In the past three years, astronomers have discovered a mysterious new type of radio source. We call these long period transients.
Author
- Iris de Ruiter
Postdoctoral Researcher in OzGrav, University of Sydney
These objects emit bright radio signals that repeat every few minutes to every few hours. We have found about a dozen examples, but we still don't understand which type of star could emit radio pulses in this peculiar way.
In new research published in Nature Astronomy today, we have discovered a new long period transient. Furthermore, we identified the stars responsible for the mysterious radio flashes - a breakthrough never achieved before.
Spoiler alert: they're not the typical "cosmic lighthouses" you might expect.
What is a cosmic lighthouse?
You may have heard of cosmic objects called pulsars - they're a type of neutron star.
Neutron stars are the remnants of extremely massive stars when they've reached the end of their life. Pulsars are rotating neutron stars; as they spin, they emit a beam of radio emission that we can detect on Earth. This is why pulsars are often called cosmic lighthouses - they "show" us a radio pulse on every rotation. We know of thousands of pulsars in our Milky Way galaxy.
You might think that sounds extremely similar to the mysterious long period transients I just described, and you'd be right.
However, the pulsars we know typically flash every second. These new objects show much slower repetition. According to theories about the evolution of neutron stars, pulsars that rotate this slowly shouldn't exist.
So, is there another option?
White dwarfs are the other suggested source of long period transients. White dwarfs are the remnants of low-mass stars (like our Sun) at the end of their life, making them the smaller sibling of neutron stars.
A cosmic detective hunt
Using the international LOFAR radio telescope in Europe, my colleagues and I discovered a new object: ILTJ1101+5521.
Ploughing through the LOFAR data , we found seven bright pulses. Taking a closer look at the timing of these pulses, we found that they arrive every two hours (every 125.52978 ± 0.00002 minutes to be exact).
This made ILTJ1101 a new example of a long period transient.
We compared the location of the radio pulses to optical catalogues, which list stars and galaxies that telescopes have observed in visible light. And there it was - we found there was a faint red star exactly at the location of our radio pulses.
However, the properties of the radio pulses indicated these radio signals couldn't be generated by this red star alone.
A hidden companion
Many stars have a stellar friend. The two stars are bound to each other and orbit each other. Known as binary stars, such pairings are incredibly common. About 50% of the stars with a mass similar to our Sun have a binary companion .
To investigate whether this was true for the red star at the location of our radio pulses, we took a spectrum. A spectrum shows how much light the star emits at each wavelength .
Each type of star emits a unique spectral " fingerprint ". Over different observations, we saw the fingerprint of the red star shift to slightly longer or shorter wavelengths. This effect is known as the Doppler effect , indicating that the star is moving away from us in one observation and moving towards us in the other. That's similar to how the pitch of an ambulance siren changes as it moves towards you and then recedes in the distance.
The only way this type of movement can be achieved is if the red star is in a binary with another star. We found that the two stars orbit each other every two hours- that's their orbital period.
It matches up perfectly with the puzzling slow repetition of the radio pulses we detected.
What is the companion?
Alongside spectra, we also had photometry measurements of ILTJ1101. Similar to the spectra, the photometry measurements show the amount of light the stars emit at different wavelengths. However, the spectra only covered a limited wavelength range, whereas the photometry measurements were taken over a much broader range of wavelengths.
From these photometry measurements we found a small excess of blue light. This light is not expected from the red star alone, and cannot be produced by a neutron star.
A white dwarf, however, perfectly fit the brief.
This is how we figured out that the radio pulses from ILTJ1101 are coming from a white dwarf in a binary system with a red star.
Mystery solved? Not quite
Does this mean all long period transients are white dwarf binaries? Probably not.
Some of these long period transients show very clear pulsar characteristics . Additionally, the periods of some long period transients are only 18 minutes , which would be extremely short for an orbital period of a white dwarf binary. There is one other long period transient that is likely to be associated with a white dwarf.
The current landscape of long period transients is sparse. We need to find more of them to get a full understanding of these mysterious objects and how they work.
However, we now know that white dwarfs, with a little help from a stellar friend, can produce radio pulses just as bright as neutron stars.
Iris de Ruiter acknowledges support through the Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav) and the CORTEX project of the research programme NWA-ORC which is (partly) financed by the Dutch Research Council (NWO).
