After surviving the supernova explosion at the end of its evolution, the core of the massive star ends up as either a neutron star or a black hole. Curiously, there is a gap in the mass distribution of these compact objects: the neutron stars observed so far have a maximum mass of 2.2 solar masses, while the smallest black holes, the least massive, start out at only three solar masses. Now researchers led by Euan Parr of Max Planck Radio Astronomy have found a compact object, with a mass of about 2.35 solar masses, that fits perfectly into this gap. This discovery is either the lowest mass black hole ever discovered, or the most massive neutron star. However, the researchers write In “Science” magazineThey cannot distinguish between these two variables.
The compact object is located in the globular star cluster NGC 1851 in the southern Dove constellation. It is the companion of the millisecond pulsar PSR J0514−4002E. Such a rapidly rotating neutron star forms when it is part of a nearby binary star system and is pulling mass from its companion. The companion can be either a white dwarf — even the remnant of a less massive star — or a regular star. From observations made by the MeerKAT radio telescope in South Africa, researchers were able to deduce the millisecond properties of the pulsar: the pulsar rotates on its axis every 5.6 milliseconds and the orbital period of the two objects is 7.44 days. The orbit eccentricity in this system is very high at 0.71.
A neutron star or black hole as a companion to a pulsar
Further analysis, which included relativistic effects due to strong gravitational forces in this system, eventually revealed that the mass of the entire system was 3,887 solar masses. The uncertainty of this value was very low at 0.004 solar masses. The pulsar itself is said to have a mass of about 1.53 solar masses, and its companion star is about 2.35 solar masses. But this companion is not a star: the star should be visible in images from the Hubble Space Telescope in the optical region of the electromagnetic spectrum. This is not the case.
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