NICER / ISS Science Nugget
for February 27, 2025

An X-ray-only pulsar: another Musketeer?

The lighthouse-like beams that emanate from rapidly spinning neutron stars, and which appear to us as pulses of radiation as they sweep past Earth, cover the entire electromagnetic spectrum, from radio waves to gamma rays. But only a very few (most notably the pulsar at the heart of the Crab Nebula) are visible at all the wavelengths we have sampled with our telescopes. Some pulsars are especially self-effacing, only seeing fit to present themselves in one electromagnetic band.

The emissions in different bands - e.g., radio vs. X-ray - typically arise from very different mechanisms, some originating at the stellar surface, others in the magnetosphere. The shapes of the beams also differ: radio emissions form narrow "pencil" beams, while X- and gamma-ray beams are thought to be broader and fan-like; the latter are more likely to be intercepted by a distance observer. Similarly, some wavelengths of radiation travel unimpeded through the interstellar medium, while others are absorbed or scattered by it. Ultimately, emission geometry and distance from the Earth play important roles in whether a given pulsar is detectable.

A paper recently published in the peer-reviewed European journal Astronomy & Astrophysics, by J. Kurpas (Potsdam Univ., Germany) and collaborators, describes a NICER study of a candidate neutron star initially seen in an all-sky survey by the eROSITA mission (eRASS). Follow-up observations of the previously unknown X-ray source, eRASSu J0657, revealed no visible-light emission, no radio counterpart, and no associated gamma-rays, all to very deep limits. Typically, only neutron stars have high ratios of X-ray to visible brightness, and this hypothesis motivated a search for X-ray pulsations through NICER's General Observer program. Data collected in November 2023 were analyzed by the team, revealing coherent pulsations at a rate of nearly 4 per second (261 millisecond period). Observations with ESA's XMM-Newton telescope four months later confirmed the pulsations and measured a slightly longer period, a result of the gradual slowing-down of the neutron star's spin. These measurements imply that J0657 is approximately 700,000 years old and has a magnetic field strength (10^12 Gauss) that is typical of most known neutron stars. Thus, J0657 is generally unremarkable compared to the rest of the known pulsar population, except that its pulsations are invisible at every wavelength other than low-energy X-rays, which are likely produced in hot spots on the neutron star's surface - there is no evidence of magnetospheric emission. Three comparable pulsars, usually called the Musketeers, with soft X-ray surface emission and relatively nearby, are cataloged. J0657 thus appears to be the newest Musketeer.