A dense object about 40,000 light years away is lighter than any black hole seen before, but heavier than any neutron star, leaving astronomers unsure what they are looking at
By Alex Wilkins
18 January 2024
An artist’s impression of a pulsar orbiting a black hole – one possible interpretation of the mysterious binary system
Daniëlle Futselaar (artsource.nl)
A strange object about 40,000 light years away is either the heaviest neutron star or the lightest black hole we have ever seen, sitting in a mysterious void of objects that astronomers have never directly observed.
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Is there an ancient black hole at the edge of the solar system?
A neutron star forms when a star has run out of fuel and collapses under its gravity, creating a shockwave called a supernova and leaving an ultra-dense core behind. According to astrophysical calculations, these cores must remain below a certain mass, around 2.2 times the mass of the sun, or they will collapse even further, creating a black hole.
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However, black holes have only been observed with a mass more than five times that of the sun, leaving a gap in scale between neutron stars and black holes. There have been some dense objects observed in this gap by gravitational wave observatories, but astronomers have never spotted them with conventional telescopes.
Now, Ewan Barr at the Max Planck Institute for Radio Astronomy in Germany and his colleagues have spotted an object of 2.5 solar masses by observing a pulsar that orbits it. A pulsar is a neutron star that spits out pulses of light at regular millisecond intervals due to an intense magnetic field.
Pulsars emit light with extreme regularity, but very massive nearby objects can warp these rhythms, as predicted by Albert Einstein’s theory of relativity. By observing the pulsar’s pulses for more than a year using the MeerKAT radio telescope in South Africa, Barr and his team were able to calculate the mass of the pulsar’s partner.