Quasar microlensing |
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Authors: | R W Schmidt and J Wambsganss |
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Institution: | 1.Astronomisches Rechen-Institut,Zentrum für Astronomie der Universit?t Heidelberg,Heidelberg,Germany |
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Abstract: | Quasar microlensing deals with the effect of compact objects along the line of sight on the apparent brightness of the background
quasars. Due to the relative motion between quasar, lenses and observer, the microlensing magnification changes with time
which results in uncorrelated brightness variations in the various images of multiple quasar systems. The amplitudes of the
signal can be more than a magnitude with time scales of weeks to months to years. The effect is due to the “granular” nature
of the gravitational microlenses—stars or other compact objects in the stellar mass range. Quasar microlensing allows to study
the quasar accretion disk with a resolution of tens of microarcseconds, hence quasar microlensing can be used to explore an
astrophysical field that is hardly accessible by any other means. Quasar microlensing can also be used to study the lensing
objects in a statistical sense, their nature (compact or smoothly distributed, normal stars or dark matter) as well as transverse
velocities. Quasar microlensing light curves are now being obtained from monitoring programs across the electromagnetic spectrum
from the radio through the infrared and optical range to the X-ray regime. Recently, spectroscopic microlensing was successfully
applied, it provides quantitative comparisons with quasar/accretion disk models. There are now more than a handful of systems
with several-year long light curves and significant microlensing signal, lending to detailed analysis. This review summarizes
the current state of the art of quasar microlensing and shows that at this point in time, observational monitoring programs
and complementary intense simulations provide a scenario where some of the early promises of quasar microlensing can be quantitatively
applied. It has been shown, e.g., that smaller sources display more violent microlensing variability, first quantitative comparison
with accretion disk models has been achieved, and quasar microlensing has been used to determine the fraction of dark matter
in a lensing galaxy for the first time. This is the quantitative beginning. The future of quasar microlensing is bright. |
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