(Images available via the links in the Notes section) An international team of astrophysicists led by the Netherlands and the UK have discovered that radio pulses lasting seconds to minutes are due to two stars coming together – rather than emissions from a single star. The results are published today (12 March) in Nature Astronomy .
In recent years, a new astronomical phenomenon has puzzled radio astronomers: researchers have detected radio pulses from the Milky Way that last from seconds to minutes. These pulses are unlike anything expected from known radio-emitting neutron stars, or pulsars*, which produce pulses on the order of milliseconds. Furthermore, these so-called long-period transients (LPTs) are periodic at timescales of 10s of minutes to hours, unlike a radio pulsar (that emits radio waves once every few seconds).
There have been a few hypotheses for the origins of these novel pulses, but evidence has been scant. Now, a new discovery led by astronomers from the Netherlands and the UK has provided vital clues to the nature of these radio bursts for the first time.
The study focused on a collection of these mysterious periodic radio signals detected in 2022. Using a novel imaging technique, the team led by Dr Iris de Ruiter (University of Amsterdam at the time of the study, now University of Sydney) with Dr Kaustubh Rajwade (Department of Physics, University of Oxford) detected several of these LPT radio pulses in data taken with the Low Frequency Array (LOFAR), an international radio telescope.
Acting like a large radio camera, the telescope could pinpoint the exact location of the radio pulse in the sky that was traced to a star-like object about 1,600 light-years away. Follow-up observations with the 6.5 m diameter Multiple Mirror Telescope in Arizona and the Hobby-Eberly Telescope in Texas (USA) showed that it is not one flashing star, but two stars that together cause the pulse.
The two stars, a white dwarf (a rather bright ember of a Sun-like star after it sheds away all the material around it) in orbit around a red dwarf (a star much smaller and lighter than the Sun), orbit a common centre of gravity every 125 minutes. The star system is located in the direction of the constellation of the Great Bear (Ursa Major).
According to the researchers, there are two possibilities for how the stars generate the unusually long radio pulses. Potentially, the radio bursts emanate from the strong magnetic field of the white dwarf, or they could be produced by the interaction of the magnetic fields of the white dwarf and its stellar companion. However, further observations are needed to clarify this.
'Thanks to this discovery, we now know that compact objects other than neutron stars are capable of producing bright radio emission,' comments Dr Rajwade who leads the effort to find the unexplained LPTs with the LOFAR telescope and helped to identify the periodic pattern between radio pulses.
'We worked with experts from all kinds of astronomical disciplines,' adds Dr de Ruiter. 'With different techniques and observations, we got a little closer to the solution step by step.'
In recent years, about ten such radio-emitting systems have been discovered by other research groups. However, these groups have not yet been able to prove whether these pulses come from a white dwarf or a neutron star.
Dr Rajwade continues to search the data from LOFAR for LPTs: 'This finding is very exciting! We are starting to find a few of these LPTs in our radio data. Each discovery is telling us something new about the extreme astrophysical objects that can create the radio emission we see. For instance, the unexpected observation of coherent radio emission from the white dwarf in this study could help probe the evolution of magnetic fields in this type of star.'
*Pulsar: stellar remains of a brilliant supernova explosion.
