NICER / ISS Science Nugget
for October 10, 2019

New X-ray pulsars point to the future of pulsar physics

NICER was designed to study the pulsed X-ray emissions of rapidly rotating neutron stars and to extract measurements of their masses and radii. With up to twice the mass of our Sun contained within an object approximately 15 miles in diameter, the interior densities of neutron stars surpass anything that can be reproduced in Earth laboratories, and lie beyond what can be explained by nuclear physics theory. Therefore, measuring their masses and radii by studying their sinusoidal X-ray pulsations is key to understanding the nature of such super-dense matter. Only a handful of such targets are currently available and routinely observed by NICER, but it is important to measure the masses and radii of a larger sample of neutron stars.

By observing multiple neutron stars known to be fast-rotating from radio observations (radio pulsars), NICER has discovered previously unknown pulsed X-ray emission. This confirms their suitability for mass and radius measurements, with NICER or with future missions such as STROBE-X (a proposed NICER successor). Five such neutron stars, with spin rates between 190 and 456 rotations per second (11,000 to 27,000 RPM), show broad, sine wave-like pulsed emission. Furthermore, the X-ray pulsations seen by NICER (black lines in the figure) are not always aligned with the radio pulsations (red lines). This result is at odds with previously known X-ray pulsars (NICER's key targets) and with our current understanding of pulsar emission that suggests that the X-ray and radio emission both originate along the star's magnetic axis.

This work, led by Dr. Sebastien Guillot (Institut de Recherche en Astrophysique et Planétologie, Toulouse, France), was accepted this week for publication in the Astrophysical Journal Letters.