Light Dawns on Dark Gamma-Ray Bursts
Posted by Darshana Sanrakshak Shambhala on January 2, 2011 at 8:24pm in Astronomy Corner
ScienceDaily (Dec. 30, 2010) — Gamma-ray bursts are among the most energetic
events in the Universe, but some appear curiously faint in visible light. The
biggest study to date of these so-called dark gamma-ray bursts, using the
2.2-meter MPG/ESO telescope at La Silla in Chile, has found that these
explosions don't require exotic explanations. Their faintness is now explained
by a combination of causes, the most important of which is the presence of dust
between the Earth and the explosion.
Gamma-ray bursts (GRBs), fleeting
events that last from less than a second to several minutes, are detected by
orbiting observatories that can pick up their high energy radiation. Thirteen
years ago, however, astronomers discovered a longer-lasting stream of less
energetic radiation coming from these violent outbursts, which can last for
weeks or even years after the initial explosion. Astronomers call this the
burst's afterglow.
While all gamma-ray bursts [1] have afterglows that
give off X-rays, only about half of them were found to give off visible light,
with the rest remaining mysteriously dark. Some astronomers suspected that these
dark afterglows could be examples of a whole new class of gamma-ray bursts,
while others thought that they might all be at very great distances. Previous
studies had suggested that obscuring dust between the burst and us might also
explain why they were so dim.
"Studying afterglows is vital to further
our understanding of the objects that become gamma-ray bursts and what they tell
us about star formation in the early Universe," says the study's lead author
Jochen Greiner from the Max-Planck Institute for Extraterrestrial Physics in
Garching bei München, Germany.
NASA launched the Swift satellite at the
end of 2004. From its orbit above the Earth's atmosphere it can detect gamma-ray
bursts and immediately relay their positions to other observatories so that the
afterglows could be studied. In the new study, astronomers combined Swift data
with new observations made using GROND [2] -- a dedicated gamma-ray burst
follow-up observation instrument, which is attached to the 2.2-metre MPG/ESO
telescope at La Silla in Chile. In doing so, astronomers have conclusively
solved the puzzle of the missing optical afterglow.
What makes GROND
exciting for the study of afterglows is its very fast response time -- it can
observe a burst within minutes of an alert coming from Swift using a special
system called the Rapid Response Mode -- and its ability to observe
simultaneously through seven filters covering both the visible and near-infrared
parts of the spectrum.
By combining GROND data taken through these seven
filters with Swift observations, astronomers were able to accurately determine
the amount of light emitted by the afterglow at widely differing wavelengths,
all the way from high energy X-rays to the near-infrared. The astronomers used
this information to directly measure the amount of obscuring dust that the light
passed through en route to Earth. Previously, astronomers had to rely on rough
estimates of the dust content [3].
The team used a range of data,
including their own measurements from GROND, in addition to observations made by
other large telescopes including the ESO Very Large Telescope, to estimate the
distances to nearly all of the bursts in their sample. While they found that a
significant proportion of bursts are dimmed to about 60-80 percent of the
original intensity by obscuring dust, this effect is exaggerated for the very
distant bursts, letting the observer see only 30-50 percent of the light [4].
The astronomers conclude that most dark gamma-ray bursts are therefore simply
those that have had their small amount of visible light completely stripped away
before it reaches us.
"Compared to many instruments on large telescopes,
GROND is a low cost and relatively simple instrument, yet it has been able to
conclusively resolve the mystery surrounding dark gamma-ray bursts," says
Greiner.
Notes:
[1] Gamma-ray bursts lasting longer than two
seconds are referred to as long bursts and those with a shorter duration are
known as short bursts. Long bursts, which were observed in this study, are
associated with the supernova explosions of massive young stars in star-forming
galaxies. Short bursts are not well understood, but are thought to originate
from the merger of two compact objects such as neutron stars.
[2] The
Gamma-Ray burst Optical and Near-infrared Detector (GROND) was designed and
built at the Max-Planck Institute for Extraterrestrial Physics in collaboration
with the Tautenburg Observatory, and has been fully operational since August
2007.
[3] Other studies relating to dark gamma-ray bursts have been
released. Early this year, astronomers used the Subaru Telescope to observe a
single gamma-ray burst, from which they hypothesised that dark gamma-ray bursts
may indeed be a separate sub-class that form through a different mechanism, such
as the merger of binary stars. In another study published last year using the
Keck Telescope, astronomers studied the host galaxies of 14 dark GRBs, and based
on the derived low redshifts they infer dust as the likely mechanism to create
the dark bursts. In the new work reported here, 39 GRBs were studied, including
nearly 20 dark bursts, and it is the only study in which no prior assumptions
have been made and the amount of dust has been directly measured.
[4]
Because the afterglow light of very distant bursts is redshifted due to the
expansion of the Universe, the light that left the object was originally bluer
than the light we detect when it gets to Earth. Since the reduction of light
intensity by dust is greater for blue and ultraviolet light than for red, this
means that the overall dimming effect of dust is greater for the more distant
gamma-ray bursts. This is why GROND's ability to observe near-infrared radiation
makes such a difference.