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Simulation of a millisecond pulsar orbiting a Sun-like star. Image: ESA/Francesco Ferraro.
Millisecond stars are densely-packed neutron stars, weighing at least 1.4 times the mass of the Sun, typically formed following a supernova event where a massive star collapses, causing the core material to be compressed. Retaining much of the initial star’s angular momentum while contracting to a small size, millisecond stars exhibit extremely high rotational speeds around their axis.
“Most millisecond stars rotate once around their axis in a few hundred milliseconds or more, meaning they complete several revolutions per second. However, in some cases, they can rotate hundreds of times per second, which translates to a few milliseconds per revolution. When that happens, they are called millisecond pulsars,” explained lead author Alessandro Ridolfi, a researcher post-doctoral fellow at the Cagliari Observatory in Italy.
Millisecond pulsars are exceedingly rare in the universe as they can only achieve such rotational speeds when they are in a binary system. In such a system, two objects orbit around each other. For millisecond pulsars, their companion is often a star similar to our Sun, but it can occasionally be a white dwarf, a black hole, or another neutron star.
“To reach rotational speeds of hundreds of revolutions per second, millisecond pulsars need to accumulate matter from their companion star over an extended period, spanning billions of years,” Ridolfi added. “These stars are akin to the super-precise clocks of the cosmos. They exhibit much more stable rotations compared to ordinary pulsars, making them suitable for high-precision experiments.”
Globular clusters – groups of stars bound together by gravitational forces, with their unique orbits around the outskirts of a galaxy – are ideal candidates for finding millisecond pulsars in the universe. They contain countless stars and are subjected to strong gravitational interactions, making them likely sites for binary systems. More than half of the known millisecond pulsars are currently located within these clusters.
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The South African Radio Astronomy Observatory’s MeerKAT radio telescope network in South Africa. Image: SARAO.
In a recent study, Ridolfi and colleagues employed the MeerKAT radio telescope network – comprised of 64 individual dishes operated by the South African Radio Astronomy Observatory (SARAO) – to focus on observing nine globular clusters encircling the Milky Way. As a result, they discovered not just one, but up to 8 new millisecond pulsars, rendering it one of the largest studies of millisecond pulsars to date.
Among these newly identified celestial bodies, the research team paid special attention to an object with an unusual orbit, named PSR J1823-3021G. It’s a millisecond pulsar with an “extremely elliptical” orbit. It’s highly probable that its initial neutron star companion had a lighter star on its orbit, but this was replaced by a more massive star due to the aftermath of a collision event. PSR J1823-3021G is also one of the largest millisecond pulsars, weighing over twice as much as our Sun, a rarity in itself.
“Among the new discoveries, we are particularly hopeful to find a binary system composed of two millisecond pulsars, or one millisecond pulsar orbiting a massive black hole,” shared Ridolfi. “However, achieving this would require more advanced radio telescopes than MeerKAT in the future.”