South Africa’s MeerKAT Radio Telescope Discovers Millisecond Pulsars Inside Globular Clusters
A pulsar is the crushed core of an exploded star, a rapidly spinning cinder that repeatedly swings a beam of light in our direction. Pulsars are among the most exotic celestial bodies known. They have diameters of about 24 kilometers, but at the same time roughly the mass of our Sun. A sugar-cube sized piece of its ultra-compact matter on the Earth would weigh hundreds of millions of tons. A sub-class of them, known as millisecond pulsars, spin up to several hundred times per second around their axes. Millisecond pulsars are strongly magnetized, old neutron stars in binary systems which have been spun up to high rotational frequencies by accumulating mass and angular momentum from a companion star. Today we know of about 200 such pulsars with spin periods between 1.4 to 10 milliseconds. These are located in both the Galactic Disk and in Globular Clusters. (Image Credit: NASA)
South Africa’s MeerKAT Radio Telescope Discovers Millisecond Pulsars Inside Globular Clusters
Using South Africa’s MeerKAT Radio Telescope, astronomers from the Italian National Institute of Astrophysics (INAF) in Rome and the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, have discovered new millisecond pulsars hidden deep inside globular clusters.
Millisecond pulsars are extremely compact stars mainly made up of neutrons and are amongst the most extreme objects in the Universe. They pack hundreds of thousands of times the mass of the Earth in a sphere with a diameter of about 24 km. They spin at very high rates and emit a beam of radio waves that hit the observer at every rotation, like a lighthouse. Millisecond pulsars spin at truly breathtaking speeds -- rates of up to 700 times per second, which means that these stars are spinning at a staggering 42,000 revolutions per minute (rpm). To put this in context, a Ferrari Formula 1 race car engine has a maximum internal engine rotational speed of 15,000 rpm.
The formation of millisecond pulsars is highly enhanced in the star-rich environments at the centers of globular clusters. “We directed the MeerKAT antennas toward nine globular clusters and we discovered new pulsars in six of them,” says Alessandro Ridolfi, lead author and a post-doctoral research fellow at INAF and MPIfR. Five of these new pulsars orbit around another star and one of these, named PSR J1823-3021G, is particularly interesting. “Because of its highly elliptical orbit, and massive companion, this system is likely the result of an exchange of partners. Following a 'close encounter,' the original partner was expelled and replaced by a new companion star,” continues Ridolfi.
Operated by the South African Radio Astronomy Observatory (SARAO), MeerKAT is the largest radio telescope in the Southern hemisphere and one of two Square Kilometer Array Observatory (SKAO) precursor instruments. Located in the Karoo desert of South Africa, the radio telescope will soon be expanded with an additional 20 dishes, bringing the total number of antennas up to 84 and becoming “MeerKAT+”. This will later be gradually integrated into the first phase of the SKAO project, whose construction is about to begin and will continue until 2027. The first scientific observations of MeerKAT+ could begin as early as 2023, during the testing phases of the telescope.
Tasha Gautam, a doctoral researcher at the MPIfR in Bonn and co-author of the paper, explains “This particular pulsar could have a high mass, more than 2 times the mass of the Sun, or it could be the first confirmed system formed by a millisecond pulsar and a neutron star. If confirmed by current additional observations, this would make this millisecond pulsar a formidable laboratory for studying fundamental physics.”
The eight new pulsars that were recently discovered are just the tip of the iceberg. The observations that led to their discovery used only about 40 of the 64 MeerKAT antennas and focused only on the central regions of the globular clusters.
"The MeerKAT radio telescope is a huge technological step forward for the research and the study of pulsars in the southern sky,” says Andrea Possenti from INAF, coordinator of pulsar observations in globular clusters for the MeerTIME collaboration. “In the next few years, MeerKAT is expected to find dozens of new millisecond pulsars, giving us a foretaste of what will then happen with the advent of the mid-frequency antennas of the SKA Observatory, which will revolutionize many fields of astrophysics, including the study of pulsars.”
Ridolfi, Gautam, and Possenti are members of the TRAnsients and PUlsars with MeerKAT (TRAPUM) collaboration, a Large Survey Proposal with a broad international collaboration of astronomers excited by the possibilities opened up by MeerKAT. For this particular work, they shared telescope time with a second Large Survey Proposal for MeerKAT, MeerTIME, which is using MeerKAT to study already known pulsars with unprecedented precision.
This work served as a testbed for the TRAPUM collaboration to better plan the fully-fledged globular cluster pulsar survey, which is currently underway and which makes use of all the current 64 dishes (thus further gaining in sensitivity). The survey will broaden the search to many more globular clusters, and will also survey their outer regions.
"Past surveys for pulsars in globular clusters have discovered bizarre and extreme binary pulsars. Hopefully, with new instruments like MeerKAT we will discover even more extreme systems that can teach us more about the basic laws of our Universe”, concludes Paulo Freire, also a co-author from the MPIfR.
Pulsars – The Most Extraordinary Physics Laboratories in the Universe
Pulsars were discovered in 1967 and that discovery earned the Nobel Prize in 1974. Pulsars are perhaps the most extraordinary physics laboratories in the Universe. In fact, research on these extreme and exotic objects has already produced two Nobel Prizes. Pulsar researchers now are poised to learn otherwise unavailable details of nuclear physics and test General Relativity in conditions of extremely strong gravity.
Neutron stars are the remnants of massive stars that exploded as supernovae. They pack more than the mass of the Sun into a sphere no larger than a medium-sized city, making them the densest objects in the Universe, except for black holes, for which the concept of density is theoretically irrelevant. Pulsars are neutron stars that emit beams of radio waves outward from the poles of their magnetic fields. When their rotation spins a beam across the Earth, radio telescopes detect that as a "pulse" of radio waves.
By precisely measuring the timing of such pulses, astronomers can use pulsars for unique "experiments" at the frontiers of modern physics.
Pulsars are at the forefront of research on gravity. Albert Einstein published his theory of General Relativity in 1916, and his description of the nature of gravity has, so far, withstood numerous experimental tests. However, there are competing theories.
"Many of these alternate theories do just as good a job as General Relativity of predicting behavior within our Solar System. One area where they differ, though, is in the extremely dense environment of a neutron star," said Ingrid Stairs, of the University of British Columbia.
In some of the alternate theories, gravity's behavior should vary based on the internal structure of the neutron star.
"By carefully timing pulsar pulses, we can precisely measure the properties of the neutron stars. Several sets of observations have shown that pulsar motions are not dependent on their structure, so General Relativity is safe so far," Stairs explained.
Recent research on pulsars in binary-star systems with other neutron stars, and, in one case, with another pulsar, offer the best tests yet of General Relativity in very strong gravity. The precision of such measurements is expected to get even better in the future, Stairs said.
Another prediction of General Relativity is that motions of masses in the Universe should cause disturbances of space-time in the form of gravitational waves and study of pulsars in binary-star systems have given indirect evidence for their existence. That work won a Nobel Prize in 1993.
Now, astronomers are using pulsars throughout our Milky Way Galaxy as a giant scientific instrument to directly detect gravitational waves.
"Pulsars are such extremely precise timepieces that we can use them to detect gravitational waves in a frequency range to which no other experiment will be sensitive," said Benjamin Stappers, of the University of Manchester in the UK.
By carefully timing the pulses from pulsars widely scattered within our Galaxy, the astronomers hope to measure slight variations caused by the passage of the gravitational waves. The scientists hope such Pulsar Timing Arrays can detect gravitational waves caused by the motions of supermassive pairs of black holes in the early Universe, cosmic strings, and possibly from other exotic events in the first few seconds after the Big Bang.
"At the moment, we can only place limits on the existence of the very low-frequency waves we're seeking, but planned expansion and new telescopes will, we hope, result in a direct detection," Stappers said.
With densities as much as several times greater than that in atomic nuclei, pulsars are unique laboratories for nuclear physics. Details of the physics of such dense objects are unknown.
"By measuring the masses of neutron stars, we can put constraints on their internal physics," said Scott Ransom of the National Radio Astronomy Observatory. "Just in the past three to four years, we've found several massive neutron stars that, because of their large masses, rule out some exotic proposals for what's going on at the centers of neutron stars," Ransom said.
The work is ongoing, and more measurements are needed. "Theorists are clever, so when we provide new data, they tweak their exotic models to fit what we've found," Ransom said.
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