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UMD Scientists Help Uncover Origins of Castaway Gamma-ray Bursts

These enigmatic bursts of energy have been traced back to previously undiscovered galaxies in the early Universe
An international team of astronomers found that certain short gamma-ray bursts (GRBs) did not originate in the vastness of intergalactic space as they initially seemed to appear. A deeper multi-observatory study instead found that these perceived castaway GRBs actually formed in remarkably distant—and therefore faint—galaxies up to 10 billion light-years away. The study was published on July 26, 2022, in the Monthly Notices of the Royal Astronomical Society.

A number of GRBs appear as lonely flashes of intense energy far from any obvious galactic home, which has raised questions about their true origins and distances. Using data from some of the most powerful telescopes on Earth and in space, including the National Science Foundation’s (NSF) Gemini telescopes, astronomers were able to solve this mystery.

“Many short GRBs are found in bright galaxies relatively close to us, but some of them appear to have no corresponding galactic home,” said Brendan O’Connor, an astronomer at the University of Maryland and George Washington University. “By pinpointing where the short GRBs originate, we were able to comb through troves of data from observatories like the twin Gemini telescopes to find the faint glow of galaxies that were simply too distant to be recognized before.”

Their discovery suggests that short GRBs, which form during the collisions of neutron stars, may have been more common in the past than expected. Since neutron-star mergers forge heavy metals, including gold and platinum, the Universe may have been seeded with precious metals earlier than expected as well.

This cosmic sleuthing required the combined power of some of the most powerful telescopes on Earth and in space, including the Gemini North telescope in Hawai‘i and the Gemini South telescope in Chile. The two Gemini telescopes comprise the International Gemini Observatory, operated by NSF’s NOIRLab. Other observatories involved in this research include the NASA/ESA Hubble Space Telescope; the Lowell Discovery Telescope in Arizona; the Gran Telescopio Canarias in Tenerife in the Canary Islands, ESO’s Very Large Telescope in Cerro Paranal in Chile and the Keck Observatory in Hawai‘i.

The researchers began their quest by reviewing data on 120 GRBs captured by two instruments aboard NASA’s Neil Gehrels Swift Observatory: Swift’s Burst Alert Telescope, which signaled a burst had been detected; and Swift’s X-ray Telescope, which identified the general location of the GRB’s X-ray afterglow. Additional afterglow studies made with the Lowell Discovery more accurately pinpointed the location of the GRBs.

The afterglow studies found that 43 of the short GRBs were not associated with any known galaxy and appeared in the comparatively empty space between galaxies.

“These hostless GRBs presented an intriguing mystery and astronomers had proposed two explanations for their seemingly isolated existence,” said O’Connor.

One hypothesis was that the progenitor neutron stars formed as a binary pair inside a distant galaxy, drifted together into interstellar space and eventually merged billions of years later. The other hypothesis was that the neutron stars merged many billions of light-years away in their home galaxies, which now appear extremely faint due to their vast distance from Earth.

“We felt this second scenario was the most plausible to explain a large fraction of hostless events,” said O’Connor. “We then used the most powerful telescopes on Earth to collect deep images of the GRB locations and uncovered otherwise invisible galaxies 8 to 10 billion light-years away from Earth.”

To make these detections, the astronomers utilize a variety of optical and infrared instruments mounted on the twin 8.1-meter Gemini telescopes. The Gemini Observatory offers the unique capability for observations from both hemispheres, which is incredibly important for GRB follow-up due to their ability to survey the entire sky. Gemini data were used to localize 17 out of 31 GRBs analyzed in their sample.

This result could help astronomers better understand the chemical evolution of the Universe. Merging neutron stars trigger a cascading series of nuclear reactions that are necessary to produce heavy metals, like gold, platinum and thorium. Pushing back the cosmic timescale on neutron-star mergers means that the young Universe was far richer in heavy elements than previously known.

“This pushes the timescale back on when the Universe received the ‘Midas touch’ and became seeded with the heaviest elements on the periodic table,” O’Connor said.

In addition to O’Connor, several UMD scientists contributed to this research, including physics Ph.D. candidate Joseph Durbak, Astronomy Associate Research Scientist Alexander Kutyrev, Astronomy Professor Sylvain Veilleux, Astronomy Adjunct Associate Professor S. Bradley Cenko, former Astronomy Associate Research Scientist Eleonora Troja, former Astronomy Postdoctoral Associate Simone Dichiara and Pradip Gatkine (M.S.’16, Ph.D. ‘20, astronomy).

This article is adapted from text provided by NSF’s NOIRLab. “

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