Quasar Microlensing

Contents

Introduction

The Einstein gravitational theory predicts that light and all other wavelengths of electromagnetic radiation are bent in the vicinity of any massive body, and it is of course everyday experience that the bending is neglegible in everyday experience. But, in astronomy where vast distances are involved, the bending can be sufficient to distort and displace the image, and in extreme cases can produce multiple images of a single distant source. This phenomenon is called gravitational lensing, and in extreme cases multiple images separated by a few arc-seconds on the sky are found for distant quasars lensed by a foreground galaxy with the mass of the Milky Way galaxy. Since discovery of the first such object by Dennis Walsh in 1979, 80 such objects have been recognized at optical and radio wavelengths. Formation of multiple images is always accompanied by magnification of the source image and amplification of the brightness.

Of course, the magnification properties depend on the mass of the lensing object. For a galaxy lens-mass of a billion suns, the observed separation of the multiple images is approximately an arcsec. But if the lensing galaxy has a grainy mass structure of a typical galaxy made of stars, the individual stars within the galaxy cause further image splitting within the main image with accompanying brightness amplification, in a process known as microlensing. Thus, if the lensed quasar image could be observed in great detail, it would be found to have brightness irregularities caused by the grainy mass distribution within the lens.

Discovery

Because it was understood at the time of the discovery of the first lens in 1979 and that measurement of a time delay associated with the different light paths contained important information about the expansion rate of the universe, several research groups attempted to measure the cosmological time delay. But, soon Chang and Refsdal realized that the microlensing effect could cause important brightness changes that would mask the intrinsic quasar brightness measurements needed to determine the time delay and the expansion rate of the universe. This prediction was confirmed when the first measurement of the time delay revealed the microlensing much as feared, the the subject of time delay estimation in the first object was in a state of confusion for 10 years after its discovery.

During this time, the Einstein Cross quasar also was found to display microlensing behavior, which seemed weaker than expected but was carefully studied even though a time delay would not be measured until 2005 (Vakulik et al. 2005). By then, the microlensing was also seen in 6 more gravitational lens systems.

An unexpected surprise was the detection of a very rapid microlensing, with weak fluctuations lasting just a day. This was immediately recognized to be the signature of an important population of rogue planets in the lens galaxy, since an important signature of small rapid positive and negative amplifications were recognized. For such rapid brightness fluctuations to occur, the source quasar must have finer structure than had been supposed, and further study is allowing new understanding of the optical emitting structures within quasars.

Preview Image

"Finding Planets: Multiple Methods Help Track Elusive Quarry" - Gravitational Microlensing.  This method derives from one of the insights of Einstein's theory of general relativity: gravity bends space. We normally think of light as traveling in a straight line, but light rays become bent when passing through space that is warped by the presence of a massive object such as a star. This effect has been proven by observations of the Sun's gravitational effect on starlight.  When a planet happens to pass in front of a star along our line of sight, the planet's gravity will behave like a lens. This focuses the light rays and causes a temporary sharp increase in brightness and change of the apparent position of the star.  Astronomers can use the gravitational microlensing effect to find objects that emit no light or are otherwise undetectable.  (Soure: Cal-Tech/JPL - Planet Quest.)