NICER / ISS Science Nugget
for May 16, 2024

A new isolated neutron star

The thousands of neutron stars known in our Galaxy make up a menagerie of different types. Some are powered by their rotational kinetic energy, others by their strong magnetic fields, either through magnetospheric processes (such as field-line reconnection) or internal heating from dissipation of fields. The spin-powered stars typically reveal themselves as radio pulsars, while X-ray emission dominates for the magnetically powered group. But there are exceptions galore, and it's unclear what relationship, if any, exists between the different types; for example, it's possible that the internally-heated objects (collectively called X-ray-dim isolated neutron stars, or XDINS) may have once been magnetars powered by their magnetospheres. Until recently, only seven XDINS were known, the smallest population sub-type of neutron stars, so finding new ones is important for understanding their unique characteristics: relatively slow rotation, thermal emission from the star's surface (with no evidence of magnetospheric activity), and spectral features in their low-energy X-ray emission that suggests exotic atomic phenomena in the presence of strong surface fields.

A survey of the X-ray sky conducted by the eROSITA instrument on the Spektrum-Roentgen-Gamma (SRG) mission has produced a handful of XDINS candidates: low-energy X-ray sources with no extended emission around them and no visible-light counterpart (to very stringent limits). Through a proposed General Observer project, NICER observed the strongest eROSITA XDINS candidate, eRASSU J1317Ð4026 (or J1317, for short) in the Spring of 2023. The proposing team also carried out observations with the ESA-NASA X-ray Multi-Mirror (XMM) Newton telescope. NICER's fast-timing capability enabled a search for a wide range of periodicities, and that analysis produced a clear detection at 12.76 seconds, consistent with the rotation rates of all other known XDINS. Given this detection and the relatively long period, within the reach of XMM-Newton's instruments, the team confirmed the pulsations in the XMM data as well. These results were recently published in the European journal Astronomy & Astrophysics by J. Kurpas (Potsdam Univ., Germany) and collaborators.

In addition to resolving the nature of this previously unknown X-ray source, and contributing a new member to the small club of XDINS, the discovery of pulsations from J1317 provides a new example of spectral features, of uncertain origin, from the surface of a likely high-magnetic field pulsar. It also sheds light on the demographics of this group: the small number of known objects may be misleading, as other evidence suggests they may be very numerous but largely invisible unless they are young and nearby. Thus, new XDINS detections help us probe a potentially large population of dark neutron stars further afield in our Galaxy.