The fusion of neutron stars results in a magnetar with the brightest kilonova ever observed

Long ago and far across the universe, a huge burst of gamma rays released more energy in half a second than the sun will produce during its entire 10 billion year lifespan.

After examining the incredibly bright burst with optical, radiographic, near-infrared, and radio wavelengths, an astrophysics team led by Northwestern University believe they have potentially spotted the birth of a magnetar.

Researchers believe the magnetar was formed by the fusion of two neutron stars, which has never been observed before. The fusion resulted in a brilliant kilonova – the brightest ever – whose light finally reached Earth on May 22, 2020. The light first came as a burst of gamma rays, called a short gamma-ray burst.

“When two neutron stars merge, the most common predicted result is that they form a heavy neutron star that collapses into a black hole in milliseconds or less,” said Wen-fai Fong of Northwestern, who has led the study. “Our study shows that it is possible that, for this particular short gamma-ray burst, the heavy object survived. Instead of collapsing into a black hole, it became a magnetar: a fast-spinning neutron star that has large magnetic fields, pouring energy into its surrounding environment and creating the very bright glow we see. ”

The research was accepted by the Astrophysics Journal and will be published online later this year.

Fong is Assistant Professor of Physics and Astronomy at the Weinberg College of Arts and Sciences in the Northwest and a member of CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics). The research involved two undergraduates, three graduate students, and three postdoctoral fellows from Fong’s lab.

‘A new phenomenon is happening’

After the light was first detected by NASA’s Neil Gehrels Swift Observatory, scientists quickly enlisted other telescopes – including NASA’s Hubble Space Telescope, Very Large Array, Observatory WM Keck and the Las Cumbres Observatory Global Telescope Network – to study the consequences of the explosion and its host galaxy.

Fong’s team quickly realized that something was wrong.

Compared to x-ray and radio observations, the near-infrared emission detected with Hubble was far too bright. In fact, it was 10 times brighter than expected.

“As the data came in, we formed a picture of the mechanism that produced the light we were seeing,” said study co-investigator Tanmoy Laskar of the University of Bath in the UK. “As we got the observations from Hubble, we had to completely change our thought process, because the information Hubble added made us realize that we had to let go of our conventional thinking and that there was a new phenomenon in progress. Then we had to figure out what it meant for the physics behind these extremely energetic explosions. “

Magnetic monster

Fong and his team discussed several possibilities to explain the unusual brightness – known as the short gamma-ray burst – that Hubble saw. Researchers believe that the short bursts are caused by the fusion of two neutron stars, extremely dense objects on the mass of the sun compressed into the volume of a large city like Chicago. While most short gamma-ray bursts likely end in a black hole, the two neutron stars that merged in this case may have combined to form a magnetar, a supermassive neutron star with a very strong magnetic field.

“You basically have these magnetic field lines that are anchored to the star that rotate at about 1,000 times per second, and that produces a magnetized wind,” Laskar explained. “These rotating field lines extract the rotational energy of the neutron star formed during fusion, and deposit that energy in the ejecta of the explosion, making the material glow even brighter.”

“We know that magnetars exist because we see them in our galaxy,” Fong said. “We believe that most of them formed during the explosive death of massive stars, leaving these highly magnetized neutron stars behind. However, it is possible that a small fraction forms in neutron star mergers. We’ve never seen evidence of this before, let alone. in infrared light, which makes this discovery special. “

Strangely Bright Kilonova

Kilonovae, which are typically 1,000 times brighter than a conventional nova, should accompany short gamma-ray bursts. Unique to the fusion of two compact objects, kilonovae glow from the radioactive decay of heavy elements ejected during fusion, producing coveted elements like gold and uranium.

“We have only one confirmed and well-sampled kilonova to date,” said Jillian Rastinejad, co-author of the article and graduate student of Fong’s lab. “So it’s especially exciting to find a potential new kilonova that looks so different. This discovery gave us the opportunity to explore the diversity of kilonovae and their remaining objects.”

If the unexpected brightness seen by Hubble was from a magnetar that deposited energy into the kilonova material, then in a few years the material ejected from the burst will produce light that appears at radio wavelengths. Tracking radio observations may ultimately prove it was a magnetar, leading to an explanation of the origin of these objects.

“Now that we have a very bright candidate, kilonova,” said Rastinejad, “I am excited about the new surprises that short gamma-ray bursts and neutron star mergers have in store for us in the future.”