For all our current proficiency at studying the cosmos, there are some basic things that are still extremely difficult to do.
One of those things is gauge distances, especially for random, transient flashes of light. And now one of those transient flashes, interpreted as a possible burst of gamma radiation from 13.4 billion light-years across the Universe, has been unmasked.
In two new papers, separate teams of astronomers have found that the flash – called GN-z11-flash – is from something much closer to home. Namely, it was sunlight reflecting off a bit of discarded rocket in Earth orbit.
In one paper, a team led by astrophysicist Charles Steinhardt of the University of Copenhagen's Niels Bohr Institute in Denmark ruled it much more likely that the signal originated within the Solar System.
In the second, a team led by astronomer Michał Michałowski of the Adam Mickiewicz University in Poland traced it to a piece of space junk in the vicinity of the flash – the discarded Breeze-M upper stage of a Russian Proton rocket.
Meanwhile, the original team that reported GN-z11-flash and speculated that it may be a gamma-ray burst, led by astronomer Linhua Jiang of the Kavli Institute for Astronomy and Astrophysics in China, has stuck by their conclusion that the signal came from much farther away.
"This is a typical problem in astronomy – it's difficult to measure distances," Michałowski told ScienceAlert.
"An object with a given recorded brightness may be a faint nearby object or a luminous distant object. In both cases they would appear equally bright for us. The object in question turned out to be a very nearby piece of space junk, but its brightness was equally compatible with a huge stellar explosion at the edge of the observable Universe."
GN-z11-flash was detected on 7 April 2017, when Jiang and his team were conducting near-infrared observations of a distant galaxy named GN-z11 using the MOSFIRE instrument on the Keck I telescope in Hawaii. In the 5.3 hours of data collected, they found a brief burst, shorter than 245 seconds, coincident with the position of the galaxy in the sky.
They ruled out a moving object, such as a satellite, since the flash occurred several hours after sundown, and ruled out known minor planets; there were none, they found, in that region of the sky. The likeliest explanation, therefore, was an ultraviolet flash associated with a gamma-ray burst from GN-z11.
Other scientists were less convinced. The levels of luck needed to make such a detection would be extremely high, especially given the scarcity of gamma-ray bursts detected in the early Universe. To date, there have been just a handful; and GN-z11-flash would be earlier than all of them, detected in just a brief, five-hour observation window.
"The extreme improbability of the transient source being a gamma-ray burst in the very early Universe requires robust elimination of all plausible alternative hypotheses," Steinhardt and his team wrote in their paper.
"We identify numerous examples of similar transient signals in separate archival MOSFIRE observations and argue that Solar System objects – natural or artificial – are a far more probable explanation for these phenomena."
Michałowski and colleagues drilled it down to a specific object. They carefully studied space debris database Space-Track, and found a discarded stage of a Proton rocket launched in 2015. This rocket, they found, was at a distance of 13,758 kilometers from Earth, and would have appeared in the MOSFIRE field of view during the time the flash was taking place.
Moreover, at that altitude, it would not have been within Earth's shadow – meaning that sunlight could indeed have bounced off it.
Jiang and colleagues are unconvinced. The profile of the flash, they said, is different from flashes from near-Earth objects, and their calculations suggest that the Breeze-M rocket stage wasn't as close to the field of view. It could, they concede, have been from an unknown rocket, but even then, the probability of this is low, they say.
"We cannot completely rule out the possibility of unknown satellites (or debris)," they wrote. "Despite this fact, our new calculations have suggested that our original conclusion remains valid."
We expect we haven't heard the last of GN-z11-flash. However, as good old Occam says, if there's a simpler explanation, that's probably going to be where the answer lands; and, as Carl Sagan pertinently noted, extraordinary claims require extraordinary evidence.
To conclusively link GN-z11-flash to GN-z11, the science community is going to want to see that extraordinary evidence. For now, Michałowski seems pretty satisfied with his team's conclusion.
"On 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 that I can push science in the right direction," he said.
"On the other hand, it's a pity that such an extraordinary discovery turned out to have such a mundane explanation."