A few years ago, scientists discovered an unusual astronomical phenomenon: Objects that sent short bursts of radio waves at distinct intervals, emanating from various parts of space.
Scientists named them “long-period radio transients.” They didn’t know much about them, but they knew what the objects were not. They were not quasars. They were not pulsars. They were a mystery, but thanks to UWM Physics & Astrophysics Professor David Kaplan and an international cohort of astronomers, the world is finally getting a glimpse of what is producing these puzzling energy bursts.
Kaplan is one of the authors of a new paper published in the prestigious journal Nature Astronomy in June. In it, he and his colleagues detailed the search for the identity of a particular long-period transient they had observed emanating from a point in the Milky Way Galaxy about 3,000 light-years away from Earth.
Here are three things you need to know about Kaplan’s work.
1. This particular long-period radio transient is a white dwarf binary star – and a “Rosetta Stone.”
The paper’s lead author is a PhD student named Kovi Rose who is part of an international team of researchers led by scientists at the University of Sydney in Australia. Kaplan works closely with this team, who uses Australia’s ASKAP radio telescope to search the sky for objects called “slow transients.” A few years ago, the team noticed a new type of object that looked like a pulsar – but there was something strange about them.
Like pulsars, the objects emitted regular bursts of energy in the form of radio waves. Unlike pulsars, which are the rapidly-spinning, dense core left behind after a star explodes, these objects emitted energy bursts at a fairly slow rate.
“Pulsars emit pulses once every 10 seconds for the slowest ones – so still relatively rapid. What they found was something that emitted pulses once about every 20 minutes,” Kaplan said. “The slowest ones pulse every six hours.”
With the support of scientists like Kaplan, Rose began tracking one of these objects, dubbed “long-period radio transients,” that put out bursts of radio waves about every 80 minutes. The scientists tracked the energy signature to a particular portion of the Milky Way Galaxy. Then, star by star, they searched for the object.
Rose found a particular white dwarf star in orbit with another larger, less dense red dwarf. The pair had some unusual variations in their radio waves, optical waves, and x-ray waves which repeated every 80 minutes, matching the radio wave intervals that Rose was tracking.
“The pulsing that we see is not coming from a spin. We think it’s coming from an orbit,” Kaplan explained. “In order for (these binary stars) to orbit once every 80 minutes, they have to be both very small and very close together. In fact, they’re probably so small and so close together that some material from one star is spilling out onto the other star, and that gives rise to a particular signature that we saw in some observations that really ties it to this class of cataclysmic variables (which have been widely studied for the past hundred years).”
That’s significant because “This system gives us a way to decode these signals. It could help us determine whether other long-period transients are more like pulsars or like white dwarf systems, acting like a stellar Rosetta stone,” Rose said in a press release about the discovery.
2. This discovery adds to our collective knowledge about space.
The universe is vast and humanity is only beginning to scratch the surface of what strange and wild things live amongst the cosmos. Every new discovery is another step that betters our understanding of our place in it.
“We’ve been studying the sky for thousands of years, but we’re still finding these new and exciting objects that we don’t understand. Sometimes they lead to new pieces of fundamental physics that turn out to be really important,” Kaplan said. For example, it was groundbreaking news when quasars and pulsars were discovered and identified, and today they have become an integral part of science curriculums. Long-period transients may be the next thing to be added to astronomy textbooks.
“Cosmic mysteries are inspirational. There’s still so much out there that we don’t understand,” Kaplan added. “There are discoveries that we make about the universe that inspire the next generation of people.”
3. This discovery has led to even more questions.
While identifying the white dwarf binary star was a significant step forward, scientists now have a new bevy of questions. For starters – are all long-period radio transients white dwarf binary stars?
“That’s the fundamental question. And the answer is, we don’t know,” Kaplan said.
The team has discovered other objects that behave similarly to Rose’s binary star, but that are not white dwarfs. Are they the same type of astronomical object, or are they different? They’ve also observed that some long-period radio transients seems to be intermittent and can “turn on” or off. How and why do they do that?
“There’s a big effort to try and discover more of these and discover them more systematically, so we can try to understand which properties are accidents and which properties are required, and to study them over time as well,” Kaplan said. “(These objects all) walk like a duck, but do they talk like a duck? We don’t know if they’re both ducks or maybe one’s a duck and the other’s a goose.”
Kaplan thinks there may be several different birds – or rather, types of stars – at work. No matter what they turn out to be, one thing is for certain: Humanity has barely scratched the surface of the mysteries of space.
By Sarah Vickery, College of Letters & Science