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  • Gravitational Lensing Technique Discerns Distant Quasar
Nov 2011
PARIS, Nov. 9, 2011 — A new twist on gravitational lensing has led to a unique view of a quasar surrounding a distant black hole.

While black holes themselves are invisible, the forces they unleash cause some of the brightest phenomena in the universe. Quasars — glowing discs of matter that orbit supermassive black holes — heat up and emit extremely bright radiation as they do so.

Using the Hubble Space Telescope, an international team of astronomers measured the quasar’s size and studied the colors (and hence the temperatures) of different parts of the disc. These observations show a level of precision equivalent to spotting individual grains of sand on the surface of the moon.

A quasar that has been gravitationally lensed by a galaxy is in the foreground, which can be seen as a faint shape around the two bright images of the quasar. Observations of one of the images show variations in color over time. This is caused by stars within the lens galaxy passing through the path of the light from the quasar, magnifying the light from different parts of the quasar's accretion disc as they move. This has allowed a team of scientists to reconstruct the color and temperature profile of the accretion disc with unprecedented precision. The level of detail involved is equivalent to being able to study individual grains of sand on the surface of the moon while standing on Earth. (Image: NASA, ESA and Jose A. Muñoz)

“A quasar accretion disc has a typical size of a few light-days, or around 100 billion kilometers across, but they lie billions of light-years away. This means their apparent size when viewed from Earth is so small that we will probably never have a telescope powerful enough to see their structure directly,” said lead investigator Jose A. Muñoz of the University of Valencia in Spain. Muñoz and his colleagues at Instituto de Astrofisica de Canarias in Spain, Ohio State University and the Harvard-Smithsonian Center for Astrophysics published their findings in Astrophysical Journal.

Until now, the minute apparent size of quasars has meant that most of our knowledge of their inner structure has been based on theoretical extrapolations rather than on direct observations. The team therefore used an innovative method to study the quasar: using the stars in an intervening galaxy as a scanning microscope to probe features in the quasar’s disc that otherwise would be far too small to see. As these stars move across the light from the quasar, gravitational effects amplify the light from different parts of the quasar, giving detailed color information for a line that crosses through the accretion disc.

The team observed a group of distant quasars that are gravitationally lensed by the chance alignment of other galaxies in the foreground, producing several images of the quasar.

Even though a black hole encircled by a quasar ordinarily is too far away to see clearly, Hubble researchers used gravitational lensing to get better results. This diagram shows three different locations: on the left, the quasar accretion disc, which is bluer in the center and redder around the edges; in the center, a spot in an intermediate galaxy that the quasar’s light is passing through; and on the right, Hubble's view in orbit around the Earth. The gravity from a star in the intermediate galaxy, as it passes through the beams of light from the quasar, deflects the differently colored beams one by one toward Hubble. The colors seen from Hubble therefore change over time as the star effectively scans across the quasar's disc. This lets astronomers directly observe the color, temperature and size of the disc with precision equivalent to seeing individual grains of sand on the surface of the Moon. (Illustration: NASA and ESA)

They spotted subtle differences in color between the images as well as changes in color over the time the observations were carried out. Part of these color differences are caused by the properties of dust in the intervening galaxies: The light coming from each lensed image has followed a different path through the galaxy, so that the various colors encapsulate spectrographic information about the material within the galaxy. Measuring the way and extent to which the dust within the galaxies blocks light at such distances is itself an important result in the study.

For one of the quasars they studied, though, there were clear signs that stars in the intervening galaxy were passing through the path of the light from the quasar. Just as the gravitational effect due to the whole intervening galaxy can bend and amplify the quasar’s light, so can that of the stars within the intervening galaxy subtly bend and amplify the light from different parts of the accretion disc as they pass through the path of the quasar’s light.

By recording the color variation, the team reconstructed the color profile across the accretion disc. This is important because the temperature of an accretion disc increases the closer it is to the black hole, and the colors emitted by the hot matter get bluer the hotter they are. This allowed the team to measure the diameter of the disc of hot matter and plot how hot it is at different distances from the center.

They found that the disc is between four and 11 light-days across (approximately 100 to 300 billion kilometers). While this measurement shows large uncertainties, it is still a remarkably accurate measurement for a small object at such a great distance, and the method holds great potential for increased accuracy in the future.

“This result is very relevant because it implies we are now able to obtain observational data on the structure of these systems, rather than relying on theory alone,” Muñoz said. “Quasars’ physical properties are not yet well understood. This new ability to obtain observational measurements is therefore opening a new window to help understand the nature of these objects.”

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The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.
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