Spitzer Sees the Infrared Echo of Neutron Star Outburst

Lori Stiles
June 6, 2005

Astronomers observing with NASA's Spitzer Space Telescope say they may have seen the infrared "echo" of a neutron star that sent a blast wave from inside supernova remnant Cassiopeia A about 50 years ago.

Energy from the blast is heating clumps of interstellar dust, tracing the blast's progress outward from its source, said Oliver Krause and George H. Rieke of the University of Arizona's Steward Observatory. Spitzer has taken two pictures, spaced a year apart, of the energized interstellar dust, glowing bright at 24 micron wavelengths but invisible at the shorter wavelengths our eyes see. The pictures show different glowing clumps. By comparing them, Krause and Rieke can estimate when the blast occurred and what caused it.

NASA is releasing the stunning Spitzer images and animations today on the Spitzer Space Telescope Website, http://www.spitzer.caltech.edu/spitzer/. Krause, Rieke and their colleagues are reporting the discovery in the June 10 issue of Science.

About 50 years ago, the neutron star appears to have shot tremendous energy in opposite directions, in paths roughly perpendicular to the direction toward Earth, Krause and Rieke said. Even if astronomers had been looking toward the Cas A supernova remnant a half century ago, they wouldn't have noticed the outburst because it was not directed toward Earth, they said.

Cassiopeia A, the youngest supernova remnant known in our galaxy, exploded 325 years ago. Spitzer took images of Cas A and surrounding sky in November 2003 while checking out Spitzer's Multiband Imaging Photometer (MIPS), the instrument that Rieke and his team developed to scan the sky at very long infrared wavelengths.

Bright, wispy filaments and knots stetched well beyond Cas A, across their entire image, which was slightly longer than the apparent diameter of Earth's moon.

Followup near-infrared observations made at the 6.5-meter Multiple Mirror Telescope Observatory at Mount Hopkins, Ariz., in May 2004 and at the Calar Alto, Spain, 3.5-meter telescope in October 2004 and January 2005 showed some of the very brightest features were moving at close to the speed of light.

The team used Spitzer to rescan Cas A in December 2004. Krause analyzed how the features had moved in a year's time, and the researchers realized that some of the infrared wisps and knots were at the wrong location to be ejecta from the supernova Cas A explosion itself 325 years ago.

"Further measurements will test this, but we think that the neutron star within Cas A had an outburst about 50 years ago," Rieke said. "We see light that is just now encountering two interstellar clouds and heating them so we see the outburst's infrared echo."

Krause, Rieke, others from UA Steward Observatory, the Max Planck Institute for Astronomy (Heidelberg), Rutgers University and Space Science Institute wrote the article being published in Science.

JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. JPL is a division of Caltech. The multi-band imaging photometer for Spitzer, which made the far infrared observations, was built by Ball Aerospace Corporation, Boulder, Colo.; The University of Arizona; and Boeing North American, Canoga Park, Calif.

Additional information about the Spitzer Space Telescope is available at http://www.spitzer.caltech.edu/spitzer/


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Oliver Krause
George H. Rieke