Observational Evidence for Quasar Microlensing

The Einstein Cross: Quadruple Quasar Q2237+0305

Since the first evidence for microlensing by Irwin et al. (1989) in this system, Q2237+0305 has been monitored by many groups (Corrigan et al. 1991; Østensen et al. 1996; Lewis et al. 1998). The most recent (and most exciting) results (Wozniak et al. 2000) show that all four images vary dramatically, going up and down like a rollercoaster in the last three years:

$\Delta m_A \approx$ 0.6 mag,

$\Delta m_B \approx$ 0.4 mag,

$\Delta m_C \approx$ 1.3 mag (and rising?),

$\Delta m_D \approx$ 0.6 mag (cf. Figure 2).

The Double Quasar Q0957+561

The microlensing results for the double quasar Q0957+561 are not as exciting. In the first few years, there appears to be an almost linear change in the (time-shifted) brightness ratio between the two images (

$\Delta m_{AB} \approx 0.25 mag$ over 5 years). But since about 1991, this ratio stayed more or less ``constant" within about 0.05 mag, so not much microlensing was going on in this system recently (Schild 1996; Pelt et al. 1998; Schmidt & Wambsganss 1998). At this moment, the possibility for some small amplitude rapid microlensing (cf. Colley and Schild 2000) cannot be excluded; however, one needs a very well determined time delay and very accurate photometry, in order to establish that. With numerical simulations and limits obtained from three years of Apache Point monitoring data of Q0957+561, and based on the Schmidt & Wambsganss (1998) analysis, we extend the limits on the masses of ``Machos" in the (halo of the) lensing galaxy: the small ``difference" between the time-shifted and magnitude-corrected lightcurves of images A and B excludes a halo of the lensing galaxy made of compact objects with masses of

$10^{-2} M_{\odot }$ (Wambsganss et al. 2000), see Figure 2.

**Figure 2:** This ``exclusion diagram'' visualizes which part of the parameter space ``quasar size''-``microlens mass'' can be excluded with what probability (indicated by height and color of the columns/bars; e.g., the white bar for a size of 10¹⁵cm and a mass of
$10^{-2} M_{\odot }$ means this parameter pair is excluded with 100% probability; light grey bars for

$10^{-1} M_{\odot }$ reflect about 80% exclusion probability). This analysis is based on a comparison between monitoring data of Q0957+561A, B and intensive numerical simulations (for more details see Wambsganss et al. 2000).
$\begin{figure} \centerline{ \epsfig{file=lego_95-97.eps, width=110mm} }\end{figure}$

Other multiple quasars/radio microlensing?

A number of other multiple quasar systems are being monitored more or less regularly. For some of them microlensing has been suggested (e.g. H1413+117, Østensen et al. 1997; or B0218+357, Jackson et al. 2000). In particular the possiblity for ``radio"-microlensing appears very interesting (B1600+434, Koopmans & de Bruyn 2000; also Koopmans, these proceedings), because this is unexpected, due to the presumably larger source size of the radio emission region. The possibility of relativistic motion of radio jets may make up for this ``disadvantage".

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Contents Page: Volume 18, Number 2

ASKAP

Public

Australia Telescope National Facility

Observational Evidence for Quasar Microlensing

The Einstein Cross: Quadruple Quasar Q2237+0305

The Double Quasar Q0957+561

Other multiple quasars/radio microlensing?