Sep. 17, 1997 New NASA Hubble Space Telescope observations of the ever-fading fireball from one of the universe's most mysterious phenomena -- a gamma-ray burst -- is reinforcing the emerging view that these titanic explosions happen far away in other galaxies, and so are among the most spectacularly energetic events in the universe.
The most recent finding from observations with Hubble's Space Telescope Imaging Spectrograph (STIS) made on Sept. 5 - nearly six months after the blast - is being reported today at the Fourth Huntsville Symposium on Gamma Ray Bursts, at the Hilton Hotel in Huntsville, Al.
"Hubble is the only telescope capable of continuing to watch the aftermath of this explosion, because it has faded to 1/500th its brightness when first discovered by ground based telescopes last March," says Andrew Fruchter of the Space Telescope Science Institute in Baltimore, Md. "These observations provide an unprecedented opportunity to better understand the catastrophe behind such incredible outbursts."
Hubble's key findings are:
1. The continued visibility of the burst, and the rate of its decline over time, support theories that the light comes from a gamma-ray burst in a "relativistic" fireball (expanding near the speed of light) located at extragalactic distances. A burst in our galaxy, at the observed brightness, would have been slowed by the interstellar medium within the first few weeks, and faded from sight by now.
2. The observations contradict earlier claims, by some astronomers that the gamma-ray burst is moving against the sky background (this offset is called proper motion). Had proper motion been detected, the gamma-ray burst would have had to be no further away than about 30,000 light-years, or about the distance to the center of the galaxy.
3. The fuzzy companion object the fireball is embedded in - as first confirmed by Hubble in March 26 observations -- has not noticeably faded. This means it is not a relatively nearby nebula produced by the explosion, but in all likelihood a host galaxy.
4. Since the burst did not occur at the center of the host galaxy, but near its edge, the gamma-ray burst phenomenon is not related to activity in the nucleus of a galaxy. The Hubble observations support the "fireball" model for a gamma-ray burst.
"These observations are consistent with colliding neutron stars creating the fireball, but do not require it. The cause of that fireball is still not determined. Though colliding neutron stars is one theoretical means of producing such a fireball it is not the only one," says Fruchter.
Hubble observations over the past six months show the fireball is fading at a constant rate, as predicted by theory. Eventually, gas plowed in front of the stellar tidal wave should build up enough resistance to bring the fireball to a halt like snow piling up in front of a plow - and it should blink out. But the fact that hasn't happened yet offers more clues to solving the gamma-ray burst mystery.
If the burst happened nearby, the resulting fireball should have had only enough energy to propel it into space for a month or so before "hitting the wall" of accumulated gas and dying out. The fact that this fireball has expanded to gargantuan size, sweeping out a bubble of space one light-year across, means the explosion was truly titanic and, to match the observed brightness, must have happened at the vast distances of galaxies.
When Hubble first acquired the fireball, on March 27 (several weeks after the initial discovery) it was at 26th magnitude. The magnitude scale is used to measure the brightness of objects in space. The lower the magnitude, the brighter the object. The unaided eye can detect objects of the 6th magnitude.
By the Sept. 5 observation, it had faded to 1/5th that brightness to 27.7 magnitude (approximately 1/500,000th) the brightness of the faintest star). The suspected host galaxy has remained at approximately 25th magnitude.
Only Hubble has the resolution and contrast capability to still distinguish the fading fireball from the now brighter host galaxy. The researchers hope for follow-up observations to continue keeping track of the burst's optical counterpart until it fades away.
The research team: Andrew Fruchter (STScI), Elena Pian (ITSR), Steve Thorsett (Princeton), Marco Tavani (Columbia), Mario Livio (STScI), Kailash Sahu (STScI), Filippo Frontera (ITSR), Larry Petro (STScI) and Duccio Macchetto (STScI).
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The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).
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