Title: A 2.9 hr Periodic Radio Transient with an Optical Counterpart
Authors: N. Hurley-Walker, S. J. McSweeney, A. Bahramian, N. Rea, C. Horváth, S. Buchner, A. Williams, B. W. Meyers, Jay Strader, Elias Aydi, Ryan Urquhart, Laura Chomiuk, T. J. Galvin, F. Coti Zelati, and Matthew Bailes
First Author’s Institution: International Centre for Radio Astronomy Research, Curtin University, Australia
Status: Published in Astrophysical Journal Letters [open access]
The radio transient sky is full of mysteries. From the familiar trilling of pulsars to the flash of fast radio bursts to, more recently discovered, long period radio transients (LPTs). While sources like pulsars tend to send out their radio signals in periods ranging from milliseconds to tens of seconds and fast radio bursts in milliseconds, these LPTs have astoundingly long periods, on the order of minutes to hours. (See this and this Astrobite for a quick refresher). We also have a good idea of what pulsars are (rotating neutron stars), while fast radio bursts and LPTs are still a mystery. The discovery from today’s paper is no slouch, boasting a period of nearly 3 hours. At the time of publishing, it is the slowest of these kinds of sources and one of the first to be definitively linked to an observed star, providing insight into what could be causing these LPTs.
One big family
It seems like every month now there is a new LPT and as the field continues to quickly grow, it’s becoming harder and harder to keep track of each new source. Strap in for some complicated names!
Starting in 2022, we have GLEAM-X J1627-52, which repeated every 18 min but was only detected for a few months. No optical or infrared companion for this source has been detected. By contrast, GPM J1839-10, which has a period of 22 min, was found to have been repeating for at least three decades. ASKAP J1935+2418 had an even longer period of 54 min. There has also been CHIME J0630+25 (P ~ 7 minutes), the closest LPT so far discovered, as well as the binary red dwarf and white dwarf (WD) system ILT J1101+5521 (P ~ 2hr).
Phew! That’s a lot of letters and numbers. Finding out more about these systems by observing them at other wavelengths has proved difficult, due to these systems residing in the plane of our Galaxy, meaning there are many stars crowded around them. This makes it a challenge to pinpoint which optical source might be associated with a given LPT (though not impossible, as with ILT J1101+5521). As such, it has also been difficult to definitively rule out different kinds of objects that might be responsible, from magnetic WDs to magnetars.
Enter GLEAM-X J0704−37, an LPT with a period for 3 hrs that has also been linked to a red dwarf.
One pulse is all it takes
GLEAM-X J0704−37 was found using the Murchison Wide Field Array in Australia. By taking 2 minute radio snapshots of the southern sky, then searching through the observations, the authors discovered a single 30 second long pulse of radio emission. Going back through archival data, they found 33 more pulses from the source, dating as far as 2013. From follow up observations by the higher frequency MeerKAT radio telescope in South Africa, they were able to more accurately determine its location and confirm that these pulses occurred every 3 hours.
They then searched through images from the Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile and the ESA Gaia space telescope, identifying a clear optical counterpart in the same location as GLEAM-X J0704−37 (Figure 1). Now that they had their star, they just needed to figure out what exactly it was. Performing spectroscopy using the Southern Astrophysical Research (SOAR) Telescope, also in Chile, they infer it is a red dwarf, an extremely common and low mass star.
Micro pulses and timing noise
There’s a lot going on with GLEAM-X J0704−37 that makes it even more mysterious. The authors try and create a timing model for the source, as is often done with other periodic sources like pulsars. They find the best fit by assuming a super-orbital period of 6.3 years (Figure 2). On top of this, there is also a quasi periodicity of 40 ms within the brightest bursts. Small scale structure within bursts have been seen in pulsars like the Crab pulsar, but these microstructures are still not widely understood and the authors say it would be a bit hasty to use this as evidence for a pulsar origin.
So what is it?
The most obvious answer would be that the radio emission is simply coming from the red dwarf itself. Some stars are known to have periodic pulsations, but these aren’t normally strongly linearly polarized like GLEAM-X J0704−37 nor would any we have detected be strong enough at this distance. (See this Astrobite for a quick guide to polarized light!)
So if not the red dwarf, then maybe there is an as yet undetected companion lurking in this system. Because of the strong linear polarization, the source is likely powerfully magnetic, which suggests a compact object like a pulsar or a WD, but neither option is exactly a perfect match.
A highly magnetized pulsar like a magnetar could explain the burst, but a magnetar’s magnetic field would have greatly decayed by now. It could be an older pulsar that’s been spun up by material from the red dwarf, but then it would be spinning too quickly to account for the 3-hour period.
The authors posit several different WD scenarios. It could be a ‘polar’ system, where material from the red dwarf accretes onto the WD, powering the radio emission. The spin period and binary period would be the same in this case (3 hours), thereby making the extra 6 year periodicity some kind of unexplained timing noise.
If the spin orbit is a bit faster, then the system might be analogous to AR-Scorpii (learn more about AR-Scorpii in this Astrobite), a WD and red dwarf ‘pulsar’ system, where the WD emits a beam of energy that sweeps across its companion, leading to electromagnetic emission from the red dwarf. But the two known WD pulsars normally have periods on the order of minutes, so too slow for GLEAM-X J0704−37.
Further observations will be needed to find this white dwarf companion, since the observations the authors have so far are not greatly constraining. More radio observations could also help constrain the timing and unveil the strange 6 year periodicity. This system poses lots of unanswered questions for LPTs, but also many opportunities for getting one step closer to their origin!
Astrobite edited by Megan Masterson
Featured image credit: K. Miller, Caltech/IPAC
