New information about long gamma-rays shatters astrophysicists' theory

Until now, it was thought they came from massive star collapses.
Fabienne Lang
Artist illustration of the long gamma-ray burst
Artist illustration of the long gamma-ray burst

Aaron M. Geller/Northwestern/CIERA and IT Research Computing Services 

Astrophysicists around the world may be shocked to learn that long gamma-ray bursts (GRBs) do not solely come from the collapse of massive stars.

A new study by astrophysicists at Northwestern University upends the long-standing belief, uncovering new evidence that at least some long GRBs can result from neutron star mergers, which were previously believed to produce only short GRBs, the university's publication reported.

It all began in December 2021 when the team detected a 50-second-long GRB (any GRB longer than 2 seconds is considered 'long'). The team searched for the GRB's afterglow but instead, it uncovered evidence of a kilonova – a rare event that only occurs after the merger of a neutron star with another compact object (either another neutron star or a black hole), per Northwestern's team.

This discovery not only challenges long-standing beliefs around GRBs but also opens novel doors for the formation of the heaviest elements in the universe.

The team's study was published in the journal Nature on December 7, 2022.

GRBs, the beliefs around them, and the new study

“When I entered the field 15 years ago, it was set in stone that long gamma-ray bursts come from massive star collapses. This unexpected finding not only represents a major shift in our understanding, but also excitingly opens up a new window for discovery,”said Northwestern’s Wen-fai Fong, senior author of the study and assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences.

The team decided to study a bright burst of gamma-ray light, named GRB211211A, that had been picked up by the Neil Gehrels Swift Observatory’s Burst Alert Telescope and the Fermi Gamma-ray Space Telescope.

The team initiated imaging with the help of the Gemini Observatory in Hawaii and the MMT Observatory in Arizona. Once they received and after examining the near-infrared images captured by the observatories, the team spotted an incredibly faint object that quickly faded.

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Supernovae don’t fade as quickly and are much brighter, so the team realized it found something unexpected that was previously believed impossible.

“There are a lot of objects in our night sky that fade quickly,” Fong said. “We image a source in different filters to obtain color information, which helps us determine the source’s identity. In this case, red color prevailed, and bluer colors faded more quickly. This color evolution is a telltale signature of a kilonova, and kilonovae can only come from neutron star mergers.”

Host galaxy

The event wasn’t the only intriguing part of the study. The GRB’s host galaxy also piqued the astrophysicists' interest. Called SDSS J140910.47+275320.8, the host galaxy is young and star-forming, almost exactly opposite of the only other known local universe host of a neutron star merger event: GW170817’s host galaxy NGC4993. 

“After the detection of GW170817 and its association with a massive, red-and-dead host galaxy, many astronomers assumed that hosts of neutron star mergers in the near universe would look similar to NGC4993,” explained Anya Nugent, a Northwestern graduate student and study co-author.

“But this galaxy is fairly young, actively star forming and not actually that massive. In fact, it looks more similar to short GRB hosts seen deeper in the universe. I think it changes our view of the types of galaxies we should watch when we’re searching for nearby kilonovae," Nugent said.

Luckily, now that the James Webb Space Telescope (JWST) is up and running, astrophysicists can look for even more clues within kilonovae. The JWST can capture images of astronomical objects, meaning it can detect specific elements coming from the object – something ground-based telescopes aren't yet capable of capturing. Using the Webb, astrophysicists might obtain direct observational evidence of heavy elements’ formation.

This new study opens the path for further observations and clearly demonstrates that even long-standing and respected beliefs can be challenged.

Abstract:

The lack of bright supernovae rules out typical core-collapse explosion, but their distance scales prevent sensitive searches for direct signatures of a progenitor system. Only tentative evidence for a kilonova has been presenteD. Here we report observations of the exceptionally bright GRB 211211A, which classify it as a hybrid event and constrain its distance scale to only 346 megaparsecs. Our measurements indicate that its lower-energy (from ultraviolet to near-infrared) counterpart is powered by a luminous (approximately 1042 erg per second) kilonova possibly formed in the ejecta of a compact object merger.