Pulsar Found Spinning So Slowly Astronomers Didn’t Even Think It Was Possible: Once Every 76 Seconds

Astronomy is advancing rapidly these days, thanks in part to how progress in one area can contribute to progress in another. For example, improvements in optics, instruments, and data processing methods have allowed astronomers to push the boundaries of optical and infrared astronomy to that of gravitational waves (GW). Radio astronomy is also progressing considerably thanks to networks such as the MeerKAT Radio Telescope in South Africa, which will soon join observatories in Australia to create the square kilometer network (SKA).

In particular, radio astronomers use new generation instruments to study phenomena such as Fast radio bursts (FRB) and neutron stars. Recently, an international team of scientists led by the University of Manchester discovered a strange neutron star emitting radios with a strong magnetic field (a “magnetar”) and an extremely slow rotation period of 76 seconds. This discovery could have important 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 Center for Astrophysics at the University of Manchester. They were joined by researchers from MeerTRAP (More Transients and Pulsars), an international consortium funded by the European Research Council (ERC) which works closely with the Max Planck Institute for Radio Astronomy (MPIfR) and several European universities and research institutes. The article describing their discovery recently appeared in natural astronomy.

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

In all the previous cases, magnetars have been observed to have rapid rotation 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 superimposed on the observations carried out by Hunting dynamic and explosive radio transients with meerKAT (ThunderKAT) team.

The two then made follow-up observations together which confirmed the position of the source and the timing of the pulses. As Dr Manisha Caleb, a former postdoctoral researcher at the University of Manchester and currently an astrophysical researcher at the University of Sydney, put it:

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

“The majority of pulsar readings don’t look for such long periods and so we have no idea how many of these sources there might be. In this case, the source was bright enough for us to detect the pulses. unique with the MeerTRAP instrument at MeerKAT.”

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

“The sensitivity provided by MeerKAT, combined with the sophisticated research possible with MeerTRAP and the ability to create simultaneous images of the sky, made this discovery possible,” added Dr Heywood, Principal Investigator at the University of Oxford and member of the ThunderKAT team who collaborated on this study. “Even then, it took an eagle’s eye to recognize it for something that was possibly a real source because it was so unusual!”

The newly discovered neutron star, designated RPS J0901-4046 (for Ppulsating Rbye Source), is a particularly interesting object that shows the characteristics of pulsars, magnetars and even fast radio bursts. This is indicated by radio emissions consistent with pulsars – which are also known to have longer orbital periods. On the other hand, the chaotic components of the sub-pulses and the polarization of the pulses are compatible with magnetars. Besides being a new type of neutron star that had only been theorized before, this discovery occurred in a well-studied part of the galaxy.

Radio surveys generally do not look for neutron stars or periods of pulses that last more than a few tens of milliseconds (i.e. millisecond pulsars). Ben Stappers, professor of astrophysics at the University of Manchester and principal investigator of the MeerTRAP project, says the discovery could mean there are many opportunities for further radio surveys in the region:

“The radio emission from this neutron star is unlike any we’ve seen before. We can see it for about 300 milliseconds, which is much longer than most other neutron stars emitting radios. There appear to be at least 7 different types of pulses, some of which exhibit a strongly periodic structure, which could be interpreted as seismic vibrations of the neutron star.These pulses could give us vital insight into the nature of the mechanism emissions from these sources.

Given the difficulty of this discovery and the collaborative effort it took to make it, detecting similar sources is likely to be difficult. However, it does imply 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 – which suggest a possible link between strongly magnetized neutron stars, ultra-long-period magnetars and fast radio bursts.

These results could help solve the lingering mystery of what causes FRBs, which have puzzled astronomers since the first one was detected in 2007 (the Lorimer Burst). This is especially true in the rare cases where the source is repeated in nature. Although the study of this energetic phenomenon has also progressed considerably, astronomers still do not know what causes them – with explanations ranging from the rotation of neutron stars and black holes to possible extraterrestrial transmissions!

Further reading: The University of Manchester, natural astronomy

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