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The Slow Radio Pulse Mystery

by John G. Cramer

Alternate View Column AV-226

Keywords:
slow, radio pulses, radio astronomy, neutron star, hot sub-dwarf 
Published in the September-October-2023 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 05/09/2023 and is copyrighted 2023 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

The low frequency radio band of the cosmic radio spectrum (about 0.1 to 2.0 GHz) observed with modern radio telescopes is relatively empty, populated mainly by periodic few-second pulsar blips and the random long-term scintillations of active galactic nuclei (active supermassive black holes doing their thing in distant galaxies).  The pulsars emitting these periodic radio bursts are rapidly rotating neutron stars that beam out radio waves.  The pulsar radio beams, containing frequencies of around 0.4 to 1.4 GHz, are emitted from the neutron star's magnetic poles and are swept around in a geometric cone as the neutron star spins on its rotational axis, rather like the beam of a lighthouse.  Earth-based radio astronomers detect these emissions as radio blips observed when a pulsar's radio beam happens to be properly aligned to sweep past the Earth-based antenna system every few seconds, like a ticking clock.

A neutron star is formed after the supernova of a medium-mass star (1.2 to 3.0 solar masses), and as it collapses it typically shrinks in size to a sphere of radius around 10 kilometers and a density of about 1017 kg/m3 (100 quadrillion times the density of water).  Because the pre-collapse star's angular momentum is conserved during collapse to a much smaller radius, its spin rate after collapse can be as high as 716 rotations per second.  This enormous spin rate gradually decreases to less than one rotation per second as the star ages and sheds it initial angular momentum over a time period of a few hundred thousand years.  Since the first pulsar discovery in 1967, radio astronomers have found over 3,300 pulsars, and that number is still growing.  The slowest pulsar discovered so far, named PSR J0250+5854, makes one rotation every 23.5 seconds.


Recently Dr. Natasha Hurley-Walker, a radio astronomer at the International Centre for Radio Astronomy Research (ICRAR) in Australia assigned her Curtin University undergraduate students the project of combing through 24 hours of recorded cosmic radio data, looking for repetitive structures.  The in-galactic-plane data was from a large sky survey called GLEAM-X (GaLactic and Extragalactic All-sky MWA eXtended) that had been recorded by the Murchison Widefield Array (MWA) , one of the most sensitive radio telescopes on Earth, located in the Australian Outback.

Her student Tyrone O'Doherty, who is now a graduate student at ICRAR, noticed a strong radio pulse in GLEAM-X records and found that it repeated.  In the low-frequency radio band he found slowly repeating radio pulses that were remarkably bright and had a period of 18.18 minutes (i.e., repeating every 1,091 seconds or around 50 times slower than the slowest known pulsar).

Hurley-Walker's group then systematically searched a broader range of GLEAM-X data, looking for more periodic transients with the observed period.  In data that were recorded between January and March 2018, they found a total of 71 of the periodic pulses.  These were 30-60 seconds wide in the frequency range 72-231 MHz.  They gave the emitting object the name GLEAM-X J162759.5-523504.3.

The source was the brightest object in the radio-sky during the brief time-span of the pulses.  Searching earlier and later data indicated that the source was not visible before it went through an active period in January 2018, became quiet for most of February, and turned back on for most of March 2018.  It was not visible after that.  In other words, the object had only two active about-30-day periods in early 2018.  A check with X-ray astronomers also showed that during the two active periods in the radio region there was no accompanying X-ray activity.

Hurley-Walker's group published their results in the January 26, 2022 issue of the journal Nature.  They reported the period of the repeating pulses to be 1,091.1690 0.0005 seconds.  The pulses had a strong linear polarization (881%), indicating the influence of a strong magnetic field, and a direction of polarization that was constant with time and with position within the pulse.  Using the observed frequency dispersion of the pulses, the group estimated the distance to the source object from Earth to be 1.3 0.5 kpc (i.e., about 4,240 light-years) and found that it was in the galactic plane.  For scale, the distance to the center of our galaxy is about 26,000 light-years, so the object must be within our galaxy.


What could be the source of the pulses?  As we mentioned above, there are no neutron-star pulsars observed to have such a long period.  Further, a calculation shows that the energy available for a radio beam from a slow-rotating neutron star would be much too small to account for the object's radio brightness.  Therefore, we must reject the possibility that it is just a slow pulsar and look elsewhere for a source process.

After the discovery of radio pulses with a regular 18-minute period was announced, there was some speculation on the Internet that this might be a techno-signal, an indication of an intentional or accidental signal from a high-tech alien civilization (what astronomers jokingly call the LGM hypothesis, i.e., Little Green Men).  Perhaps the pulses came from some high technology broadcast device located on the surface of a spinning planet, a moon, or an asteroid.  Perhaps it was mounted on a space station that was rotating every 18 minutes.  Perhaps it was from a satellite with an 18-minute orbit around a star or planet.  The strength and periodicity suggest a beam from a spinning or orbiting source, but the value of 18 minutes is not easy to explain.

While a spinning asteroid might have a rotation period of 18 minutes, a massive planet would be unlikely to have a planetary "day" that was so short.  Further, objects in low-Earth-orbit have a orbital period of about 100 minutes or longer, so an 18 minute satellite orbit would require a planet at least 6 times more massive than Earth.  A large 2001-doughnut-style rotating space station rotating every 18.18 minutes would have to have a radius of 179 km (i.e., much too large) to have an internal centrifugal pseudo-gravity of 1 g.  Thus, even for the LGM hypothesis, the 18-minute period is a problem.

Moreover, there is evidence against the LGM hypothesis.  Hurley-Walker's group observed that the signal was spread across a wide range of frequencies.  They take this as an indication that the signal is likely to be from a natural process and not an artificial signal, since SETI surveys expect and look for narrow-band signals.  Further, the strong linear polarization suggests that the source object had a very strong magnetic field, which is unlikely for an artificial source.


Fortunately, there is now a fairly plausible natural explanation for the long period, provided by Harvard astronomer Avi Loeb and his colleague Dani Maoz.  They have suggested that the emitting object is not a neutron star, but rather a "hot sub-dwarf", i.e., the remnant collapsing core of a burned-out Sun-like star that is too low in mass to form a supernova as it starts to cool and shrink to a white dwarf.  Such a remnant typically will end up as a white dwarf with around half the mass of the Sun and about the size of the Earth.  However, in the early hot sub-dwarf phase of collapse it would be about a third of the radius of the Sun, or equivalently thirty times larger than Earth.  Loeb and Maoz suggested that the 18-minute object is a such a hot sub-dwarf that is at beginning of its initial descent to becoming a white dwarf.  It rotates on its axis in 18 minutes, a period common to hot sub-dwarfs, and it has a strong magnetic field trapped by the collapse.  Like a regular pulsar, it shines twin radio beams from its magnetic poles as it rotates.

From calculations they find that the hypothesized sub-dwarf would have to rotate at close to the maximum possible speed (just below centrifugal break up) and that its radiated power would have to be close to the highest possible value (just below destruction by radiation pressure).  To be at these maximum values, it would have to be spun up and powered by the accretion of infalling matter from a companion star.  Thus, the GLEAM-X object may be just a long-period pulsar-like object in a binary orbit, which we happen to see at a very early stage of white dwarf formation, rather than a later stage and more massive neutron star.

This scenario suggests a testable prediction: if the sub-dwarf is in a close orbit with a companion star, the Doppler shift from its orbital velocity should slightly modulate the emitted radiation.  Loeb and Maoz suggest that the GLEAM-X data should be reanalyzed to search for such modulations of its period and spectrum, from which the characteristics of the hypothesized binary system might be extracted.


Is the Loeb-Maoz explanation plausible?  It seems reasonable, but there is one thing about it that bothers me about it: the fact that such a long-period pulsar object has been briefly observed only once.  Our galaxy has very many Sun-like stars in the right mass range, many of which are members of binary systems and many of which should be reaching the burn-out stage and forming hot sub-dwarfs.  Why, then, have we not seen more of these long-period pulsar-like objects in the 55 or so years that radio astronomers have been searching for, detecting, and studying the radio emissions of pulsars.  I think a statistical analysis of observation likelihood is needed.


John G. Cramer's 2016 nonfiction book describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at:
       
http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402 .

SF Novels:  John's 1st hard SF novel Twistor is available online at: https://www.amazon.com/Twistor-John-Cramer/dp/048680450X.   His 2nd and 3rd novels, Einstein's Bridge and its new sequel Fermi's Question, are now available as eBooks from Baen Books at: https://www.baen.com/einstein-s-bridge.html and https://www.baen.com/fermi-s-question.html .

Alternate View Columns Online: Electronic reprints of 226 or more of "The Alternate View" columns written by John G. Cramer and previously published in Analog are currently available online at:  http://www.npl.washington.edu/av .


References:

Slow Periodic Radio Pulses:

N. Hurley-Walker, et al, A radio transient with unusually slow periodic emission, Nature 60, 526-530 (2022).

Abraham Loeb and Dan Maoz, A Hot Subdwarf Model for the 18.18 minute Pulsar GLEAM-X. preprint ArXiv 2202.04949v1 (2022).


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