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NICER / ISS Science Nugget
for October 7, 2021



NICER Enables New Limits on Gravitational-wave Emission from Mountains on Neutron Stars

The LIGO-Virgo-KAGRA (LVK) collaboration is a global ground-based effort to detect and understand sources of gravitational radiation -- the propagation of space-time ripples -- and to better characterize the extreme phenomena responsible for the emission of gravitational waves (GWs). A leading candidate for producing continuous GWs is the class of accreting millisecond X-ray pulsars (AMXPs), neutron stars that spin at hundreds of rotations per second and that are powered by mass transfer from orbiting companion stars. Since the start of its science mission, NICER has prioritized the study of AMXPs, including discovering several new ones. In a new paper submitted for peer-reviewed publication (preprint available), the LVK team has used NICER's published results on AMXPs to constrain GW emission, and thus to probe the neutron stars themselves.

Rapidly spinning neutron stars are expected to emit GWs if they are not axisymmetric (e.g., because of "mountains" on their surfaces) or if their interiors are prone to mechanical oscillation modes. Detection of these GWs in LVK data requires knowledge of the pulsar's spin rate and correction for its orbital motion, if known. NICER's X-ray observations provide this information, in particular during the episodic accretion-induced outbursts of AMXPs. As a result, LVK has produced stringent upper limits on the ellipticity and oscillation amplitudes from 20 AMXPs (see figure). In one case, for the 400.6 Hz pulsar known as SAX J1808.4-3658, LVK data analyzed over a 24-day outburst in August 2019 yielded a candidate GW signal with a false-positive probability of 2% -- insufficient to claim a robust detection but a tantalizing hint that future GW observations with X-ray timing input may yield a first-ever measurement of gravitational radiation from a spinning neutron star.


Estimates of the probability density functions (blue traces) of constraints on ellipticity (epsilon95%; left panel) and dimensionless r-mode oscillation amplitude (alpha 95%; right panel) from gravitational-wave emission upper limits for 20 accreting millisecond pulsars. The black dashes in both panels correspond to the individual estimates of epsilon 95% or alpha 95% for each pulsar.



Figure: Estimates of the probability density functions (blue traces) of constraints on ellipticity (ε95%; left panel) and dimensionless r-mode oscillation amplitude (α95%; right panel) from gravitational-wave emission upper limits for 20 accreting millisecond pulsars. The black dashes in both panels correspond to the individual estimates of ε95% or α95% for each pulsar.



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