Long ago and far across the universe, an enormous burst of gamma rays unleashed more energy in a half-second than the Sun will produce over its entire 10-billion-year lifetime. In May of 2020, light from the flash finally reached Earth and was detected by NASA’s Neil Gehrels Swift Observatory. Scientists quickly enlisted a slew of facilities, including NASA’s Hubble Space Telescope, to study the explosion’s aftermath.
The astronomers were baffled by what they found – the near-infrared emission from the short gamma-ray burst (GRB), as seen by Hubble, was 10 times brighter than expected. These results challenge conventional theories of what happens in the aftermath of a neutron star merger.
“By quickly capturing light from the event at radio to X-ray wavelengths and combining that data with physics, we found new evidence for what happens when two neutron stars collide,” said study co-investigator, Dr Tanmoy Laskar from the Department of Physics at the University of Bath. “We once thought these mergers produced black holes, but now it appears this may not always be the case.”
Dr Wen-fai Fong from Northwestern University in Evanston, Illinois and lead author of the study, added: “These observations did not fit traditional explanations for short gamma-ray bursts. Given what we knew about the radio and X-rays from this blast, it just didn’t match up. The near-infrared emission that we found with Hubble was way too bright. In terms of trying to fit the puzzle pieces of this gamma-ray burst together, one puzzle piece was not fitting correctly.”
Short gamma-ray bursts are thought to be caused by the collision of two neutron stars – extremely dense objects about the mass of the Sun, compressed into the size of a small city. A neutron star is so dense that on Earth, one teaspoonful would weigh a billion tons.
Neutron star collisions are both very rare and extremely important. They are thought to be a main source of the universe’s heavy elements, including gold and uranium.
Accompanying such collisions, scientists expect to see a kilonova. Kilonovae are optical and infrared glows that result from the radioactive decay of heavy elements, and they are unique to mergers involving neutron stars.
The team studying the phenomenon discussed several possibilities to explain the unusual brightness witnessed by Hubble.
“As the data were coming in, we were forming a picture of the mechanism that was producing the light we were seeing,” said Dr Laskar. “When we got the Hubble observations, we had to completely change our thought process. The information that Hubble added made us realise we had to discard our conventional wisdom as there was a new phenomenon going on. The most exciting part was puzzling out what that could mean for the physics powering these extremely energetic explosions.”
The two neutron stars that merged in this case may have combined to form a magnetar – a massive neutron star with a very powerful magnetic field. If so, this could be new evidence that neutron star collisions make magnetars rather than collapsing into black holes. Until now, scientists have believed the merger of two neutron stars always resulted in a black hole.
“You basically have these magnetic field lines that are anchored to the star that are whipping around at about a thousand times a second, and this produces a magnetized wind,” explained Dr Laskar. “These spinning field lines extract the rotational energy of the neutron star formed in the merger, and deposit that energy into the radioactive debris from the blast, causing the material to glow even brighter.”
If the extra brightness seen by Hubble did indeed come from a magnetar depositing energy into the kilonova material, then the team would expect the material thrown from the blast (the ejecta) to produce light that shows up at radio wavelengths. Follow-up radio observations, which are currently underway, may ultimately prove the veracity of the magnetar theory.
“With its amazing sensitivity at near-infrared wavelengths, Hubble really sealed the deal with this burst,” said Dr Fong. “Amazingly, Hubble was able to take an image only three days after the burst. Through a series of later images, Hubble showed that a source faded in the aftermath of the explosion. This is as opposed to being a static source that remains unchanged.
"With these observations, we knew we had not only nabbed the source, but we had also discovered something extremely bright and very unusual. Hubble’s angular resolution was also key in pinpointing the position of the burst and precisely measuring the light coming from the merger.”
NASA’s James Webb Space Telescope, which is scheduled to launch on 31 October 2021, will be particularly well-suited for this sort of observation.
Professor Edo Berger from the Harvard University in Cambridge in Massachusetts, USA, and principal investigator of the Hubble programme, said: "Webb will completely revolutionise the study of similar events. With its incredible infrared sensitivity it will not only detect such emission at even larger distances, but it will also provide detailed spectroscopic information that will resolve the nature of the infrared emission."
The team’s findings appear in an upcoming issue of The Astrophysical Journal.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.
Video credit: NASA, ESA, and D. Player (STScI)