Extrasolar Moons as Gravitational Microlenses Christine Liebig

CHAPTER 4. CHOICE OF SCENARIOS 41
−∆mag
3
2
1
Triple Lens Binary Lens
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
t/tE
(a) Rsource = 2R⊙. While not as pronounced
as with a solar sized source, the tell-tale signs
of a caustic crossing are still visible.
−∆mag
3
2
1
Triple Lens Binary Lens
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
t/tE
(c) Rsource = 10R⊙. The caustic crossing features
are no longer distinguishable.
−∆mag
3
2
1
Triple Lens Binary Lens
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
t/tE
(b) Rsource = 5R⊙. The planetary double
peak is still noticeable, but the caustic crossing
features are starting to wash out.
−∆mag
3
2
1
Triple Lens Binary Lens
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
t/tE
(d) Rsource = 20R⊙. The planet still reveals
itself as a bump on the slope of the Paczyński
curve.
Figure 4.11: Testing the effect of an increased source radius Rsource. Compare also
figure 4.9.
4.2.5 Sampling rate
When observing a gravitational lensing event, data points are acquired by taking
frames of the object. Typical exposure time has a duration of 30 to 300 seconds.
Higher observing frequency equals better coverage of the resulting light curve. A
constant observing rate facilitates understanding of the planetary population. In real
observations sampling is constrained by numerous factors. Usually, one or two dozen
events must be covered at a reasonable rate during follow-up observations, because
it is never certain which event will evolve to show those interesting anomalies that
we look for. Ground-based observations will always be weather-dependent. Then
there is the Pacific gap; more of a problem, when the nights are getting shorter in
the southern hemisphere.
A normal microlensing event is seen as a transient brightening that lasts for
about a month. A planet will alter the light curve for hours or days; a small planet
or a moon only for minutes or hours. This duration is inversely proportional to the