Dolores Beasley

Headquarters, Washington, DC

(Phone: 202/358-1753)

Steve Roy

Marshall Space Flight Center, Huntsville, AL

(Phone: 256/544-6535)

Dr. Wallace Tucker

Harvard-Smithsonian Center for Astrophysics, Cambridge, MA

(Phone: 617/496-7998)

RELEASE: 00-66

Using NASA’s Chandra X-ray Observatory, a team of scientists
has attacked one of astronomy’s oldest and thorniest problems,
determining the distance to a cosmic object.

The scientists measured the distance to an X-ray source by
observing the delay and smearing (filtering) out of X-ray signals
traversing 30,000 light years of interstellar gas and dust.
Chandra “opened a new world,” said Peter Predehl of the Max-Planck
Institute, Garching, Germany, the lead author on a report to be
published in the European journal Astronomy and Astrophysics.

“Geometrical distance measurements are of particular
importance for astronomy. Now we have a new method that works for
distant sources,” Predehl said. One of the most crucial pieces of
information needed in astronomy is the distance to the stars and
galaxies. This information also is among the most difficult to
obtain because, with rare exceptions, astronomers cannot measure
distance directly and must use a variety of ingenious but
uncertain techniques.

This new method relies on the scattering of X-rays by
interstellar dust grains between a source and the Earth. Although
the scattering material is different, the dust produces a halo,
much like the halo around a traffic light on a foggy night.

“When the light switches from red to green (or vice versa),
the halo around the light is also slightly delayed,” Predehl
explained. “No one would use this delay for determining the
distance to the traffic light, of course (the delay is only a few
billionths of a second). But if
the ‘traffic light’ is 30,000 light years away, the delay is on
the order of 15 minutes. Using the excellent and unprecedented
resolution of the Chandra observatory, we can distinguish between
light that was 30,000 years on its way and other light that needed
only a few minutes more.”

Other members of the team included Vadim Burwitz and Joachim
Trumper, also of the Max-Planck Institute, and Frits Paerels of
Columbia University, New York, NY. Trumper and a colleague
proposed this method 27 years ago, but it could not be applied
until an X-ray observatory with Chandra’s unique capability was
available.

The X-ray source Cygnus X-3 acts like a cosmic traffic light,
or more appropriately, lighthouse. Its X-ray emission varies
regularly with a 4.8-hour period, as a neutron star or black hole
circles a nearby companion star. The radiation from the halo is
delayed and smeared out, so the variations are damped. For the
inner part of the halo, the damping is small, whereas for the
outer part, the periodic variation is completely washed out. By
observing the time delay and variations at different parts of the
halo, the distance to the source can be determined.

Seismologists use a similar method based on the propagation
of sound waves through the Earth to determine the epicenters of
earthquakes or to locate unusual geological formations.

Predehl and colleagues observed Cygnus X-3 for 3 1/2 hours
with Chandra using the Advanced CCD Imaging Spectrometer (ACIS).
By analyzing the time variations in the halo, the astronomers
determined that the distance to Cygnus X-3 is 30,000 light years,
within about 20 percent accuracy. The accuracy was limited by the
short observing time, which was less than the full 4.8-hour period
of variation. The team hopes to refine this estimate in the near
future, as data from a longer observation of the source becomes
available.

The X-ray scattering method of measuring cosmic distances
depends on the fact that X-rays, because of their high energies,
are scattered through small angles by dust grains. It cannot be
used with optical telescopes because visible light photons have
lower energy and are scattered through much larger angles by the
dust grains.

In principle, the method also could work for nearby galaxies,
such as the Small and Large Magellanic Clouds and the Andromeda
Nebula. If so, it would help astronomers in their quest to
understand the size and age of the universe, since it would
provide an independent estimate of the size of the first steps on
the cosmic distance ladder.

The ACIS instrument was built for NASA by the Massachusetts
Institute of Technology, Cambridge, MA, and Pennsylvania State
University, University Park. NASA’s Marshall Space Flight Center
in Huntsville, AL, manages the Chandra program. TRW, Inc., Redondo
Beach, CA, is the prime contractor for the spacecraft. The Chandra
X-ray Center at the Harvard-Smithsonian Center for Astrophysics
controls science and flight operations from Cambridge, MA.

-end-

NOTE TO EDITORS: Images to illustrate this release,
including high resolution digital versions of the X-ray image
(JPG, 300 dpi TIFF), and information on Chandra’s progress can be
found at:

http://chandra.harvard.edu and http://chandra.nasa.gov