Astronomers are puzzled over an enigmatic companion to a pulsar

Circling around a pulsar in our galaxy is a mysterious entity that is either a very heavy neutron star, one of the lightest black holes ever discovered, or an exotic and never-before-seen quasi-stellar object.

The new finding comes from the MeerKAT Radio Telescope in South Africa, which carefully monitored 13 millisecond pulsars in a dense cluster of stars 40,000 light years from Earth. These pulsars are a type of neutron star that quickly spin, rotating in fractions of a second, while sending out powerful beams of radiation like a cosmic lighthouse.

For some pulsars, those beams flash past our planet with a regularity that rivals an atomic clock. By hunting for tiny variations in the beams’ arrival on Earth, researchers can deduce the existence of anything perturbing the pulsar’s motion.

The ticks of one particular pulsar, known as PSR J0514−4002E, revealed that it has an invisible companion weighing between about 2.1 and 2.7 times the mass of the sun, researchers report in the Jan. 19 Science. That potentially makes it too heavy to be a neutron star, astronomer Ewan Barr of the Max Planck Institute for Radio Astronomy in Bonn, Germany, and colleagues say. Neutron stars are thought to collapse into black holes once they reach around two to three times the sun’s mass (SN: 7/22/22). But because nobody knows exactly where that dividing line rests, nor precisely what happens once this limit is reached, the researchers can’t definitively say what this object is. 

Researchers have discovered a handful of similar entities before, including one found using the LIGO and Virgo gravitational wave detectors in 2020 (SN: 6/23/20). Barr and colleagues speculate that this new object formed when two lighter neutron stars crashed together. By studying the pulsar ticks more closely, the researchers hope to determine the hidden entity’s true nature and use it to probe matter in similarly extreme objects.