The classification and definitive analysis of the 39 events detected by Virgo and LIGO in the third observation period (which ran from April to October 2019) was published today on the ArXiv online archive. Most of these are black hole mergers, the characteristics of which, however, question some established astrophysical models and open up new scenarios. A likely merger of neutron stars and two probable 'mixed' neutron star-black hole systems were also detected in the same period.

It took a year of work and complex analysis by the researchers of the Virgo and LIGO scientific collaborations to complete the study of all of the gravitational-wave signals that were recorded by the Virgo interferometer, installed at the European Gravitational Observatory, in Italy, and the two LIGO detectors, in the US, during the data-taking period - called 'O3a' - which ran from the 1st of April to the 1st of October, 2019. Events included: 36 mergers of black holes; a likely merger of a binary system of neutron stars; and two systems that were most likely composed of a black hole and a neutron star. Among these, four "exceptional events" have, during the last year, already been published, but the catalogue released today provides, for the first time, a complete picture of the extraordinarily large number of recorded gravitational-wave signals and their sources. It represents a wealth of observations and data on the physics of black holes, barely imaginable until only a few years ago.

"Since the end of the O2 observing run in August 2017, many efforts have been made to upgrade many of the technical components and different sectors of the detector, in order to boost the Virgo sensitivity across the whole frequency range", said Ilaria Nardecchia, a researcher at the University of Roma Tor Vergata and member of the Virgo Collaboration. "We reaped the benefits of our work because we doubled the sensitivity of the detector!"

Indeed, between September 2017, and April 2019, the sensitivity of the three detectors has been significantly improved. This has led, for example, to Virgo becoming capable of observing a volume of the universe almost ten times larger than in the previous observational run (O2).

"Observations with Advanced Virgo and LIGO have exceeded expectations. As well as opening a new and exciting phase in the history of human observation of the cosmos, we are seeing events that either lacked observational evidence until now, or go beyond our current understanding of stellar evolution", said Ed Porter, directeur de recherche CNRS at APC-Paris, and member of the Virgo Collaboration. "Just five years after the first detection of gravitational waves, we can say that gravitational astronomy is a concrete reality."

The detection of gravitational signals allows us, in fact, for the first time, to closely observe the dynamics of extraordinary mergers of black holes and neutron stars, which release bursts of energy equivalent to several solar masses in gravitational waves. This allows us to study, as never before, the physics of black holes, the cosmic phenomena that generate them and even the characteristics of the largest populations of black holes. Actually, the results of the present catalogue raise serious questions about the validity of some of the astrophysical scenarios and models, which until now seemed the most plausible.

In particular, the masses of black holes, presented in the O3a catalogue, question various theoretical and observational limits on the mass ranges of black hole populations. Some observations, for example, indicate the presence of compact objects (which could be either black holes or neutron stars) exactly in the gap between the mass of the heaviest neutron stars and that of the lightest black holes observed by astronomers to date. This gap could therefore narrow or even disappear. Other observed black holes have a mass with a value between 65 and 120 solar masses; a range forbidden by stellar evolution models. According to these models, the very massive stars, beyond a certain threshold, are completely disrupted by the supernova explosion, due to a process called pair instability, and leave behind only gas and cosmic dust. The existence of black holes in the range prohibited by pair instability suggests other mechanisms of black hole formation, such as the merger of smaller black holes or the collision of massive stars, but may also indicate the need to revise our description of the final stages of the lives of stars.

The publication of the O3a catalogue is the conclusion of complex work involving many phases and covering detector calibration, data characterisation and data analysis. The catalogue for each observation run is only published once researchers have the final validated dataset, thus making it possible to estimate the physical parameters (such as distance, mass and spins) of the black-hole and neutron-star mergers, as well as a confident estimate of their margins of error. Of the 39 events presented in this latest catalogue, 26 were announced immediately after detection, while 13 are reported for the first time in the paper published today. These add to the 11 gravitational-wave events reported by LIGO and Virgo for the previous runs (O1 and O2). In addition to the LIGO-Virgo events catalogue, three other articles have also been released today on the arXiv server: the global analysis of the astrophysical properties of the gravitational-waves sources; new tests of the theory of general relativity; and the search for gravitational-wave signals coincident with gamma-ray bursts."

"These papers are very important and represent a further step forward in a long and exciting journey", said Giovanni Losurdo, INFN researcher and spokesperson for the Virgo Collaboration. "We are already looking forward to the results of the second part of the third observation period (O3b). The very high number of events still to analyse and understand promises that the next catalogue will be as exciting, if not more so, than this one. Meanwhile, we are striving to implement a substantial upgrade of the Virgo detector, aiming to pursue the next run, in 2022, again with a considerably improved sensitivity."

Citizen-science projects for gravitational-wave data-analysis

Two citizen-science projects, Gravity Spy for LIGO and the European project, REINFORCE for Virgo, allow everyone to contribute to the identification of spurious signals and therefore to the discovery of new gravitational-waves signals, by collaborating directly with researchers involved in the analysis of the data of the three interferometers.

In fact, although external as well as internal noise sources are minimised, the data taken by the interferometers are still plagued by some disturbances. In some cases, these are monitored by witness sensors and are then subtracted from the data in real-time. Nevertheless, the identification of other noises is more problematic and requires off-line dedicated analysis in order to flag them. This is the case with glitchy noises; those that are generated, for instance, by light scattered off the main laser beam and that then recombine with it. The careful studies required to claim a true gravitational-wave signal explain why the LIGO and Virgo Collaborations issue alerts of a candidate event to the scientific community soon after it has been measured. This can then either be confirmed by subsequent analysis and hence considered a true signal or not. Thanks to Gravity Spy and REINFORCE, citizen scientists can help researchers in this complex analysis work by directly accessing the data detected by the LIGO and Virgo interferometers.

Link to papers:

• Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run
• Population properties of compact objects from the second LIGO–Virgo Gravitational-Wave Transient Catalog
• Tests of General Relativity with Binary Black Holes from the second LIGO–Virgo Gravitational-Wave Transient Catalog
• Search for Gravitational Waves Associated with Gamma-Ray Bursts detected by Fermi and Swift during the LIGO-Virgo Run O3a

Image caption: The image shows sky localisations for the different LIGO-Virgo detections that are included in the O3a catalogue. Each localisation - represented by shaded areas on the map - is deduced on the basis of information provided by the three detectors in the network. The day and time of arrival on Earth, a scientific name and the time it took the signal to reach the Earth from wherever in the Universe it was generated, are all recorded. The smaller the shaded area in the sky map, the better the signal has been localised. Localisation is crucial in enabling follow-up searches with different messengers, such as light or neutrinos.

• The announcement is made in light of the recent approval of the preliminary draft of the Spanish government’s budget which announces an investment of more than five billion euros in science
• The signatories demand a national pact that includes long-term strategies aimed at promoting frontier science and business innovation
• The document aims to add and provide strategic solutions to the recent Pact for Science and Innovation that the Government has just announced

SOMMa, ASEICA and AseBio, entities that together account for almost ten thousand researchers across the public and private sector, dozens of research centres and nearly 300 leading Spanish companies in the biotechnology sector, join their voices to urge the political class to transform the country. More than 40 organisations supporting stronger support for science and innovation have signed a document that calls for Spain to reach and exceed 2.5% investment in R&D by 2027 and radically change its current economic model.

The signatories consider this is a crucial moment of putting R&D at the centre of Spain’s future strategy for a sustainable and resilient recovery. It is a unique window of opportunity presented by yesterday’s preliminary draft of the state’s new general budget, the European Reconstruction Plan, the "Green Deal" and the missions of the new Horizon Europe Framework Program of the European Union.

Furthermore, the public has been able to grasp the need to have a solid research capacity for the challenges we are currently facing. Despite this, Spain invests just 1.24% of its GDP in R&D, a figure much lower than the EU average (2.12%), and far from countries such as Germany, Denmark or Austria (around 3%).

The call for action proposes a broad range of administrative and legal recommendations, as well as the implementation of strategic actions so that science and innovation act as the engines for the recovery of the country. The transformation of the economic model would counterbalance the dependence of sectors heavily affected by the current pandemic, giving Spain new opportunities and a stronger position for the future. Crystallizing the great Spanish potential in R&D would lay the foundations for a solid recovery through a sustainable, competitive economic model based on providing high added value.

“The current context has abruptly exposed the shortcomings of our economic model. Spain is thought to be one of the advanced economies most affected by the pandemic, making it essential to change the foundations of our economic model before we can regrow. This is a turning point which we cannot back away from, as it is only through a new economic model that we can ensure the future of Spain. Taking urgent action through the state’s general budget is just the starting point. A state pact for R&D is necessary now more than ever." Luis Serrano, president of SOMMa and director of the Centre for Genomic Regulation.

“Covid-19 has revealed what we have been warning for a long time: the urgent need to invest in science and innovation to ensure the health of our citizens and to develop an economy based on knowledge, not entertainment. We must take the bull by the horns: we need stable long-term plans and short-term solutions to meet these challenges. Let's say it once again: research is not a luxury, it is the only way we have to ensure the health and quality of life of our fellow citizens. And this does not depend on ideologies, it is a project that cuts across all divides”. Xosé Bustelo, President of ASEICA.

“This appeal to our political representatives, to the administration and to society itself, is nothing but a joint demand for a long-term strategy that promotes science and innovation in our country and places them at the heart of its strategy. Innovative companies and entities are committed to this effort if we have an adequate and stable framework that allows us to work collaboratively with the rest of the agents of the R & D & I ecosystem and thus contribute to the transformation of our production model. " Ion Arocena, CEO of AseBio.

The appeal led by SOMMa, ASEICA and AseBio centres around three groups of measures, the first of which focuses on the strengthening of basic and translational frontier science. The signatories demand a simplification of expense management and associated bureaucracy, an increase and optimization of investment, new talent recruitment programs, and mechanisms that favour the stability of research projects promoted by public organizations.

The second tier of recommendations propose measures that strengthen innovation and promote the transition to a sustainable economy with high added value. It is proposed to promote public-private cooperation and innovative business fabric, as well as the creation of a patronage-fundraising law. It also calls for a need to undertake a profound reform of the aid model for business R&D and a legal framework that minimizes uncertainties and provides stability and security to the R&D system.

Finally, the third tier of recommendations calls for new mechanisms to increase synergies between the academic and business sectors. Key to this will be the development of a long-term national strategy that includes the autonomous communities, increasing the capacity to transfer the knowledge of universities and research institutes into innovative solutions and the creation of new technology-based companies. Finally, the signatories appeal to cultivate the value of science as a reference for citizenship, the business community and political action.

You can find the full document with the list of signatories here (in Spanish).

• Virgo and LIGO have announced the detection of an extraordinarily massive merging binary system: two black holes of 66 and 85 solar masses, which generated a final black hole of around 142 solar masses.
• The remnant black hole is the most massive ever detected with gravitational waves. It lies in a range of mass (from 100 to 1000 solar masses) within which a black hole has ever before been observed, either via gravitational waves or electromagnetic observations and may help to explain the formation of supermassive black holes.
• Moreover, the most massive component of the binary system lies in a mass range forbidden by stellar evolution theory and challenges our understanding of the final stages of massive stars life.

The scientists of the international collaborations running the Advanced Virgo detector at the European Gravitational Observatory (EGO) in Italy and the two Advanced LIGOs, in the US, have announced the detection of a black hole of around 142 solar masses, which is the final result of the merger of two black holes of 66 and 85 solar masses. Both the primary components and the remnant lie in a range of mass never observed before, either via gravitational waves or with electromagnetic observations. The final black hole is the most massive ever detected with gravitational waves. The gravitational-wave event was detected by the three interferometers of the global network on the 21st of May, 2019. The signal (named GW190521) has been analysed by scientists. Two scientific papers reporting the discovery and its astrophysical implications have been published today on Physical Review Letters and Astrophysical Journal Letters respectively.

“The signal observed on May 21 of the past year is a very complex one and, since the detected system is so massive, we only observed it for a short time: about 0.1 s”, says Nelson Christensen, directeur de recherche CNRS at ARTEMIS in Nice, France and member of the Virgo Collaboration. “This doesn’t look much like a chirp, which is what we typically detect: it is more like something that goes ‘bang’ and the system that generated it is the most massive that LIGO and Virgo have detected until now.” Indeed, the analysis of the signal, based on a powerful suite of state-of-the-art computational and modelling tools, revealed a large amount of information about the different stages of this unique merger.

The breaking of the mass record of the Virgo and LIGO observational runs is just one of the several special features that make the detection of this exceptional merger an unprecedented discovery. A crucial aspect, which particularly drew the attention of astrophysicists, is that the remnant belongs to the class of so-called ‘intermediate-mass black holes’ (from a hundred up to a hundred thousand solar masses). The interest in this black-hole population is linked to one of the most fascinating and challenging puzzles for astrophysicists and cosmologists: the origin of supermassive black holes. These giant monsters, millions to billions of times heavier than the sun and often at the centre of galaxies, may arise from the merger of ‘smaller’ intermediate-mass black holes.

Until today, very few intermediate-mass black hole candidates have been identified through electromagnetic observations alone and the remnant of GW190521 is the first observation of an intermediate-mass black hole via gravitational waves. It is of even greater interest, owing to it being in the range from 100 to 1,000 solar masses, which has represented for many years a sort of "blackhole desert", because of the paucity of candidate events within this range.

The components and the dynamics of the merging binary system of GW190521 offer also other extraordinary astrophysical insights. In particular, the most massive component challenges the models describing the collapse of the heaviest stars, at the end of their lives, into black holes. According to these models, the very massive stars are completely disrupted by the supernova explosion, due to a process called pair instability, and leave behind only gas and cosmic dust. Therefore, astrophysicists would not expect to observe any black hole in the mass range between about 60 and 120 solar masses: exactly the range of mass in which the most massive component of GW190521 lies. Hence, this detection opens new perspectives on the study of massive stars and supernova mechanisms.

“Several scenarios predict the formation of black holes in the so-called pair-instability mass gap: they might result from the merger of smaller black holes or from the collision of massive stars or even from more exotic processes”, says Michela Mapelli, professor at Padova University, member of INFN Padova and of the Virgo Collaboration. “However, it is also possible that we have to revise our present understanding of the final stages of the star's life and the resulting mass constraints on black-hole formation. Either way, GW190521 is a major contribution to the study of the formation
of black holes.”

In fact, the Virgo and LIGO GW190521 detection highlights the existence of black-hole populations that have never been observed before or are unexpected and, in so doing, raises intriguing new questions about their formation mechanisms. Despite the unusually short duration of the signal, which limits our ability to infer the astrophysical properties of the source, the most advanced analyses and models currently available suggest that the initial black holes had strong spins, that is to say, they were rotating rapidly.

“The signal shows hints of precession, a rotation of the orbital plane produced by spins with large magnitude and particular orientation”, states Tito Dal Canton, CNRS researcher at IJCLab in Orsay, France, and member of the Virgo Collaboration. “The effect is weak and we cannot claim it is definitely present, but if true, it would support the hypothesis that the progenitor black holes arose and lived in a very shaky and crowded cosmic environment, like a dense star cluster or the accretion disk of an active galactic nucleus.”

Several different scenarios are still compatible with the shown results and even the hypothesis that the progenitors of the merger might be primordial black holes has not been discarded by scientists. We actually estimate that this merger occurred about 7 billion years ago, a time close to the ancient
ages of the Universe. With respect to previous gravitational-wave detections, the observed GW190521 signal is very short in time and more difficult to analyse. Due to the more complex nature of this signal other more exotic sources have been considered, and these possibilities are described in an accompanying publication. However, these possibilities are disfavoured with respect to the source being a binary black hole merger.

“The observations made by Virgo and LIGO are shedding light on the dark universe and defining a new cosmic landscape”, states Giovanni Losurdo, Virgo spokesperson and head of research at Istituto Nazionale di Fisica Nucleare in Italy “And today, once again, we announce an unprecedented discovery. We keep improving our detectors to enhance their performance and look further and further into the Universe.”

Additional information

The Virgo Collaboration is currently composed of approximately 580 members from 109 institutes in 13 different countries, including Belgium, France, Germany, Greece, Hungary, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Monaco and Japan. 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 VirgoCollaboration 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.

LIGO is funded by the National Science Foundation (NSF) and operated by Caltech and MIT, which conceived of LIGO and led the project. Financial support for the Advanced LIGO project was led by the NSF, with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Approximately 1,300 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 athttps://my.ligo.org/census.php

Read the articles

Properties and Astrophysical Implications of the 150 M Binary Black Hole Merger GW190521

- The international study counts on the participation of researchers from the Institute of Cosmos Sciences of the University of Barcelona, the Institute of High Energy Physics and the Autonomous University of Madrid

The Sloan Digital Sky Survey (SDSS) publishes a comprehensive analysis of the largest three-dimensional map of the Universe ever created, which fills the most significant voids of our exploration on the history of cosmos.

Our knowledge on the Universe includes both the ancient and recent history of its expansion, but there were voids corresponding to 11,000 million years between both periods. For five years, scientists from SDSS have worked to discover what happened during that period of time, and used the information to get one of the most important advances in the cosmology of the last decade.

The new results come from one of programmes in SDSS, the international collaboration Extended Baryon Oscillation Spectroscopic Survey (eBOSS), in which more than a hundred astrophysicists take part. Three Spanish researchers played an important role in the analysis that was presented today: Héctor Gil Marín, from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB); Andreu Font Ribera, from the Institute of High Energy Physics (IFAE), and Santiago Ávila, from the Autonomous University of Madrid. The new results feature the detailed
measurements of more than two million galaxies and quasars, which cover 11,000 million years of cosmic time.

Thanks to the study of the radiation of the cosmic microwave background (CMB), and the measures of the quantity relate to the elements that were created after the Big Bang, we know how the Universe was like at the beginning. We also know the history of the expansion of the Universe over billions of years, thanks to the maps of galaxies and measurements of the distances between them, including those in
phases prior to SDSS.

“The eBoss analysis and the previous experiments in SDSS show the history of the expansion of the Universe over the largest amount of time studied so far”, notes Héctor Gil Marín, from ICCUB. The researcher has led the analysis of these galaxy maps, measuring the expansion rhythm and the growth of structures of the Universe from 6,000 million years ago. These measurements help merging the early and late physics, which generates a complete image of the expansion of the Universe over time.

The obtained map shows filaments and voids that define the structure of the Universe from the moment it was only 300,000 years old. With this map, researchers look for patterns in the distribution of galaxies, which provide information on these key parameters of the Universe, which eBOSS could measure with a precision over 1%.

The map is the result of more than twenty years of efforts to map the Universe through the telescope from the Alfred P. Sloan Foundation. The cosmic history it reveals shows that the expansion of the Universe started accelerating about 6,000 million years ago, and it has increased since then. This accelerated expansion may be so due to a mysterious compound in the Universe, called dark matter, which is consistent with Einstein’s general relativity theory, but difficult to conciliate with our current knowledge of particle physics.

When combining the observations from eBOSS with studies on the early Universe, researchers obtained an image with some incompatibilities. The measurement of the current rate of expansion of the Universe (Hubble’s constant) is about 10% less compared to the value found when measuring the rate of expansion using the distance to near galaxies.

“The high precision of the data makes it unlikely for this mismatch to result from chance”, notes Andreu Font Ribera, IFAE researcher in Barcelona, who led the interpretation of results. “The great variety of data in eBOSS leads to the same conclusion in several ways”, he adds.

There is not a widely accepted explanation for this discrepancy in the measures of expansion rates, but an interesting possibility is that a previously unknown way of matter or energy of the early Universe would have left a mark in the expansion we observe now.

These results have seen the light today with the publication of more than twenty science articles in ArXiv, documents that describe, over more than five hundred pages, the analysis of the latest data in eBOSS. With this summit, the key objectives of the study are reached.

The different groups in the eBoss team, located in universities worldwide, have focused on different aspects of the analysis. Researchers have analysed red and massive galaxies to obtain the part of the map dating from 6,000 million years ago. For further galaxies, they used younger blue galaxies. Last, they used quasars –lightning galaxies that lighten as a consequence of the absorbed matter through a supermassive blackhole in its nucleus– to obtain the map of the Universe from 11,000 million years ago and previous periods of time. To reveal the patterns of the Universe, they conducted an analysis of every measurement, in order to rule out potential pollutants.

“We measured the statistical properties of these maps of galaxies and deduced the rate at which the Universe expands over time”, says Santiago Ávila, from the Autonomous University of Madrid (UAM), who carried out new methods to simulate computer galaxy maps like the ones in this study. Ávila adds that “in combination with additional data from the microwave cosmic background and observations of
supernovas, we estimated the geometrical curve of the Universe is in fact, plain, and we measured the rate of local expansion with a precision over 1%”.

Following the path of SDSS, researchers are already working on the next generation of telescopes to reveal eBOSS. It will begin at the end of the year with the Dark Energy Spectroscopic Instrument (DESI), which will observe ten times more galaxies and quasars than eBOSS thanks to a new instrument in the Kitt Peak National Observatory (Arizona, United States). At the same time, the European Space Agency plans the launch of the Euclid satellite by 2022. This is the satellite with a unique telescope to provide a complementary view of the Universe. These
instruments, which count on participation from Spain, will provide data with a precision that has never been seen so far, which enables us to solve the enigma of the dark matter and discordance between the rate of expansion of the local and early Universe. Or, perhaps, they will reveal more surprises.

About the Sloan Digital Sky Survey

Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is www.sdss.org.

SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, the Chilean Participation Group, the French Participation Group, Harvard-Smithsonian Center for Astrophysics, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, the Korean Participation Group, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatório Nacional / MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University.

Links to the results

• Description of the project:
  https://www.sdss.org/surveys/eboss/
• Summary of SDSS/eBOSS data: https://www.sdss.org/science/final-bao-and-rsd-measurements/

• Summary of the cosmology interpretation:
  https://www.sdss.org/science/cosmology-results-from-eboss/

CONTACT:

Héctor Gil Marín

hectorgil@icc.ub.edu

675 43 63 06

Junior Leader La Caixa Fellow

Institute of Cosmos Sciences, University of Barcelona

• The detection of a Gamma ray burst by MAGIC telescopes allows us to study whether the speed of light in vacuum is a constant in nature.

• The results, published in the journal Physical Review Letters, show that photons of different energies emitted about 4.5 billion years ago reach Earth with a time difference of less than a minute, thus putting a limit on the hypothesis that the speed of photons depend on their energy.

• Researchers from the Institute for High Energy Physics (IFAE), the Autonomous University of Barcelona (UAB), the Institute of Cosmos Sciences of the University of Barcelona (ICCUB), the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas ( CIEMAT), the Universidad Complutense de Madrid (UCM) and the Instituto de Astrofísica de Canarias (IAC), are participating in the research.

In 2019, the MAGIC telescopes detected the first Gamma Ray Burst at very high energies [1, 2]. This was the most intense gamma-radiation ever obtained from such a cosmic object. But the GRB data have more to offer: with further analyses, the MAGIC scientists could now confirm that the speed of light is constant in vacuum – and not dependent on energy. So, like many other tests, GRB data also corroborate Einstein’s theory of General Relativity. The study has now been published in Physical Review Letters.

Einstein's general relativity (GR) is a beautiful theory which explains how mass and energy interact with space-time, creating a phenomenon commonly known as gravity. GR has been tested and retested in various physical situations and over many different scales, and, postulating that the speed of light is constant, it always turned out to outstandingly predict the experimental results. Nevertheless, physicists suspect that GR is not the most fundamental theory, and that there might exist an underlying quantum mechanical description of gravity, referred to as quantum gravity (QG). Some QG theories consider that the speed of light might be energy dependent. This hypothetical phenomenon is called Lorentz invariance violation (LIV). Its effects are thought to be too tiny to be measured, unless they are accumulated over a very long time. So how to achieve that? One solution is using signals from astronomical sources of gamma rays.

Gamma Ray Bursts, the most violent explosions in the universe

Gamma-ray bursts (GRBs) are powerful and far away cosmic explosions, which emit highly variable, extremely energetic signals. They are thus excellent laboratories for experimental tests of QG. The higher energy photons are expected to be more influenced by the QG effects, and there should be plenty of those; these travel billions of years before reaching Earth, which enhances the effect.

GRBs are detected on a daily basis with satellite borne detectors, which observe large portions of the sky, which allows them to detect and locate GRBs almost instantaneously when they occur, and send alerts to telescopes around the world, including MAGIC telescopes, to participate in their observation and study. On January 14, 2019, after receiving an alert from the GRBs detector on the Swift satellite, the MAGIC telescope system detected the first GRB in the domain of teraelectronvolt energies (TeV, 1000 billion times more energetic than the visible light), hence recording by far the most energetic photons ever observed from such an object. Multiple analyses were performed to study the nature of this object and the very high energy radiation.

Marc Ribó, Tenure track lecturer from the Institute of Cosmos Sciences (ICCUB) and Deputy Coordinator of Physics of the MAGIC Collaboration, tells us: "One of the most positive aspects revealed by the detailed study of the GRB190114 is that it is a more or less common GRB. This is good news because it means that we will probably detect more. Our detection opens a new phase in the search for LIV effects on cosmic gamma-ray sources observations.”

Naturally, the MAGIC scientists wanted to use this unique observation to hunt for effects of QG. At the very beginning, they however faced an obstacle: the signal that was recorded with the MAGIC telescopes decayed monotonically with time. While this was an interesting finding for astrophysicists studying how GRBs are produced, it was not favorable for LIV testing. Daniel Kerszberg, a researcher at IFAE in Barcelona said: “when comparing the arrival times of two gamma-rays of different energies, one assumes they were emitted instantaneously from the source. However, our knowledge of processes in astronomical objects is still not precise enough to pinpoint the emission time of any given photon”. Traditionally the astrophysicists rely on recognizable variations of the signal for constraining the emission time of photons. A monotonically changing signal lacks those features. So, the researchers used a theoretical model, which describes the expected gamma-ray emission before the MAGIC telescopes started observing. The model includes a fast rise of the flux, the peak emission and a monotonic decay like that observed by MAGIC. This provided the scientists with a handle to actually hunt for LIV.

Testing the quantum nature of space-time

A careful analysis then revealed no energy-dependent time delay in arrival times of gamma rays. Einstein still seems to hold the line. “This however does not mean that the MAGIC team was left empty handed”, said Giacomo D’Amico, a researcher at Max Planck Institute for Physics in Munich; “we were able to set strong constraints on the QG energy scale”. The limits set in this study are comparable to the best available limits obtained using GRB observations with satellite detectors or using ground-based observations of active galactic nuclei.

The limits on quantum gravity that have been obtained in this work are compatible with those already existing to date, and are the first to be obtained by observing the most energy GRB emission that can occur. With this pioneering study, the MAGIC team has established a starting point for future research in the search for measurable effects of the quantum nature of space-time.

In contrast to previous works, this was the first such test ever performed on a GRB signal at TeV energies. With this seminal study, the MAGIC team thus set a foothold for future research and even more stringent tests of Einstein’s theory in the 21st century. Oscar Blanch, spokesperson of the MAGIC collaboration, concluded: "This time, we observed a relatively nearby GRB. We hope to soon catch brighter and more distant events, which would enable even more sensitive tests."

MAGIC and the MAGIC collaboration

MAGIC (Major Atmospheric Gamma Imaging Cherenkov) is a system of two 17 meter diameter telescopes located at 2200 meters above sea level at the Observatorio El Roque de los Muchachos (ORM), in the Canary island of La Palma, Spain. The pioneering telescopes are designed to detect very high energy gamma-rays in the energy range from 30 GeV to more than 50 TeV, using the imaging atmospheric Cherenkov technique. The MAGIC telescopes are run by an international collaboration of around 280 people from 12 countries, including scientists, engineers, technicians and other staff.

The Spanish community has been involved in MAGIC since its inception. The members of MAGIC are currently Researchers from the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas ( CIEMAT), the Instituto de Astrofísica de Canarias (IAC), the Institute for High Energy Physics (IFAE), the Autonomous University of Barcelona (UAB), the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Universidad Complutense de Madrid (UCM). The Institut d’Estudis Espacials de Catalunya (IEEC) participates in this project through researchers from the ICCUB and CERES-UAB units. In addition, the MAGIC data center is the Port d'Informació Científica (PIC), a collaboration of the IFAE and the CIEMAT.

[1]: https://doi.org/10.1038/s41586-019-1750-x

[2]: https://doi.org/10.1038/s41586-019-1754-6

When the most massive stars die, they collapse under their own gravity and leave behind black holes; when stars that are a bit less massive die, they explode in supernovas and leave behind dense, dead remnants of stars called neutron stars. For decades, astronomers have been puzzled by a gap that lies between neutron stars and black holes: the heaviest known neutron star is no more than 2.5 times the mass of our sun, or 2.5 solar masses, and the lightest known black hole is about 5 solar masses. The question remained: does anything lie in this so-called mass gap?

Now, in a new study from the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector in Europe, scientists have announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap. The object was found on August 14, 2019, as it merged with a black hole of 23 solar masses, generating a splash of gravitational waves detected back on Earth by LIGO and Virgo. A paper about the detection has been accepted for publication in The Astrophysical Journal Letters.

Figure 1.This graphic shows the masses for black holes detected through electromagnetic observations purple), the black holes measured by gravitational-wave observations (blue), the neutron stars measured with electromagnetic observations (yellow), and the neutron stars detected through gravitational waves (orange). GW190814 is highlighted in the middle of the graphic as the merger of a black hole and a mystery object around 2.6 times the mass of the sun. Image credit: LIGO-Virgo/ Frank Elavsky & Aaron Geller (Northwestern)

"We've been waiting decades to solve this mystery" says Vicky Kalogera, a professor at Northwestern University. "We don't know if this object is the heaviest known neutron star, or the lightest known black hole, but either way it breaks a record."

"This is going to change how scientists talk about neutron stars and black holes," says co-author Patrick Brady, a professor at the University of Wisconsin, Milwaukee, and the LIGO Scientific Collaboration spokesperson. "The mass gap may in fact not exist at all but may have been due to limitations in observational capabilities. Time and more observations will tell."

The cosmic merger described in the study, an event dubbed GW190814, resulted in a final black hole about 25 times the mass of the sun (some of the merged mass was converted to a blast of energy in the form of gravitational waves). The newly formed black hole lies about 800 million light-years away from Earth.

Before the two objects merged, their masses differed by a factor of 9, making this the most extreme mass ratio known for a gravitational-wave event. Another recently reported LIGO-Virgo event, called GW190412, occurred between two black holes with a mass ratio of about 4:1.

"It's a challenge for current theoretical models to form merging pairs of compact objects with such a large mass ratio in which the low-mass partner resides in the mass gap. This discovery implies these events occur much more often than we predicted, making this a really intriguing low-mass object," explains Kalogera. "The mystery object may be a neutron star merging with a black hole, an exciting possibility expected theoretically but not yet confirmed observationally.

However, at 2.6 times the mass of our sun, it exceeds modern predictions for the maximum mass of neutron stars, and may instead be the lightest black hole ever detected."

When the LIGO and Virgo scientists spotted this merger, they immediately sent out an alert to the astronomical community. Dozens of ground- and space-based telescopes followed up in search of light waves generated in the event, but none picked up any signals. So far, such light counterparts to gravitational-wave signals have been seen only once, in an event called GW170817. The event, discovered by the LIGO-Virgo network in August of 2017, involved a fiery collision between two neutron stars that was subsequently witnessed by dozens of telescopes on Earth and in space. Neutron star collisions are messy affairs with matter flung outward in all directions and are thus expected to shine with light. Conversely, black hole mergers, in most circumstances, are thought not to produce light.

According to the LIGO and Virgo scientists, the August 2019 event was not seen by light-based telescopes for a few possible reasons. First, this event was six times farther away than the merger observed in 2017, making it harder to pick up any light signals. Secondly, if the collision involved two black holes, it likely would have not shone with any light. Thirdly, if the object was in fact a neutron star, its 9-fold more massive black-hole partner might have swallowed it whole; a neutron star consumed whole by a black hole would not give off any light.

"I think of Pac-Man eating a little dot," says Kalogera. "When the masses are highly asymmetric, the smaller neutron star can be eaten in one bite."

How will researchers ever know if the mystery object was a neutron star or a black hole? Future observations with LIGO and possibly other telescopes may catch similar events that would help reveal whether additional objects exist in the mass gap.

"This is the first glimpse of what could be a whole new population of compact binary objects," says Charlie Hoy, a member of the LIGO Scientific Collaboration and a graduate student at Cardiff University. "What is really exciting is that this is just the start. As the detectors get more and more sensitive, we will observe even more of these signals, and we will be able to pinpoint the populations of neutron stars and black holes in the universe."

"The mass gap has been an interesting puzzle for decades, and now we've detected an object that fits just inside it," says Pedro Marronetti, program director for gravitational physics at the National Science Foundation (NSF). "That cannot be explained without defying our understanding of extremely dense matter or what we know about the evolution of stars. This observation is yet another example of the transformative potential of the field of gravitational- wave astronomy, which brings novel insights with every new detection."

“Thanks to improvements in the Virgo / EGO observatory, in data analysis techniques and in dynamic astrophysical models, areas where the Institut de Ciències del Cosmos de la Universitat de Barcelona (ICCUB) has a relevant role, we hope to be able to detect more events like GW190814 that allow us to understand the exact nature of these intriguing astrophysical objects ”, explains Jordi Portell, coordinator of the Virgo group at ICCUB.

Additional information about the gravitational-wave observatories:

LIGO is funded by the NSF and operated by Caltech and MIT, which conceived of LIGO and lead the project. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Approximately 1,300 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 550 members from 106 institutes in 12 different countries including 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.

On April the 25th, 2019, the network of gravitational-wave (GW) detectors formed by the European Advanced Virgo, in Italy, and the two Advanced LIGO, in the US, detected a signal, named GW190425. This is the second observation of a gravitational-wave signal consistent with the merger of a binary-neutron-star system after GW170817. GW190425 was detected at 08:18:05 UTC; about 40 minutes later the LIGO Scientific Collaboration and the Virgo Collaboration sent an alert to trigger follow-up telescope observations.

The source of GW190425 is estimated to be at a distance of 500 million light years from the Earth. It is localized in the sky within an area about 300 times broader than was the case for the BNS observed by LIGO and Virgo in 2017, the famous GW170817, which gave birth to multi-messenger astrophysics. However, unlike GW170817, no counterpart (electromagnetic signals, neutrinos or charged particles) has been found to date.

There are a few explanations for the origin of GW190425. The most likely is the merger of a BNS system. Alternatively, it might have been produced by the merger of a system with a black hole (BH) as one or both components, even if light BHs in the mass-range consistent with GW190425 have not been observed. Yet, on the basis solely of GW data, these exotic scenarios cannot be ruled out. The estimated total mass of the compact binary is 3.4 times the mass of the Sun. Under the hypothesis that GW190425 originated from the merger of a BNS system, the latter would have been considerably different to all known BNS in our galaxy, the total mass range of which is between 2.5 and 2.9 times the mass of the Sun. This indicates that the NS system that originated GW190425 may have formed differently than known galactic BNSs.

"After the surprise of the initial results", says Alessandro Nagar of the Istituto Nazionale di Fisica Nucleare (INFN) of Turin, Italy, "we have finally reached a reliable understanding of this event. Although predicted theoretically, heavy binary systems like those that might have originated GW190425 may be invisible through electromagnetic observations."

"While we did not observe the object formed by the coalescence, our computer simulations based on general relativity predict that the probability that a BH is formed promptly after the merger is high, about 96%", says Sebastiano Bernuzzi of the University of Jena, Germany.

Gamma-ray bursts (GRBs) are brief and extremely powerful cosmic explosions, suddenly appearing in the sky, about once per day. They are thought to result from the collapse of massive stars or the merging of neutron stars in distant galaxies. They commence with an initial, very bright flash, called the prompt emission, with a duration ranging from a fraction of a second to hundreds of seconds. The prompt emission is accompanied by the so-called afterglow, a less brighter but longer-lasting emission over a broad range of wavelengths that fades with time. The first GRB detected by the MAGIC telescopes, known as GRB 190114C, reveals for the first time the highest energy photons measured from these objects.

This ground-breaking achievement by MAGIC provides critical new insight for understanding the physical processes at work in GRBs, which are still mysterious. The photons detected by MAGIC must originate from a process hitherto unseen in the afterglows of GRBs, clearly distinct from the physical process that is known to be responsible for their emission at lower energies.

MAGIC detection and multi-wavelength observations of GRB 190114C

On January 14th, 2019, a GRB was discovered independently by two space satellites: the Neil Gehrels Swift Observatory and the Fermi Gamma-ray Space Telescope. The event was named GRB 190114C, and within 22 seconds, its coordinates in the sky were distributed as an electronic alert to astronomers worldwide, including the MAGIC Collaboration, which operates two 17m diameter Cherenkov telescopes located in La Palma, Spain. Since GRBs appear at unpredictable locations in the sky and then rapidly fade, their observation by telescopes such as MAGIC requires a dedicated follow-up strategy.

An automatic system processes in real time the GRB alerts from satellite instruments and makes the MAGIC telescopes point rapidly to the sky position of the GRB. The telescopes were designed to be very light and capable of fast repointing: despite the weight of 64 tons each, they can reach and start observing any given position in the sky in just about 25 seconds. MAGIC was able to start the observation of GRB 190114C just 50 seconds after the beginning of the GRB.

The analysis of the resulting data for the first tens of seconds reveals emission of photons in the afterglow reaching teraelectronvolt (TeV) energies, that is, a trillion times more energetic than visible light. During this time, the emission of TeV photons from GRB 190114C was 100 times more intense than the brightest known steady source at TeV energies, the Crab Nebula. In this way, GRB 190114C became the record setter as the brightest known source of TeV photons. As expected for GRB afterglows, the emission faded quickly with time, similar to the afterglow emission that had been known at lower energies. The last glimpses were seen by MAGIC half an hour later.

For the very first time, the unambiguous detection of TeV photons from a GRB was announced by the MAGIC Collaboration to the international community of astronomers just a few hours after the satellite alerts, after a careful check of the preliminary data. This facilitated an extensive campaign of multi-wavelength (MWL) follow-up observations of GRB 190114C by over two dozens of observatories and instruments, providing a full observational picture of this GRB from the radio band to TeV energies. In particular, optical observations allowed a measurement of the distance to GRB 190114C. It was found that this GRB is located in a galaxy from which it took 4.5 billion years for the light to reach the Earth.

Highest energy photons from a newly revealed emission process

Although TeV emission in GRB afterglows had been predicted in some theoretical studies, it had remained observationally elusive for a long time, despite numerous searches at TeV energies over the past decades with various instruments, including MAGIC. What physical mechanism is behind the production of the TeV photons finally detected by MAGIC? Antonio Stamerra, the Deputy Spokesperson of the MAGIC collaboration, points out: "These energies are much higher than what can be expected from synchrotron radiation, caused by high-energy electrons spiraling in magnetic fields. This process is understood to be responsible for the emission that had been previously observed at lower energies in GRB afterglows. These new results, together with the very comprehensive MWL data, provide the first unequivocal evidence for an additional, distinct emission process in the afterglow”. Lara Nava, scientist associated with the MAGIC collaboration, adds: “ From our study, the most likely origin of the TeV emission is the so-called inverse Compton process, where a population of photons are significantly kicked up in energy by colliding with high energy electrons”.

“After more than 50 years since GRBs were first discovered, many of their fundamental aspects still remain mysterious”, says Razmik Mirzoyan, the Spokesperson of the MAGIC Collaboration. “The discovery of gamma-ray emission from GRB 190114C in the new, TeV window of the electromagnetic spectrum shows that the GRB explosions are even more powerful than thought before. The wealth of new data on GRB 190114C acquired by MAGIC and the extensive MWL follow-up observations now offer important clues to unravel some of the mysteries concerning the physical processes at work in GRBs”.

A comparative study of all previous GRB observations by MAGIC suggests that GRB 190114C was not a particularly unique event except for its relative proximity (light took 4.5 billion years to reach the Earth), and that the successful detection owes to the excellent performance of the instrument. "MAGIC has opened a new window to study GRBs", says Susumu Inoue, the coordinator of the MAGIC Transients working group most involved in the project. "Our results indicate we may be able to detect many more GRBs at TeV energies. This will pave the way for a much deeper understanding of these fascinating cosmic explosions."

Advanced Virgo and the two Advanced LIGO detectors have been taking science data continuously since the 1st of April, 2019, when they began their third observation period, named O3. Together, they form the most sensitive global gravitational-wave observatory to date.

During the past six months, the network has operated with all three of the interferometers active concurrently for 44% of the time. Candidate signals have been identified from as far as 17 billion light years away, as was the case with the candidate event of the 6th of July, 2019 (more information is available here).

O3 has proven very rich in terms of alerts. A total of 31 candidate events have been recorded so far, and the LIGO-Virgo Collaboration has issued several public alerts, which are freely accessible at the Gravitational Wave Candidate Event Database. The alerts facilitate follow-up observations by other telescopes (e.g. electromagnetic and neutrino) and enhance the extraordinary potential of multi-messenger observations, pioneered with the GW170817 event.

During the first and second observation runs - O1 and O2 - ten mergers of binary black holes and one merger of a binary neutron star were identified by the LIGO-Virgo Collaboration (more information here). Since the beginning of O3, the LIGO-Virgo Collaboration has identified approximately one binary merger candidate a week. Preliminary results suggest that the majority are mergers of binary black holes. Detailed analysis is ongoing to understand the properties of all the candidates.

"There have been so many triggers!", says Giuseppe Greco - post-doctoral researcher at the University of Urbino and collaborator at the Istituto Nazionale di Fisica Nucleare (INFN) in Italy - enthusiastically. "It was really exciting to issue so many candidate events. They triggered an extraordinary effort by scientists from all over the world".

"After the alerts are sent, work continues to fully assess whether they are true gravitational-wave detections and to extract all available physics information from the data. The quest for the discovery of a new type of source is extremely motivating", says Marie-Anne Bizouard, CNRS researcher at the Observatoire Côte d’Azur, in France, and Burst Source Group co-chair. "Compact binary system mergers are not the only gravitational-wave source the data analysis groups are working on, day and night, either."

Nicolas Arnaud, a CNRS researcher, currently seconded to the European Gravitational Observatory (EGO), also points out that, "Candidate gravitational-wave events keep the Detector Characterization Group permanently on the look-out. Alerts can pop up at any time, including nights and weekends. Each time Virgo is part of such a trigger, we need to quickly assess the quality of our data. This is one of the inputs required to decide whether the alert will go public or should be retracted. Shifters on duty for a week use tailored automated software and growing expertise to vet the events."

During O3, the interferometers have been left almost undisturbed, in order to maximise the amount of data collected. For Advanced Virgo, the only interruptions have been to allow for weekly maintenance and calibration and sum to an average of around 20 hours per week.

The Virgo and LIGO Scientific Collaborations have, however, agreed to pause O3 for a month, as of the 1st of October. For Advanced Virgo, the break will be used to improve the performance achieved during the O3 run, both in terms of sensitivity to gravitational-waves and duty cycle - the extent to which the interferometers are taking useful science data. O3 will then restart on the 1st of November and will run until the 30th of April, 2020.

"After six months of continuous data-taking, the interferometer needs a check-up", says Matteo Tacca, researcher at Nikhef in The Netherlands and the Virgo Commissioning Coordinator. "Acquiring data for such a long period is not only exciting for gravitational-wave searches, but it is also helpful from the instrumental point of view.

"We can analyse the data that have been produced by the instrument while it was undisturbed, in order to better understand its behaviour. We already have some useful indications for further studies, which may help to improve the instrument. During this break, we will make some hardware upgrades to fix a few issues we have found relating to the stability of the machine. We will also have the opportunity to hunt for some technical noise sources that impact upon the detector sensitivity."

When the O3 pause concludes at the start of November, Advanced Virgo and LIGO should be even better placed to continuously acquire useful science data for another 6 months.

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.

EGO-Virgo Media Contacts:

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LIGO Media Contacts:

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Ricardo Rodriguez

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IGFAE Outreach team

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