Neutron Star Collision Observed In Millimeter Light For The First Time

The merger of two neutron stars has two crucial emissions: gravitational waves and a short burst of gamma rays. Such an energetic event can leave a powerful afterglow that astronomers can track. For the first time, researchers were able to capture this emission in millimeter light, and the gamma-ray burst (GRB) was one of the brightest on record.

The short-duration GRB, called GRB 211106A, was spotted in X-rays and the team was able to follow it up with the Atacama Large Millimeter/submillimeter Array (ALMA). As reported in The Astrophysical Journal Letters, the afterglow was one of the most energetic on record, with the upper limit for the energy being five times that the Sun will release over its whole lifetime.

“This short gamma-ray burst was the first time we tried to observe such an event with ALMA,” said Northwestern’s Professor Wen-fai Fong, principal investigator of the ALMA program. 

“Afterglows for short bursts are very difficult to come by, so it was spectacular to catch this event shining so brightly. After many years of observing these bursts, this surprising discovery opens up a new area of study, as it motivates us to observe many more of these with ALMA and other telescope arrays in the future.”

The event took place in a galaxy 20 billion light-years away, well beyond the capabilities of gravitational wave detectors. Still many of our light detectors, from gamma-rays to radio waves, were able to follow up this event. In gamma-rays, the bursts last for just a few tenths of a second but in other wavelengths, the effect can be detectable even days after the burst was emitted.

“These mergers occur because of gravitational wave radiation that removes energy from the orbit of the binary stars, causing the stars to spiral in toward each other,” lead author Dr Tanmoy Laskar, an Excellence Fellow at Radboud University, explained. 

“The resulting explosion is accompanied by jets moving at close to the speed of light. When one of these jets is pointed at Earth, we observe a short pulse of gamma-ray radiation or a short-duration GRB.”

Only half a dozen short GRBs have been detected in radio waves, so the first detection in millimeter wavelengths is important. On its own, the measurement gives information about the environment around the jet. Together with X-rays, it allows for an estimate of the energy of the event.

With radio waves, it is possible to estimate the opening angle of the jet. That provides a measurement of the rates of these events in the universe. Visible light observations with Hubble showed the field of galaxies where it might have come from, but it was ALMA that allowed the team to pinpoint the short GRB to a specific one.

“The Hubble observations revealed an unchanging field of galaxies,” Laskar added. “ALMA’s unparalleled sensitivity allowed us to pinpoint the location of the GRB in that field with more precision, and it turned out to be in another faint galaxy, which is further away. That, in turn, means that this short-duration gamma-ray burst is even more powerful than we first thought, making it one of the most luminous and energetic on record.”