A radio burst was pulsing from the Milky Way. Astronomers traced it to a dead star
Over the past decade, scientists have detected a puzzling phenomenon: radio pulses coming from within our Milky Way galaxy that would pulse every two hours, like a cosmic heartbeat. The long radio blasts, which lasted between 30 and 90 seconds, appeared to come from the direction of the Ursa Major constellation, where the Big Dipper is located.
Now, astronomers have zeroed in on the surprising origin of the unusual radio pulses: a dead star, called a white dwarf, that is closely orbiting a small, cool red dwarf star. Red dwarfs are the most common type of star in the cosmos.
The two stars, known collectively as ILTJ1101, are orbiting each other so closely that their magnetic fields interact, emitting what's known as a long period radio transient, or an LPT. Previously, long radio bursts were only traced to neutron stars, the dense remnants left after a colossal stellar explosion.
But the discovery, described in a study published Wednesday in the journal Nature Astronomy, shows the movements of stars within a stellar pair can also create rare LPTs.
'We have for the first time established which stars produce the radio pulses in a mysterious new class of 'long period radio transients,'' said lead study author Dr. Iris de Ruiter, a postdoctoral scholar at the University of Sydney in Australia.
The unprecedented observations of such bright, long radio bursts from this binary star system are just the beginning, astronomers say. The discovery could help scientists better understand what types of stars are capable of producing and sending radio pulses across the cosmos — and in this case, reveal the history and dynamics of two entwined stars.
Locked in a stellar dance
To solve the Milky Way mystery, de Ruiter devised a method to identify radio pulses lasting seconds to minutes within the archives of the Low-Frequency Array telescope, or LOFAR, a network of radio telescopes throughout Europe. It's the largest radio array that operates at the lowest frequences detectable from Earth.
De Ruiter, who developed her method while she was a doctoral student at the University of Amsterdam, uncovered a single pulse from observations made in 2015. Then, focusing on the same patch of sky, she found six more pulses. All of them appeared to originate from a faint red dwarf star. But de Ruiter didn't think the star would be able to produce radio waves by itself. Something else had to be instigating it.
The pulses differed from fast radio bursts, which are incredibly bright, millisecond-long flashes of radio waves. Almost all FRBs originate from outside our galaxy, and while some of them repeat, many appear to be one-off events, de Ruiter said. Fast radio bursts are also much more luminous.
'The radio pulses are very similar to FRBs, but they each have different lengths,' said study coauthor Charles Kilpatrick, research assistant professor at Northwestern University's Center for Interdisciplinary Exploration and Research in Astrophysics, in a statement.
'The pulses have much lower energies than FRBs and usually last for several seconds, as opposed to FRBs which last milliseconds. There's still a major question of whether there's a continuum of objects between long-period radio transients and FRBs, or if they are distinct populations.'
De Ruiter and her colleagues conducted follow-up observations of the red dwarf star using the 21-foot (6.5-meter) Multiple Mirror Telescope at the MMT Observatory on Mount Hopkins in Arizona, as well as the LRS2 instrument on the Hobby-Eberly Telescope, located at the McDonald Observatory in the Davis Mountains in Texas.
The observations showed the red dwarf was moving back and forth rapidly, and its motion matched the two-hour period between radio pulses, Kilpatrick said. The back-and-forth motion was due to another star's gravity tugging on the red dwarf. The researchers were able to measure the motions and calculate the mass of the companion star, which they determined to be a white dwarf.
The team found that the two stars, located 1,600 light-years from Earth, were pulsing together as they orbited a common center of gravity, completing one orbit every 125.5 minutes.
Deciphering mysterious pulses
The research team believes there are two possible causes behind the pulses. Either the white dwarf has a strong magnetic field that routinely releases the pulses, or the magnetic fields of the red dwarf star and the white dwarf interact as they orbit.
The team has planned to observe ILTJ1101 and study any ultraviolet light that may be emanating from the system, which could reveal more about how the two stars have interacted in the past. De Ruiter also hopes the team can observe the system in radio light and X-rays during a pulse event, which could shed light on the interaction between the magnetic fields.
'At the moment the radio pulses have disappeared completely, but these might turn back on again at a later time,' de Ruiter said.
The team is also combing through LOFAR data in search of other long pulses.
'We are starting to find a few of these LPTs in our radio data,' said study coauthor Dr. Kaustubh Rajwade, a radio astronomer in the department of physics at the University of Oxford, in a statement. 'Each discovery is telling us something new about the extreme astrophysical objects that can create the radio emission we see.'
Other research groups have found 10 long radio pulse-emitting systems over the past couple of years, and they are trying to determine what creates them because the pulses, all of which originate in the Milky Way, 'are unlike anything we knew before,' de Ruiter said.
Unlike the short bursts produced by pulsars, or rapidly spinning neutron stars, LPTs can last anywhere from a few seconds to nearly an hour, said Natasha Hurley-Walker, radio astronomer and associate professor at the Curtin University node of the International Centre for Radio Astronomy Research in Australia. Hurley-Walker was not involved in the new study.
'Looking back, transient radio sources have stimulated some of the most exciting discoveries in astrophysics: the discovery of pulsars and therefore neutron stars, the discovery of FRBs which have unlocked the capacity to measure the otherwise invisible matter between galaxies, and now the discovery of LPTs, where we're only at the tip of the iceberg in terms of what they will tell us,' Hurley-Walker said via email. 'What's fascinating to me is that now that we know these sources exist, we're actually finding them in historical data going back decades — they were hiding in plain sight.'
Scanning the sky with powerful radio telescopes will only lead to more incredible findings, she said.
'The biggest would most likely be the discovery of technosignatures via SETI,' Hurley-Walker said of signals that could be created by intelligent life, which is something the SETI Institute has sought out for decades.
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