rg lt r
  lt 7 rg) is enhanced for inclined Kerr disks (due
  to Doppler boost)
• When the disk is face-on, emission from inner
  part is weak, and the spectrum is not very
  different from the Schwarzschild case

12
Application of Kerr disk spectra

• GRO J1655-40
• i70?, d3.2 kpc, Tcol/Teff1.7 fixedM16 M? and
  M3.5?1017 g s-1with a0.998Kerr disk model
  works to solve too-small mass problem
• a0.68 to 0.88 plausible (Gielinski et al. 2001)
• 450 Hz QPO (Strohmayer 2001) supports a standard
  (geometrically thin) disk around a spinning black
  hole (Abramowicz and Kluzniak 2001)

13
Application of Kerr disk spectra

• IC342 Source 1
• face-on Kerr disk (d4Mpc, Tcol/Teff1.7,a0.998)
  M29 M? and L14 LEdd
• edge-on (i 80?) Kerr disk (a0.998)M 355 M?
  and L0.9 LEdd
• Super-Eddington problem may be solved only if the
  disk is highly inclined
• Kerr disk model is not plausible for ULX because
  disk inclination should be random

14
Origin of hard disk spectra of ULX

• Optically thick ADAF
• Strong disk comptonization

15
From recent study of Galactic black hole
candidates

• Abramowicz et al. (1995)

Comptonized disk
T ? r -0.75
16
From recent study of Galactic black hole
candidates

• Standard optically thick disk
• Gravitational energy release ? Radiation
• T( r)? r -0.75, Rin const., Ldisk ? Tin4

Disk Oscillation

• Disk instability
• Energy release ? Comptonizing plasma
• Disk compotonization

• Optically thick ADAF disk
• Energy release ? Advection
• T( r)? r -0.5, Ldisk saturates

17
Optically thick ADAF disk (slim disk)
Watarai et al. (2001) Standard disk ?Slim disk
when L LEdd Ldisk saturates at high
Tin IC342 spectral change explained well
18
Strong disk comptonization

• IC342 source 1, Schwarzschild disk with M100 M?,
  LLEdd (Tin 0.6 keV)
• Put comptonizing corona with y(4kTe/mc2)te0.5 ?
  soft photons comptonized and appear in higher
  energy band
• observed hard spectrum can be explained

19
Summary

• Standard accretion disk model around Kerr black
  hole can explain the hard spectra of Galactic
  superluminal jet sources (highly inclined system)
  
• This model is not suitable for ULX, as accretion
  disks are not preferentially inclined
• Optically thick ADAF model or strong disk
  comptonization may work for ULX