You’ll be a limbo star. How (s)low can you go?

Title: A radio transient with unusually slow periodic emisison

Authors: N. Hurley-Walker, X. Zhang, A. Bahramian, S. J. McSweeney, T. N. O’Doherty, P. J. Hancock, J. S. Morgan, G. E. Anderson, G. H. Heald & T. J. Galvin  

First Author’s Institution: International Centre for Radio Astronomy Research (ICRAR), Curtin University, Bentley, Western Australia, Australia

Status: Nature [Accepted — Closed Access]

All around the limbo clock, hey let’s do the limbo rock! Limbo lower now. Limbo lower now. How (s)low can you go? 

Probably not as slow as the source discovered by today’s authors! This newly discovered object is rotating with a period of 18.18 minutes. While that’s a lot faster than the rotation of the Earth (with a period of one day), it’s a lot slower than the rotation period of radio sources such as pulsars (neutron stars which emit brilliant, sweeping beams of light like a lighthouse) and magnetars (neutron stars with extra strong magnetic fields), for which periods are ~seconds or milliseconds. And this source seems to be a lot more similar to pulsars and magnetars than it is to our own Earth. So what exactly is going on here? Have we discovered a new type of object, or just a special case of an already known one?

We’ve got a detection, folks!

The initial discovery of this unusually long-period source included two detections within a 24 hour period with the Murchison Widefield Array (MWA), a low-frequency radio telescope in Western Australia. The MWA was being used to survey the Galactic plane as a part of the GLEAM-X survey (the Galactic and Extragalactic All-sky MWA Extended Survey). GLEAM-X is a follow-up of the GLEAM survey, which was an all-sky low-frequency radio survey aimed at studying things like radio galaxies, active galactic nuclei, supernova remnants, and pulsars (just to name a few).

And another 71 detections!

After the initial two detections, the authors did a detailed search of older GLEAM-X data and found an additional 71 pulses from the same sky location between January and March of 2018. The pulses (shown below in Fig. 1) are exceptionally long, lasting for 30 to 60 seconds. As a comparison, bursts from pulsars and fast radio bursts (extragalactic bursts of radio energy) typically only last for ~milliseconds

Figure 1: A handful of the pulses seen from this source. The top three pulses look fairly “smooth” while there is structure within some of the later pulses. The magenta bursts were detected at an observing frequency of 215 MHz while the cyan bursts were detected at 88 MHz.

Interestingly, many of the pulses seem to have sub-pulses – i.e., pulses WITHIN the main pulse. Some even seem to have sub-pulses with widths < 0.5 seconds! However, the author’s do not have data on timescales shorter than 0.5 seconds, so they are unable to explore any of these really short sub-structures in detail (although they use them to deduce that this source is a compact object!). 

In addition to the ultra-long period of 18.18 minutes, the source also very weirdly “shut off” (i.e., it produced no detectable radio emission) for a 26 day interval after having been “on” for 30 days. It was then on for another 30 days before shutting off again. It has yet to turn back on (at least at a level detectable by the MWA and within the times of the observations). 

So what might it be?

The authors are able to use the dispersion measure of the source, a proxy for distance, to determine that the source is located in our own Galactic backyard, at a distance of 4200 light years. Additionally, the short sub-pulses with durations less than 0.5 seconds imply that this source is a compact object, since the maximum diameter of the object is equal to the variation time*speed of light.

As you can see in Fig. 2 below, our mysterious source (whose official name is GLEAM-X J162759.5-523504.3) is most similar to pulsars, RRATs (Rotating RAdio Transients), and another transient radio source, GCRT 1745, who’s got a 77 minute periodicity with 10 minute-long bursts. Unfortunately, there is no known progenitor for GCRT 1745, although there was some speculation that it might be an ultra-long period magnetar. 

Figure 2: Luminosity vs. (frequency times burst width) for a variety of astrophysical sources. GLEAM-X J162759.5-523504.3 (shown with a solid black diamond) is closest to GCRT 1745 (shown with a brown circle), RRATs (shown with red dots), and pulsars (shown with blue dots).

Due to the speculation that GCRT 1745 might be an ultra-long period magnetar – in addition to the similarity of GLEAM-X J162759.5-523504.3 to pulsars/RRATs – the authors explore a magnetar origin for their new source. Some of the most striking evidence for a magnetar origin has to do with the source’s luminosity. Emission from radio pulsars is often fueled by the fact that their rotation speeds are decreasing, thus converting rotational energy into other forms of energy. Pulsars typically emit a very small fraction of this “spin-down energy” in the radio band. However, the spin-down energy of GLEAM-X J162759.5-523504.3 is much SMALLER than the extrapolated luminosity of the detected radio emission…. so there must be another source of energy fueling the detected emission! One such possibility: magnetic energy, as in the case of a magnetar. 

The windows of radio emission (i.e. 30 days on, 26 days off, 30 days on) are also more consistent with a magnetar picture than a pulsar picture. However, magnetars are also typically detected in the X-ray band. While the authors are able to get some observing time with the Swift X-ray Telescope, they do not detect anything… BUT, there’s still hope! Their X-ray limits cannot constrain the predicted X-ray emission from this source, and so they can’t rule out the possibility that X-ray emission is in fact produced. 

Admittedly, this seems to be placing a lot of hope in the magnetar category. What about some other sources? The authors also discuss the possibility that our mystery source might be a white-dwarf with really high spin-down luminosity. 

Even if this source doesn’t turn out to be an ultra-long period magnetar, it’s still the first source with a minute-long periodicity, opening the door for searches for other similar sources! And, with ever more detections, we can hopefully start to draw a better picture of unknown radio transients such as GLEAM-X J162759.5-523504.3 and GCRT 1745.

Astrobite edited by Ryan Golant

Featured image credit: ESO/L. Calçada

About Alice Curtin

I’m a PhD student at McGill University studying Fast Radio Bursts and pulsars using the Canadian Hydrogen Mapping Experiment (CHIME). My work mainly focuses on characterizing radio frequency interference, investigating possible relationships between gamma-ray bursts and FRBs, and using pulsars as calibrators of future radio instruments. When not doing research, I typically find myself teaching physics to elementary school students, spending time with friends, or doing something active outside.

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