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The glow after the fusion of neutron stars was explained by a “superluminal” jet

K. Mooley et al. / Nature

The “superluminal” motion of the radio wave source, as well as the smooth growth and sharp decrease in the intensity of the radiation emitted after the fusion of two neutron stars (event GW170817), can be explained using the “cocoon + jet” scenario in which the jet is directed at an angle of about 20 degrees with respect to to the Earth and begins to dominate only 60 days after the merger. This scenario was confirmed by observations with an extremely long-range interferometer made by astronomers from the USA, Australia, Israel and Sweden. The article was published in  Nature .

On the morning of August 17, 2017 (08:41 on the time of the eastern coast of the USA), the gravitational observatories LIGO and Virgo registeredthe gravitational wave GW170817, which occurred when two massive objects merged. Just two seconds later, the Fermi and INTEGRAL telescopes recorded a bright burst of gamma radiation coming from the same sky area. Analysis of the data confirmed that the coincidence was not accidental – both signals were emitted when two neutron stars with a mass of about 1.1 and 1.6 times the mass of the Sun located at a distance of about 100 million light-years from the Earth merge. This is the first event in history, during which gravitational and electromagnetic components of the radiation of the same object were recorded. Before that, all registered gravitational waves always came “alone”, since their sources were mergers of black holes.

Understand what really happened after the merger of neutron stars, it was quite difficult. Astronomers have registered not only gamma radiation, but also radiation in the optical, ultraviolet, infrared, X-ray and radio bands. In particular, the intensity of radiation in the X-ray and radio bands continued to increase for another 150 days after the merger, and then relatively quickly came to naught.

To explain this “afterglow,” many theoretical models have been developed , including the “cocoon with a muted jet ” model and the “cocoon with structured jet” model. Preliminary data collected during the first 107 days after the merger by terrestrial radio telescopes VLA and ATCA and uGMRT satellite pointed to a “muffled” version; more about this result can be found in our  news “Radio emission from fused neutron stars strengthened” a cocoon “of stellar matter.” Nevertheless, the scientists noted in the article that the result obtained is not completely reliable, but it can easily be rechecked using an ultra-long-range radiointerferometer(VLBI) – a system of several ground-based telescopes and one space telescope, working synchronously.

Now the same group presented the result of the refined measurements. This time, astronomers used the HSA (High Sensitivity Array) system, which includes VLA and  GBT terrestrial telescopes , as well as ten VLBA telescopes . Using this system, scientists measured the observed position of the source two times, 75 and 230 days after the outbreak. It turned out that during the time elapsed between the measurements, the source center shifted by almost 0.003 arc seconds, which corresponds to the motion with the speed β ≈ 4 c , where c  is the speed of light. A similar “superluminal” motion occurs when the source moves with an ultrarelativistic velocity at an angle θ to the observation line: in the case whenthe gamma factor of the source is γ, the maximum observed source velocity can reach β = γ c if θ = 1 / γ. This speed is characteristic of the nuclei of active galaxies and microquasars, but very rarely is achieved by other objects. Until that moment, only one similar object was known to scientists – GRB 030329 gamma-ray burst, which had a speed of β ≈ 3-5 c .

Considering different scenarios after the fusion of neutron stars, numerically simulating them on a computer and comparing the results with observational data, scientists chose the option that was most likely realized in practice. It turned out that the best data obtained is explained by the “cocoon + jet” model, in which the radiation of the cocoon dominates for the first 60 days, and the relativistic jet breaking through the cocoon for the remainder of the time.The gamma-factor of the jet slowly decreases from γ = 4 on day 75 to γ ​​= 3 at 230 days; The angle of the jet to the axis of rotation of the object is approximately 4 degrees, and the angle of inclination to the line of observation is 20 degrees. The total energy released during the entire emission of radiation (both cocoon and jet) is 10 43  joules in order . Qualitatively, the model is similar to the “jet + cocoon” model (“structured jet”), described earlier, but differs from it in the angles of observation and opening of the jet. It is due to the small viewing angle that the apparent speed of the jet exceeds the speed of light.

The uniqueness of the event GW170817 made it possible to test a lot of theoretical models that predicted unusual effects – for example, with the help of it scientists confirmed that our space is four-dimensional, imposed a limiton the mass of gravitons and  refined the limiting mass of neutron stars. More information about the importance of this event for science, as well as the history of discovery can be found in the material “The Birth of Gold” .

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