DURHAM, N.H. -- A University of New Hampshire
researcher and her colleague have uncovered new
information about gamma-ray bursts by looking at
the bursts a little longer and a little smarter than
anybody else -- using 20-year-old data to do it.
Alanna Connors, research scientist at the UNH Institute for the Study of Earth, Oceans and Space, and Geoff Hueter of Far Point Technologies, have discovered that gamma ray bursts -- one of the more mysterious happenings in the universe -- do not simply fade after the initial burst. Instead, they fade slowly, then experience a resurgence before slowing dying out.
Connors and Hueter discuss their findings in the July 1 edition of the Astrophysical Journal, describing how they used data dating back to 1978 to dispute current popular theories concerning gamma-ray bursts. In fact, they conclude that a wealth of old data -- 30 years of it, much of it published -- may seriously constrain present theory.
"These are incredibly exciting times for trying to understand what these bright blasts of gamma-rays are trying to tell us," says Connors. "There are many new, exciting broad-band observations coming out at a rapid pace, but to really understand the driving physics, these need to be integrated with the understanding one gains from historic data as well."
The past 18 months have indeed witnessed grand excitement in the field of gamma-ray bursts. For the first time, scientists have been able to gauge the distance from the Earth of these elusive explosive bursts. Connors points out at least some bright bursts are among the most distant objects we can detect.
Despite their distance, scientists have been able to theorize how these bursts might occur. "The overall pattern of decay seen hours or days after the burst has provided dramatic preliminary support for cosmological fireball models -- in which an entire star or exotic binary system (containing neutron stars or black holes) completely annihilates, spewing their energy into a strong ultra-relativistic wind of very hot, expanding shells of plasma."
But what would be seen, Connors and Hueter wondered, if instead of swinging sensors into position hours or days after the gamma-ray burst, they could have stared at the position of the explosive event during the burst itself and for several hours afterwards?
By chance, the HEAO 1 satellite did just that 20 years ago, May 8, 1978. With these measurements -- and their powerful new kind of software algorithm -- Connors and Hueter succeeded in not only detailing its spectra at high resolution during the burst, but also the rare observation of the transition from "burst" to "afterglow." Both look different than the fireball models currently accepted, they found.
"Let me show you an analogy - with sound," says Connors, balancing the base of a bowl-shaped bell in one hand. "The motion of this wooden mallet represents the explosive fireball wind, as it slams either into itself or the surrounding interstellar material," she continues, striking the mallet along the rim of the bowl.
"Hear that bright, sharp sound as it bangs together? Those loud, high sound frequencies are analogous to the very intense high frequency/high energy light frequencies of the gamma-ray burst itself. That longer-lasting lower pitch/lower frequency ringing that slowly fades is analogous to the burst afterglow."
Connors admits it's not a perfect analogy. It would be better if the bell would expand as it continues to reverberate, so you could hear the pitch go down, the peak frequency descending as the fireball shell expands. You could even get a feel of how the lowering of the peak frequency is linked to the fading of the afterglow intensity.
According to the 1978 data, one sees first the two bright peaks of the burst itself, each showing the well-known spectral evolution -- a whistling down from high to low frequencies -- characteristic of many gamma-ray bursts. The whole thing fades back down to background after about 70 seconds. Then in the softest, lowest energy channels, one sees a slow resurgence. About seven minutes after burst onset, it peaks at about one to two orders of magnitude fainter than the burst. There is irregular variability on top of that as it decays away again on very roughly a half hour time-scale.
As Connors points out, "This does not fit the original afterglow scenario of a slowly fading power-law decay. Nor does the shape of its evolving spectrum. It raises the question, Is there a real distinction between the burst and afterglow?" In fact, Connors says, had one looked closely at X-ray spectra of other gamma-ray bursts, going at least as far back as 1981, one would have seen others that also clearly did not match current theories.
The hope is that present day X-ray instruments may already be able to see the soft resurgent "transition" from burst to afterglow, especially if looked at with the same kind of painstaking approach Connors and Hueter developed for these older data.
"And can some of the rapid response optical or even radio systems catch this resurgent variability within minutes to an hour the burst starts?" asks Connors. "Some very simple extrapolation indicates it may be bright enough."
By Carmelle Druchniak
UNH News Bureau