# Presentation: 038701

## X-ray Observation of Black Hole Accretion Disks

- Ken Ebisawa (NASA/GSFC, USRA)

## Galactic Black Hole Candidates Spectral States and Accretion Disk

- High/soft state (1038erg/s)
- 2-10 keV X-ray spectrum is thermal (ultra-soft), power-law hard-tail above ~10keV
- Optically thick and geometry thin disk
- Low/hard state(1037erg/s)
- 2-10 keV X-ray spectrum is power-law
- Optically thin and geometry thick disk
- In this talk, I will concentrate on observation of *optically thick accretion disk* spectra

## Accretion disk solution

- Abramowicz et al. (1995)

- Optically thick
- ADAF

*Standard disk* is observationally established
*Slim disk* seems to be observed in Galactic BHC and ULXs

## Standard Accretion Disk Model

- Gravitational energy is released as thermal emission (Shakura and Sunyaev 1973)
- Comptonization is significant in the inner region
- Local emission shows Wien peak with Tcol(r)
- Tcol/Teff ~1.7-1.9 independent of radius and disk luminosity (Shimura and Takahra 1995; Ross and Fabian 1996).
- Standard accretion disk spectrum looks like super-position of blackbody spectra
- multi-color disk-blackbody approximation works (*diskbb* in xspec)

## Optically thick accretion disk in the Soft state

- Theoretical optically thick accretion disk spectrum
- (Shimura and Takahara 1995)

**Local emission stongly affected by**
** ****Compton scattering**

**T****col****: color temperature**
**T****eff****: effective temperature**
**T****col**** ~ 1.9 T****eff**

## High state of Galactic Black Hole Candidates

- Inner disk radius is constant at ~3 Rs (last stable orbit in Schwarzschild metric)
- X-ray observation  disk size Rs=2GM/c2
- Black hole mass may be constrained from X-ray energy spectral analysis

## Optically thick accretion disk in the Soft state

- LMC X-3 GINGA
- (Ebisawa et al. 1993)

**Intensity vs. Hardness correlation**

**Optically thick accretion disk**
**L****disk**** ******** R****in****2**** T****in****4**

## Optically thick accretion disk in the Soft state

- LMC X-3 GINGA
- (Ebisawa et al. 1993)

**Spectral fitting with optically **
**thick accretion disk **
**with Schwarzschild metric,**
**R****in**** = 3 Schwarzschild radii**

**Distance, inclination fixed,**
**M, dM/dt free parameters.**

**M remarkably constant**

**M~6 (****(T****col****/T****eff****)****/1.9)****2**** **M** **

**Consistent with optical results**

## Relativistic effects

- Innermost disk radius = Last stable orbit
- 3Rs in Schwarzschild metric
- ~0.5 Rs in extreme Kerr case
*Kerr disk can be hotter than Schwarzschild disk*
- Doppler effect, gravitational redshift, light bending
*Disk spectra calculation...*
- Schwarzschild disk spectrum is not very different from the Newtonian spectrum
*Face-on* Kerr disk spectrum is not very difference either
*Inclined* Kerr disk spectrum is significantly hardened
- Hard spectra of microquasars may be explained

## Laor, Netzer and Piran (1990)

- Kerr disk

- Scwarzschild disk

- Inclined Kerr disk is brighter in high energies!

- cos i

- cos i

- Comparison of Schwarzschild and Kerr disk spectra

- 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

- Ebisawa et al. (2003)

## Application of Kerr disk spectra to GRO J1655-40

- GRO J1655-40 and GRS1915+15 (microquasars) are highly inclined systems, and known to have unusually high disk temperature (e.g., Zhang, Cui and Chen 1997)
- Kerr disk model suggested instead of Schwarzschild disk
- GRO J1655-40 spectral fitting
- i=70, d=3.2 kpc, Tcol/Teff=1.7 fixed
- Schwarzschild disk model  M=1.8 M (Ebisawa et al. 2003)
- Too small compared to M=7 M from optical observation
- Kerr disk model with a=0.68 to 0.88 is consistent with M=7 M (Gierlinski et al. 2001; Ebisawa et al. 2003)
*Inclined Kerr disk model works to solve the “too hot a disk” problem!*
- 450 Hz QPO (Strohmayer 2001) also supports presence of the  disk around a spinning black hole (Abramowicz and Kluzniak 2001)

## Optically thick ADAF disk (slim disk)

- Standard disk breaks when Ldisk  > LEdd
- Energy advection will be dominant
*Slim disk* solution
*Slim disk luminosity can exceed L**Edd**!*
*May explain ULXs naturally*
*~5 L**Edd** of ~20 *M black hole* ****~10**40** ergs/s*

## Slim disk can exceed LEdd!

- Optically thick and geometrically thick (slim) disk

- For standard disk h/r~0.1, Ldisk < LEdd
- For slim disk h/r ~ 1
- 	 Slim disk luminosity can be ~ 10 LEdd

## Characteristics of the Slim disk  

_**Temperature profile**_**(****Watarai et al.2000****)**

**Standard disk**

- Characteristics of the Slim disk

- Radial dependence of the disk temperature changes
*p* = 0.75  0.50

- Inner disk radius can be smaller and temperature higher

**Slim disk ****(optically thick ADAF) **

- Standard disk
- (radiation dominant, T  r -0.75)

***T *****increase**

## Slim disk in XTEJ1550-56

**Multicolor disk blackbody with T(r)~ r****‐p**

_***p***__**-free disk model**_

**Slim disk is observed in Galactic BHC**
**when luminosity is extremely high**

**Smaller *****p***** gives systematically better fits!**

- Kubota and Makishima (2004)

## Slim disk in  GRS1915+105

**Yamaoka (2001)**

**Standard disk model fit **
**T(r)********r ****-0.75**

**High T****in**

**T(r)********r ****-0.5**** favored**

****** ****slim disk**

**Disk oscillation between the **
**standard disk and slim disk**

## Slim disk in ULX (IC342 Source1)

- Standard disk fit

- Slim disk model fit by Watarai et al. (2001)

- Mass varies
- Unreasonable, bad model!

- Constant mass (~23M), only mass accretion rate changes
- Reasonable, better model!

- Ebisawa et al. (2003)

## Caveat in ULX spectral modeling

*Phenomenological* “standard disk+power-law model” may mislead to very low disk temperature and large disk normalization (radius)
- Unexplained power-law dominates
- “intermediate” mass black hole accounts for soft excess
- Tin < 0.1 keV, M>100 M
- Slim disk with Comptonization (e.g. Kawaguchi 2003) can explain intrinsically hard ULX spectra
- Disk component dominates
- power-law tail component hardly required
- Reasonable stellar black hole mass (M < 40 M )

## Standard disk and slim disk comparison for M81 X-9

- Standard disk

- Slim disk

- Power-law

- Power-law

- Application of Kawaguchi (2003) slim disk model to XMM spectra (Foschini et al. 2004)

- M~150 M

- M~18 M

- Unexplanied power-law component is
- dominant, Tin ~0.3 keV, large Rin

- Slim disk explains most of the spectrum,weak hard-tail required

## Standard disk and slim disk comparison for NGC1313 X-2

- Standard disk

- Slim disk

- Power-law

- Power-law

- Tin~0.27 keV
- M~120 M

- M~4 M

- Application of Kawaguchi (2003) slim disk model to XMM spectra (Foschini et al. 2004)

## Likely 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 may not exist in our Galaxy, but not uncommon in external galaxies

## Conclusion

- Standard accretion disk model is established for Galactic BHC when Ldisk  LEdd
- Slim disk seems to be observed in Galactic BHC and ULXs  when Ldisk  LEdd
- ULXs are presumably slim disk around massive stellar black holes (< 40 M ) shining at super-Eddington luminosities
*Theoretical work on slim disk spectra and application to observed spectra urgently needed*

- Acknowlegement to my colleagues: *A. Kubota, P. Zycki, K. Watarai, K.Yamaoka, L. Foschini*, and *T. Kawaguchi*