October 6, 2006
Mug Shots of Supernovas Reveal Two Key Findings
Scientists using NASA's Swift satellite have observed two dozen recent star explosions, called supernovae, quickly after the event and have discovered never-before-seen properties, some of which run counter to prevailing theories.
In one observation, they have confirmed the origin of Type Ia supernovae, an important class of explosions used to measure distances and dark energy. In other observations they have found new mechanisms to produce X-rays and ultraviolet light.
The findings have emerged from a concerted effort by the Swift team to capture images of ordinary supernovae as rapidly as possible. Stefan Immler of NASA's Goddard Space Flight Center, Greenbelt, Md., led the analysis of these supernovae.
"For many supernovae, we are getting to the scene of the crime to investigate the explosion within hours to days, as opposed to the typical delay time of days to weeks," said Immler. "We are finding clues about how stars explode that would have disappeared had we turned our telescopes to the site just a few days later."
Swift's primary goal to study gamma-ray bursts, the most energetic explosions in the universe. Most gamma-ray bursts last no longer than about ten seconds, so speed is essential to detect and study the bursts. Swift has three telescopes: A gamma-ray telescope to detect the burst, and X-ray and ultraviolet/optical telescopes to provide rapid follow-up observations, all the while broadcasting the burst location to other observatories.
"These same properties---speed, agility and multi-wavelength capability---are proving to be crucial in unraveling the supernova mystery as well," said Swift Principal Investigator Neil Gehrels of Goddard.
There are several types of supernovae, that differ in their origins and light characteristics. In our galaxy, a supernova will occur only once or twice a century. In the nearby universe, however, dozens of stars will explode, and they are close enough to study in detail with ground- and space-based telescopes. Collectively, the Swift supernova project provides a comprehensive picture of supernovae in their host galaxies, which Immler has arranged in a massive poster reminiscent of police mug shots. Two supernovae stand out.
An explosion called SN 2005ke is the first Type Ia supernova detected in X-ray wavelengths, and it is much brighter in the ultraviolet than expected. Type Ia are called "standard candles" and are used by astronomers to measure distances in the universe, because each Type Ia shines with a known luminosity. Immler's team says it has the first observational evidence to support one theory about the origin of these supernovae.
The two theories for the origins of a Type Ia are: an explosion of a white dwarf in orbit around another white dwarf or, an explosion of a white dwarf in orbit around a red giant star. The dense white dwarf can accumulate gas donated from the companion. When the dwarf reaches the critical mass of 1.4 solar masses, a thermonuclear explosion ensues.
Immler's group has found direct evidence in the X-ray and ultraviolet light of SN 2005ke that the white dwarf, now obliterated, was indeed orbiting a red giant. The scientists detected shock waves from the explosion ramming into gas from a red giant and found no evidence of a second white dwarf. This observation may help astronomers understand the birthplaces and evolution of these supernovae, so crucial to the field of cosmology and dark energy.
The second supernova is SN 2006bp, a Type II, which is the core collapse of a massive star once it runs out of fuel. Here, Immler's team observed the explosion in detail less than a day after the event, a record for any space-based telescope. The team found that X-rays are present directly after the explosion, a surprise discovery, and that these X-rays fade within days. This means they have simply been missed in previous supernova observations because X-ray observatories don't turn to the explosion typically for at least a week.
The X-rays speak directly about the chemical content and immediate surroundings of the exploded star, and they show the presence of hot gas heated by the explosion in the direct vicinity of the star. The Swift observation implies that the star's wind did not blow a cavity around the star before the explosion, as is commonly thought.
Swift, launched in November 2004, is a NASA mission in partnership with the Italian Space Agency and the Particle Physics and Astronomy Research Council, United Kingdom, and is managed by Goddard Space Flight Center. Penn State University personnel control science and flight operations from the Mission Operations Center. Immler, a member of the Swift Science Center, receives funding through the Universities Space Research Association.
Researchers are presenting their findings today at the High Energy Astrophysics Division of the American Astronomical Society meeting in San Francisco.