Astronomy is advancing rapidly today, thanks in part to the way progress in one area can contribute to progress in another. For example, improved optics, instruments and data processing methods have allowed astronomers to push the boundaries of optical and infrared astronomy to gravitational wave (GW) astronomy. Radio astronomy is also making significant progress thanks to arrays such as the MeerKAT radio telescope in South Africa, which will work in the near future with observatories in Australia to Square kilometer array (SKA).
In particular, radio astronomers use next-generation instruments to study phenomena such as: 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 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 of the Jodrell Bank Center for Astrophysics at the University of Manchester. They were joined by researchers from the LakeTRAP (More Transients and Pulsars) group, 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†
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Neutron stars are the extremely dense remnants of massive stars that collapsed by gravity and shed their outer layers in a supernova. These stars often have very fast rotations, and their powerful magnetic fields cause them to emit tight beams of radiation that hurtle 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 fast 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 sensed 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. Like dr. Manisha Caleb, a former postdoctoral researcher at the University of Manchester and a current astrophysical researcher at the University of Sydney, said:
“Amazingly, we detect radio emission from this source only 0.5% of its rotational period. This means that it is very coincidental that the radio beam crossed the Earth. It is therefore likely that there are many more of these very slow-spinning sources in the Milky Way, which has important implications for how neutron stars are born and age.
“Most pulsar studies don’t look very long for periods, so we have no idea how many of these sources there are. In this case, the source was bright enough to detect the single pulses with the MeerTRAP instrument at MeerKAT.”
“The sensitivity that MeerKAT offers, combined with the advanced search possible with MeerTRAP and the ability to capture simultaneous images of the sky, made this discovery possible,” added Dr. Heywood, a senior researcher at the University of Oxford and a member of the ThunderKAT team who contributed to this study. “Even then it took an eagle eye to spot it for something that may have been a real source because it looked so unusual!”
The newly discovered neutron star, designated PSR J0901-4046 (in front of pulsating Radio source), is a particularly interesting object that exhibits features of pulsars, magnetars, and even fast radio bursts. This is indicated by the radio emissions consistent with pulsars – which are also known for their longer orbital times. In contrast, the chaotic subpulse components and the polarization of the pulses are consistent with magnetars. Not only was this discovery a new type of neutron star that had been theorized before, but it happened in a well-studied part of the galaxy.
Radio surveys do not usually look for neutron stars or pulse periods that last longer than a few tens of milliseconds (ie, pulsars of milliseconds). Ben Stappers, professor of astrophysics at the University of Manchester and principal investigator of the MeerTRAP project, says this discovery could mean there are plenty of opportunities for new radio explorations in the region:
“The radio emission from this neutron star is unlike anything we’ve seen before. We can watch it for about 300 milliseconds, which is much longer than for most other radio-emitting neutron stars. There seem to be at least 7 different pulse types, some of which show a highly periodic structure, which can be interpreted as seismic vibrations of the neutron star. These pulses can give us a vital insight into the nature of the emission mechanism for these sources.”
Given how challenging this discovery was and the concerted effort it took to make it, it’s likely difficult to detect similar sources. However, this implies that there could be a larger population of undiscovered 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 link between highly magnetized neutron stars, ultra-long-period magnetars and fast radio bursts.
These results could help solve the enduring mystery of what causes FRBs, which have puzzled astronomers since the first was discovered in 2007 (the Lorimer Burst). This is especially true in the rare cases where the source has repeated itself in nature. While the study of this energetic phenomenon has also progressed significantly, astronomers still don’t know what causes it – with explanations ranging from rotating neutron stars and black holes to possible extraterrestrial transmissions!
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