Researchers have discovered the most luminous kilonova candidate to date. Its infrared emission, observed by the Hubble Space Telescope, is 10 times brighter than expected.
Short gamma-ray bursts are incredibly powerful emissions of radiation that last less than two seconds. They are believed to be the product of a collision between neutron stars, resulting in the formation of a black hole — but apparently not all the time.
"These observations do not fit traditional explanations for short gamma-ray bursts," lead author Wen-fai Fong, from Northwestern University, said in a statement. “Given what we know about the radio and X-rays from this blast, it just doesn't match up. The infrared emission that we’re finding with Hubble is way too bright. In terms of trying to fit the puzzle pieces of this gamma-ray burst together, one puzzle piece is not fitting correctly."
The brightness of the kilonova's infrared emission provided the team with an important clue on what might have happened. The collision left behind a magnetar, an extremely magnetic neutron star. The finding is reported in The Astrophysical Journal.
Neutron stars are the remnants of the extremely dense nucleus of stars that went supernova, packing the mass of our Sun in a sphere not much bigger than a city; nothing but black holes are denser. Neutron star collisions are important for the chemical enrichment of the universe. Elements such as gold, platinum, and uranium are formed during these events — and their formation gives rise to a short gamma-ray burst.
When the short gamma-ray burst 200522A was detected on May 22, 2020, several telescopes were pointed at the event, tracking its afterglow. Hubble’s capabilities were key to the discovery.
"Hubble really sealed the deal in the sense that it was the only one to detect infrared light," explained Fong. "Amazingly, Hubble was able to take an image only three days after the burst. You need another observation to prove that there is a fading counterpart associated with the merger, as opposed to a static source. When Hubble looked again at 16 days and 55 days, we knew we had not only nabbed the fading source, but that we had also discovered something very unusual. Hubble’s spectacular resolution was also key in disentangling the host galaxy from the position of the burst and to quantify the amount of light coming from the merger."
Researchers expect to study these dramatic events in more detail with the improved sensitivity of future observatories such as the James Webb Space Telescope.