Brief gamma-ray bursts traced further into the distant universe

Brief gamma-ray bursts traced further into the distant universe

A team of astronomers led by Northwestern University has compiled the most comprehensive inventory yet of the galaxies that produce short gamma-ray bursts (SGRBs).

Using multiple highly sensitive instruments and sophisticated galaxy modelling, researchers located the galactic home of 84 SGRBs and examined the properties of 69 of the identified host galaxies. Among other things, they discovered that about 85% of the examined SGRBs come from young, actively star-forming galaxies.

The astronomers also found that more SGRBs appeared earlier when the Universe was much younger – and at greater distances from the centers of their host galaxies – than previously known. Surprisingly, several SGRBs have been sighted far outside their host galaxies – as if they had been “kicked out,” a finding that begs the question of how they were able to travel so far away.

“This is the largest catalog of SGRB host galaxies that has ever existed, so we expect it to be the gold standard for many years to come,” said Anya Nugent, a Northwest PhD student who led the study, which focused on the modeling of host galaxies. “Creating this catalog and finally having enough host galaxies to see patterns and draw significant conclusions is exactly what the field needed to advance our understanding of these fantastical events and what happens to stars after they die.” “

The team published two articles detailing the new catalog today (Nov. 21) in The Astrophysical Journal. Because SGRBs are among the brightest explosions in the universe, the team calls their catalog BRIGHT (Broadband Repository for Investigating Gamma-ray burst Host Traits). All BRIGHT data and modeling products are publicly available online for community use.

Nugent is a graduate student in physics and astronomy at Weinberg College of Arts and Sciences in Northwestern and is a member of the Center for Interdisciplinary Investigation and Research in Astrophysics (CIERA). She is being advised by Wen-fai Fong, an assistant professor of physics and astronomy at Weinberg and a key member of CIERA, who led a second study focused on SGRB host observations.

Benchmark for future comparisons

When two neutron stars collide, they produce short bursts of intense gamma-ray light known as SGRBs. While gamma rays last only a few seconds, optical light can last for hours before dropping below the limit of detection (an event called the afterglow). SGRBs are among the most luminous explosions in the universe, with at most a dozen being detected and located each year. They currently represent the only way to study and understand a large population of merging neutron star systems.

Since NASA’s Neil Gehrel’s Swift Observatory first spotted an SGRB afterglow in 2005, astronomers have spent the past 17 years trying to understand which galaxies produce these strong flares. Stars within a galaxy can provide insight into the environmental conditions required for the production of SGRBs and may link the mysterious outbursts to their origins in neutron star mergers. So far, only one SGRB (GRB 170817A) has a confirmed neutron star merger origin – as it was discovered just seconds after the binary neutron star merger (GW170817) was observed by gravitational wave detectors.


Number of gamma-ray bursts in new record catalog

“In a decade, the next generation of gravitational-wave observatories will be able to detect neutron star mergers at the same distances as we do with SGRBs today,” Fong said. “Thus, our catalog serves as a benchmark for future detections of neutron star mergers.”

“The catalog can really make an impact beyond a single class of transients like SGRBs,” said Yuxin “Vic” Dong, study co-author and astrophysics graduate student at Northwestern. “Given the wealth of data and results presented in the catalogue, I believe a wide variety of research projects will make use of it, perhaps in ways we have not yet thought of.”

Insight into neutron star systems

To create the catalog, the researchers used multiple high-sensitivity instruments at the WM Keck Observatory, the Gemini Observatories, the MMT Observatory, the Large Binocular Telescope Observatory, and the Magellan Telescopes at Las Campanas Observatory to capture deep images and spectroscopy of some of the faintest galaxies in identified from survey of SGRB hosts. The team also used data from two of NASA’s major observatories, the Hubble Space Telescope and the Spitzer Space Telescope.

“This is the largest catalog of SGRB host galaxies that has ever existed, so we expect this to be the gold standard for many years to come.” —Anya Nugent, PhD student in astrophysics

Before these new studies, astronomers characterized host galaxies of just a few dozen SGRBs. The new catalog includes four times the number of existing samples. With the benefit of a much larger dataset, the catalog shows that SGRB host galaxies can be either young or star-forming gold old and near death. This means that neutron star systems form in a variety of environments, and many of them have short timescales from formation to merger. Because neutron star mergers produce heavy elements such as gold and platinum, the catalog’s data will also improve scientists’ understanding of when precious metals were first formed in the Universe.

“We suspect that the younger SGRBs we’ve found in younger host galaxies come from binary star systems that formed in a ‘burst’ of star formation and are so closely linked that they can merge very quickly,” Nugent said. “Long-standing theories suggest that there must be ways to rapidly merge neutron stars, but until now we have not been able to observe them. We find evidence of older SGRBs in much older galaxies and believe that the stars in these galaxies either took longer to form a binary star or were a binary star system that was further separated. So it took longer to merge them.”

potential of JWST

With the ability to detect the faintest host galaxies from very early times in the Universe, NASA’s new infrared flagship observatory, the James Webb Space Telescope (JWST), is poised to further advance the understanding of neutron star mergers and their temporal past started.

“I am excited to have the opportunity to use JWST to dig deeper into the causes of these rare, explosive events,” said Nugent. “JWST’s ability to observe faint galaxies in the Universe could uncover additional SGRB host galaxies currently evading detection, and perhaps even reveal a missing population and connection to the early Universe.”

“I started observing for this project 10 years ago, and it’s been so satisfying to be able to pass the torch on to the next generation of researchers,” Fong said. “It is one of the greatest joys of my career to see years of work come to life in this catalog thanks to the young researchers who really took this study to the next level.”

The Short GRB host galaxies I and Short GRB host galaxies II studies were supported by the National Science Foundation (award numbers AST-1814782 and AST-2047919), the David and Lucile Packard Foundation, the Alfred P. Sloan Foundation, and the Research Corporation for Scientific Advancement.

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