GRB 130603B, SDS J112848.22+170418.5
Credit: NASA, ESA, N. Tanvir (University of Leicester), A. Fruchter (STScI), and A. Levan (University of Warwick . More Images
NASA's Hubble Space Telescope has provided the strongest evidence yet
that short-duration gamma-ray bursts are triggered by the merger of
two small, super-dense stellar objects, such as a pair of neutron stars
or a neutron star and a black hole.
The definitive evidence came from Hubble observations in
near-infrared light of the fading fireball produced in the aftermath of
a short gamma-ray burst (GRB). The afterglow reveals for the first
time a new kind of stellar blast called a kilonova, an explosion
predicted to accompany a short-duration GRB.
A kilonova is about 1,000 times brighter than a nova, which is caused
by the eruption of a white dwarf. Such a stellar blast, however, is
only 1/10th to 1/100th the brightness of a typical supernova, the
self-detonation of a massive star.
Gamma-ray bursts are mysterious flashes of intense high-energy
radiation that appear from random directions in space. Short-duration
blasts last at most a few seconds, but they sometimes generate faint
afterglows in visible and near-infrared light that continue for several
hours or days.
The afterglows have helped astronomers determine that GRBs lie in
distant galaxies. The cause of short-duration GRBs, however, remains a
mystery. The most popular theory is that astronomers are witnessing the
energy released as two compact objects crash together. But, until now,
astronomers have not gathered enough strong evidence to prove it, say
researchers.
A team of researchers led by Nial Tanvir of the University of
Leicester in the United Kingdom has used Hubble to study a recent
short-duration burst in near-infrared light. The observations revealed
the fading afterglow of a kilonova explosion, providing the "smoking
gun" evidence for the merger hypothesis.
"This observation finally solves the mystery of the origin of short
gamma-ray bursts," Tanvir said. "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. This result now appears to provide definitive proof
supporting that scenario."
Astrophysicists have predicted that short-duration GRBs are created
when a pair of super-dense neutron stars in a binary system spiral
together. This event happens as the system emits gravitational
radiation, tiny ripples in the fabric of space-time. The energy
dissipated by the waves causes the two objects to sweep closer together.
In the final milliseconds, as the two objects merge, the death spiral
kicks out highly radioactive material. This material heats up and
expands, emitting a burst of light. This powerful
kilonova blast emits as much visible and near-infrared light every
second as the Sun does every few years. A kilonova lasts for about a
week.
In a recent science paper Jennifer Barnes and Daniel Kasen of the
University of California, Berkeley, and the Lawrence Berkeley National
Laboratory presented new calculations predicting how kilonovas should
look. They predicted that the same hot plasma producing the radiation
will also act to block the visible light, causing the gusher of energy
from the kilonova to flood out in near-infrared light over several days.
An unexpected opportunity to test this model came on June 3 when
NASA's Swift Space Telescope picked up the extremely bright gamma-ray
burst, cataloged as GRB 130603B, in a galaxy located almost 4 billion
light-years away. Although the initial blast of gamma rays lasted just
one-tenth of a second, it was roughly 100 billion times brighter than
the subsequent kilonova flash.
The visible-light afterglow was detected at the William Herschel
Telescope and its distance was determined with the Gran Telescopio
Canarias, both located in the Canary Islands.
"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. The calculations suggested that the light would
most likely be brightest in near-infrared wavelengths about 3 to 11
days after the initial blast. The researchers needed to act quickly
before the light faded, so they requested Director's Discretionary
Observing Time with Hubble's Wide Field Camera 3.
On June 12-13 Hubble searched the location of the initial burst,
spotting a faint red object. An independent analysis of the data from
another research team confirmed the detection. Subsequent Hubble
observations three weeks later, on July 3, revealed that the source had
faded away, therefore providing the key evidence it was the fireball
from an explosive event.
"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 Andrew Fruchter of the Space Telescope Science Institute in
Baltimore, Md., a member of Tanvir's research team. "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."
In addition to confirming the nature of short GRBs, the discovery has
two important implications. First, the origin of many heavy chemical
elements in the universe, including gold and platinum, has long been a
puzzle. Kilonovas are predicted to form such elements in abundance,
spraying them out into space where they could become part of future
generations of stars and planets.
Second, the mergers of compact objects are also expected to emit
intense gravitational waves, first predicted by Albert Einstein.
Gravity waves have not yet been discovered, but new instruments under
development may make the first detections within a few years. "Now it
seems that by hunting for kilonovas, astronomers may be able to tie
together the events giving rise to both phenomena," Tanvir said.
The team's results will appear online on Aug. 3 in the journal Nature.
CONTACT
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-339-4514
dweaver@stsci.edu / villard@stsci.edu
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-339-4514
dweaver@stsci.edu / villard@stsci.edu
