Mysterious signal emanating from our galaxy could be one of the rarest known objects

(ORDO NEWS) — The mysterious, repeating radio signal in the Milky Way that has puzzled astronomers may be such a rare object that only one such object has been tentatively identified.

A signal called GLEAM-X J162759.5-523504.3 could be a white dwarf radio pulsar, according to an astrophysicist Jonathan Katz of Washington University in St. Louis, posted on the arXiv preprint server and not yet peer-reviewed.

“From the early days of pulsar astronomy, there have been speculations that a rotating magnetic white dwarf might exhibit pulsar-like activity,” Katz writes in his paper.

“The recently discovered periodic radio transition object GLEAM-X J162759.5-523504.3 is a candidate for the first true white dwarf pulsar.

It has a period of 18.18 minutes (1091 s), and its pulses exhibit low-frequency (72-215 MHz) radiation with a brightness temperature ∼ 1016 K, which suggests coherent radiation. It has no binary companion to interact with. Thus, it meets the criteria for a classical pulsar, although its period is hundreds of times longer than any of them.”

When a star dies, various outcomes are possible: after ejecting its outer material and core, no longer supported by the external fusion pressure, it collapses under its own gravity.

If the mass of the precursor star exceeds the mass of the Sun by about 30 times, the core collapses into a black hole.

A progenitor star with 8 to 30 times the mass of the Sun gives rise to a neutron star about 20 kilometers (12 miles) across and about 1.4 times the mass of the Sun.

The core of the progenitor star, eight times the mass of the Sun, collapses into a white dwarf, collecting a mass 1.5 times that of the Sun into a ball the size of the Earth and the Moon.

Pulsars are a subset of neutron stars. These are neutron stars that rotate insanely fast and at such an angle that beams of bright radio waves emanating from the magnetic poles rush past the Earth with each rotation – on a scale from seconds to milliseconds. (Here’s how it sounds in the audio.)

Scientists wondered if this behavior could be observed in white dwarf stars, and in 2016 they seem to have come close to it by observing a star called AR Scorpii. Trapped in a red dwarf star binary, AR Scorpii flares up every few minutes.

However, as Katz points out, its binary orbit is closer than that of neutron star pulsars in binary systems, and the periodic signal lacks coherence. This means that the physical processes that generate the signal can be very different from traditional radio pulsars.

This brings us back to GLEAM-X J162759.5-523504.3, located about 4,000 light years from Earth. From January to March 2018, data collected by the Murchison Wide Field Array in the Australian desert showed it pulsing brightly for about 30 to 60 seconds every 18.18 minutes – one of the brightest objects in the sky at low frequency radio.

It doesn’t match the profile of any known astronomical object, but the research team that discovered it thought it might be a hypothetical object known as an ultra-long-period magnetar. This is a neutron star with an unusually powerful magnetic field, but the explanation still did not quite fit.

“No one expected to directly detect such a star because we didn’t expect them to be that bright,” explained then astrophysicist Natasha Hurley-Walker of the Curtin University node of the International Center for Radio Astronomy Research (ICRAR) in Australia. “Somehow it converts magnetic energy into radio waves much more efficiently than anything we’ve seen before.”

A pulsar was considered as a possible option, but there are two main problems: the first is a long period of rotation, and the second is that the pulses were too bright for a neutron star pulsar. Both of these problems, according to Katz, are solved if the object is a white dwarf.

If so, this would be the first white dwarf discovered that shares the physics and emission mechanism of traditional radio pulsars.

This means that GLEAM-X J162759.5-523504.3 may become a promising target for optical observations; although white dwarfs are very dim and we may not be able to see visible light at that distance. However, given the possibility, it’s worth a try.

In addition, astronomers could study other white dwarfs to see if they match any of the properties of GLEAM-X J162759.5-523504.3.

“If it were bright enough, optical observations could also determine its magnetic field spectroscopically or polarimetrically,” Katz explained.

“Fast rotating, highly magnetized white dwarfs would be promising targets for low-frequency radio observations to determine if any of them are white dwarf pulsars.”

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