Astronomers Find First Observational Evidence for "Hypernova" Explosion
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453April 12, 1999
Charleston, S.C. -- Astronomers at Northwestern University and University of Illinois have detected the first observational evidence for the remnants of a hypernova, an explosion a hundred times more energetic than an exploding star (supernova) and the possible source of powerful gamma ray bursts, the most energetic events known in the Universe other than its creation as a result of the Big Bang.
The detection of hypernova remnants could provide better understanding of the dynamics of star explosions and begin to unlock the mystery of the gamma ray burst (GRB) phenomenon. Gamma ray bursts are remote, elusive flashes of invisible, high-energy radiation that appear from random directions in space and typically last a few seconds. During that short period, the most powerful bursts are releasing more energy than the rest of the entire Universe. Satellites detect about one GRB each day.
Q. Daniel Wang, research assistant professor of physics and astronomy at Northwestern University, identified two hypernova remnants in galaxy M101, also known as the Pinwheel galaxy for its beautiful spiral shape. These remnants were previously classified as supernova remnants, two shell-like nebulae or clouds. Wang's subsequent detection and analysis of X-ray light from these nebulae pointed to a more energetic process. He presents these results at the meeting of the High Energy Astrophysics Division of the American Astronomical Society in Charleston, S.C., April 12, 1999.
One nebula that Wang observed, MF83 with a radius greater than 430 light-years, is one of the largest supernova remnants known. The other, NGC5471B, is rapidly expanding at a velocity of at least 100 miles per second. Both have X-ray luminosities about ten times brighter than the brightest supernova remnants known in our galaxy. Wang calculated the energy required to produce the two remnants based on their size, expansion velocity, and the light signature of the X-ray radiation. He then concluded that both remnants likely resulted from hypernovae.
"These are two of the most unusual remnants known,'' Wang said. "We see that they are bright in X-ray even at a distance of 25 million light years away. They must be from spectacular explosions.''
Much of Wang's calculations were based on the work of Dr. You-Hua Chu of the Astronomy Department at University of Illinois and her collaborators, who made detailed optical observations of these two remnants.
Hypernovae were first proposed by Dr. Bohdan Paczynski of Princeton University in 1998 as a way to explain GRBs, which were initially discovered by U.S. military Vela satellites in the 1960s. With an explosion energy orders of magnitude greater than a typical supernova, hypernovae are likely related to the formation of black holes, possibly due to the collapse of a massive star and/or their mergers with extremely dense objects, such as neutron stars.This scenario has become particularly attractive because of the evidence that GRBs appear close to massive star-forming regions where such activity is likely to occur.
But astronomers still know little about the true nature of GRBs or hypernovae. "I suspect GRBs may well be just a tip of an iceberg, as we have no clue why some explosions generate so much gamma-ray emission,'' Paczynski said.
For example, the brightest GRB known, the recently observed GRB 990123, represents more energy than any one star could produce. One explanation is that the light astronomers saw in this burst, which theoretically left its source in all directions, was instead focused into a beam as it shot through space. Just as a flashlight is brighter when pointed directly in your eyes, astronomers might have measured a concentrated form of energy due to what is known as the beaming effect.
"By studying remnants of GRBs or hypernovae in nearby galaxies, we can calculate the explosion energy without the beaming effect," Wang said. "We can also examine the environment of the explosions to infer their true nature."
The environment of NGC5471B, for example, is a star-forming region. The event that produced NGC5471B, therefore, was most likely the collapse of a massive star. Indeed, M101 is one of a few nearby galaxies with vigorous ongoing star formation. This explains why one single galaxy could contain two relatively rare hypernova remnants with ages less than about a million years.
Wang's observations were based on data from the High Resolution Imager onboard the German-operated ROSAT satellite (a German/US/UK joint venture). His work will appear in an upcoming issue of the Astrophysical Journal Letters. Chu's observations were from NASA's Hubble Space Telescope and the four-meter telescope at Kitt Peak National Optical Astronomical Observatory, sponsored by NSF.
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