Scientists confirm that two neutron stars merging into each other do not only generate gravitational waves. They also emit short gamma-ray bursts.
In 2017, an international collaboration of astronomers announced that they detected gamma-ray flashes associated with the coalescence of two neutron stars. The announcement was exactly what the Oregon team predicted a few months earlier.
In a new paper published in the Physical Review Letters, astrophysicists at the Oregon State University explore the connections that link gamma rays to neutron star mergers and gravitational waves.
What Are Gamma Rays?
Gamma rays are electromagnetic waves with the shortest wavelength in the spectrum. They are the most powerful waves and can generate as much energy in a couple of seconds as the sun can in tens of billions of years.
Gamma rays are classified into long duration and short duration waves. The former is released when a massive star dies, which leads to a black hole, and lasts anywhere from two seconds to a few minutes. The latter has long been surmised to come from the union of neutron stars and are said to last up to a couple of seconds.
Neutron stars are extremely dense compact stars that were once the center of a massive star. If one were to cut a piece of a neutron star the size of a sugar cube, it would have a mass heavier than a billion tons.
Neutron Star Mergers Cause Short Gamma-Ray Bursts
Scientists working at the Laser Interferometer Gravitational-Wave Observatory and the Virgo team at European Gravitational Observatory spotted gravitational waves following the collision of neutron stars in August 2017. Gravitational waves, a breakthrough discovery made in 2015, are ripples in the fabric of space-time that are caused by an enormous release of energy.
Three months after the discovery, the collaborators announced that X-ray/gamma-ray flashes that were associated with the release of gravitational waves and light from the preceding kilonova.
Davide Lazzati, a theoretical astrophysicist at the College of Science, says the discovery could allow scientists to investigate neutron stars and gravitational waves in new ways.
"The gamma rays allowed for a precise localization of where the gravitational waves are coming from," Lazzati explained.
The announcement is an affirmation of what Lazzati and his team predicted in the summer of 2017: that the merger of two neutron stars generated gamma rays and they could be detected even if the beams did not point toward the Earth.
Prediction Comes To LIfe
Gamma rays are beams of electromagnetic waves that are parallel to each other. One can liken one ray to the light of a lighthouse. There is no difficulty spotting it if the beam is pointed directly toward the observer.
On the other hand, gravitational waves point in all directions. They can easily be detected from whatever reference point.
Lazzati's team postulated that short gamma-ray bursts create a cocoon around the original beam. The cocoon emits wavelengths that are not as strong as the first beam, but it can be detected by an observer even if the beam does not point directly toward them.
In the observations in the following months, the LIGO and Virgo teams detected behavior that is consistent with the prediction.
"More radiation came after the burst of gamma rays: radio waves and X-rays," Lazzati said. "The observation is exactly the behavior we predicted."