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NICER / ISS Science Nugget
for May 4, 2023



May the Nuclear Force be with 4U 1730-22

Beginning in late 2021 and through August 2022, NICER observed two outbursts from the accreting neutron star binary system 4U 1730-22, which had been quiescent for some 50 years. These observations have yielded a number of interesting findings, among which is the discovery of so-called millihertz quasi-periodic oscillations (mHz QPOs), recently published by G. Mancuso (Argentine Institute for Radioastronomy) and collaborators in the peer-reviewed U.K. journal Monthly Notices of the Royal Astronomical Society.

The surface of a neutron star offers an exotic stage on which we are able to watch physics play out, providing insights into the conditions in these unique environments. When a neutron star draws matter from a low-mass companion star, the accumulated matter is compressed to exceptional densities at high temperatures, and undergoes nuclear fusion reactions as a result. Thermonuclear burning of hydrogen and helium on the surface is thought to be continuous at high rates of mass accretion, cataclysmic (manifesting as "Type I burst" explosions) at lower rates, and "marginally stable" in a narrow band between the two. The ignition - primarily of helium - and quenching of the marginally stable process manifests as bouts of contrasting higher- and lower-temperature thermal emission that typically lasts about 100 seconds. Such millihertz QPOs are seen to disappear immediately after a Type I burst, suggesting depletion of the thermonuclear fuel in these explosive events (of which NICER detected 17 from 4U 1730 during its 2021-2022 outbursts).

NICER detected mHz QPOs in 35 individual observations of 4U 1730, with frequencies ranging from 4.5 to 8.1 millihertz; on average, only 2% of the observed emission participated in this oscillatory behavior. The fluctuations occur only in a narrow band of the system's luminosity, which tracks the rate of mass accretion from the companion star. This behavior provides evidence for a degree of confinement of the nuclear flame on the neutron star surface that remains difficult to explain in detail, although a leading hypothesis is that the star's rapid rotation may set up turbulent mixing of the accreted material and preferentially cause ignition to occur primarily near the equator. Only a handful of examples of mHz QPO behavior exist to date, so 4U 1730 adds to a small but valuable dataset that enables exploration of the extreme conditions on neutron star surfaces.


Two examples of marginally stable nuclear burning on the surface of the neutron star in the low-mass X-ray binary 4U 1730-22, observed with NICER in May (left) and February (right) 2022. The rate of photon detections in the energy range 1 to 5 keV fluctuates approximately every 143 seconds (left) and 156 seconds (right); these quasi-periodic signals are also evident in the

Figure: Two examples of marginally stable nuclear burning on the surface of the neutron star in the low-mass X-ray binary 4U 1730-22, observed with NICER in May (left) and February (right) 2022. The rate of photon detections in the energy range 1 to 5 keV fluctuates approximately every 143 seconds (left) and 156 seconds (right); these quasi-periodic signals are also evident in the "periodograms" (inset plots) as peaks at 7 and 6.4 mHz, respectively. (Figure credit: Mancuso et al. 2023)



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