Scientists detect the optical counterpart of the latest gravitational waves of LIGO / Virgo

A team of scientists, using the Dark Energy Camera (DECam), the main observation tool of the Dark Energy Survey (DES), has registered one of the first optical images of the collision of two neutron stars, discovered by the collaborations LIGO and Virgo by observing gravitational waves. It is the first time that a well-confirmed collision between two neutron stars is detected and it is also the first time that a cosmic source is detected simultaneously in gravitational and electromagnetic waves.

DES scientists joined forces with a team of astronomers based at the Smithsonian Center for Astrophysics (CfA) in Harvard, and together they have worked using several observatories around the world to confirm their original data. The images taken with DECam captured the flash of one kilonova -a explosion similar to a supernova, but on a smaller scale-that occurs when two collapsed stars (called neutron stars) collide with each other, creating heavy radioactive elements.

This particularly violent fusion occurred 130 million years ago in a galaxy near ours (NGC 4993), it is the source of the gravitational waves detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo collaborations on 17 August. This is the fifth source of gravitational waves detected. The first one was discovered in September 2015, whereby the three founding members of the LIGO collaboration were awarded the Nobel Prize in Physics two weeks ago.

This last event supposes the first detection of gravitational waves caused by the collision of two neutron stars and, consequently, the first one that has a visible source. The previous detection was due to the collision of two black holes, which can not be observed with telescopes. This collision of neutron stars occurred relatively close to Earth, so that within a few hours after receiving the news from LIGO / Virgo, scientists were able to point their telescopes in the direction of the event and get an image clear of the light emitted in the collision.

One of the most important records of this kilonova was obtained with DECam, the main instrument of the DES project. This is one of the most powerful digital imaging devices that exist. It was built and tested at Fermilab, and is mounted on the White Telescope, 4 meters, belonging to the National Science Foundation, and located at Cerolo Tololo Observatory in Chile. The images of DES are processed at the National Center for Supercomputing Applications of the University of Illinois, in Urbana-Champaign. DES collaboration has an important participation of Spanish scientists and engineers, who have responsibilities in all aspects, from science to the maintenance of measurement instruments.

The scientists of LIGO / Virgo work with dozens of astronomical collaborations around the world, including DES, and who have the role of providing images of the areas of the sky where the gravitational waves detected are originated. The DES and CfA team has been preparing for an event like this for more than two years, forging connections with other astronomical collaborations and launching a protocol to mobilize rapidly every time a new source is detected. In this way, after a few hours of receiving information about the location in the sky, the team had reserved time in several observatories, including the NASA Hubble Space Telescope and the Chandra X-ray space observatory. The result is a very rich data set that covers the entire electromagnetic spectrum, from radio waves to X-rays.

To add even more emotion to observation, this latest gravitational wave detection correlates with a gamma ray explosion detected by NASA's Fermi Space Telescope and later in X-rays by the ESA Comprehensive Telescope. The combination of all these detections is like seeing a ray and listening to the corresponding thunder for the first time, and opens a world of new scientific discoveries.

This event also provides a unique and completely new way of measuring the pace of expansion of the universe, the Hubble constant. As astrophysicists use supernovae as standard candles (objects with a known intrinsic brightness) to measure cosmic expansion, kilonovas can be used as standard sirens (objects whose intensity in gravitational waves is known). LIGO / Virgo scientists can use this fact to measure the distance to these events, while optical tracking with DES and other telescopes results in red shift or recession speed. The combination of both measures allows scientists to determine the current pace of expansion. This new type of measure is complementary to others that do DES in their mission to advance in the understanding of dark energy, the mysterious substance responsible for the current acceleration in the expansion of the universe.

According to Juan García-Bellido, one of the people in charge of the kilonova analysis in DES, "the group of gravitational waves of the mapped DES has been working for at least two years for the optical tracking of an event like this. Hours after the collision of the two neutron stars, DECam independently discovered the source in the visible and near infrared in the galaxy NGC4993, from which we know very well its position in the sky and its displacement to the red, which has Allowed, among other things, to determine the rhythm of expansion of the universe. It's exciting to see live how 70 different experiments are coordinated to be able to make an accurate measurement of one of the most violent events in the universe, one kilonova or short gamma-ray burst. "

DES recently began the fifth and last year of its mission to map a very wide area of ​​the southern sky with an unprecedented detail. DES scientists will use this data to learn more about the effect of dark energy over the last eight billion years of history in the universe, and in this process will measure 300 million galaxies, 100,000 galaxy clusters and 3,000 supernovae .

The DES-Spain group, formed by CIEMAT, IEEC / CSIC, IFAE and UAM / IFT, has contributed to building DECam, the camera with which these observations have been made. In particular, he designed, constructed and validated the electronics, and set up the guiding system, among other contributions. He has also supported the optical tracking program of gravitational waves, participates in the scientific analysis and publications associated with this discovery and is one of the founding partners of DES collaboration, with funding from MINECO, IEEC, CSIC and Generalitat de Catalunya. Catalonia.

The Dark Energy Survey is a collaboration of more than 400 scientists from 26 institutions in seven countries. Funds for DES projects have been provided by the U.S. Department of Energy Office of Science, U.S. National Science Foundation, Ministry of Economy, Industry and Competitiveness of Spain, Science and Technology Facilities Council of the United Kingdom, Higher Education Funding Council for England, ETH Zurich for Switzerland, National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign , Kavli Institute of Cosmological Physics at the University of Chicago, Center for Cosmology and AstroParticle Physics at Ohio State University, Mitchell Institute for Fundamental Physics and Astronomy at Texas A & M University, Funding for Studies and Projects, Fundação Carlos Chagas Filho de Amparo à Pesquisa do State of Rio de Janeiro, National Council for Scientific and Technological Development and Ministry of Science and Technology, Deutsche Forschungsgemeinschaft, and collaborating institutions, whose list can be found at http://www.darkenergysurvey.org/collaboration.

Image

LEFT: Image of the NGC4993 galaxy taken by DES (Dark Energy Survey) on August 18, 2017, in which the arrow points to the position of the first optical detection of the explosion (Kilonova) resulting from the shock of two neutron stars . This explosion was detected hours before as a gravitational wave by the interferometers LIGO and VIRGO and as a gamma-ray outbreak by the Fermi satellite. RIGHT: Image of the same galaxy taken 14 days later, in the no longer sees trail of the explosion. The institutions of DES-Spain have contributed to the construction of the chamber (DECam) with which these observations have been made, participating in the publications of this discovery and are founding partners of DES collaboration.

Contacts

IEEC-CSIC
Dr. Enrique Gaztañaga, Research Professor at the CSIC, gazta@ice.csic.es

IFAE
Dr. Ramon Miquel, Director of the IFAE and Professor of Research ICREA, ramon.miquel@ifae.es

CIEMAT
Dr. Eusebio Sánchez, CIEMAT Scientific Researcher, eusebio.sanchez@ciemat.es

IFT-UAM / CSIC
Dr. Juan García-Bellido, Professor of the UAM and member of the IFT, juan.garciabellido@uam.es