A neutron star in the X-ray binary system 4U 1820-30 spins 716 times per second, making it one of the fastest-spinning objects ever observed, according to an analysis of data gathered by NASA’s Neutron Star Interior Composition Explorer (NICER) between 2017 and 2022.
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An artist’s depiction of the X-ray binary system 4U 1820-30 in the core of the globular cluster NGC 6624. Image credit: NASA.
4U 1820-30 is located about 26,000 light-years away from Earth in the constellation of Sagittarius.
This X-ray binary system is part of a metal-rich globular cluster called NGC 6624.
It consists of two stars — a neutron star and a white dwarf companion. The latter orbits the neutron star once every 11 minutes, making it the system with the shortest known orbital period.
4U 1820-30 typically displays short X-ray bursts, only lasting for 10-15 seconds. This is likely because the ignited helium-rich fuel burns out rapidly on its surface.
“Due to its intense gravity, the neutron star pulls material away from its companion star,” said Dr. Gaurava Jaisawal from DTU Space and colleagues.
“When enough material has accumulated on its surface, a violent thermonuclear burst occurs on the neutron star, similar to an atomic bomb.”
The astronomers used NASA’s NICER X-ray telescope, mounted outside of the International Space Station, to observe 4U 1820-30.
“We were studying thermonuclear explosions from this system and then found remarkable oscillations, suggesting a neutron star spinning around its centre axis at an astounding 716 times per second,” Dr. Jaisawal said.
“If future observations confirm this, the 4U 1820-30 neutron star would be one of the fastest-spinning objects ever observed in the Universe, matched only by another neutron star called PSR J1748-2446.”
Between 2017 and 2021, NICER detected 15 thermonuclear X-ray bursts from 4U 1820-30.
It was one of these bursts that showed a signature known as ‘thermonuclear burst oscillations,’ occurring at a frequency of 716 Hz.
These burst oscillations match the spin frequency of the neutron star itself, meaning that it rotates on its axis at a record-high rate of 716 times per second.
“During the bursts, the neutron star becomes up to 100,000 times brighter than the Sun, releasing an immense amount of energy,” said DTU Space researcher Dr. Jerome Chenevez.
“So we are dealing with very extreme events, and by studying them, we get new insights into the exiting life cycles of binary star systems and the formation of elements in the Universe.”
The findings appear in the Astrohysical Journal.
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Gaurava K. Jaisawal et al. 2024. A Comprehensive Study of Thermonuclear X-Ray Bursts from 4U 1820-30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation. ApJ 975, 67; doi: 10.3847/1538-4357/ad794e
