A Pulsar has Been Found Turning so Slowly Astronomers Didn't Even Think it was Possible: Once Every 76 Seconds

Astronomy is progressing rapidly these days, thanks in part to how advances in one area can contribute to progress in another. For instance, improved optics, instruments, and data processing methods have allowed astronomers to push the boundaries of optical and infrared to gravitational wave (GW) astronomy. Radio astronomy is also advancing considerably thanks to arrays like the MeerKAT radio telescope in South Africa, which will join with observatories in Australia in the near future to create the Square Kilometer Array (SKA).

In particular, radio astronomers are using next-generation instruments to study phenomena like Fast Radio Bursts (FRBs) and neutron stars. Recently, an international team of scientists led by the University of Manchester discovered a strange radio-emitting neutron star with a powerful magnetic field (a “magnetar”) and an extremely slow rotational period of 76 seconds. This discovery could have significant implications for radio astronomy and hints at a possible connection between different types of neutron stars and FRBs.

The research was led by astrophysicists Manisha Caleb, Ian Heywood, and Benjamin Stappers from the Jodrell Bank Centre for Astrophysics at the University of Manchester. They were joined by researchers from the MeerTRAP (More Transients and Pulsars) group, an international consortium funded by the European Research Council (ERC) that collaborates closely with the Max-Planck Institut für Radioastronomie (MPIfR) and multiple European universities and research institutes. The paper that describes their discovery recently appeared in Nature Astronomy.

Neutron stars are the extremely dense remnants of massive stars that have undergone gravitational collapse and shed their outer layers in a supernova. These stars often have very fast spins, and their powerful magnetic fields cause them to emit tight beams of radiation that sweep across the sky (hence the term “magnetar”). Astronomers are currently aware of about 3,000 pulsars in the Milky Way galaxy, and the timing of their pulses is used as a sort of “astronomical beacon” (or “cosmic lighthouse”).

In all previous cases, magnetars have been observed to have rapid rotational periods. But in this case, the team observed what appeared to be an “ultra-long period magnetar,” a theoretical class of neutron stars with extremely strong magnetic fields. The source was initially detected thanks to a single pulse observed by the MeerTRAP instrument piggybacking on observations led by The HUNt for Dynamic and Explosive Radio transients with meerKAT (ThunderKAT) team.

The two then conducted follow-up observations together that confirmed the position of the source and the timing of the pulses. As Dr. Manisha Caleb, a former postdoctoral researcher from the University of Manchester and a current astrophysical researcher at the University of Sydney, said:

“Amazingly we only detect radio emission from this source for 0.5% of its rotation period. This means that it is very fortuitous that the radio beam intersected with the Earth. It is therefore likely that there are many more of these very slowly spinning sources in the Galaxy which has important implications for how neutron stars are born and age.

“The majority of pulsar surveys do not search for periods this long and so we have no idea how many of these sources there might be. In this case the source was bright enough that we could detect the single pulses with the MeerTRAP instrument at MeerKAT.” 

The three types of neutron stars and the characteristics that define them. Credit: NASA/JPL-Caltech

“The sensitivity that MeerKAT provides, combined with the sophisticated searching that was possible withMeerTRAP and an ability to make simultaneous images of the sky, made this discovery possible,” added Dr. Heywood, a senior researcher with the University of Oxford and a member of the ThunderKAT team who collaborated on this study. “Even then, it took an eagle eye to recognize it for something that was possibly a real source because it was so unusual looking!”

The newly-discovered neutron star, designated PSR J0901-4046 (for Pulsating Radio Source), is an especially interesting object that shows characteristics of pulsars, magnetars, and even fast radio bursts. This is indicated by the radio emissions that are consistent with pulsars – which are also known for having longer orbital periods. In contrast, the chaotic sub-pulse components and the polarization of the pulses are consistent with magnetars. In addition to being a new type of neutron star that was only theorized previously, this discovery occurred in a well-studied part of the galaxy.

Radio surveys don’t usually search for neutron stars or pulse periods that last more than a few tens of milliseconds (i.e., millisecond pulsars). Ben Stappers, a professor of astrophysics at Manchester University and the Principal Investigator of the MeerTRAP project, says that this discovery could mean that there are plenty of opportunities for new radio surveys in the region:

“The radio emission from this neutron star is unlike any we have ever seen before. We get to view it for about 300 milliseconds, which is much longer than for the majority of other radio emitting neutron stars. There seem to be at least 7 different pulse types, some of which show strongly periodic structure, which could be interpreted as seismic vibrations of the neutron star. These pulses might be giving us vital insight into the nature of the emission mechanism for these sources.”

Given how challenging this discovery was and the collaborative effort it took to make it, detecting similar sources is likely to be difficult. However, this implies that there could be a larger population of undetected long-period neutron stars just waiting to be discovered. This discovery also raises the possibility of a new class of radio transients – ultra-long period neutron stars – that suggest a possible connection between highly-magnetized neutron stars, ultra-long period magnetars, and fast radio bursts.

These results could help resolve the enduring mystery of what causes FRBs, which astronomers have puzzled over since the first was detected in 2007 (the Lorimer Burst). This is especially true in the rare instances where the source has been repeating in nature. While the study of this energetic phenomenon has also advanced considerably, astronomers are still unsure what causes them – with explanations ranging from rotating neutron stars and black holes to possible extraterrestrial transmissions!

Further Reading: The University of Manchester, Nature Astronomy