Australia Telescope National Facility

X-Ray Microlensing of Bright Quasars
Shin Mineshige, Atsunori Yonehara, Rohta Takahashi, PASA, 18 (2), in press.

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Introduction

It is widely believe that black-hole accretion works as a central engine producing quasar activity, although our understanding of the basic flow structure is still in a stage far from being satisfactory. One reasons is that we are unable to resolve accretion-flow structure with any existing telescopes. There exists, however, one potentially useful method to investigate the structure of quasar accretion disks; that is the technique by using microlensing (Chang & Refsdal 1979, 1984; Blandford & Hogg 1985). Broad-band photometry will be able to detect the color changes, thereby revealing the structure of quasar accretion disks. Here, we elucidate the theory of microlens diagnostics on quasar. We present expected microlens light variations of luminous quasars based on the disk-corona model by Kawaguchi, Shimura, & Mineshige (2000) and compare the results with those of other accretion flow models.

The ideal source for this purpose is Q 2237+0305, the so-called Einstein Cross (Huchra et al. 1985). The Einstein-ring radius on the source plane is

$r_{\rm E} \sim 1.5\times 10^{15} ({M_{\rm lens}}/{M_\odot})^{1/2} {\rm m}$ , whereas a caustic crossing length over the quasar image plane during a time t is,

$r_{\rm cross} \sim 6.9\times 10^{11} v_{1000}({t}/{1~{\rm d}})$ m, where

$v_{1000} \equiv v_{\rm t}/1000$ km s^-1 with $v_{\rm t}$ being the transverse velocity of the lens object on the lens plane. Fortunately, this crossing length is comparable to the Schwarzschild radius for a $10^8 M_\odot$ black hole,

$r_{\rm g}\simeq 3\times 10^{11}$ m, and is much smaller than $r_{\rm E}$ . Namely, due to a finite source-size effect, we are able to resolve the source structure on scales much less than the Einstein-ring radius. By frequent observations we can resolve the disk structure with a good spatial resolution (e.g. Wambsganss, Paczynski, Schneider 1990).

**Figure 1:** Left panels: Spectral energy distribution in the rest frame of a standard disk (top), an optically thin ADAF (middle), and a disk-corona model (bottom), respectively, all with the contributions by individual concentric rings being plotted with thin lines. Right panels: Caustic crossing light curves of the standard disk (top), the ADAF (middle), and the disk-corona model (bottom), respectively. We set
$\Delta F_\nu = 0$ outside the caustic.
$\begin{figure} \begin{center} \psfig{file=fig1.eps,height=12.8cm}\end{center}\end{figure}$

Next Section: Accretion Flow Models and
Title/Abstract Page: X-Ray Microlensing of Bright
Previous Section: X-Ray Microlensing of Bright
Contents Page: Volume 18, Number 2

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