The Virgo and LIGO detectors are ready to start the new Observing run called O3.The hunt for gravitational waves is set to start on April 1st.
3rd Observations Run
The European Virgo detector, based in Italy at the European Gravitational Observatory (EGO), and the NSF-funded LIGO twin detectors, located in the states of Washington and Louisiana (USA), will start to take data next week. This one-year collaboration will register science data continuously, and the three detectors will operate as a global observatory. Since August 2017, the end of the second observation run O2, the two collaborations have intensively worked on their interferometers to improve the sensitivity and reliability.
In 2015, after LIGO began observing for the first time in an upgraded program called Advanced LIGO, it soon made history by making the first direct detection of gravitational waves. The ripples traveled to Earth from a pair of colliding black holes located 1.3billion light-years away - a discovery which led to the award of the 2017 Nobel Prize in Physics. Since then, the LIGO-Virgo detector network has uncovered nine additional blackhole mergers and one explosive smashup of two neutron stars.
“With our three detectors now operational at a significantly improved sensitivity, the global LIGO-Virgo detector network is expected to make several new detections. Moreover it will allow precise triangulation of the sources of gravitational waves. This will be an important step towards our quest of multi-messenger astronomy”, says Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and VU University Amsterdam, who is the spokesperson for the Virgo collaboration.
The detector sensitivity is commonly given in terms of the distance at which it can observe the merger of a binary neutron star system. “During O2 Advanced Virgo could observe neutron star events up to a distance of 88 million light years, and Virgo sensitivity has now improved by a factor of 2 over O2”, says Alessio Rocchi, researcher at INFN and Virgo’s commissioning coordinator.
The scientific outcome of O3 is expected to be tremendous, and it will potentially reveal new exciting signals coming from new sources, such as the merger of mixed binaries made by a black hole and a neutron star. O3 will also target long lasting gravitational waves produced for instance by spinning neutron stars which are not symmetric with respect to their axis. However, thedetection of such signals, as well as those from core collapse supernova and other sources, is still an enormous challenge. Nevertheless, thanks to the upgrades of Virgo and LIGO, signals for the merger of binary black holes -such as for GW150914, the first gravitational-wave event ever detected- are expected to become quite common, up to one per week. Scientists also expect to observe perhaps up to tens of binary neutron star mergers, such as GW170817 which opened the era of multi-messenger astronomy as well as providing insights into binary evolution, nuclear physics, cosmology and fundamental physics.
Updating and testing
Since August 2017 both LIGO and Virgo have been updated and tested. Scientists have improved their offline and online data analysis and developed further the procedures for releasing Open Public Alerts: these will within minutes notify the physics and astronomy community when a potentia lgravitational-wave event is observed.
Virgo has fully replaced the steel wires which were used in O2 to suspend the four main mirrors of the 3 km long interferometer: the mirrors are now suspended with thin fused-silica (‘glass’) fibers, a procedure which has allowed toincrease the sensitivity in the low-medium frequency region, and has a dramatic impact in the capabilities to detect mergers of compact binary systems.
A second major upgrade was the installation of a more powerful laser source, which improves the sensitivity at high frequencies. Last but not least, squeezed vacuum states are now injected into Advanced Virgo thanks also to a collaboration with the Albert Einstein Institute in Hannover. This technique takes advantage of the quantum nature of light and improves the sensitivity at high frequencies.
Squeezing is a major upgrade also implemented in the two LIGO interferometers in the US for this next observation run. Moreover, the laser power has been doubled tomore precisely measure the effect of passing gravitational waves. Other upgrades were made to LIGO’s mirrors at both locations, with a total of five of eight mirrors being swapped out for better-performing versions.
Network of international contributions
During O3 the LIGO-Virgo Collaboration will continue to communicate new findings to the scientific community as well as to the general public. Furthermore, scientists will keep on extracting all possible physics results from the data. The global LIGO-Virgo network will provide prompt localizations of gravitational-wave signals and will release confident events publicly through the Open Public Alert system. This will maximize the science that the entire scientific community can do with the gravitational-wave detections and to minimize the chance of missing any electromagnetic or neutrino counterparts.
The Japanese detector KAGRA is expected to join the global LIGO-Virgo network in the last part of O3, extending the detection and pointing capabilities of the global network.
Five groups in Spain are contributing to LIGO-Virgo gravitational wave astronomy, in areas that range from theoretical modelling of the astrophysical sources to improving the detector’s sensitivity for current and future runs. Two groups, in UIB and IGFAE-USC, are within the LIGO Scientific Collaboration; the University of Valencia (UV), ICCUB and IFAE are Virgo members. After the wonderful discoveries brought by the first two observing runs, the Spanish LIGO-Virgo Groups are eagerly looking forward to the imminent O3 run.
A research team from the Institute (ICCUB) will help processing and analyzing vast amounts of data from O3 more efficiently and reliably. Massive data handling and cutting-edge instrumentation and electronics expertise is being transferred to Virgo, knowledge emerging from the successful ICCUB participation in high-energy physics LHCb and large astronomical surveys as Gaia. In this way, multidisciplinary ICCUB experts will contribute to the detection and analysis of gravitational waves, providing both instrumentation and software. Furthermore, they will contribute with data analysis and their large scientific know-how especially in the field of cosmology.
The gravitational physics group at UIB will follow a broad scientific program to study gravitational waves emitted from black holes and neutron stars. The team will continue leading searches for continuous wave signals from unknown neutron stars, as well as for the transient signals emitted after the merger of two neutron stars. Models of the gravitational wave signal from merging black holes are an essential part of the data analysis process, and the UIB is involved in the development of one of the two key models used so far, the value of which will be tested in making new discoveries.
The IGFAE Gravitational Wave group at the University of Santiago de Compostela is currently significantly upgrading the PyCBC detection pipeline to maximize the reach of binary searches in the new O3 network. The group will also be involved in deducing information about the populations of gravitational-wave sources as a whole, including hints that the dozens of likely new binary black hole detections will give on the formation and evolution of these so far mysterious systems.
The Valencia Virgo Group is anticipating the promise of O3 to increase the number of detections of binary neutron star systems and, perhaps, yield the first observations ever of systems yet undetected such as mixed black hole - neutron star mergers and (however unlikely due to the low gravitational-wave amplitude and event rate) core-collapse supernova explosions. Astrophysical sources like neutron stars and supernova progenitors are the main focus of the Valencia Virgo Group, regarding wave form modelling through numerical relativity simulations, parameter estimation, and data analysis.
The High Energy Physics Institute IFAE was already playing an important role in the commissioning of the interferometer prior to O3 started. This energetic involvement will continue in aspects related to operations and the upgrade of the interferometer. IFAE is working on the construction of new baffles instrumented with photo sensors around the testmasses in the suspended areas, allowing for a much more efficient alignment andfine-tune of the parameters of the interferometer during operations.
LIGO is funded by NSF and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Scienceand Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Nearly 1300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at https://my.ligo.org/census.php.
The Virgo Collaboration is currently composed of approximately 350 scientists, engineers, and technicians from about 70 institutes from Belgium, France, Germany ,Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN)in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration groups can be found at http://public.virgo-gw.eu/the-virgo-collaboration/.
More information is available on the Virgo website at http://www.virgo-gw.eu.
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