In space, no one can hear you scream, but only if you're human. Stars, on the other hand, can send out a shout that reverberates across the universe.
British astronomers have detected the remains of such an event, a distant powerful gamma-ray burst that sent out a booming X-ray echo in the form of two concentric radiation rings propagating through space. It's the first time astronomers have seen an X-ray echo from a gamma-ray burst.
"We were quite fortunate with this GRB," said Simon Vaughan, an astronomer with Britain's University of Leicester, who led the study. "We caught it pretty fast."
Gamma-ray bursts, or GRBs, are the most powerful explosions in the universe, although much of their origin remains a mystery to astronomers. They can last up to a few minutes or less than a second, and while some occur in tandem with a supernova explosion, only those stars capable of collapsing into a black hole seem to be able to make a GRB, researchers said.
X-rays through an interstellar fog
Vaughan's team recorded the pair of rings emanating from the burst, known as GRB 031203, six hours after it exploded, which took just 30 seconds. The echo is a delayed effect caused by X-rays that were scattered as they passed through interstellar dust on their way out into space. As X-ray photons scattered, they illuminated nearby dust particles much like the headlights of a car illuminate fog.
The expansion effect, however, is really an optical illusion, especially since it looks to observers as if it's occurring at about 1,500 times the speed of light. "It's an optical trick of light, though quite fun," Vaughan told Space.com.
He said the X-rays are travelling just as fast as other forms of light. The echo rings only appear to expand because the X-rays scattered by dust further from GRB 031203 take longer to reach Earth than those X-rays scattered closer to the line of sight. There appear to be two rings because the light is passing through two separate sheets of interstellar dust.
Astronomers can use the X-ray echo effect to locate exactly where those sheets of interstellar dust sit, which can help determine where stars, or even planets, may eventually form.
Vaughan said his team is still processing data but believes the nearest dust sheet to Earth is about 2,900 light-years away in the Gum nebula, a large bubble of ionized gas formed from repeated supernova explosions. The second dust sheet sits about 4,500 light-years away. One light-year is the distance light travels in a year, about 6 trillion miles, or 9.5 trillion kilometers.
It's all about the view
When it comes to detecting an X-ray echo like that seen in Vaughan's study, it's all about location.
In the past, astronomers have seen similar echoes in the visible light range during supernova explosions. Some observers have even detected slight X-ray echoes from binary-star systems, but the stars in those systems are so bright they nearly drown our their halos, Vaughan added.
GRB 031203 was originally detected by the European Space Agency's Integral satellite, an instrument built to find GRBs, on Dec. 3. The find was then relayed to astronomers on Earth via a GRB alert network, and six hours later Vaughan and his colleagues used ESA's XMM-Newton X-ray satellite to study the aftereffects of GRB 031203.
While the strongest, and most direct, emissions from a GRB are from gamma rays — hence the name "gamma-ray burst" — the delayed X-ray echo is also useful because give astronomers an idea of the X-ray brightness of GRB 031203 when it exploded.
"XMM-Newton's measurements are thus crucial to better understand the nature of the burst," Fred Jansen, project scientist for XMM-Newton, explained in a written statement. "The more details we gather of the burst, the more we learn how black holes are made."
By chance, astronomers said, GRB 031203 happened to go off within the plane of Earth's own Milky Way galaxy, which puts much interstellar dust in the its light path toward Earth. The alignment gave Vaughan's team a clear view of the fading "afterglow" X-ray emissions of the GRB's post-explosion core, as well as the echo rings.
"A combination of rapid observing and the direction that this thing went off meant we stood a good chance of detecting it," Vaughan said. "But the chances of us finding another [GRB] like this are slim."
A Swift approach to GRBs
On the average, researchers with both the Integral and XMM-Newton satellites follow up on about one gamma-ray burst a month. The explosions themselves occur on an almost daily basis, but only those GRBs whose gamma rays are pointed directly at Earth can be detected.
Later this year, NASA's Goddard Space Flight Center is set to launch its Swift spacecraft, a dedicated GRB observatory that will be able to home in on a burst up to 90 seconds after detection. Once a GRB is identified, Swift should start taking measurements with three onboard instruments, which together cover the X-ray, gamma ray and optical wavelengths.
"When Swift is launched, it's going to be chasing something like a GRB every other day," Vaughan said, "which should increase our chances of seeing more X-ray rings."
The Swift mission is an international collaboration and part of NASA's Medium Explorer program. The spacecraft is expected to launch in May 2004 and detect more than 200 gamma-ray bursts during its projected two-year lifespan.