Burst Behind the Sun Reveals Magnetic Charm of Distant Explosion

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.

RHESSI

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.

"Just absolutely astounding," said Dr. Donald Lamb of the University of Chicago, a veteran gamma-ray-burst hunter, at a meeting in Nashville last week where the result was presented.

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.

According to Dr. Steven Boggs of the University of California, Berkeley, the lead scientist on the discovery, 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.

"The polarization is telling us that the magnetic fields themselves are acting as the dynamite, driving the explosive fireball we see as a gamma-ray burst," Boggs said.

This is a big clue to the origin of gamma-ray bursts. It implies that at least some gamma-ray bursts might be created by the collapse of a massive star into a black hole. A black hole itself has no magnetic field, but the local magnetic field can thread through the black hole. If rapidly spinning, the black hole will wind up the local field like a string on a top. The energy density in the tightly wound, compressed field would eventually get so high that the field would rebound outward in a massive fireball, dragging matter with it.

RHESSI was doing what it was built for -- snapping pictures of solar flares when it caught an extremely bright gamma-ray burst in the background, over the edge of the Sun. RHESSI is not a gamma-ray burst satellite, but it does have an instrument capable of measuring polarization at X-ray and gamma-ray energies.

The December burst occurred at the right time and in right place, and Boggs knew the right thing to do. His analysis, which took several months, appeared in the May 22, 2003, issue of Nature.

Scientists may need to wait to see such polarization in another gamma-ray bursts. Although these bursts are common, appearing somewhere randomly on the sky at least once a day, they last only for a few seconds to a minute, making them hard to study. Also, dedicated gamma-ray burst satellites, such as HETE and the soon-to-be-launched Swift satellite, do not have instruments that can efficiently measure polarization. A gamma-ray observatory built by the European Space Agency, called Integral, is now in orbit making observations and may be able to detect burst polarization if, like RHESSI, it is looking at the right spot at the right time.

RHESSI's discovery may be the "ship" that launched a thousand satellite proposals. With the realization that light from distant, celestial objects can be so polarized, astronomers are proposing new telescopes crafted to detect high-energy polarization. The long process of careful planning has begun.

In the meantime, astronomers keeping there fingers crossed, hoping to get lucky.

Additional Links

Visit the RHESSI Home Page. (http://hesperia.gsfc.nasa.gov/hessi/)