New NASA Hubble Space Telescope observations of the ever-fading fireball fromone of the universe's most mysterious phenomena -- a gamma-ray burst -- isreinforcing the emerging view that these titanic explosions happen faraway in other galaxies, and so are among the most spectacularlyenergetic events in the universe.
The most recent finding from observations with Hubble's Space TelescopeImaging Spectrograph (STIS) made on Sept. 5 - nearly six months after theblast - is being reported today at the Fourth Huntsville Symposium onGamma Ray Bursts, at the Hilton Hotel in Huntsville, Al.
"Hubble is the only telescope capable of continuing to watch theaftermath of this explosion, because it has faded to 1/500th itsbrightness when first discovered by ground based telescopes last March,"says Andrew Fruchter of the Space Telescope Science Institute inBaltimore, Md. "These observations provide an unprecedentedopportunity to better understand the catastrophe behind such incredibleoutbursts."
Hubble's key findings are:
1. The continued visibility of the burst, and the rate of its decline overtime, 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 astronomersthat the gamma-ray burst is moving against the sky background (thisoffset is called proper motion). Had proper motion been detected, thegamma-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 firstconfirmed by Hubble in March 26 observations -- has not noticeablyfaded. This means it is not a relatively nearby nebula produced by theexplosion, but in all likelihood a host galaxy.
4. Since the burst did not occur at the center of the hostgalaxy, but near its edge, the gamma-ray burst phenomenon is not relatedto activity in the nucleus of a galaxy. The Hubble observations supportthe "fireball" model for a gamma-ray burst.
"These observations are consistent with colliding neutron stars creatingthe fireball, but do not require it. The cause of that fireball is stillnot determined. Though colliding neutron stars is one theoretical meansof producing such a fireball it is not the only one," says Fruchter.
Hubble observations over the past six months show the fireball is fadingat a constant rate, as predicted by theory. Eventually, gas plowed infront of the stellar tidal wave should build up enough resistance tobring 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 offersmore clues to solving the gamma-ray burst mystery.
If the burst happened nearby, the resulting fireball should have hadonly 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 thisfireball has expanded to gargantuan size, sweeping out a bubble of spaceone light-year across, means the explosion was truly titanic and, tomatch the observed brightness, must have happened at the vast distancesof galaxies.
When Hubble first acquired the fireball, on March 27 (several weeks after the initial discovery) it was at 26th magnitude. The magnitude scale isused to measure the brightness of objects in space. The lower the magnitude,the brighter the object. The unaided eye can detect objects of the 6thmagnitude.
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 stilldistinguish the fading fireball from the now brighter host galaxy. Theresearchers hope for follow-up observations to continue keeping track ofthe burst's optical counterpart until it fades away.
The research team: Andrew Fruchter (STScI), Elena Pian (ITSR), SteveThorsett (Princeton), Marco Tavani (Columbia), Mario Livio (STScI), KailashSahu (STScI), Filippo Frontera (ITSR), Larry Petro (STScI) and DuccioMacchetto (STScI).
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The Space Telescope Science Institute is operated by the Association ofUniversities for Research in Astronomy, Inc. (AURA) for NASA, undercontract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation betweenNASA and the European Space Agency (ESA).
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