The merging of two neutron stars allows the first simultaneous study in light and gravitational waves

After centuries of studying the universe in electromagnetic waves - what we call light -, the detection in 2015 of gravitational waves opened a new window to the cosmos. The origin of this new emission was in the fusion of two black holes, objects that do not emit light and can only be studied through its gravitational influence. Now, an international study has allowed, for the first time, to observe an object in light and gravitational waves: a merging of two neutron stars that has inaugurated a new era in the observation of the universe.

A UNIQUE PHENOMENON

On August 17, at 12:41:04, LIGO instrument detected the transient event of gravitational waves named GW170817, the fifth in history. Two seconds later, Fermi and INTEGRAL satellites detected a gamma-ray burst (GRB), which research groups from around the world began to study, including three from the Institute of Astrophysics of Andalusia (IAA-CSIC). As a result, numerous articles are published this week that provide a complete picture of the phenomenon, occurring in the galaxy NGC 4993, about one hundred and thirty million light-years.

"After the detection of optical light with the robotic telescope Javier Gorosabel at the Spanish station BOOTES-5 (Mexico), we participated in an observation campaign that allowed us to study the phenomenon for fifteen days covering from ultraviolet to near-infrared -says Alberto Castro-Tirado, principal investigator of the ARAE group (Robotic Astrophysics and High Energies) of the Institute of Astrophysics of Andalusia (IAA-CSIC) that participates in three articles on the event. This way could be identified the kilonova associated with the gravitational wave emitting source in the galaxy NGC 4993, and whose origin is in the merging of two neutron stars".

Neutron stars are very compact, fast-rotating objects that emerge when a very massive star ejects its envelope in a supernova explosion. "Nearly three decades ago a fusion of two neutron stars was predicted to produce a short burst of gamma rays (GRB), gravitational waves and a kilonova, a phenomenon similar to a supernova but whose energy comes in part from the decay of radioactive species. Thanks to the studies of GW170817 this scenario has been confirmed" says Christina Thöne, co-principal investigator of the IAA-CSIC HETH group (High Energy Transients and their Hosts), which participates in six articles on GW170817.

THE ORIGIN OF HEAVY ELEMENTS

This phenomenon has also made possible to establish a clear relationship between the merging of neutron stars and the production of chemical elements. Virtually all the chemical elements that we know have an astronomical origin, and they occurred either in stages very close to the big bang, in which the hydrogen and the helium were formed, or in the stars, both through the fusion of elements in the nucleus (producing carbon, nitrogen or iron) and explosive events (where lead or copper are generated).

However, there are discrepancies about the so-called r-process (or rapid process), which takes place in explosive stellar events and is responsible for the production of half of the elements heavier than iron, including uranium and the plutonium. Although supernovas were originally believed to be the source of these elements, the latest studies favored neutron star mergers as the main producers of the heaviest elements.

"We have observed what could be considered the last message of two stars that possibly died about ten billion years ago, but which has allowed us to study the heavy elements that form in these environments and confirm that the neutron star mergers are one of the sources of the elements of the r-process", says the researcher.

The discovery and study of both the gravitational waves and light of this phenomenon has revealed many of the physical processes involved and establish a unique body of knowledge of a celestial object: gravitational waves have revealed the mass, rotation, distance and position in the sky, while electromagnetic waves have allowed to study its surroundings (an aging galaxy that possibly merged with another in its recent past), as well as the hydrodynamics and the formation of elements in the ejected material. Even this study has allowed us to obtain an independent value of the Hubble constant, which measures the rate of expansion of the universe.

"The electromagnetic counterpart was exceptionally weak for an event of this type, and we were very lucky to detect it. This discovery is one of the most important in astronomy in probably a decade", says Antonio de Ugarte, co-investigator principal of the HETH group of the IAA-CSIC involved in the studies.

The Institute of Astrophysics of Andalusia has taken part in different observational campaigns of this object, that cover practically all the wavelengths and use the most advanced astronomical facilities, among them the European VLBI Network (through the group RJB, Relativistic jets and blazars), the Hubble space telescope, the Chandra satellite or the Very Large Telescope.

GRAVITATIONAL WAVES

Gravitational waves are undulations in the structure of spacetime, the "fabric" that makes up the universe and which we can imagine as a tensile elastic mesh. A mesh that, in the presence of matter, curves. This curvature in the geometry of spacetime due to the presence of matter is the cause of the gravitational effects that govern the movement of bodies (both that of the planets around the Sun and that of galaxy clusters).

Einstein predicted, in his general theory of relativity (1916), the existence of gravitational waves, a phenomenon associated with objects that generate the most extreme gravitational environments, such as binary systems of black holes and neutron stars. These systems would generate distortions in spacetime that, like the waves produced by a stone in the water, propagate from the origin to the speed of light, carrying valuable information about the objects that produce the waves and about the nature of gravity .

Currently there are two large facilities dedicated to the search and analysis of gravitational waves, LIGO in the United States, and Virgo in Italy. The first two detections of gravitational waves were made by LIGO, while the third was the result of the LIGO-Virgo collaboration. "Now, the challenge is to add more detections of gravitational wave sources, but also to find their light counterparts. In this sense, my research group at the IAA has signed a unique collaboration agreement in Spain to detect these counterparts", says Alberto Castro-Tirado (IAA-CSIC).

Twelve researchers from the Institute of Astrophysics of Andalusia have participated in the study of GW170817: Alberto Castro-Tirado, Binbin Zhang, Juan Carlos Tello, Youdong Hu y Ronan Cunniffe (ARAE - Robotic Astrophysics and High Energies -); Christina Thöne, Antonio de Ugarte, Alex Kann, Luca Izzo, Zach Cano y Gabriella Hodosan (HETH - High Energy TransientS and their Hosts -); Iván Agudo (RJB -Relativistic Jets and blazars-).