This ultra-long period radio signal can’t make up its mind

Title: An emission-state-switching radio transient with a 54-minute period

Authors: M. Caleb, E. Lenc, D. L. Kaplan, T. Murphy, Y. P. Men, R. M. Shannon, L. Ferrario, K. M. Rajwade, T. E. Clarke, S. Giacintucci, N. Hurley-Walker, S. D. Hyman, M. E. Lower, Sam McSweeney, V. Ravi, E. D. Barr, S. Buchner, C. M. L. Flynn, J. W. T. Hessels, M. Kramer, J. Pritchard & B. W. Stappers

First Author’s Institution: University of Sydney, Camperdown, Australia

Status: Published in Nature Astronomy [open access]

The past couple decades have revealed an entire zoo of radio sources in the night sky, from powerful fast radio bursts that shine for just a few milliseconds, to the constant thrum of pulsars, the rapidly rotating, ultra-dense, and ultra-magnetic remnants of massive stars. One of the most striking recent additions to this radio zoo are a handful of mysterious sources that emit with extremely long periods. Take for instance GLEAM-X J1627-52 and GPM J1839-10, which have a period of 18 and 21 minutes respectively, and most recently, ASKAP J1935+2148, which has a stunningly long period of 54 minutes, nearly three times that of GLEAM-X J1627-52 and GPM J1839-10. For comparison, typical pulsar periods range from milliseconds to tens of seconds (See Figure 3). While radio astronomers have offered up two possible explanations for what kinds of objects could create these types of ultra-long period signals, namely magnetic white dwarfs (MWD) and pulsars, none seem to neatly fit into these categories. (See this astrobite to learn about past efforts to constrain GLEAM-X J1627-52 and GPM J1839-10). The authors of today’s paper find similar issues interpreting ASKAP J1935+2148. They are able to rule out MWDs, but cannot totally explain what could be creating these mysterious signals.

Chance Encounter

The first observation of ASKAP J1935+2148 was a happenstance detection made while searching for a gamma ray burst using the Australian Square Kilometer Array Pathfinder (ASKAP) on October 15th, 2022. It was then detected in four follow up observations, as well as a single weak detection in ASKAP archival data from three days prior. After November 5th, 2022, the source was no longer detected using ASKAP. So where did it go? 

Enter MeerKAT (originally the Karoo Array Telescope prior to its expansion into MeerKat). This array of 64 telescopes in South Africa was used to follow up at a higher frequency (1,284 MHz compared to 288 MHz at ASKAP). Two radio pulses, the characteristic blips indicating a signal rising above the background noise, were detected over two observations. All the observed pulses from both telescopes can be seen in Figure 1, ordered by the date of observation. You might notice the stark contrast between the MeerKAT and ASKAP pulses. This hints at one of the most remarkable things about ASKAP J1935+2148 (other than its ultra-long period), namely the dramatic change in its emission properties over time. 

Figure 1: Pulse profiles from both the ASKAP and MeerKAT observations. The MeerKAT pulses are notably much weaker and narrower than the ASKAP pulses, which the authors attribute to a change in emission mode intrinsic to the source, rather than being a result of differences between the two telescopes. Each pulse is aligned in phase, meaning it arrives when expected based on the period of the source. Image Credit: Figure 1 in today’s paper.

A Polarizing Topic

The authors identify 3 distinct modes to describe ASKAP J1935+2148’s emission. The main difference between the first two modes is how long they last, how strong they are, and their polarization. A radio wave’s polarization describes the orientation of its electric and magnetic fields. The first mode is marked by larger pulses with 10-50 second widths and high levels of linear polarization, which is when the electric field of the radio wave is confined to a single plane. (See Figure 2 for an example). 

The second mode was seen during the MeerKAT observations, and is differentiated by its much fainter (26 times fainter!) pulses and smaller pulse width of approximately 370 milliseconds. These pulses are also 70% circularly polarized, compared to the greater than 90% linear polarization seen in the strong pulses of the first emission mode. Circular polarization occurs when the electric field can be decomposed into two plane waves, each 90 degrees out of phase with the other. The total electric field vector will then appear to trace out a circle if observed in the direction of the source. 

The polarization of a radio wave can tell us a lot about the magnetic fields of the local environment that birthed it. Compact objects like neutron stars can sustain wildly powerful magnetic fields, billions and billions times greater than that of Earth. The changing polarization of ASKAP J1935+2148 suggests its magnetic field may be contributing to the change in its emission modes.

Figure 2: Diagram showing linear and circular polarization of a light wave. Both polarizations are special modes of elliptical polarization. Pulsars tend to be highly polarized, due to electrons being relativistically accelerated along curved magnetic field lines. Image Credit: Hyperphysics

So these MeerKAT pulses are 26 times weaker, and 135 times more narrow (again see Figure 1) compared to those detected by ASKAP! Are we even sure they are from the same source? What’s causing this stark contrast in emission mode? 

First of all, it’s important to note that MeerKAT is five times more sensitive than ASKAP and so it is not unusual that it would pick up much fainter pulses than ASKAP. The fact that the sky position and rotation measure (which indirectly measures the magnetic field of the “stuff” between us and the source) agree for both sets of observations also helps confirm the pulses originate from the same source. So the differences in modes observed is more likely due to differences happening at the source itself. 

What about the third mode? Because ASKAP J1935+2148 is periodic, the arrival time of future pulses can be predicted with amazing accuracy, down to hundreds of milliseconds. The authors find that there is not always a pulse observed when expected, providing evidence that the source may sometimes turn off or be quenched in some way. They estimate the signal is in this quenched mode 40-50% of the time. 

So to recap: Three modes, two of which show large differences in strength, duration, and polarization. What could be causing these different modes?

Walking the line

Let’s take a look at the two types of objects that have been suggested as sources for these ultra-long period transients: magnetic white dwarfs (MWD) and pulsars. Isolated MWDs have yet to be observed emitting in the radio, and the estimated magnetic field strength for a MWD that could produce ASKAP J1935+2148 would need to be larger than any previously observed (though it does still lie below the theoretical maximum). To fully rule out MWDs, the authors also constrain the radius of the source based on its period and this estimated magnetic field strength, finding that even the most conservative scenario could not be satisfied by a MWD. 

So if not a MWD, what about a pulsar? The periodic nature of ASKAP J1935+2148 and its narrow pulse width relative to its period, suggests the generation of relativistic particles is powering the radio emission, like in pulsars. Furthermore, the known pulsar PSR J1107-5907 already exhibits similar emission mode switching like ASKAP J1935+2148, meaning there are similarities in the existing neutron star population. But, the observed radio luminosity is still much larger than the one inferred by applying a pulsar model, so it’s unclear what could be powering the signal. The authors point to some more exotic mechanisms, such as energy released by breaking and reconnection of the pulsar’s magnetic field lines, as is theorized to happen in the Sun’s corona, but there is no clear preferred explanation. 

ASKAP J1935+2148, like GLEAM-X J1627-52 and GPM J1839-10, challenges our existing neutron star models and understanding of the existing population. It differentiates itself from past long period transients due to the varied emission modes. Interestingly, as seen in Figure 4, it also falls below the pulsar death line, the line marking the point at which pulsars are expected to no longer produce observable radio emission. ASKAP J1935+2148 falls squarely below this line, calling into question our understanding of how pulsars evolve over time. How could a pulsar have such a strong magnetic field at this point in its lifecycle? What is the source of its radio emission and what causes it to quench? How many of these kinds of sources should we expect to detect? These are just some of the questions left to be answered in the burgeoning field of ultra-long period radio transients! 

Figure 3: The classic period vs. period derivative chart for the known pulsar and pulsar-like population. The three long-period sources discussed in this paper, GLEAM-X J1627-52, GPM J1839-10, and ASKAP J1935+2148 all fall below the pulsar death line, the line differentiating the parameter space (colored in gray) where pulsars are no longer expected to emit radio waves that we could detect. Image Credit: Extended Data Fig. 1 from today’s paper.

Astrobite edited by Pranav Satheesh

Featured image credit: CSIRO

About Magnus L'Argent

Magnus is a first year Master's student and Trottier Fellow at McGill University. When not searching for new pulsars, fast radio bursts, and other radio transients, he enjoys going on hikes, reading sci-fi, and watching movies.

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