LIGO Sheds Light on Cosmic Event
An analysis by the international LIGO (Laser Interferometer
Gravitational-Wave Observatory) Scientific Collaboration has excluded one previously
leading explanation for the origin of an intense gamma ray burst occurring
last winter. Gamma ray bursts are among the most violent and energetic events
in the universe and scientists have only recently begun to understand their
origins.
The LIGO project, which is funded by the National Science
Foundation, was initially designed by the California Institute of Technology
and the Massachusetts Institute of Technology for the detection of cosmic gravitational
waves and for the development of gravitational wave observations as an astronomical
tool. Research is carried out by the LIGO Scientific Collaboration, a group
of 580 scientists at universities around the U.S. and in eleven foreign countries,
including LSU. The LIGO Scientific Collaboration interferometer network also
includes the GEO600 interferometer located in Hannover, Germany designed and
operated by scientists from the Max Planck Institute for Gravitational Physics
and partners in the UK.
Each of the L-shaped LIGO interferometers (including the 2
km and 4 km detectors in Hanford and a 4-km instrument in Livingston, Louisiana)
uses a laser split into two beams which travel back and forth down long arms
in evacuated beam tubes. The beams are used to monitor the distance between
precisely figured mirrors. According to Albert Einstein's 1916 theory of general
relativity, the separation of the mirrors changes very slightly when a gravitational
wave – a distortion in space-time produced by massive accelerating objects
that propagates outward through the universe – passes by. The interferometer
is constructed in such a way that it can detect a change of less than a thousandth
the diameter of an atomic nucleus in the lengths of the arms relative to each
other.
On February 1, 2007, the Konus-Wind, INTEGRAL, MESSENGER,
and SWIFT gamma ray satellites measured a short but intense outburst of energetic
gamma rays originating from the direction of M31, the Andromeda galaxy, located
2.5 million light years away. The majority of such short (less than 2 seconds
in duration) gamma ray bursts (GRBs) are thought to emanate from the merger
and coalescence of two massive but compact objects, such as neutron stars or
black hole systems. They can also come from astronomical objects known as soft
gamma ray repeaters, which are less common than binary coalescence events and
emit less energetic gamma rays.
During the intense blast of gamma rays, known as GRB070201,
the 4-km and 2-km gravitational wave interferometers at Hanford were in science
mode and collecting data. They did not, however, measure any gravitational
waves in the aftermath of the burst.
That non-detection was significant.
The burst had occurred along a line of sight that was consistent
with it originating from one of Andromeda's spiral arms, and a binary coalescence
event – the merger of two neutron stars or black holes, for example – was
considered among the most likely explanations. Such a monumental cosmic event
occurring in a nearby galaxy should have generated gravitational waves that
would be easily measured by the ultra-sensitive LIGO detectors. The absence of
a gravitational wave signal meant GRB070201 could not have originated in this
way in Andromeda. Other causes for the event, such as a soft gamma ray repeater
or a binary merger from a much further distance, are now the most likely contenders.
LIGO's contribution to the study of GRB070201 marks a milestone
for the project, says Caltech's Jay Marx, LIGO's executive director: "Having
achieved its design goals 2 years ago, LIGO is now producing significant scientific
results. The non-detection of a signal from GRB070201 an important step towards
a very productive synergy between gravitational wave and other astronomical
communities that will contribute to our understanding of the most energetic
events in the cosmos."
"This is the first time that the field of gravitational wave
physics has made a significant contribution to the gamma ray astronomical community,
by searching for GRBs in a way that electromagnetic observations cannot," adds
David Reitze, a professor of physics at the University of Florida and spokesperson
for the LIGO Collaboration.
Up until now, Reitze says, astronomers studying GRBs relied
solely on data obtained from telescopes conducting visible, infrared, radio,
x-ray, and gamma ray observations. Gravitational waves offer a new window into
the nature of these events.
"We are still baffled by short GRBs. The LIGO observation
gives a tantalizing hint that some short GRBs are caused by soft gamma repeaters.
It is an important step forward," says Neil Gehrels, the lead scientist of
the SWIFT mission at NASA's Goddard Space Flight Center.
"This result is not only a breakthrough in connecting observations
in the electromagnetic spectrum to gravitational wave searches, but also in
the constructive integration of teams of complementary expertise. Our findings
imply that multi-messenger astronomy will become a reality within the next
decade, opening a wonderful opportunity to gain insight on some of the most
elusive phenomena of the universe," says Szabolcs Márka, an Assistant
Professor of Physics at Columbia University in New York.
At LSU, there is a large and active group of scientists who
are members of the LIGO Scientific Collaboration, including Professors
Joseph Giaime, head of the LIGO Livingston Observatory, and Gabriela Gonzalez,
recently recognized by an award from the American Physical Society for
her contributions to the field of gravitational waves. The students and research scientists in the LSU group
were critical contributors to the search of gravitational waves coincident
with the very strong Gamma Ray Burst 070201, working on the calibration
and characterization of the data taken at the time with the Hanford
detectors, and for all of the LIGO detectors during the Fifth Scientific
Run.
The next major construction milestone for LIGO will be the
beginning of the Advanced LIGO Project, which is expected to start in 2008.
But Advanced LIGO, which will utilize the infrastructure of LIGO, will be 10
times more sensitive than the current LIGO. Advanced LIGO will incorporate
advanced designs and technologies for mirrors and lasers that have been developed
by the GEO project, and which have allowed the GEO detector to achieve enough
sensitivity to participate in this discovery despite its smaller size.
The increased sensitivity will be important because it will
allow scientists to detect cataclysmic events such as black-hole and neutron-star
collisions at 10-times-greater distances.
Last Updated: March 3, 2017
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