Our discussion of microlensing assumed a single star and a circularly symmetric potential about it. This is usually a good approximation for stars in our galaxy. However, when the star is in another galaxy and the source is a background quasar (Figs. 7.19, 7.20), it is necessary to include the gravitational effects of the galaxy’s other stars and its dark matter. Recast the microlensing analysis (Sec. 7.6.3) in the potential formulation of Eq. (7.98) and add external magnification and external shear contributions to ψ that are proportional to θ_x² + θ_y² and θ_x² − θ_y², respectively. The latter will break the circular symmetry, and structurally stable caustics will be formed. Explore the behavior of these caustics as you vary the strength and sign of the magnification and shear contributions. Plot a few flux variations that might be observed.

Figure 7.19

Figure 7.20

Transcribed Image Text:

t = tgeo + tlens = deff 2c (0 - 3)² - = defft C (7.98)