Puzzled scientists using the Hubble Space Telescope have seen a violent explosion billons of light-years that lasted just half a second.
The brightest infrared light from a short gamma-ray burst ever seen, the “bizarre glow” was spotted using the orbiting telescope in May 2020. It was 10 times brighter than it was thought possible.
The glow from a short gamma-ray burst called GRB 200522A was spotted by astronomers three days after. It was so powerful that it unleashed more energy in a split-second than our Sun will produce over its entire 10 billion-year lifetime.
A paper about the discovery of the radio afterglow in the center of a young, star-forming galaxy has been accepted for publication in The Astrophysical Journal.
What is a short gamma-ray burst?
Short gamma-ray bursts like these are rated to be among the most explosive events in the known Universe. It’s thought that they are caused by a super-rare event—the merging of two neutron stars.
A neutron star is a pretty extreme celestial objects that’s born of a supernova. Very small and extremely dense, they’re thought to be what’s leftover after a massive star explodes in a supernova.
They’re comprised of tightly-packed neutrons—a sub-atomic particle with no net electrostatic charge—and are thought to contain the Universe’s heavy elements, such as uranium and gold.
What is a kilonova?
After a neutron star merger, scientists expect to see a “kilonova”—an explosion 1,000 times brighter than a regular nova. An optical and infrared glow from the radioactive decay of heavy elements, a kilonova can only be caused by the merger of either two neutron stars or the merger of a neutron star and a black hole.
However, a kilonova is not what the scientists saw. So what was it?
NASA’s Hubble Sees Unexplained Brightness from Colossal Explosion
NASA, ESA, STScI, W. Fong (Northwestern)
What did the scientists see?
Scientists were able to detect light across the entire electromagnetic spectrum—a rare event in itself—from five billion light years away.
“These observations do not fit traditional explanations for short gamma-ray bursts,” said Wen-fai Fong, assistant professor of physics and astronomy at Northwestern University. She is lead author of the study. “Given what we know about the radio and X-rays from this blast, it just doesn’t match up ... the near-infrared emission that we’re finding with Hubble is way too bright.”
So what caused the unusually bright explosion? Their theory is that they saw a magnetar being created.
What is a magnetar?
It’s a supermassive neutron star with a very powerful magnetic field, which may have been the result of the merger of the two neutron stars. It’s possible that the resulting magnetar energised the material ejected during the kilonova explosion, which made it glow even brighter.
The birth of a magnetar from a neutron star merger has never definitively been seen before.
“We know that magnetars exist because we see them in our galaxy,” said Fong. “We think most of them are formed in the explosive deaths of massive stars, leaving these highly magnetized neutron stars behind.”
“However, it is possible that a small fraction form in neutron star mergers,” said Fong. “We have never seen evidence of that before, let alone in infrared light.”
That’s what makes this discovery so special.
How astronomers measure the brightness of a distant object
Since the short gamma-ray burst took place many millions of light-years from our own galaxy, the event’s apparent brightness as seen from Earth orbit wasn’t that impressive. The researchers used the W. M. Keck Observatory on Maunakea in Hawaii to pinpoint the precise distance of its host galaxy, thereby calculating the burst’s intrinsic brightness.
“Just as the brightness of a light bulb when it reaches your eye depends on both its luminosity and its distance from you, a burst could be really bright because either it is intrinsically luminous and distant, or not as luminous but much closer to us,” said Fong. “With Keck, we were able to determine the true brightness of the burst and thus the energy scale. We found it was to be much more energetic than we originally thought.”
First detected by NASA’s Neil Gehrels Swift Observatory, the the explosion’s aftermath and its host galaxy was then studied by Hubble, the Very Large Array, the W.M. Keck Observatory and the Las Cumbres Observatory Global Telescope network
What happens next?
During the next few years it should be possible to detect light from this huge explosion at radio wavelengths, so the scientists plan to make follow-up observations, perhaps even with NASA’s upcoming James Webb Space Telescope (JWST), which will have uniquely sensitive infrared capabilities.
“We can’t wait to combine the power of Keck and JWST along with other facilities as a team to go after even more enigmatic events like these,” said John O’Meara, Keck Observatory Chief Scientist. “This study shows that we have much left to learn.”
“It’s amazing to me that after 10 years of studying the same type of phenomenon, we can discover unprecedented behavior like this,” said Fong. “It just reveals the diversity of explosions that the universe is capable of producing, which is very exciting.”