Shin Mineshige, Atsunori Yonehara, Rohta Takahashi, PASA, 18 (2), in press.
Next Section: Inhomogeneous Disk Structure
Title/Abstract Page: X-Ray Microlensing of Bright
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Accretion Flow Models and Microlens Variations
We here consider three representative flow models: the standard disk, the optically thin ADAF, and a composite disk-corona model.
In the standard disk model (Shakura & Sunyaev 1973) viscous heating is balanced with radiative cooling. From the basic equations we can uniquely determine effective temperatures as a function of radius for given black-hole mass, M, and mass-flow rate, 
. Here, we set
, and is determined so as to reproduce the observed V magnitudes of the Einstein Cross in the absence of a microlensing. The resultant total spectrum and the local spectrum emitted from each concentric ring is displayed with thick and thin lines, respectively, in Figure 1 (left top). There are two prominent spectral features: (1) Since the disk emits blackbody radiation with temperature of 
K, emitted photon energy is restricted to optical to UV ranges. (2) Brightness distribution simply reflects the depth of the gravitational potential well. Consequently, the disk is hotter inside and cooler outside.
Microlens light curves reflect such radial changes of the radiation spectra. The inner hot and outer cool character of the standard disk yields a smaller effective size of the region emitting shorter-wavelength radiation, causing more rapid variations in shorter-wavelength radiation than longer-wavelength one, as is depicted in the right top panel of Figure 1 (see also Yonehara et al. 1998).
The next solution is an optically-thin advection-dominated accretion flow (ADAF, Ichimaru 1977; Narayan & Yi 1995; Abramowicz et al. 1995). In this model, it is advective energy transport (energy flow carried by accreting material) that is balanced with viscous heating. In contrast with the standard disk, in which accretion energy efficiently goes into radiation energy, accretion energy of gas in ADAF turns to its internal energy, with little fraction being radiated (Ichimaru 1977). In a word, an ADAF is faint hot flow, which contrasts a cool and bright standard disk. In Figure 1 (left middle) we display the spectral energy distribution of a typical ADAF, calculated by Manmoto, Kusunose, & Mineshige (1997). Two unique spectral features are: (1) Emitted photon energy is spread over a large frequency range, from radio (via synchrotron) to hard X-
rays (via inverse Compton). (2) Radiation energy does not reflect the depth of the potential well, where photons are emitted.
The left middle panel of Figure 1 shows that the emission is dominated by that from the innermost part within
. This is because large magnetic-field and electron energy densities are achieved only in that compact region, thus efficient synchrotron radiation in radio being possible there (radio photons are Compton up-scattered to produce optical and X-rays). Its consequence is that microlens of ADAF produces rather abrupt changes both in the optical and soft X-ray fluxes as are displayed in the right middle panel of Figure 1.
Since the optically thin ADAFs are too faint to explain the luminosity of bright quasars, we need an alternative model for making high-energy emission possible from luminous AGNs. We, finally, consider a composite disk-corona model by Kawaguchi et al. (2000), since they could, for the first time, reproduce the observed broad-band spectral properties of quasars. The calculated spectrum is shown by the thick line of Figure 1 (left bottom). According to this model, the big blue bump is by thermal emission from the disk body at small radii, the soft X-ray excess is inverse-Compton scattering of the soft photons from the inner disk, and the hard X-rays are bremsstrahlung radiation from the coronae at large radii. The contributions of the individual rings are also displayed by thin lines in Figure 1.
Importantly, soft X-rays are only from the vicinity of the black hole, as in the case of ADAF, while hard X-rays are from rather wide areas. Such unique emission properties produce interesting features in the multi-wavelength microlens light curves (see the right bottom panel of Figure 1). Soft X-ray radiation shows a relatively sharp peak around the caustic crossing time, while hard X-ray variations are rather smooth. These features are closely related to distinct emission mechanisms in different X-ray energy bands, as described above.
Next Section: Inhomogeneous Disk Structure
Title/Abstract Page: X-Ray Microlensing of Bright
Previous Section: Introduction
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