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Largest gamma-ray burst ever seen signals death of a star


Carl W. Akerlof, a physicist at the University of Michigan, uses two optical telescopes like this one to track the afterglow of an enormous gamma-ray blast.

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It was a blast so huge, so intense, that instruments on a satellite designed to detect such things registered it as an impossible anomaly and wouldn't do their job.

Thirty seconds later, George Ricker's beeper went off. The Massachusetts Institute of Technology researcher was just getting up. It was 6:37 a.m., Saturday, March 29. Dr. Ricker quickly went to his computer and learned that the NASA High-Energy Transient Explorer satellite his MIT team designed and built was receiving some pretty weird signals.

It was soon clear that Dr. Ricker and a team of physicists and astrophysicists all over the globe were witnessing the biggest blast of gamma rays ever recorded, from a source closer than any seen before.

The burst lasted a little over 30 seconds, but if you had the power to see the invisible gamma rays hurl toward Earth, your retina would have scorched with a light brighter than the entire universe. As the electromagnetic waves disappeared, there remained an afterglow a trillion times brighter than the sun. It lasted for more than two hours. Astronomers continue watching this distant point in the constellation Leo, tracking glowing optical light and pulses of infrared and radio waves.

No one wants to look away, because a supernova may be next on the cosmological menu.

This is what it looks like when stars die.

Some 2 billion light years away from Dr. Ricker's computer, a giant star was moving through the melodrama of stellar death. The outer layers of the star - the hydrogen and helium that kept it burning - were all but gone. Now gravity was in charge. The rapidly spinning core collapsed in on itself, forming a black hole.

The collapse unleashed shockwaves of energy that ripped the star apart. The energy shot from the poles at near light speed in cone-shaped bursts. One of those cones pointed to Earth, sending the gamma waves that first alerted scientists. Gamma waves are the most energetic part of the electromagnetic spectrum that includes x-rays, ultraviolet rays, visible light, and radio waves.

Carl W. Akerlof, a physicist at the University of Michigan, waits for moments like these. He's leader of the Robotic Optical Transient Search Experiment. This international group of researchers hopes to pinpoint the source of gamma-ray bursts and watch the light waves that follow in the visible spectrum. So far, it has telescopes operating in Texas and Australia.

Gamma ray bursts are a frequent phenomenon in the universe, but their sources - often billions of light years away - are notoriously hard to find. Even if a detecting satellite narrows down the potential source to a tiny bit of sky, at these distances, even narrow bands of the heavens are full of things to look at.

The system is set up so that the satellite signals the telescopes and points them in the direction of the gamma ray source seconds after gamma rays are detected. But this time, the sheer size of the blast convinced the satellite sensors that the whole thing was a mistake. Scientists had to transfer the information to telescopes themselves about an hour after the initial blast.

No one knew if that would be too late, but it was worth a gamble. In 1998, Dr. Akerlof's team managed to find a source of gamma radiation 12 billion light years away, and they used nothing more sophisticated than a very big camera lens.

Their present telescopes are 100 times more powerful. The hour delay didn't rob them of observation time at all. Both of Dr. Akerlof's telescopes were able to track the gamma ray source, now glowing in the visible spectrum, as were others around the world.

“What's been fascinating, is while the object decreased in brightness similar to previous events, it became brighter a day or so later. It seems to bop up and down,'' Dr. Akerlof said.

Although energy from the star is decaying, it's not the steady drop seen in most other events of this kind. Three times it temporarily pulsed a little brighter. Scientists theorize these bright pulses occur as energy from the star collides with material the star threw off earlier.

This pulsing may not be the end of the action.

“In a few days we may see a major supernova bump,'' said Jay Norris, a gamma ray expert at Goddard Space Flight Center in Greenbelt, MD. “That would have the supernova fanatics jumping up and down.''

“I expect a supernova in roughly a week from now,'' Dr. Ricker said. But that's not the only theory. Others believe the gamma radiation event was the result of a supernova that took place in the past.

Watching what happens will help researchers fill in the details of stellar death, and learn what comes first, the gamma rays or the supernova.

What made it all possible, Dr. Ricker said, is a detector on the NASA satellite “about the size of a coffee cup.'' Think of it as a high-tech sundial for invisible lightwaves.

It uses an x-ray detector covered by mesh. The device calculates the direction of the gamma wave source by measuring the shadow cast when waves strike the mesh.

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