Microlensing and the structure of active galactic nucleus accretion disks

Rapid variability has been reported in two of the four gravitationally lensed images of Q2237+0305, and this is attributed to microlensing caused by the intervening stars. The associated constraints on the source size and properties are studied and compared with a variety of stationary accretion disk models. It is found that graybody disks have a half-power radius r~ 2 x 10^16^(V/V_L_)^-1.7^(L_L,46_)^0.5^(ɛ/0.1)^-0.5 ^cm, where V is the frequency, L_L,46_ is the luminosity per log frequency in units of 10^46^ ergs s^-1^ at the Lyman limit V_L_, and ɛ is the emissivity relative to a blackbody. The reported microlensing variation in Q2237 + 0305 requires the disk size to be over 3 times smaller than a blackbody disk of similar luminosity, implying that the optical emission is either nonthermal or optically thin. An exploration of nonstationary disk models including orbiting, transient hot spots leads to a similar conclusion. Implications for models of active galactic nucleus optical continua are briefly discussed.

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