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Despite all of our current skills in studying the cosmos, there are some basic things that are still extremely difficult to do.
One of those things is measuring distances, especially for random and transient flashes of light. And now, one of those transient flashes, interpreted as a possible burst of gamma radiation 13.4 billion light years across the Universe, has been unmasked.
In two new articles, separate teams of astronomers have discovered that the flash – called GN-z11-flash – is coming from something much closer to home. Namely, it was sunlight reflecting off a bit of an abandoned rocket in Earth orbit.
In an article, a team led by astrophysicist Charles Steinhardt of the Niels Bohr Institute at the University of Copenhagen in Denmark estimated that the signal was much more likely to come from the solar system.
In the second, a team led by astronomer Michał Michałowski from Adam Mickiewicz University in Poland tracked it down to a piece of space debris near the flash – the discarded Breeze-M upper floor of a Russian Proton rocket.
Meanwhile, the original team that reported the GN-z11 flash and speculated that it could be a gamma-ray burst, led by astronomer Linhua Jiang from the Kavli Institute of astronomy and astrophysics in China, concluded that the signal came from much further away.
“It’s a typical problem in astronomy – it’s difficult to measure distances,” Michałowski told ScienceAlert.
“An object with a given recorded brightness could be a weak near object or a bright distant object. Either way, they would appear just as bright to us. The object in question turned out to be very near space debris, but its brightness was also consistent with a huge star explosion at the edge of the observable Universe. “
The GN-z11 flash was detected on April 7, 2017, when Jiang and his team were making near infrared observations of a distant galaxy named GN-z11 using the MOSFIRE instrument at the Keck I Telescope in Hawaii. During the 5.3 hours of data collected, they found a brief burst, less than 245 seconds, coinciding with the galaxy’s position in the sky.
They excluded a moving object, such as a satellite, since the flash occurred several hours after sunset, and excluded known minor planets; there was none, they found, in that region of the sky. The most likely explanation was therefore an ultraviolet flash associated with a gamma-ray burst of GN-z11.
Other scientists were less convinced. The levels of luck needed to perform such a detection would be extremely high, especially given the rarity of gamma-ray bursts detected at the start of the Universe. To date, there have been only a handful; and GN-z11-flash would be earlier than all, detected in a brief five-hour observation window.
“The extreme improbability that the transient source is a gamma ray burst at the very beginning of the Universe necessitates a robust elimination of all plausible alternative hypotheses,” Steinhardt and his team wrote in their paper.
“We identify many examples of similar transient signals in separate archival MOSFIRE observations and claim that solar system objects – natural or man-made – are a much more likely explanation for these phenomena.”
Michałowski and his colleagues drilled it to a specific object. They carefully studied the Space-Track space debris database and found an abandoned stage of a Proton rocket launched in 2015. This rocket, they discovered, was 13,758 kilometers from Earth. and would have appeared in the MOSFIRE field of view during the hour the flash occurred.
Also, at this altitude, it would not have been in Earth’s shadow, meaning sunlight could have bounced off it.
Jiang and his colleagues are not convinced. The flash profile, they said, is different from NEO flashes, and their calculations suggest that the Breeze-M rocket stage was not as close to the field of view. It could, they concede, be from an unknown rocket, but even then the likelihood of it happening is low, they say.
“We cannot completely rule out the possibility of unknown satellites (or debris),” they wrote. “Despite this fact, our new calculations suggested that our initial conclusion remains valid.”
We think we haven’t heard the latest from GN-z11-flash. However, as good old Occam says, if there is a simpler explanation, this is probably where the answer lands; and, as Carl Sagan has aptly noted, extraordinary claims require extraordinary evidence.
To conclusively link the GN-z11-flash to the GN-z11, the scientific community will want to see this extraordinary evidence. For now, Michałowski seems quite happy with his team’s conclusion.
“On the one hand, the existence of a gamma-ray burst in such a distant galaxy would have important consequences on our understanding of the formation of the first stars and galaxies, so I was happy to be able to push science in the right direction” , he said.
“On the other hand, it’s a shame that such an extraordinary discovery turned out to have such a trivial explanation.”
All three articles were published in Nature astronomy. They can be found here, here and here.
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