Astronomers using a telescope array in South Africa have spotted a mysterious yet massive object within what astronomers term the “mass gap” between neutron stars and black holes, that also shares a binary orbit with a neutron star.
The fact that the mysterious object lies in this mass gap leads them to believe it is either the largest neutron star ever observed, the least massive black hole, or something else entirely. The discovery could have significant applications for understanding the “uncertain physics” underlying a range of massive cosmic objects.
Objects of this size are typically categorized as “astrophysical compact objects,” and they generally come in one of two varieties: black holes or neutron stars. Still, there is an enormous mass gap between the smallest black hole and the largest neutron star. For example, the largest neutron stars range between 2.2 to 2.5 solar masses, while black holes smaller than 5 solar masses are considered extremely rare. The result is a mass gap where these objects simply should not be.
As a result, discovering a compact astronomical object situated within this gap is a major event. Astronomers Ewan Barr and Arunima Dutta from the Max-Planck-Institut für Radioastronomie, who made the discovery, say that previous objects in this mass gap have also been spotted, but “the nature of these objects and the mechanisms through which they formed remain unknown.”
The astronomers made the discovery while scanning a globular cluster known as NGC 1851 using the Karoo Array Telescope (MeerKAT) in South Africa. There, they spotted a pulsar in a binary orbit with an unknown compact object with a mass that landed in the lower range of the mass gap.
“The nature of these mass gap objects is unknown, as is the formation of their host binary systems,” the researchers write in the study detailing their findings. They also point out that the companion’s mass of 2.09 to 2.71 (solar masses) is in the mass gap, “indicating either a very massive NS (Neutron Star) or a low-mass BH (Black Hole).”
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