A team of astronomers led by Northwestern University has developed the most comprehensive inventory to date of the galaxies where short gamma-ray bursts (SGRBs) originate.
Using several highly sensitive instruments and advanced galaxy modeling, the researchers determined the galactic homes of 84 SGRBs and examined the characteristics of 69 of the identified host galaxies. Among their findings, they found that about 85% of the SGRBs studied come from young, actively star-forming galaxies.
The astronomers also found that more SGRBs occurred in earlier times, when the universe was much younger — and at greater distances from the centers of their host galaxies — than previously known. Surprisingly, several SGRBs were spotted far beyond their host galaxies — appearing to have been “thrown out,” a finding that raises questions about how they were able to travel so far away.
“This is the largest catalog of SGRB host systems ever, so we expect this to be the gold standard for many years to come,” he said. Anya Nugent, a graduate student from the Northwest who led the study, focused on modeling host systems. “Building this catalog and finally having enough host galaxies to see patterns and draw significant conclusions is exactly what the field needed to further our understanding of these fantastic events and what happens to stars after they die.”
The team published two papers today (Nov. 21) in The Astrophysical Journal describing the new catalog. Because SGRBs are among the brightest explosions in the universe, the team calls its catalog BRIGHT (Broadband Repository for Investigating Gamma-ray Burst Host Traits). All of BRIGHT’s data and modeling products are publicly available online for community use.
Nugent is a graduate student in physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences and a member of the Center for interdisciplinary research and research in astrophysics (CIERA). She is advised by Wen-fai Fongan assistant professor of physics and astronomy at Weinberg and a key member of CIERA, who led a second study focusing on SGRB host observations.
Benchmark for future comparisons
When two neutron stars collide, they generate momentary flashes of intense gamma rays known as SGRBs. While the gamma rays last only seconds, the optical light can last for hours before fading below detection levels (an event called afterglow). SGRBs are some of the brightest explosions in the universe, with at most a dozen 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 Gehrels Swift Observatory first discovered a SGRB afterglow in 2005, astronomers have spent the past 17 years trying to understand which galaxies produce these powerful outbursts. Stars in a galaxy could provide insight into the environmental conditions required to produce SGRBs and link the mysterious outbursts to their neutron star fusion origins. So far, only one SGRB (GRB 170817A) has a confirmed origin of the neutron star merger – as it was detected just seconds after gravitational wave detectors observed the binary neutron star merger (GW170817).
number of gamma-ray bursts in new record-breaking catalog
“A decade from now, the next generation of gravitational wave observatories will be able to detect neutron star mergers at the same distances as we do SGRBs today,” Fong said. “Thus, our catalog will serve as a benchmark for comparison with future detections of neutron star mergers.”
“The catalog could really have an impact beyond just a single class of transients like SGRBs,” he said Yuxin “Vic” Dong, study co-author and astrophysics Ph.D. student at Northwest. “With the wealth of data and results presented in the catalog, I think several research projects will make use of it, perhaps even in ways we haven’t thought of yet.”
Understanding neutron star systems
To create the catalog, the researchers used several highly sensitive instruments from 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 imaging and spectroscopy of some of the faintest galaxies. identified in the survey of SGRB hosts. The team also used data from two of NASA’s Great Observatories, the Hubble Space Telescope and Spitzer Space Telescope.
“This is the largest catalog of SGRB host systems ever, so we expect this to be the gold standard for many years to come.” – Anya Nugent, astrophysics graduate student
Prior to these new studies, astronomers characterized host galaxies of only a few dozen SGRBs. The new catalog is four times the number of existing samples. With the advantage of a much larger dataset, the catalog shows that SGRB host galaxies can be both young and star-forming or old and imminent death. This means that neutron star systems form in a wide variety of environments and many of them have a fast time frame from formation to merging. Because mergers of neutron stars create heavy elements such as gold and platinum, the catalog’s data will also give scientists more insight into when precious metals were first created in the universe.
“We suspect that the younger SGRBs we found in younger host galaxies come from binary stellar systems formed in a ‘burst’ of star formation and bound so tightly that they can merge very quickly,” Nugent said. “Long-standing theories have suggested that there must be ways to rapidly merge neutron stars, but so far we have not been able to observe them. We find evidence for older SGRBs in the galaxies that are much older and believe that the stars in those galaxies either took longer to form a binary system or were a binary system that was more separated, hence it took longer to merge them.”
Potential of JWST
With the ability to detect the faintest galaxies from very early times in the universe, NASA’s new flagship observatory, the James Webb Space Telescope (JWST), is poised to advance understanding of neutron star mergers and how far back in time they began.
“I’m extremely excited about the opportunity to use JWST to dig deeper into the homes of these rare, explosive events,” Nugent said. “JWST’s ability to observe faint galaxies in the universe could reveal more SGRB host galaxies that are currently escaping detection, perhaps even revealing a missing population and a link to the early universe.”
“I started observing for this project a decade ago and it was so satisfying to pass the torch 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 have really taken this study to the next level.”
The studies, “Short GRB host systems I” and “Short GRB host systems IIwere 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.