Another space record: biggest blast in our galaxy
Posted Feb. 18, 2005
Courtesy Center for Astrophysics
World Science staff
Forget “Independence Day” or “War of the Worlds.” A monstrous cosmic explosion last December showed that Earth is in more danger from real-life space threats than from hypothetical alien invasions, researchers say.
The explosion wasn’t the most powerful ever recorded: that honor goes to a burst reported last June from a distant cluster of galaxies named MS 0735.6+7421 (see World Science of last Jan. 5, “Astronomers find biggest space blast ever observed.”)
But it was the brightest from Earth’s perspective, because it was so close to us, astronomers said. And it was the most powerful ever recorded that occurred in our galaxy.
The explosion was of a type known as a gamma-ray flare. These bursts are invisible to the naked eye because they emit gamma rays, a type of radiation more energetic than visible light. Calling the blast “bright” means not that human eyes could see it, but that it was bright to telescopes designed to detect gamma rays.
NASA’s newly launched Swift satellite and other observatories observed the event, which arose from neutron star SGR 1806-20 in the constellation Sagittarius, about 50,000 light years away. A light year is the distance light travels in a year. The flare, briefly outshining the full moon, disrupted the Earth’s ionosphere, a part of the atmosphere extending from 70 kilometers (43 miles) to 400 kilometers (250 miles) above the surface.
If it had occurred within 10 light-years of us, it would destroy the much of the ozone layer, causing extinctions due to increased radiation, astronomers said.
“Astronomically speaking, this explosion happened in our backyard. If it were in our living room, we’d be in big trouble!” said Bryan Gaensler of the Harvard-Smithsonian Center for Astrophysics, lead author on a paper describing observations of the event. Gaensler headed one of two teams reporting on this eruption at a conference today at NASA headquarters.
Astronomers said the flare, detected on December 27, came from an isolated neutron star, an extremely compact type of star. A spoonful of neutron star material is thought to weigh as much as all the cars on Earth.
“This might be a once-in-a-lifetime event for astronomers, as well as for a neutron star,” said David Palmer of Los Alamos National Laboratory, lead author on a paper describing space-based observations of the burst. “We know of only two other giant flares in the past 35 years, and this December event was 100 times more powerful.”
Neutron stars form from collapsed stars. They are dense, fast-spinning, highly magnetic, and only about 15 miles wide. SGR 1806-20 is a unique neutron star called a
magnetar, with an ultra-strong magnetic field capable of stripping information from a credit card at a distance halfway to the Moon. Only about 10 magnetars are known among the many neutrons stars in the Milky Way.
“Fortunately, there are no magnetars anywhere near the earth. An explosion like this within a few trillion miles could really ruin our day,” said graduate student Yosi Gelfand of the Center for Astrophysics, a co-author on one of the papers.
Astronomers said the powerful magnetic field generated the gamma-ray flare in a violent process called magnetic reconnection. This is a sudden rearrangement of magnetic field lines, the invisible lines that are traced out and become visible when you drop iron filings near a magnet. In magnetic reconnection, field lines snap and reconnect to new field lines when they’re under too much tension. This releases energy like a burst dam.
A comparable process, on a smaller scale, creates solar flares, said Gaensler.
Using radio telescopes, he and his team detected material ejected by the blast at a velocity three-tenths the speed of light. The extreme speed, combined with the close-up view, yielded changes in a matter of days.
Spotting such a nearby gamma-ray flare offered scientists an incredible advantage, allowing them to study it in more detail than ever before. “We can see the structure of the flare’s aftermath, and we can watch it change from day to day. That combination is completely unprecedented,” said