Millihertz Quasi-periodic Oscillations in 4U 1636-536: Putting Possible Constraints on the Neutron Star Size

TL;DR

This study analyzes millihertz quasi-periodic oscillations in the neutron star system 4U 1636-53, providing constraints on neutron star size and ruling out certain equations of state, through phase-resolved spectral analysis.

Contribution

It offers the first direct measurement of the emission region size during mHz QPOs, constraining neutron star radii and informing equations of state.

Findings

Oscillations originate from variable surface emission area.

Lower limit on neutron star radius is 11.0 km.

Rules out equations of state favoring smaller neutron star radii.

Abstract

Based on previous studies of quasi-periodic oscillations in neutron star LMXBs, mHz quasi-periodic oscillations (QPO) are believed to be related to `marginally stable' burning on the neutron star (NS) surface. Our study of phase resolved energy spectra of these oscillations in 4U 1636-53 shows that the oscillations are not caused by variations in the blackbody temperature of the neutron star, but reveals a correlation between the change of the count rate during the mHz QPO pulse and the spatial extend of a region emitting blackbody emission. The maximum size of the emission area $R^2_{\mathrm{BB}}=216.7^{+93.2}_{-86.4}$km$^2$, provides the direct evidence that the oscillations originate from a variable surface area constrained on the NS and are therefore not related to instabilities in the accretion disk. The obtained lower limit on the size of the neutron star (11.0 km) rules out equations of state that prefer small NS radii. Observations of mHz QPOs in NS LMXBs with NICER and eXTP will reduce the statistical uncertainty in the lower limit on the NS radius, which together with better estimates of the hardening factor and distance, will allow improving discrimination between different equations of state and compact star models. Furthermore, future missions will allow us to measure the peak blackbody emission area for a single mHz QPO pulse, which will push the lower limit to larger radii.