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Microlensing in the Einstein Cross

Mid-Infrared Imaging of the Einstein Cross QSO

Eric Agol , Stuart Wyithe, Barbara Jones, Omer Blaes, Chris Fluke, PASA, 18 (2), in press.
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Microlensing in the Einstein Cross

One way to distinguish dust from synchrotron is to try to measure the size of the infrared emission region as dust emission has a low brightness temperature, $\sim 10^3$K, while synchrotron has a high brightness temperature, > 108 K. The dust size is estimated to be

\begin{displaymath}\nonumber R_{\rm dust} = 3\times10^{18} \left({L \over 2\time... ... K}}\right)^2 \left({\epsilon\over 0.1}\right)^{-1/2}{\rm cm}, \end{displaymath} б 

where L is the QSO luminosity, Ts is the dust sublimation temperature, and $\epsilon$ is the dust emissivity. The synchrotron source size is estimated as

\begin{displaymath} R_{\rm synch}\sim10^{14}h^{-1}\left({F_\nu\over1{\rm mJy}}\r... ...4}\left({\lambda\over100\mu{\rm m}}\right)^{5/4} \,\,{\rm cm}, \end{displaymath} (1)

where F is the flux at the self-absorption turnover frequency $\lambda$, and B is the magnetic field strength.

The Einstein Cross QSO, 2237+0305, provides an astronomical laboratory for measuring the size of the QSO emission region. The optical depth to microlensing is high $\sim 0.5$ for all images, so the optical images fluctuate continuously and independently. If the source is larger than an Einstein radius

\begin{displaymath} R_{\rm E}= 1.1\times10^{17}M^{1/2}h^{-1/2}{\rm cm} \end{displaymath} (2)

then the fluctuations will be greatly reduced since the magnification pattern will be smoothed from averaging over the entire source. Conveniently, the Einstein radius divides the size scales of the dust and synchrotron emission models,

Rdust>RE > Rsynch, so that if synchrotron is correct, microlensing fluctuations will be observed as in the optical, while if dust is correct, the fluctuations will be negligible.

Next Section: Observations
Title/Abstract Page: Mid-Infrared Imaging of the
Previous Section: Motivation
Contents Page: Volume 18, Number 2

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