NICER / ISS Science Nugget
for May 1, 2025

Tiny mountains

A rapidly spinning neutron star that is not perfectly spherical - or simply bulging along its equator, like the Earth - should emit gravitational radiation. A shape deformation as small as a local "mountain" swept around with the star's rotation can produce continuous and long-lived gravitational "spacetime ripples," unlike the transient gravitational-wave signals seen by the LIGO (U.S.) and Virgo (Europe) ground-based facilities since 2015 from the mergers of black holes and neutron stars in distant binary systems. The gravitational waves (GWs) produced by a neutron star are expected at frequencies equal to once or twice its spin frequency; only the fastest-spinning objects, with frequencies greater than about 10 Hz, are potentially detectable given the sensitivities of the current-generation LIGO/Virgo and KAGRA (Japan) detectors (shaded curve in the accompanying figure). The most sensitive searches for continuous GWs are those that target pulsars for which accurate spin frequency measurements, which can evolve slowly over time, are made with electromagnetic timing observations - in X-rays, gamma-rays, and at radio wavelengths - such as those made by NICER. The availability of such measurements substantially reduces the parameter space over which a GW search must be performed.

A recent peer-reviewed paper by the LIGO/Virgo/KAGRA (LVK) collaboration, published in The Astrophysical Journal, establishes upper limits on the strength of GW emission from 45 pulsars using the first eight months (May 2023 to January 2024) of the most sensitive GW data yet collected. For each pulsar, the rotational kinetic energy loss over time is estimated given the rate at which it is observed to be slowing down its spin, together with a canonical estimate of a neutron star's moment of inertia. For a given pulsar, an upper limit on GW emission that falls below the spin-down energy loss rate (or "luminosity") is especially valuable: it constrains what fraction of the energy is lost to producing GWs and, thereby, any deformation of the star away from axisymmetry - the size of any mountain on that neutron star. For example, the Crab pulsar has the lowest fraction of its energy going into GWs, at under 0.006 percent, and J0437-4715 (for which NICER has independently measured a radius) has the smallest mountain at less than 0.1 mm. Of the 45 pulsars searched, 29 have upper limits on GW emission below the spin-down luminosity, and of these 29, eight required NICER observations to enable their GW searches, including J0537-6910 (highlighted). NICER continues to monitor these and additional pulsars, especially during LVK data-collection windows, to enable ever-deeper GW searches leading eventually - we hope - to the first detection of continuous ripples in spacetime from a nearby star, and insights into its internal structure.