"If you look with your eyes at the sky you basically see the stars, and things from night to night are pretty much the same," says Arvind Parmar, Integrals acting project scientist. "If you had gamma ray eyes, youd see a very different universe; youd see a very violent, changing universe."
Since humans see in white light and gamma rays cannot penetrate Earth's atmosphere, however, gamma ray observatories must be put in space.
The European Space Agency (ESA), with academic and industry partners, developed four instruments for Integral: a visible light camera and X-ray monitor complement two gamma ray instruments.
Integral's primary gamma ray camera has a 30 arc-second resolution, comparable to spotting an individual in a crowd from more than three-quarters of a mile (1.3 kilometers) away. Integral also carries a spectrometer that, like a prism which disperses white light into its constituent colors, separates incoming gamma rays by distinct energies. This can help astronomers measure the redshift of the rays, and thus how far away the sources are.
Some processes, such as radioactive decay, also emit gamma rays of specific energies, so astronomers will use the spectrometer data to infer the types of reactions occurring in the sources.
All four of the instruments point in the same direction and observe at the same time. One of the advantages of this alignment, Parmar says, is in observing gamma ray bursts. GRBs last fractions of a second to a few minutes and can release in seconds the same amount of energy as the Sun in 10 billion years. They fade quickly, so it is hard to observe them as they occur, but they also occur at least once a day. The multi-wavelength instruments on Integral should help scientists detect the events, pinpoint their location, and activate a worldwide network of telescopes ready to observe new GRBs within 30 seconds, Parmar says.
"We'll have another weapon in the arsenal of people studying gamma ray bursts," Parmar says.
GRBs were discovered in the 1960s by military satellites meant for monitoring nuclear bomb testing around the globe. Scientists have since developed many viable theories for their occurrence. They include the collision of two neutron stars (the incredibly dense and small remnants of some supernovae) or a neutron star with a black hole, the formation of a black hole, and hypernova, "which is basically the collapse of a very massive spinning star," Parmar says.
"Often, when a new phenomenon is discovered, you think there's only one cause of it," Parmar adds. "And then as you investigate further, you find that the phenomena could come from different types of events."
While black holes are potential GRB sources, they are also known to emit gamma rays in different situations. Integral will point its array of instruments at both massive black holes at the centers of galaxies, such as the Milky Way, and smaller "stellar" black holes scattered throughout the universe.
"The best way sometimes of understanding something, such as the laws of physics, is to go to the very extreme, where the laws start to break down," Parmar says. "In the case of astronomy, they start to break down near black holes."
In the case of stellar black holes, they seem to undergo irregularly spaced multiple gamma ray flare-ups. These transient events, Parmar says, may be due to black holes that, through gravity, are consuming companion stars and occasionally reach a point where they must release energy. These events last longer than GRBs and tend to emit weaker particles, but astronomers do not fully understand them.
They hope to learn more with Integral. As the observatory detects these transient bursts, ESA will activate a system similar to that for GRBs.
Another of the major mysteries Integral will investigate is the formation of heavy elements in the Milky Way. Compton detected areas of relatively recent activity in the Milky Way, and Integral will study them in more detail.
"We know stars produce heavy elements, but the details of how that stuff gets out of the stars into the interstellar medium and becomes the fuel for the next generation of stars is unclear," Parmar says.
To cut down on costs, the mission planners reused the service module design from the XMM-Newton space telescope, with a few modifications to fit the Russian Proton rocket on which Integral will be launched. The Russians will pay for the launch, which will take place at the Baikonur Cosmodrome in Kazakhstan, in exchange for observing time.
The first science data should be available in early 2003, Parmar says. And there are many in the astronomy community waiting for that data. When ESA invited astronomers to submit proposals for Integral observing time, they received 19 times the amount available.
"I've never known one as high as that," Parmar says.
While the astronomers who applied were interested in learning more specific events, its not possible to predict what all Integral might reveal.
"There's bound to be something completely unexpected out there that's going to be very, very interesting," Parmar says.
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