Astronomers at Northwestern University and University of Illinois have detected the first observational evidence for the remnants of hypernovae, explosions a hundred times more energetic than supernovae and the possible source of powerful gamma ray bursts (GRB), making them the most energetic events known in the Universe other than 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 GRB phenomenon. GRBs are elusive flashes of high-energy radiation that appear about three times a day from random directions in space and last typically a few seconds. During such a brief period, a burst can be more luminous than the rest of the entire Universe.
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. Wang presents these results at the meeting of the High Energy Astrophysics Division of the American Astronomical Society in Charleston, S.C., April 12, 1999. The work will also appear in the May 20 issue of the Astrophysical Journal Letters.
One nebula that Wang observed, MF83 with a radius of over 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 an order of magnitude brighter than the brightest supernova remnants known in our Galaxy.
Wang calculated the energy required to produce the two remnants based on their size, expanding 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 Professor 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 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 canonical supernova, hypernovae are likely related to the formation of black holes, possibly due to the collapse of massive stars and/or their mergers with dense, or compact, objects. 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, assuming the radiation astronomers saw left its source in all directions. One explanation for all this energy is that the radiation was actually confined into a beam during the explosion. This concentration of energy is known as the beaming effect, which may significantly affect the energy estimate of a GRB.
"By studying remnants of GRBs or hypernovae in nearby galaxies, we can calculate their explosion energies without the beaming effect," Wang said. "We can also examine the environment of the explosions to infer their true nature."
The environment of NGC5471B is a star-forming region. The event that produced NGC5471B, Wang said, was therefore 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.
Although a hypernova remnant may last tens of million years, it does not shine as gloriously as the initial explosion. In fact, the two hypernova remnants cannot be seen in a typical visual picture in optical light. But they are visible in X-ray and in emission lines from some specific atomic transitions.
"We have used a combination of tools to uncover these remnants," said Wang, "just like doing archaeology."
Wang's observations were based primarily on data from the German/US/UK joint mission ROSAT X-ray satellite, while Chu's observations were from the Hubble Space Telescope and the 4-m telescope at Kitt Peak National Optical Astronomical Observatory.
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