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.
X-ray pulsed emission of the five NICER-discovered (or -confirmed) X-ray
pulsars (black), with their radio pulsations over-plotted. The brightest
one, PSR J0614-3329, adds to the list of pulsars suitable for NICER's
key science goal of measuring the masses and radii of neutron stars.
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.