Virgo–Polgraw


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

16.10.17 - Discovery marks first cosmic event observed in both gravitational waves and light.

For the first time, scientists have directly detected gravitational waves — ripples in space and time — 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 space-based observatories. Read more

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Virgo congratulates Rainer Weiss, Barry C. Barish and Kip S. Thorne for being awarded the Physics Nobel Prize 2017!

03.10.17 - The Virgo collaboration warmly congratulates Rainer Weiss, Barry C. Barish and Kip S. Thorne on the award of the 2017 Nobel prize in physics ''for decisive contributions to the LIGO detector and the observation of gravitational waves''. The first detection of gravitational waves was announced by the LIGO Scientific Collaboration and the Virgo Collaboration on February 11 2016, five months after the observation of the GW150914 signal, generated by the coalescence of two stellar mass black holes located more than a billion light-years away.

I am delighted that this year’s Nobel prize has gone to our gravitational wave research, says Jo van den Brand, from Nikhef and VU University Amsterdam, the spokesperson of the Virgo Collaboration. The detection of these minute wrinkles in spacetime constitutes an extraordinary achievement. It is the start of a new chapter in our study of the Universe.

Since the first discovery, three more gravitational waves generated two colliding black holes have been detected. The most recent of these detections, on August 14, 2017, was the first one with three detectors at the same time, namely the two Advanced LIGO detectors and the upgraded Advanced Virgo instrument, which jointly operated for 4 weeks starting August 1, 2017.

This first measurements of gravitational waves confirm an important prediction of Albert Einstein's general relativity from 1915. This is the culmination of decades of work, both on the theoretical and experimental sides. Having a three-detector global network opens new prospects for multi-messenger astronomy, adds Federico Ferrini, director of the European Gravitational Observatory (EGO) where the Virgo detector is located.


The LIGO-Virgo global network of three interferometers opens a new era for gravitational wave science by jointly observing a black-hole merger

27.09.17 - 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 historically first time when the polarizations of gravitational waves have been assessed.The three-detector observation was made on August 14, 2017 at 10:30:43 UTC. The detected gravitational waves were emitted during the final moments of the merger of two black holes with masses about 31 and 25 times the mass of the Sun and located about 1.8 billion light-years away. The newly produced spinning black hole has about 53 times the mass of our Sun. This means that about 3 solar masses were converted into gravitational-wave energy during the coalescence.

The Virgo detector joined the Network Observing Run 2 (O2) on August 1, 2017 at 10:00 UTC, after the multi-year Advanced Virgo upgrade program, and months of intense commissioning to improve its sensitivity. The real-time detection was triggered with data from all three LIGO and Virgo instruments.  Even though Virgo is at present less sensitive than LIGO, two independent search algorithms based on all the information available from the three detectors demonstrated the evidence of a significant signal in the Virgo data as well.

Overall, the Universe volume which is likely to contain the source shrinks by more than a factor 20 when moving from a two-detector network to a three-detector network. The sky region for GW170814 has a size of only 60 square degrees, more than 10 times better than for the two LIGO interferometers alone; in addition, the accuracy with which the source distance is measured benefits from the addition of Virgo. Being able to point to a smaller volume is important as many compact object mergers – for example when neutron stars are involved – are expected to produce broadband electromagnetic emission in addition to gravitational waves. The precision pointing information enabled 25 facilities to perform follow-up observations based on the LIGO-Virgo detection but no counterpart was identified – as expected for black holes.

Virgo doesn’t respond in the exactly same way to passing gravitational waves as the LIGO detectors because of its orientation on Earth, meaning that one can test another prediction of general relativity, which is concerned with polarizations of gravitational waves. Polarization describes how space-time is distorted in the three different spatial directions as a gravitational wave propagates. Initial tests based on the transient GW170814 event compare extreme cases: on the one hand, pure general relativity-allowed polarizations; on the other hand, pure polarizations forbidden by Einstein’s theory. The analysis of the data shows that Einstein’s prediction is strongly favored.

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A very exciting LIGO-Virgo Observing run draws to a close on the 25th of August

25.08.17 - A very exciting LIGO-Virgo Observing run draws to a close on the 25th of August. The Virgo and LIGO Scientific Collaborations have been observing since November 30, 2016 in the second Advanced Detector Observing Run 'O2', searching for gravitational-wave signals, first with the two LIGO detectors, then with both LIGO and Virgo instruments operating together since August 1, 2017. Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public (see also this link for updates)


VIRGO joins LIGO for the “Observation Run 2” (O2) data-taking period

01.08.17 - Today, Tuesday August 1st 2017 at 11 CEST, the VIRGO detector based in Europe has officially joined “Observation Run 2” (O2) and is now taking data alongside the American-based twin LIGO detectors. This major step forward for the VIRGO Collaboration is the outcome of a multi-year upgrade program, whose primary goal was to significantly improve the detector performance in terms of sensitivity. More informations.


GW170104: Third direct detection of gravitational waves

01.06.17 - The Advanced LIGO detectors registered in the 4th of January 2017 a merger of a black hole binary system of masses approx. 30 and 20 Solar masses and a formation of a remnant black hole of 49 solar masses and spin parameter of about 0.64, at a distance of about 880 Mpc (corresponding to a redshift z = 0.18). It is likely that at least one of the black holes was spinning opposite the direction of the binary orbit. This is the first detection to show evidence for such a spin configuration. The detection was also used to test the general theory of relativity and to estimate the graviton mass.

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Polgraw group research topics

  • Observations of electromagnetic waves in coincidence with gravitational waves. The EM Follow-up project managed the LIGO-Virgo collaboration is the search for optical transitions associated with candidates for gravitational waves. Many of potentially detectable gravitational wave sources (merging binary system of neutron stars, supernovæ, ...), may also emit electromagnetic radiation. Simultaneous observations of gravitational and electromagnetic waves may provide very interesting scientific results. The EM Follow-up project is run jointly by the LIGO-Virgo and astronomical partners (over 60 teams).

    The Pi of the Sky project takes an active part in the search for optical transients associated with candidates for gravitational waves in the data gathered by the Advanced LIGO and Advanced Virgo. A network of robotic Pi of the Sky telescopes (CFT Sciences, UW, NCBJ) consists of two observatories placed in different hemispheres. The southern hemisphere observatory is located in San Pedro de Atacama (SPdA) in Chile. The northern hemisphere observatory is located is near Huelva (Instituto National de Technica Aeroespecial - INTA) in Spain. Pi of the Sky telescopes have a very large field of view, which allows for a quick sky scans in search for optical flares.

    Analysis of the data collected by Pi of the Sky telescopes is a joint project of the Pi of the Sky and of the Polgraw research groups. Pi of the Sky has already participated in a LIGO-Virgo Looc-Up project (2009-2010), the aim of which was to search for optical flares associated with the candidates for gravitational waves found in the data of the previous generation LIGO and Virgo detectors. The results of the Looc-Up project have been published in A&A 539, A124 (2012) and ApJS 211 (2014) 7.

  • Construction of Advanced Virgo. Our engineers had worked together to create the components of the active damping system seismic, SAT. In cooperation with Smart Instruments we have developed, also for the incoming Advanced Virgo, the controllers and actuators for the testing of thermal compensation of the mirror curvature, using the Thermally Deformable Mirror (TDM) method, and the Central Heating Residual Aberration Correction System (CHRAC). We are currently working on developing a new, modular control system for seismic attenuators, which is an improvement of currently used systems. We have recently obtained a grant for the development and implementation of the data collection system of measurement and signal processing for the calculation and Newtonian noise reduction for the Virgo interferometer.

  • Periodic gravitational waves. The deformation of the mass distribution of a rotating compact stars, caused by the elastic strain or non-axisymmetric distribution of the magnetic field is a promising source of periodic gravitational waves (with a frequency proportional to the frequency of rotation of the star). Currently we know more than 2000 pulsars: neutron stars that rotate about their axes and equipped with a strong magnetic field. Some of them are located in binary systems. The total number of neutron stars in the galaxy is estimated to be approx. one hundred million stars. To explore this vast population of mostly electromagnetically invisible objects group, the Virgo-Polgraw is developing a set of codes for the statistical data analysis. We are searching for gravitational waves from known sources (eg. Crab and Vela pulsars), as well as use modern techniques of parallel computing (MPI, OpenMP, graphics accelerators such as graphical processing units, GPU) on the largest computer clusters in the world to conduct a search of the entire sky in all frequencies in the range of sensitivity of the LIGO and Virgo detectors for unknown sources (see the Polgraw All-sky project site and the documentation).

  • Modelling of astrophysical sources of gravitational waves - binary systems. Binary systems consisting of compact objects: neutron stars or black holes are among the most important sources of gravitational waves for the LIGO and Virgo interferometric detectors. Such systems emit characteristic gravitational waves of amplitude increasing with time and frequency. The standard method used to detect these waves in the noise of the detector is called the matched filter method. This method can be used, if one knows how a priori the characteristic behavior of the signal. The collection of such pre-prepared signals is called a filter bank. The design of the filter bank for the signal coming from the coalesced binary system demands for a sufficiently good accuracy solution to the relativistic two-body problem, which is finding the motion and gravitational radiation of two bodies interacting gravitationally with accordance to the general theory of relativity. There are two classes of methods for solving this problem: analytical methods involving perturbative solutions of Einstein's equations using post-Newtonian expansions, and numerical methods.

    The computational costs associated with the numerical solutions of Einstein's equations describing the compact binary systems are so large that it is impossible in the near future to construct a filter bank for detection of signals from merging binary systems based on the numerical results only. Numerical results obtained so far allowed for the construction of a number of filters and their comparison to the filters based on the post-Newtonian results: "numeric" and "post-Newtonian" filters agree very well in the area corresponding to the adiabatic phase of the quasicircular orbits (in this phase the orbits of the bodies form a densely wound coils of slowly decreasing sizes). Therefore, one solution is to use a filter hybrid, which is a combination of filters based on the approximate post-Newtonian results (describing the early phase of the evolution of the system) with a numerically-calculated filters associated with late phase evolution (or phase related to the merging of objects and the ringdown oscillations ie. quasinormal modes of the Kerr black hole)

    We have derived, using the canonical formalism of Arnowitt, Desser and Misner, Hamiltonians for the system of two slow-moving compact bodies in the third and fourth order of the post-Newtonian expansion (third order corresponds to corrections of the order of (v/c)6 with respect to the Newtonian gravity, where v is a typical orbital speed in the system, and c is the speed of light, while the fourth post-Newtonian order describes the corrections up to (v/c)8). We have also found corrections related to the impact of the angular momentum of the bodies (called spins) on their orbital motion (this kind of phenomenon is called spin-orbit coupling).