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 a rate of hundreds of rotations per second. They emit a beam of radio waves that are detected by the observer at every rotation, like a lighthouse. The formation of these objects is highly enhanced in the star-rich environments at the centres of globular clusters.
“We directed the MeerKAT antennas toward nine globular clusters, and we discovered new pulsars in 6 of them!,” says the lead author, Alessandro Ridolfi, 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.
Tasha Gautam, doctoral researcher at the MPIfR in Bonn and co-author of the paper, explains: “This particular pulsar could have a high mass, more than two 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 are just the tip of the iceberg: the observations that led to their discovery used only about 40 of the MeerKAT 64 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 survev their outer regions.
Operated by SARAO, MeerKAT is the largest radio telescope in the Southern hemisphere and one of two SKA Observatory precursor instruments in South Africa. Located in the Karoo desert, 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 will start this year and will continue until 2027. The first scientific observations of MeerKAT+ could begin as early as 2023, during the testing phases of the telescope.
TRAPUM is one of the Large Survey Proposals running on MeerKAT and is an international collaboration, led by the University of Manchester and the MPIfR, and includes institutions such as INAF, the National Radio Astronomy Observatory (NRAO) and the SARAO.
MeerTIME is also a Large Survey Proposal running on MeerKAT, led by the Swinburne University of Technology, and integrating several Australian institutions as well as INAF, University of Manchester, MPIfR, NRAO and SARAO.