The merger of neutron stars produces a magnetar with the brightest kilonova ever observed

Long ago and throughout the universe, a huge explosion of gamma rays released more energy in half a second than the sun will produce during its 10 billion years of life.

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

Researchers believe the magnetar was formed by the merger of two neutron stars, something that has never been observed before. The merger produced a brilliant kilonova, the brightest ever seen, whose light finally reached Earth on May 22, 2020. The light first came as a gamma-ray burst, called a short gamma-ray burst.

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

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

Fong is an assistant professor of physics and astronomy at Weinberg College of Arts and Sciences in Northwestern and a member of the CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics). The research involved two undergraduate students, three graduate students and three postdoctoral fellows from Fong’s laboratory.

‘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, WM Keck Observatory, and 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-rays and radio observations, the near-infrared emission detected with Hubble was too bright. In fact, it was 10 times brighter than expected.

“As the data was coming in, we were forming an image of the mechanism that was producing the light we were seeing,” said study co-investigator Tanmoy Laskar of the University of Bath in the UK. “When we got the Hubble observations, we had to completely change our thinking process, because the information Hubble added made us realize that we needed to discard our conventional thinking and that there was a new phenomenon going on. Then we have had to figure out what that 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 a brief gamma-ray burst, that Hubble saw. Researchers think that short bursts are caused by the merger of two neutron stars, extremely dense objects about the mass of the sun compressed into the volume of a large city like Chicago. While most of the short gamma-ray bursts likely result 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 powerful magnetic field.

“Basically you have these magnetic field lines that are anchored to the star that are moving at around 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 in the fusion and deposit it in the ejecta of the explosion, making the material shine even brighter.”

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

Oddly bright Kilonova

Kilonovae, which are typically 1,000 times brighter than a classic nova, should accompany short bursts of gamma rays. Unique to the fusion of two compact objects, kilonovae glow with the radioactive decay of heavy elements ejected during fusion, producing coveted elements such as gold and uranium.

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

If the unexpected brightness seen by Hubble came from a magnetar that deposited energy into the kilonova material, then, within a few years, the material ejected from the burst will produce light that occurs at radio wavelengths. Follow-up radio observations may eventually show that it was a magnetar, leading to an explanation of the origin of such objects.

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