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.”
Other team members include Michael Eracleous (Penn State), Eric Agol
(University of Washington), and Sarah Gallagher (UCLA). NASA’s Marshall
Space Flight Center, Huntsville, Ala., manages the Chandra program for
NASA’s Office of Space Science, Washington. Northrop Grumman of Redondo
Beach, Calif., formerly TRW, Inc., was the prime development contractor for
the observatory. The Smithsonian Astrophysical
Observatory controls science and flight operations from the Chandra X-ray
Center in Cambridge, Mass.
Additional information and images are available at:
and
