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EI2GYB > ASTRO 11.10.21 12:00l 84 Lines 8746 Bytes #999 (0) @ WW
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Subj: Did Astronomers See a Distant, Dying Star? Or an Earth-boun
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Did Astronomers See a Distant, Dying Star? Or an Earth-bound Satellite?
What seemed a lucky break - the discovery of a gamma-ray burst in the most distant known galaxy - might instead be the flash of passing space debris. As satellites fill low-Earth orbit, such events might become common.
Last year, astronomers caught a surprising flash of light from the most distant galaxy known - GN-z11, which existed just 400 million years after the Big Bang. The team proposed, after ruling out other possibilities, that what they had seen was the fading afterglow of a dying star. The event would have heralded the earliest example of star death in the known universe.
That is, if the light came from a star at all. Two studies appearing October 4th in Nature Astronomy make the case that what the original team saw was not a natural phenomenon but the chance passing of a manmade object in Earth orbit.
The Flash
Linhua Jiang (Peking University, China) led a team in observing GN-z11 in near-infrared light using the Multi-Object Spectrometer for Infrared Exploration (MOSFIRE) on the Keck I telescope in Hawai'i.
The galaxy is so faint that the team was planning to combine more than 100 images before measuring the spectrum. But during one of these individual exposures, which lasted 179 seconds, something happened. A bright source of light crossed the slit, apparently right at the center of the galaxy.
Jiang's team checked whether other nearby objects could have photobombed their observations and came up blank. They concluded they had seen the ultraviolet afterglow of a long gamma-ray burst (GRB), the violent destruction of one of the universe's earliest stars.
But there were curiosities in this discovery. For one, the spectrum wasn't quite what one would expect for a GRB, though it was still within the realm of possibility. Curiouser still was how incredibly lucky the team had to be: Based on the galaxy's star formation rate, it ought to host between 1 and 200 GRBs every million years. The chance of Jiang's team actually catching one of them during one of their exposures was 1 in 10 billion.
What Are the Chances?
Immediately after the study's publication in Nature, other astronomers began putting theory papers on the astronomy preprint archive that examined the conclusion of seeing such an unlikely event.
Scientists don't like flukes - they prefer repeatable, testable things. That's not to say unlikely things don't happen. But when something with a near-zero chance of occurring actually comes to pass, it makes one think: Maybe that kind of thing isn't so rare after all.
"You shouldn't believe you got a 1 in 10 billion lucky break," says Charles Steinhardt (Cosmic Dawn Center, Denmark). "You should instead believe that you've misunderstood the physics of an area where we know we need to learn a lot about the physics."
Indeed, many astronomers were putting their heads together to consider what it was we've misunderstood. But as Steinhardt talked the discovery over with colleagues down the hall, he realized that in fact the probability of having found a GRB was so low that that other really unlikely things might in fact be more probable.
Together with his colleagues, Steinhardt posted on the arXiv in January that perhaps the flash had been a manmade object in Earth orbit. However unlikely, he argued, it was still worth considering. (That paper appears today in Nature Astronomy's Matter Arising.)
Then, just a month later, Michal Michalowski (Adam Mickiewicz University, Poland) and colleagues posted on the arXiv that they had found the object: the Breeze-M upper stage of a Russian Proton rocket. (Their paper also appears today in Nature Astronomy's Matter Arising.)
"Our calculations showed that the [rocket booster] crossed through the slit that covered the distant galaxy GN-z11," Michalowski explains.
Jiang's team was actually aware of this exact booster; they had used a website popular with amateur astronomers, Calsky.com, to calculate its trajectory and found that it passed outside the telescope's field of view.
"CalSky.com was a really useful website for determining when things in the sky (moonrise, Jupiter set, satellite passes, ISS crossing in front of the Moon) would occur from an observer's location," says S&T's Kelly Beatty. "It shut down in October 2020 due to lack of funding."
Both Jiang's and Michalowski's groups tried to contact the former owners of Calsky, but to no avail. It remains unclear why the groups' calculated trajectories for the booster were different by several arcminutes.
But Michalowski is certain the booster is to blame for the flash, adding that his team has double-checked the calculations: "In addition to our own software, we used three publicly available satellite orbit software packages: JPL Horizons, OREKIT, and SkyField. The resulting orbits agree with our calculations within 0.1 km (1.5 arcsec)."
While Jiang's team replied to the newly published studies maintaining the validity of their results, they also make clear that they never conclusively claimed the flash was a GRB. "We just reported this event and provided our most probable interpretation," Jiang says.
"I do think the original paper by Jiang and colleagues was a reasonable thing," Steinhardt says. "Ultimately, when you see something weird as an observer, what do you do? You put it out there for the community to give their best guesses as to what it is."
Satellite Interference
This wasn't the first incident of a manmade object posing as an astronomical phenomenon. In the mid-1980s, Sky & Telescope reported on multiple observers who'd seen mysterious, repeated flashes in the constellation Perseus. Ultimately, astronomers determined that the unfortunately named "Perseus Flasher" was in fact merely the glints of sunlight off satellites.
Nor will this be the last of manmade interference with the skies - far from it. The stuff in Earth orbit is increasing exponentially. Most notable has been the Starlink network, which will ultimately be made up of more than 12,000 satellites in low-Earth orbit.
At the same time, technological developments in astronomy are enabling telescopes to take fast, repeated exposures of the sky, turning single snapshots into videos. Astronomers can watch supernovae as they happen, catch Near-Earth objects moving between frames. But they'll also increasingly find their observations hindered, even blocked as satellites and space debris streak across images.
Jonathan McDowell (Center for Astrophysics, Harvard & Smithsonian) has led a team in developing software and databases, including a "more robust" version of Calsky, that will help astronomers deal with this problem.
"We're focusing on mainly a couple applications," he explains. "One is to figure out at what time tonight can I observe this object and not get 'Starlinked.' And another is, ok, I got streaked, what can I retrieve from my data." He adds that astronomers will also be able to use this software in cases like the GN-z11 flash, to determine if/when a satellite or other debris has interfered with a past observation.
But software isn't a cure-all. If low-Earth orbit fills to capacity, it may become difficult for telescopes to avoid satellites. And while it's possible in most cases to remove a trail, it's nearly impossible to look "underneath" it to see the stars and galaxies.
More troubling, it's possible a satellite could interfere with an observation without astronomers ever knowing it. Satellite and true astronomical source become particularly difficult to tease apart for transient events and for spectroscopy - both exemplified by the GN-z11 flash.
"We're in this world now where every paper you write on observational astronomy is going to have to ask the question, 'Is this real or is this a satellite?'" McDowell says. "It's not always going to be possible to tell."
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