LIGO and Virgo open a new hunting season


two technicians with equipment

Engineers Hugh Radkins and Betsy Weaver work inside the detector's vacuum system at the LIGO Hanford Observatory in Washington, USA, during recent hardware upgrades. With the upgrades completed, the two US LIGO facilities, in partnership with the Virgin unit in Italy, are ready to begin the third stage of observation. [Image: LIGO/Caltech/MIT/Jeff Kissel] [Enlarge image]

April 1, 2019– Just over three years after the first detection of gravitational waves (GWs), the GIG laser observers and the Virgo laser interferometer begin their third observation today.

Known in the GW community simply as "O3," the one-year observation is likely to produce a crop of new astronomical observations – the result of a 40% improvement in the already impressive sensitivity of the two LIGO facilities in the United States. United States and a near doubling of the sensitivity of the Virgin installation in Italy. The O3 period also saw the long awaited streaming of the KAGRA GW observatory in Japan. And in a new twist, LIGO / Virgo Scientific Collaboration (LSC) will be making data on possible near-real-time GW detections available.

Increasing Laser Power

The O3 race will add to the impressive chain of milestones achieved in the first two GW observation executions. These include the detection of gravitational waves from ten binary black-hole fusions as well as the collision of a pair of ultra-dense neutron stars. The latest detection – coordinated with observations of more traditional optical telescopes, X-rays and gamma rays in an innovative example of "multi-meshed astronomy" – has resulted in a fresh harvest of new scientific information.

the front of the baffle

Alena Ananyeva, a LIGO member, at LIGO Livingston Observatory, installing "dampers" to control diffused light. [Image: LIGO/Caltech/MIT/Matt Heintze] [Enlarge image]

LSC scientists are confident that LIGO and Virgo observatories will record observations at an even faster rate on O3 as a result of the technical improvements implemented since the last observation, O2, in August 2017 (see "Gravitational Waves: The next"). Step, "OPN, May 2018).

The improvements include a doubling of the laser power of the facility lasers, which are combined in an L-shaped Michelson interferometer configuration with 3 to 4 km long interferometer arms to capture the subtle perturbations in spacetime that can signal a gravitational passage. wave. Also installed in the update round were diffuse light suppressors, or "dampers", designed to control the light scattered inside the huge interferometers.

Hammering the noise

In addition to laser power, other recent updates have focused on efforts to increase sensitivity by eliminating and eliminating noise sources across multiple subsystems. At LIGO, this included the enormous engineering challenge of swapping a number of 40 kg mirrors, or test masses, exquisitely suspended at each end of the laser interferometer arms. As a passing gravitational wave moves through spacetime, small movements in these mirrored mirrors result in infinitesimal changes in the arm lengths that are read in the interferometric signal. New improved mirror versions include improved coatings to reduce thermal noise.

Virgo Mirror Installation

A view of one of Virgo's mirrors, or test masses, behind when a researcher removes the safety stops from the mirror after the recent update. (The mirror is reflective in the infrared range used by the Virgo interferometer, but transparent to visible light.) [Image: EGO/Virgo Collaboration/Perciballi] [Enlarge image]

At Virgin, meanwhile, steel wires that suspend the main mirrors have been replaced by cast-silica versions that reduce vibrational noise and increase the facility's ability to pick up low- and medium-frequency GWs. And both LIGO and Virgo will now use a trick of quantum mechanics, the injection of a "squeezed" state of light into the photodetector, to reduce uncertainties in photon arrival times attributable to quantum vacuum fluctuations.

These and other technical improvements were partially developed and matured in another facility, the GEO 600, a smaller GW observatory in Europe that served as a vital testbed for technologies to sharpen the power of observation of large sites. The GEO 600 will also participate in the O3 race.

Sampling more of the cosmos

equipment installation

Researchers install the source of light squeezed into a clean room in the Virgin's gravitational wave detector. [Image: © H. Lück/B. Knispel/Max Planck Institute for Gravitational Physics] [Enlarge image]

Recent sensitivity enhancements will allow the global GW network to sample a much expanded cosmos for evidence of high-energy astronomical events. In O3, for example, the sensitivity of LIGO, in the wake of recent updates, should enable it to detect binary neutron star fusions at a distance of 550 million light years – over 190 million light-years more than in O2.

This, coupled with an eight-fold expansion of the volume of space now visible to Virgo, could increase the rate of detection of black-hole binary collisions anywhere, from a few events per month to a few per week, and binary star merge neutrons between one per year and one per month. There is also the possibility of catching more exotic and previously inaccessible events, such as a black hole and a neutron star.

Instant access to data

The public will have almost immediate access to this discovery harvest through new software developed by LSC scientists. The software will be able to send open public alerts within five minutes of GW detection, according to Sarah Antier, a postdoctoral researcher at Université Paris Diderot, France.

This will allow rapid public access to parameters such as signal type, sky position and estimated distance for a given GW event. These parameters, in turn, will enable both professional as well as amateur astronomers, who examine several slices of the electromagnetic spectrum, to quickly train their instruments in the right stretch of the sky to follow the GW observation.

KAGRA on the way?

Construction KAGRA

A part of one of the arms of the KAGRA interferometer, under construction in 2015. [Wikimedia Commons/Christopher Berry]

The ability to locate GW sources quickly and accurately could receive another boost at the end of the one-year O3 period with the long-awaited debut of KAGRA. A 3km GW underground laser observatory, whose design includes cryogenically cooled suspended sapphire test masses at 20K, KAGRA has been under construction in Japan since 2010. But its development has been plagued by the continuing difficulty in banning attributable vibrational noise cryopreservation. cooling equipment and even infiltration of water into the underground installation.

In January of this year, however, the KAGRA team finally reported a successful 10-day test of the interferometer at cryogenic temperatures. With this significant milestone behind, the team is optimistic that the facility will be able to make its first scientific observations by the end of 2019.


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