Accretion Disk Spectra of Ultra-luminous X-ray Sources and Galactic

superluminal jet sources

Ken Ebisawa (INTEGRAL Science Data Center, NASA/GSFC) Piotr Zycki (Copernicus Center), Aya Kubota (ISAS), T. Mizuno (Stanford), K. Watarai (Kyoto)

ApJ, Nov 10 issue, 2003

“Too-hot disk” problem in ULX and superluminal jet sources

• ULX energy spectra– Thermal spectrum, like standard optically thick

accretion disk – Disk temperature too high for given luminosity

and mass, assuming Schwarzschild black hole (Okada et al. 1998; Makishima et al. 2000)

• Same problem in Galactic superluminal jet sources GRS1915+105 and GRO J1655-40 (Zhang, Cui and Chen 1997)

• Standard Schwarzschild disk model fit gives too large luminosity

or too small mass

• Observed accretion disks are “too hot” compared to what is expected from reasonable mass and luminosity

Schwarzschild disk best-fit

M=1.8 M

L= 0.4 LEdd

M=9.4 M

L= 11 LEdd

M=100 M

L= LEdd

M= 7 M (real mass)L= 0.1 LEdd

How to explain the “too-hot” accretion disk?

• Standard accretion disk around Kerr black hole might explain the hard disk spectra (Zhang, Cui and Chen 1997; Makishima et al. 2000)– Rin = 3 Rs (Schwarzschild) 0.5 Rs (extreme

Kerr)– Higher disk temperature possible than the

Schwarzschild case

In the Schwarzschild metric, Rin = 3 Rs = 6 GM/c2

Max disk color temperature for Schwarzschild black hole

Tcol ~ 1.3 keV ((Tcol/Teff)/1.7) (M/MEdd)1/4 (M/7M)-1/4

In the Kerr disk, inner radius smaller, temperature can be higher

..

Standard accretion disk temperature

• Relativistic effects and inclination dependence of the disk spectra

• When the disk is face-on, relativistic effects do not change disk spectra significantly

• Edge-on Kerr disk has very hard spectrum– Due to doppler boosts in the innermost region

Kerr disk

Schwarzschild disk

Newtonian disk

Application of Kerr disk spectra to GRO J1655

• GRO J1655-40– i=70, d=3.2 kpc, Tcol/Teff=1.7 fixed

– M=16 M with a=0.998 (extremely Kerr)

– M=7 M suggests a=0.68 to 0.88 (Gierlinski et al. 2001)

– Inclined Kerr disk model works to solve the “too-small mass problem”!

– 450 Hz QPO (Strohmayer 2001) supports presence of the disk around a spinning black hole (Abramowicz and Kluzniak 2001)

Application of Kerr disk spectra to ULX

• IC342 Source 1– face-on Kerr disk (d=4Mpc, Tcol/Teff=1.7,a=0.998)

M=29 M and L=14 LEdd

• Super-Eddington problem not solved

– edge-on (i= 80) Kerr disk (a=0.998)M= 355 M and L=0.9 LEdd

• Super-Eddington problem may be solved, but unreasonably large mass required

– Standard Kerr disk model not plausible for ULX• disk inclination should be random• ~350 M black hole unlikely

• Adjusting inclination angle does not give reasonable mass and luminosity

• Standard disk model cannot explain the ULX energy spectra

Fitting IC342 spectrum with different inclination angle

Slim disk

Optically thin

Optically thick

Abramowicz et al. (1995)

Standard diskSoft state

Advection Dominated Accretion Flow (ADAF)

Hard state

Slim disk

Optically thick ADAF disk (slim disk) is stable

unstable

Luminosity of slim disk

– Optically thick and geometrically thick disk

For standard disk h/r~0.1, Ldisk < LEdd

For slim disk h/r ~ 1 Slim disk luminosity can be ~ 10 LEdd

~10

Slim disk for ULXs

Watarai et al. (2001)

Ldisk saturates at high Tin

(due to advection)

IC342 spectral change explained well

slim diskstandard disk(Ldisk∝Tin

4)

Slim disk model for ULX

• Fitting ASCA IC342 Source 1 spectrum with Watrai’s slim disk model (face-on, Tcol/Teff = 1.7, pseudo-Newtonian potential)

– M=23 M , Ldisk ~ 6 LEdd

– Super Eddington luminosity allowed with slim disk

– Slim disk model fit successful with reasonable mass and disk luminosity!

Spectral variation of IC342 Source1• Standard disk fit • Slim disk fit

• Mass varies– Unreasonable, bad model!

• Consistent mass, only mass accretion rate change– Reasonable, better model!

Origin of ULXs

• Universal luminosity function of X-ray binaries(Grimm et al. 2003)

• Luminosity cut-off ~1040 erg/s• Presumably, ULXs are X-ray

binaries with possible maximum stellar-size black holes

• ~40 M black holes theoretically possible (Fryer 1999)

• Such a massive black holes do not exist in our Galaxy, but not uncommon in external galaxies

ULX

Summary

• “Too-hot” disk problem in ULXs and Galactic super-luminal jet sources– Observed disk temperature is too high for Standard

Schwarzschild disk

• Standard Kerr disk spectra can explain the hard spectra of Galactic super-luminal jet sources– Galactic super-luminal jet sources are edge-on systems

• Slim disk can explain L>LEdd, high disk temperature, and spectral variation of ULXs

• ULXs are black holes with a few tens M with super-Eddington luminosities