Gravity changes used for early detection of earthquakes and determination of their magnitude

Researchers from IPGP, CNRS, Caltech and Paris Diderot University have observed changes in the gravitational field produced by earthquakes before the arrival of seismic waves (propagating with speeds between 3 to 10 km/s). These gravity changes (which are conceptually different from the recently detected gravitational-waves) can be used as a new potential information carrier, helping in the early detection of major earthquakes or tsunamis and in the determination of their magnitude.

This work, based on data from the 2011 Tohoku Japanese earthquake (magnitude 9.1), opens new perspectives in the seismology. Further work is required to improve the detection of this weak gravity signal, especially for smaller earthquakes. New instruments can benefit from the technology developed in the field of the detection of gravitational waves coming from distant Universe. You can learn more (below) on this joined work between seismologists and astrophysicists published in Science on December 1st.

Press release

Science article

 

LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars

17 August 2017 – 12:41:04 UTC: For the first time, scientists have directly detected gravitational waves in addition to light from the spectacular collision of two neutron stars.

This marks the first time that a cosmic event has been viewed in both gravitational waves and light. The discovery was made using the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO); the Europe-based Virgo detector; and some 70 ground- and spacebased observatories.

Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovas. As these neutron stars spiraled together, they emitted gravitational waves that were detectable for about 100 seconds; when they collided, a flash of light in the form of gamma rays was emitted and seen on Earth about two seconds after the gravitational waves. In the days and weeks following the smashup, other forms of light, or electromagnetic radiation — including X-ray, ultraviolet, optical, infrared, and radio waves — were detected.

The image shows the localization of the gravitational-wave (from the LIGO-Virgo 3-detector global network), gamma-ray (by the Fermi and INTEGRAL satellites) and optical (the Swope discovery image) signals from the transient event detected on the 17th of August, 2017. The colored areas show the sky localization regions estimated by the gamma-ray observatories (in blue) and by the gravitational-wave detectors (in green). The insert shows the location of the apparent known galaxy NGC4993: on the top image, recorded almost 11 hours after the gravitational-wave and gamma-ray signals had been detected, a new source (marked by a reticle) is visible: it was not there on the bottom picture, taken about three weeks before the event.

 

The observations have given astronomers an unprecedented opportunity to probe a collision of two neutron stars. For example, observations made by the U.S. Gemini Observatory, the European Very Large Telescope, and the Hubble Space Telescope reveal signatures of recently synthesized material, including gold and platinum, solving a decadeslong mystery of where about half of all elements heavier than iron are produced.

Theorists have predicted that when neutron stars collide, they should give off gravitational waves and gamma rays, along with powerful jets that emit light across the electromagnetic spectrum. The gamma-ray burst detected by Fermi, and soon thereafter confirmed by the European Space Agency’s gamma-ray observatory INTEGRAL, is what’s called a short gamma-ray burst; the new observations confirm that at least some short gamma-ray bursts are generated by the merging of neutron stars — something that was only theorized before. “For decades we’ve suspected short gamma-ray bursts were powered by neutron star mergers,” says Fermi Project Scientist Julie McEnery of NASA’s Goddard Space Flight Center.

“Now, with the incredible data from LIGO and Virgo for this event, we have the answer. The gravitational waves tell us that the merging objects had masses consistent with neutron stars, and the flash of gamma rays tells us that the objects are unlikely to be black holes, since a collision of black holes is not expected to give off light.”

At the moment of collision, the bulk of the two neutron stars merged into one ultradense object, emitting a “fireball” of gamma rays. The initial gamma-ray measurements, combined with the gravitational-wave detection, also provide confirmation for Einstein’s general theory of relativity, which predicts that gravitational waves should travel at the speed of light.

You can read this full paper written by Jennifer Chu, MIT News Office below:

LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars

Watch this short video on the nytimes.com:

Detecting a Kilonova Explosion

And to know more, here are some recent publications:

Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral

Straight to the source: the LIGO-Virgo global network of interferometers opens a new era for gravitational wave science

The Virgo collaboration and the LIGO Scientific Collaboration report the three-detector observation of gravitational waves. This result highlights the scientific potential of a global network of gravitational wave detectors, by delivering a better localization of the source and access to polarizations of gravitational waves.
The two Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA and the Virgo detector, located at the European Gravitational Observatory (EGO) in Cascina, near Pisa, Italy, detected a transient gravitational-wave signal produced by the coalescence of two stellar mass black holes.

 

Localisation of the Gravitational Wave source.

In yellow : localisation from the 2 LIGO detectors.

In green : localisation using datas from 3 detectors (LIGO et Virgo).

In purple : more precise localisation after detailed data analysis.

© Collaboration LIGO-Virgo

 

GW170814

The press release is here.

 

The full publication:

GW170814: A Three-Detector Observation of Gravitational Waves
from a Binary Black Hole Coalescence
B. P. Abbott etal.
(LIGO Scientific Collaboration and Virgo Collaboration)

 

VIRGO joins LIGO for a data-taking period

On August 1st 2017, the VIRGO detector based in Europe has officially joined the American-based twin LIGO detectors for a simultaneous data-taking period. The improved sensitivity of the VIRGO detector now allows confirming a detection and locating sources of gravitational waves with greater accuracy.

After the completion of this observational period, the VIRGO operation will proceed to further improve the sensitivity of the detector and to gain more knowledge about the main sources of noise that are currently limiting mesurements.

“Today, for the first time, we have a network of three second generation detectors capable of localizing the source of a gravitational-wave signal. This is a major achievement and the best is yet to come: the sensitivity of the involved detectors will progressively improve and more detectors are expected to join in the next years, opening exciting perspectives for the multi-messenger investigation of our universe”, concludes Giovanni Losurdo from INFN Pisa former leader of the “Advanced VIRGO” project.

The full LSC VIRGO announcement here.

Image: Virgo aerial view – ©EGO-VIRGO

Gravitational wave mission LISA selected by ESA Science Program Committee

ESA (the European Space Agency) has chosen the Laser Interferometer Space Antenna (LISA) for its third large-class mission (L3) in the agency’s Cosmic Vision science program. The LISA trio of satellites is meant to study gravitational waves in space.

This is a major milestone in the development of LISA. The next big step will be the adoption, expected around 2021/2022.

In France, the APC Laboratory has a central role in LISA, since today the laboratory is coordinating, with the support of CNES and other partner laboratories, the two main French contributions considered for LISA: setting up of the Scientific Data Treatment Centre, and the Performance Management, Integration & Payload Tests.

For more information:

ESA web

NASA web

Photo Lisa concept
AEI/Milde Marketing/Exozet

Gravitational waves strike the Earth again: GW151226

The LIGO and Virgo collaborations announced today, June 15, at the 228th meeting of the American Astronomical Society in San Diego, a new gravitational wave event. Simultaneously, an article is published in Physical Review Letters.

The event was observed on December 26, 2015 at 3.38.53 UTC in the two LIGO detectors of Livingstone and Hanford (1.1 millisecond later). The event, interpreted as the merger of two black holes, is not as bright as the one announced last February, and thus the signal is not as spectacular:

GW151226

One of the reasons is that the black holes are not as massive as in the “discovery” event GW150914: the two masses are 14 and 8 solar masses, and the final black hole mass is 21 solar masses. The analysis is using templates of mergers predicted by theory, and comparing the signal with them. The signal to noise ratio (the quantitative way physicists express the fact that the signal stands out of the background “noise” in the detector) is computed to be 13, to be compared with 24 in the case of GW150914.

The distance of the event is estimated to be 1.4 billion light years.

Because the signal lasts longer in the detector than the first event observed, the LIGO-Virgo collaboration could determine that one of the initial back holes (and the final one) was spinning.

In the press conference at the AAS meeting, the collaboration reminded everyone of another event (already mentioned in the original discovery paper) which is presumably another black hole event merger. They call it LVT151012, where LVT stands for LIGO Virgo Trigger (it was observed on October 12 2015). The signal to noise ratio is 9.7 and the collaboration does not feel confident enough to call it a discovery. If it corresponded to a black hole merger, the mass of the final black hole would be 35 solar masses.

If the announcement of February 11 was closing one hundred years of quest for gravitational waves, the announcement of today clearly opens the era of gravitational waves astronomy.

We will have a great occasion to talk about this new discovery in the hangout held this Thursday 16 June, as a conclusion of the Gravity! course second session.

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