Gravitational Lensing: Microlensing and Caustics
Interactive gravitational lensing. The image plane is the main view: the lens (single point mass or a binary), its critical curve, the source-plane caustic, and the lensed images found from the lens equation $\beta = \theta - \sum_i m_i (\theta - z_i)/|\theta - z_i|^2$. Drag or let the source drift; the diagnostic strip shows the total magnification, the single-lens Paczynski bump $A(u) = (u^2+2)/(u\sqrt{u^2+4})$ or the steep caustic-crossing spikes of a binary lens. Merges the former microlensing-event and lensing-caustics playgrounds.
WHAT TO TRY
- Drag the source toward the lens: the two images stretch along the Einstein ring and brighten, and the magnification A(t) climbs the Paczynski peak. At closest approach the images nearly merge into a ring.
- Switch to the binary lens: the single smooth bump breaks into sharp caustic-crossing spikes as the source crosses the diamond caustic, the signature that flags a planet in real microlensing surveys.
- Set a small impact parameter and let the source drift: the light curve sharpens to a tall narrow peak, since the peak magnification scales inversely with how close the source passes the lens.
- Use the preset events for a quick tour: High-mag and Grazing show the tall and shallow point-lens bumps, while Binary caustic and Planet drop the source across the caustic so the light curve spikes the way a real planetary anomaly does.