Astronomers using NASA's Chandra X-ray Observatory have discovered a burst of X-rays and never-before observed short-term cyclical pulsing from a white dwarf star that has just undergone a thermonuclear explosion. These observations are helping scientists to better understand explosions that occur on the surfaces of white dwarfs in a special kind of binary star system.

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

The observations of Nova Aquila were reported at the "Two Years of Science with Chandra" symposium by an international team led by Sumner Starrfield of Arizona State University. They observed the exploding star, a nova, four times from April 2000 through October 2000. Nova Aquila was first detected by optical astronomers to become bright in the constellation of Aquila in December of 1999.

Although this star is at a distance of about 6000 light years it could be seen with the naked eye for about a month. "During this time it was about 100 thousand times brighter than our own Sun." said team member R. Mark Wagner of the University of Arizona.

Nova explosions occur on a white dwarf star (a star which used up all its nuclear fuel and then shrunk to the size of the Earth) that is orbiting a normal size star. Strong gravity tides from the white dwarf drag hydrogen gas off the normal star and this gas falls onto the white dwarf.

Once enough hydrogen gas is present on the surface it becomes hot enough for nuclear fusion reactions to start and gradually intensify until an explosion results.

Novas can occur in binary star systems where a normal star closely orbits a white dwarf -- a Sun-like star that has collapsed to about the size of the Earth. Explosions erupt when gas is pulled from the companion star onto the white dwarf's surface, creating an explosive layer of dense, hot hydrogen.

It takes more than 100 thousand years for enough hydrogen to fall onto the white dwarf and accumulate as a thick layer on the surface. When the layer exceeds its critical mass, thermonuclear fusion of hydrogen to helium occurs, creating a cosmic-sized hydrogen bomb blast. The outer layers of the white dwarf are blown away, producing a nova outburst that can be observed for a period of months as the material expands into space.

"Our first Chandra observations showed that the expanding gas was hot and nearly opaque" said Joachim Krautter of the State Observatory in Heidelberg, Germany. He also noted, " that it was not until 8 months after the explosion that the expanding gases cleared enough for us to see through them to the underlying star on which the explosion occurred."

"We found two important results in our Chandra observations: an increase in brightness in X-rays over 15 minutes, and a 40-minute cycle in brightness which lasted throughout our 7 hours of observations before and after this outburst", said Starrfield. "Although we observed nova explosions with previous X-ray satellites, we have never seen a nova perform in this way. We will use these variations to better understand the outer layers of the white dwarf."

"At least as important, our Chandra observations told us that thermonuclear fusion reactions were still occurring on the surface layers of the white dwarf -- eight months after discovery the explosion was not over" said Robert D. Gehrz of the University of Minnesota.

The Chandra X-ray data allowed astronomers to observe the white dwarf star itself soon after the explosion and to measure its temperature. They found that the white dwarf was heated to about 300,000 degrees Celsius and was pulsating with a 40-minute period.

"The cyclical changes in brightness suggest that we are observing the outer layers of the white dwarf expanding and shrinking over a 40 minute period", said Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics." Nova Aquila is one of the hottest stars ever found to go through such periodic changes in brightness."

This international team of astronomers is also trying to understand the details of the 30 earth masses of gas blown into space at speeds of more than 4 million miles per hour. This gas will eventually combine with other gas in the galaxy to form new stars.

"There is chemical evidence that some of the aluminum in our own solar system came from nova explosions. Some nova explosions eject gas that is very enriched in radioactive aluminum that once existed in our solar system but has since decayed" said team member Margarida Hernanz of the Institute for Space Studies of Catalonia.

The primary purpose of the Chandra observation was to analyze the composition of the white dwarf surface using the X-ray spectrum. This part of the project is being led by Peter Hauschildt of the University of Georgia who said "We are seeing the atmosphere of the white dwarf far earlier in the explosion than previously thought possible and our observations are providing superb data that must be carefully studied."

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), 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, MA. 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.