A jet detected from the merging of two neutron stars

An international team of astronomers, with the participation of researchers from the Institute of Astrophysics of Andalusia (IAA-CSIC), has demonstrated the existence of a jet of matter emerging from the merging of two neutron stars, moving at speeds close to that of light. Scientists have used radiotelescopes located on five continents for the detection. The results are published in the scientific journal Science.

"In August 2017, two neutron stars collided and merged into a single object, producing gravitational waves that were detected with the LIGO and Virgo observatories", explains Miguel Pérez-Torres, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who participates in the study. "The fusion of these two neutron stars -very dense stars, with masses similar to the Sun and the size of the city of Madrid- happened in a galaxy located 130 million light years from Earth. It was the first time that light was detected along all the wavelengths of the electromagnetic spectrum associated with a phenomenon related to the production of gravitational waves", says the researcher.

Astronomers have observed the event and its subsequent evolution throughout the entire electromagnetic spectrum, from gamma rays to radio waves. Two hundred days after the merging, the observations obtained by combining radio telescopes from Europe, Asia, Africa, Oceania and America have shown the existence of a jet of matter that emerged as a result of the merging, moving at speeds close to that of light.
"The fusion of these two neutron stars has allowed for the first time to correctly associate the detection of gravitational waves with one of the most powerful explosions in the universe, the gamma-ray bursts, confirming various scientific theories that have been under discussion for decades", says Iván Agudo, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who participates in the study.

After the merging, a significant amount of material was expelled into space, forming an envelope that has been observed by astronomers during all this time. However, there were several issues that astronomers could not solve with the data obtained so far. "We expected that some of this material would be ejected in the form of a jet moving at a speed close to that of light, but it was not clear if this jet could or could not pass through the envelope around the merger", explains Agudo.

EXPANDING BUBBLE OR JET OF MATTER
There were two possible scenarios: that the jet could not break the envelope and, therefore, created a feature similar to an expanding bubble, or that the jet broke the envelope and continued to move through space without the corset of the bubble. Only images in radio with a great sensitivity and detail could distinguish one case from the other. This required the use of very long-base interferometry (VLBI), a technique that  combines radiotelescopes located all around the Earth.

The authors of the study carried out observations of this region of the sky on March 12, 2018 using 32 radio telescopes belonging to the European VLBI network (EVN, which connects telescopes from Spain such as Yebes, in Guadalajara, Italy, Germany, Sweden, the Netherlands , Poland, Latvia, United Kingdom, Russia, China and South Africa), e-MERLIN in the United Kingdom, the Australian long-distance network (LBA, with antennas in Australia and New Zealand) and the very long-term network (VLBA) of U.S.

The data from all these telescopes were sent to the JIVE institute in the Netherlands, where they were combined to produce the final images, which reached such a level of detail to distinguish a person walking on the surface of the Moon. Following the same analogy, the existence of the expanding bubble would appear with the size of a truck on the Moon, while a jet would have a much smaller size. "Comparing the simulated and real images, we found that only the possibility of the jet was compatible with the observed object", explains Miguel Pérez-Torres (IAA-CSIC).

The team also determined that this jet showed as much energy as that produced by all the stars in our Galaxy for a whole year. "These results confirm the existence of a jet of particles that pierced the envelope and propagated at speeds close to that of light", adds Pérez-Torres (IAA-CSIC). In the following years, several of these mergers of two neutron stars will be discovered.
"The results obtained also suggest that more than 10% of these fusions should produce jets that get through the initial envelope and therefore could be observed," explains Iván Agudo. This type of observations will allow to clarify the physical processes that take place in one of the most powerful events that occur in the universe.