The Strange Galactic Radio Source That Kept Returning

A source appears in a radio survey, brightens, disappears and later returns. That sequence is enough to make an astronomer pay attention, especially when its other properties refuse to fit neatly into a familiar category.

The object designated ASKAP J173608.2-321635 attracted that attention in observations towards the centre of the Milky Way. Its long name records a position in the sky; the interesting part is how the signal behaved.

The Milky Way beside a schematic of a polarized radio signal — Separate sky imagery and a conceptual radio-wave illustration from the 2021 report; the wave graphic is not a photograph of the source. Source.

The discovery paper’s own description

Lead researcher Ziteng Wang described the new source in the passage quoted by the feature:

“We have presented the discovery and characterization of ASKAP J173608.2-321635: a highly-polarized, variable radio source located near the Galactic Center and with no clear multi-wavelength counterpart. ASKAP J173608.2-321635 may represent part of a new class of objects being discovered through radio imaging surveys.”

The paper also explained why more frequent observations were needed:

“Given that ASKAP J173608.2-321635 is typically not detected and can turn off on timescales from several weeks to as quickly as a day, our sparse sampling (12 epochs over 16 months) suggests that there could be other similar sources in these fields… Increasing the survey cadence and comparing the results of this search to other regions will help us understand how truly unique ASKAP J173608.2-321635 is and whether it is related to the Galactic plane, which should ultimately help us deduce its nature.”

Galactic Center Radio Transients were a comparison to investigate, not a settled identification. The discussion considered several familiar kinds of source and the ways the available data did or did not fit them. Polarization describes an organized property of electromagnetic radiation; it should not be reduced to an unexplained signal being physically “twisted” by dust. The central uncertainty in the 2021 story remains part of its scientific value.

Wang’s shorter reaction deserves its place beside the technical description: “We’ve never seen anything like it.” The paper’s comparison of possible sources considered X-ray binaries, gamma-ray bursts and supernovae. Their usual radio spectra, polarization or fading timescales did not fit this object well. Rejecting those familiar explanations narrowed the search; it did not establish one alternative identity or prove that every possible neutron-star explanation was impossible.

What made the observations unusual?

In its October 2021 research announcement, the University of Sydney described a highly polarised radio source whose brightness varied dramatically. Lead author Ziteng Wang and colleagues detected it using CSIRO’s ASKAP telescope in Western Australia and investigated it with other instruments.

Six detections over nine months in 2020 were followed by efforts to find counterparts at other wavelengths. MeerKAT in South Africa later caught the source again, with a faster disappearance than seen in the earlier observations.

The published account did not provide a settled identification. The point was that the combination of properties required further explanation.

A radio signal is still light

Radio astronomy observes electromagnetic radiation at wavelengths our eyes cannot see. A signal in this context does not imply a message, an intelligent sender or sound travelling through space.

Polarisation adds information about the radiation’s orientation. Together with changes in brightness, timing and observations at other wavelengths, it helps researchers compare possible physical sources.

Repeat observations do important work

An intermittent object presents a practical difficulty: another telescope can look in the right direction while the source is too faint to detect. A non-detection must therefore be interpreted alongside the timing and sensitivity of the observation.

This makes a survey more than a collection of sky photographs. Revisiting the same area can reveal behaviour that a single, beautiful image would miss.

For a different radio mystery connected to a more specific physical source, see the fast radio burst associated with a Galactic magnetar.

Keep the original uncertainty attached

The 2021 result is best understood as a documented discovery of unusual behaviour. It should not be promoted into evidence for extraterrestrial technology or described as a newly observed event today.

How scientists evaluate possible evidence for life offers a useful companion. In both cases, the rewarding question is what another measurement could tell us next.