Long before experiments could detect gamma-rays emitted by cosmic sources, scientists had known that the Universe should be producing such high energy photons. Hard work by several brilliant scientists had shown us that a number of different processes which were occurring in the Universe would result in gamma-ray emission. These processes included cosmic ray interactions with interstellar gas, supernova explosions, and interactions of energetic electrons with magnetic fields. In the 1960s, we finally developed the ability to actually detect these emissions and we have been looking at them ever since!
Gamma-rays coming from space are mostly absorbed by the Earth's atmosphere. So gamma-ray astronomy could not develop until it was possible to get our detectors above all or most of the atmosphere, using balloons or spacecraft. The first gamma-ray telescope carried into orbit, on the Explorer XI satellite in 1961, picked up fewer than 100 cosmic gamma-ray photons. These appeared to come from all directions in the Universe, implying some sort of uniform "gamma-ray background". Such a background would be expected from the interaction of cosmic rays (very energetic charged particles in space) with gas found between the stars.
Additional gamma-ray experiments flew on the OGO, OSO, Vela, and Russian Cosmos series of satellites. However, the first satellite designed as a "dedicated" gamma-ray mission was the second Small Astronomy Satellite (SAS-2) in 1972. It lasted only seven months due to an electrical problem, but provided an exciting view into the high-energy Universe (sometimes called the 'violent' Universe, because the kinds of events in space that produce gamma-rays tend to be explosions, high-speed collisions, and such!). In 1975, the European Space Agency launched a similar satellite, COS-B, which operated until 1982. These two satellites, SAS-2 and COS-B, confirmed the earlier findings of the gamma-ray background, and also detected a number of point sources. However, the poor resolution of the instruments made it impossible to identify most of these point sources with individual stars or stellar systems.
So what are gamma-rays and what can they tell us about the cosmos? Gamma-rays are the most energetic form of electromagnetic radiation, with over 10,000 times more energy than visible light photons. If you could see gamma-rays, the night sky would look strange and unfamiliar. The familiar sights of constantly shining stars and galaxies would be replaced by something ever-changing. Your gamma-ray vision would peer into the hearts of solar flares, supernovae, neutron stars, black holes, and active galaxies. Gamma-ray astronomy presents unique opportunities to explore these exotic objects. By exploring the universe at these high energies, scientists can search for new physics, testing theories and performing experiments which are not possible in earth-bound laboratories.
Sometimes astronomers plan for years to make a crucial scientific discovery, building a telescope to precise specifications, launching it into space, and conducting a series of long, careful surveys of stars and galaxies.
And sometimes they just get lucky.
For a gamma-ray burst that occurred on December 6, 2002, it was a little of both.
Gamma-ray bursts are the most powerful explosions known in the universe, likely culminating in the creation of a black hole, yet their origins still remain a mystery. During a chance observation, NASA's RHESSI satellite made one of the most important discoveries about these bursts in the past decade.
The satellite, called the Reuven Ramaty High-Energy Solar Spectroscopic Imager in full, detected for the first time that the light from these distant bursts can be polarized. This was big news to scientists, because it speaks of the underlying mechanics of the explosion.
Polarized light, familiar to most of us as the reflected glare blocked by Polaroid sunglasses, is light with its magnetic and electric fields vibrating primarily in one direction. Usually the light waves hitting our eyes are vibrating randomly in all directions. The December gamma-ray burst was about 80 percent polarized. That's a lot. Theorists had expected only 2 to 3 percent polarization. Some great force must have been present to polarize the light.
Gamma-ray bursts must originate from a region of highly structured magnetic fields, stronger than the fields at the surface of a neutron star -- until now, the strongest magnetic fields observed in the universe.
In the meantime, astronomers and scientists keeping there fingers crossed, hoping to get lucky.
~Maiya Wenzel
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