August 4, 2013
Hubble Finds Fireball Following Gamma Ray Burst
redOrbit Staff & Wire Reports - Your Universe Online
Researchers using the Hubble Space Telescope have discovered new evidence that the merger of two small, super-dense stellar objects is responsible for producing short-duration gamma ray bursts, according to research published Saturday in a special online edition of the journal Nature.
Hubble observed the fading fireball from one of these events last month, following a short gamma ray burst (GRB) in a galaxy almost 4 billion light-years from Earth, they explained. Experts had predicted that a kilonova would accompany a short-duration GRB, but this was the first time the phenomenon had been witnessed.
"This observation finally solves the mystery of the origin of short gamma ray bursts," lead researcher Nial Tanvir of the UK's University of Leicester said in a statement.
"Many astronomers, including our group, have already provided a great deal of evidence that long-duration gamma ray bursts (those lasting more than two seconds) are produced by the collapse of extremely massive stars. But we only had weak circumstantial evidence that short bursts were produced by the merger of compact objects," he added. "This result now appears to provide definitive proof supporting that scenario."
The kilonova, which has been dubbed the "smoking gun" evidence supporting the theory that the short-duration bursts are caused by the merger of stellar objects such as a pair of neutron stars, is said to be approximately 1,000 times brighter than a nova caused by the eruption of a white dwarf.
Astrophysicists had predicted that this type of GRBs were created when a pair of super-dense neutron stars in a binary system spiral together, and during the final few milliseconds during which they merge, they eject highly radioactive material. That material, the researchers explain, heats up and expands, emitting a burst of light.
The resulting kilonova emits roughly as much visible and near-infrared light every second as the Sun does every few years, and lasts for approximately one week. Recently, researchers Jennifer Barnes and Daniel Kasen of the University of California, Berkeley, and the Lawrence Berkeley National Laboratory published a study describing how kilonovas should look.
An unexpected chance to study their hypothesis came on June 3, when NASA's Swift mission detected an extremely bright gamma-ray burst, (GRB 130603B) in a galaxy located nearly four billion light-years away. While the initial blast of gamma rays lasted just one-tenth of a second, investigators report that it was approximately 100 billion times brighter than the kilonova flash that followed.
"We quickly realized this was a chance to test Barnes' and Kasen's new theory by using Hubble to hunt for a kilonova in near-infrared light," Tanvir said. They calculated that the light would be at its brightest in near-infrared wavelengths about three to 11 days after the initial event, so they quickly requested permission to use Hubble's Wide Field Camera 3 to search for the location of the initial burst. One June 12-13, they spotted a faint red object, and subsequent observations three weeks later confirmed that it had faded away and was likely a fireball from the blast.
"Previously, astronomers had been looking at the aftermath of short-period bursts largely in optical light, and were not really finding anything besides the light of the gamma-ray burst itself," explained researcher Andrew Fruchter of the Space Telescope Science Institute (STSCI). "But this new theory predicts that when you compare near-infrared and optical images of a short gamma-ray burst about a week after the blast, the kilonova should pop out in the infrared, and that's exactly what we're seeing."