NICER / ISS Science Nugget
for November 7, 2024

The fastest-spinning nuclear-powered pulsar?

4U 1820-30 is an ultracompact X-ray binary, a neutron star and a helium-rich white dwarf orbiting each other every 11.2 minutes. For reasons not well understood, approximately every six months the neutron star draws matter from its companion in an accretion episode lasting a few days, as material crosses a gravitational tipping point between the two stars through a process known as Roche lobe overflow. When sufficient helium fuel accumulates on the surface of the neutron star, a violent, unstable thermonuclear explosion is triggered by the extreme density and temperature. The resulting X-ray bursts are so intense that, for a brief moment, the neutron star can become up to 100,000 times more luminous than the Sun, releasing an enormous amount of energy.

Between 2017 and 2021, NICER observations of 4U 1820-30 detected 15 thermonuclear X-ray bursts. In analysis recently published by G. Jaisawal (Technical Univ. of Denmark) and collaborators in The Astrophysical Journal, one of these bursts was seen to exhibit a rare feature known as burst oscillations, occurring at a frequency of 716 Hz with 3 sigma statistical significance. Studies of other X-ray bursting systems have shown that such burst oscillations are representative of the neutron star's spin frequency. If future observations confirm the observed oscillation, 4U 1820-30 would be the fastest-spinning accretion-powered pulsar known, and match the record spin rate of another neutron star, the radio pulsar J1748-2446ad in the globular cluster Terzan 5.

The researchers also performed detailed time-resolved spectroscopy of all 15 X-ray bursts. The results indicate that these are exceptionally powerful bursts, even momentarily pushing the neutron starŐs outer layer, or photosphere, from hundreds to a thousand kilometers above the stellar surface through strong radiation pressure. The published study not only enhances our understanding of the extreme conditions around neutron stars but also highlights the impact of burst emission on the accretion environment.