NASA’s Chandra X-ray Observatory has discovered a giant outburst of X-rays
and unusual cyclical pulsing from a white dwarf star that is closely orbiting
another star — the first time either of these phenomena has been seen in
X-rays. The observations are helping scientists better understand the
thermonuclear explosions that occur in certain binary star systems.

The observations of Nova Aquila were reported today at the “Two Years of
Science with Chandra” symposium by an international team led by Sumner
Starrfield of Arizona State University.

“We found two important results in our Chandra observations. The first was
an underlying pulsation every 40 minutes in the X-ray brightness, which we
believe comes from the cyclical expansion and contraction of the outer layers
of the white dwarf,” said Starrfield. “The other result was an enormous
flare of X-rays that lasted for 15 minutes. Nothing like this has been seen
before from a nova, and we don’t know how to explain it.”

Novas occur on a white dwarf (a star which used up all its nuclear fuel and
shrank to roughly the size of the Earth) that is orbiting a normal size star.
Strong gravity tides drag hydrogen gas off the normal star and onto the white
dwarf, where it can take more than 100,000 years for enough hydrogen to
accumulate to ignite nuclear fusion reactions. Gradually, these reactions
intensify until a cosmic-sized hydrogen bomb blast results. The outer layers
of the white dwarf are then blown away, producing a nova outburst that can be
observed for a period of months to years as the material expands into space.

“Chandra has allowed us to see deep into the gases ejected by this giant
explosion and extract unparalleled information on the evolution of the
white dwarf whose surface is exploding,” said Jeremy Drake of the Harvard-
Smithsonian Center for Astrophysics.

The brightening of Nova Aquila was first detected by optical astronomers in
December 1999. Although this star is at a distance of more than 6,000 light
years, it could be seen with the naked eye for about a month, during which
it was about 100,000 times brighter than our own Sun.

Chandra observed the nova, so-called because early astronomers believed
they heralded the appearance of a new star, four times from April 2000
through October 2000. “Our first Chandra observations showed that the
expanding gas around Nova Aquila was hot and nearly opaque,” said Joachim
Krautter of the State Observatory in Heidelberg, Germany. “When we looked
months later with Chandra, the expanding gases cleared enough for us to
see through them to the underlying star on which the explosion occurred.”

The latter Chandra X-ray data revealed the cyclical changes in brightness
are due to the white dwarf expanding and shrinking over a 40-minute period.
They also showed that the temperature on the surface of the white dwarf was
300,000 degrees Celsius, making Nova Aquila one of the hottest stars ever
observed to undergo such pulsations.

“The observations told us that thermonuclear fusion reactions were still
occurring on the surface layers of the white dwarf — more than eight
months after the explosion first began!” said Robert Gehrz of the
University of Minnesota.

Other members of the team are Howard Bond (Space Telescope Science
Institute), Yousaf Butt (Harvard-Smithsonian Center for Astrophysics), Koji
Mukai (Goddard Space Flight Center), Peter Hauschildt (University of
Georgia), Margarida Hernanz (Institute for Space Studies, Catalonia, Spain),
Marina Orio (University of Wisconsin and the Torino Observatory in Italy),
and Charles Woodward (University of Minnesota).

Chandra observed Nova Aquila for a total of 10 hours with the High Resolution
Camera (HRC) and the Advanced CCD Imaging Spectrometer (ACIS). The HRC was
built for NASA by the Smithsonian Astrophysical Observatory, Cambridge, Mass.
The ACIS instrument was built for NASA by the Massachusetts Institute of
Technology, Cambridge, and Pennsylvania State University, University Park.
NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra
program. TRW, Inc., Redondo Beach, Calif., is the prime contractor for the
spacecraft. The Smithsonian’s Chandra X-ray Center controls science and
flight operations from Cambridge, Mass.

Images associated with this release are available on the World Wide Web at:


Chandra observed Nova Aquila, an outburst caused by a thermonuclear explosion
on the surface of a white dwarf star, four times from April 2000 through
October 2000. In the October data astronomers detected a dramatic flare of
X-rays and cyclical 40-minute pulsations — the first time either of these
phenomena had been seen in X-rays. The pulsations are thought to come from
the contraction and expansion of the outer layers of the white dwarf, but
the cause of the 15 minute X-ray flare remains a mystery.

The artist’s illustration depicts a classical nova binary system just before
an explosion on the surface of the white dwarf. Classical novas occur in a
system where a white dwarf closely orbits a normal, companion star. In this
illustration, gas is flowing from the large red, companion star into a disk
and then onto the white dwarf that is hidden inside the white area. As the
gas flows ever closer to the white dwarf, it gets increasingly hotter, as
indicated by the change in colors from yellow to white. When the explosion
occurs, it engulfs the disk of gas and the red companion star.

Credit: CXC/M. Weiss