Using two Maunakea Observatories in Hawaii – W. M. Keck Observatory and the international Gemini Observatory, a Program of NSF’s NOIRLa, astronomers have discovered the second-most distant confirmed short gamma-ray burst (SGRB).
Based on observations, astronomers have confirmed that the object is at 10 billion light-years away. Meanwhile, it is squarely in the epoch of cosmic high noon when the universe was in its “teenage years” and rapidly forming stars.
Astronomers think that SGRB at such an early time could alter theories about their origins, particularly the length of time it takes two neutron stars to merge and produce these powerful explosions, as well as the rate of neutron star mergers in the young universe.
Kerry Paterson, a postdoctoral associate at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), said, “This was a fascinating object to study. Our research now suggests neutron star mergers could occur surprisingly quickly for some systems — with neutron star binaries spiraling together in less than a billion years to create an SGRB.”
Within just a few hours after NASA’s Neil Gehrels Swift Observatory detected the object and broadcast a worldwide alert, Paterson’s team quickly pointed the Gemini North and Keck I telescopes toward the location of the SGRB.
They then used the Gemini Multi-Object Spectrograph, followed by Keck Observatory‘s Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) instrument to evaluate the very faint afterglow of the object, which is named GRB181123B because it was the second burst discovered on November 23, 2018.
Wen-fai Fong, assistant professor of physics and astronomy at Northwestern University and co-author of the study, said, “It was unreal. I was in New York with my family and had finished having a big Thanksgiving dinner. Just as I had gone to sleep, the alert went off and woke me up. While somewhat of a nuisance, you never know when you’ll land a big discovery like this! I immediately triggered the Gemini observations and notified Kerry. Thankfully, she happened to be observing at Keck that night and was able to rearrange her original observing plan and repoint the telescope towards the SGRB.”
Paterson said, “It was such an adrenaline rush to be at Keck when the SGRB alert went off and personally move the telescope towards the object to capture data mere hours after the burst.”
To pinpoint the distance of GRB181123B, the team obtained spectra of its host galaxy through follow-up observations using Keck Observatory’s DEep Imaging and Multi-Object Spectrograph (DEIMOS).
Paterson said, “Once we obtained the optical spectrum from DEIMOS, it was clear this event was one of the most distant SGRBs measured, which further fueled our investigation to determine its precise distance.”
With a distance calculated at a cosmological redshift of 1.754, the data confirmed the object is the most distant high-confidence SGRB with an optical afterglow detection ever found.
Scientists identified specific patterns in the spectrum, along with the colors of the galaxy from the three observatories. This enabled them to precisely constrain the distance and solidify it as one of the most distant SGRBs to date in 16 years of Swift operations.
After identifying the host galaxy, scientists also determined the significant properties of the parent stellar population within the galaxy that produced the SGRB.
Fong said, “Performing ‘forensics’ to understand the local environment of SGRBs and what their home galaxies look like can tell us a lot about the underlying physics of these systems, such as how SGRB progenitors form and how long it takes for them to merge. We certainly did not expect to discover an extremely distant SGRB, as they are very rare and faint, but we were pleasantly surprised! This motivates us to go after everyone that we possibly can.”
- K. Paterson et al. Discovery of the optical afterglow and host galaxy of short GRB181123B at z=1.754: Implications for Delay Time Distributions in The Astrophysical Journal Letters. arXiv:2007.03715