Bizarre Magnification Effect
A careful analysis of observations
by NASA's Chandra X-ray Observatory of a rare quadruple quasar has
uncovered evidence that possibly a single star in a foreground
galaxy magnified X-rays coming from the quasar. This discovery
gives astronomers a new and extremely precise probe of the gas flow
around the supermassive black hole that powers the quasar.
"If our interpretation is correct, then we are seeing details
around this black hole that are 50,000 times smaller than either
the Hubble Space Telescope or Chandra could see under ordinary
circumstances," said George Chartas of Penn State University in
University Park, and lead author of a recent article on the
Cloverleaf quasar in The Astrophysical Journal.
The Cloverleaf quasar is a single object about 11 billion light
years from Earth that appears as four images produced by a process
known as gravitational lensing. If one or more galaxies lie
along the line of sight to a more distant quasar, the gravitational
field of the intervening galaxies can bend and magnify the light
from the quasar and produce multiple images of it. The four
images of the Cloverleaf quasar have been produced by one or more
intervening galaxies.
One of the images (A), in the Cloverleaf is brighter than the
others in both optical and X-ray light. Chartas and his
colleagues found the relative brightness of this image was greater
in X-ray than in optical light. The X-rays from iron atoms were
also enhanced relative to X-rays at lower energies. Since the
amount of brightening due to gravitational lensing does not vary
with the wavelength, this means that an additional object has
magnified the X-rays.
The increased magnification of the X-ray light can be explained
by gravitational microlensing, an effect which has been used to
search for compact stars and planets in our galaxy. Microlensing
occurs when a star or a multiple star system passes in front of
light from a background object.
If a single star or a multiple star system in one of the
foreground galaxies passed in front of the light path for the
brightest image, then that image would be selectively magnified.
The X-rays would be magnified much more than the visible light, if
they came from a smaller region around the black hole than the
visible light. The enhancement of the X-rays from iron ions would
be due to this same effect.
The analysis indicates that the X-rays are coming from a very
small region, about the size of the solar system, around the
supermassive black hole. The visible light is coming from a region
ten or more times larger. The angular size of these regions at a
distance of 11 billion light years is tens of thousands times
smaller than the smallest region that can be resolved by the Hubble
Space Telescope.
"The significance of the detection of microlensed X-rays from
the Cloverleaf quasar lies in the extremely small region that is
enhanced by the microlens," said Chartas. "This gives us the
ability to make strong tests of models for the flow of gas around a
supermassive black hole."