Astronomers from Denmark, while observing a binary X-ray system, discovered signs of a record-breaking characteristic in the central partner – a neutron star around which the white dwarf revolves. One of the recorded thermonuclear flares at the facility was accompanied by an intensity fluctuation at a frequency of 716 Hz. This means that the neutron star rotates around its axis at a frequency of 716 revolutions per second, which is an absolute record today.
NICER on the ISS. Image source: NASA
The dual 4U 1820-30 system is one of the most attractive candidates for monitoring. It is 26 thousand light years away from Earth in the direction of the center of the Milky Way and is located in the constellation Sagittarius. The white dwarf is very close to the neutron star and makes one revolution around it in 11 minutes. The diameter of the neutron star is approximately 12 km, and its mass is 1.4 times that of the Sun. This system is called a buster. The neutron star pulls the mass of the companion, and when it accumulates to a critical level, a thermonuclear explosion occurs, accompanied by an X-ray flash.
The system was monitored by NASA’s NICER X-ray telescope installed on the ISS. The DTU Space team from the Technical University of Denmark created the guidance system for this instrument. Of the 15 recorded thermonuclear explosions, one indicated a possible rotation speed of the neutron star – 716 revolutions per second. Previously, only one neutron star with the same rotation speed was discovered – the radio pulsar PSR J1748−2446ad. However, the record rotation speed of 4U 1820-30 remains to be confirmed in future observations. Nevertheless, the data obtained bring even more clarity to the nature of neutron stars.