The impact of surface temperature inhomogeneities on quiescent neutron star radius measurements

TL;DR

This study investigates how undetected hot spots on neutron stars can bias radius measurements from X-ray spectra, emphasizing the importance of pulsation constraints for accurate neutron star characterization.

Contribution

The paper models hot spot effects on neutron star spectra and pulsations, providing new constraints on how undetected hot spots influence radius estimates.

Findings

Hot spots can bias radius measurements downward by up to 28%.

Pulsation limits help constrain hot spot properties and their spectral effects.

Improved pulsation constraints are crucial for accurate neutron star radii.

Abstract

Fitting the thermal X-ray spectra of neutron stars (NSs) in quiescent X-ray binaries can constrain the masses and radii of NSs. The effect of undetected hot spots on the spectrum, and thus on the inferred NS mass and radius, has not yet been explored for appropriate atmospheres and spectra. A hot spot would harden the observed spectrum, so that spectral modeling tends to infer radii that are too small. However, a hot spot may also produce detectable pulsations. We simulated the effects of a hot spot on the pulsed fraction and spectrum of the quiescent NSs X5 and X7 in the globular cluster 47 Tucanae, using appropriate spectra and beaming for hydrogen atmosphere models, incorporating special and general relativistic effects, and sampling a range of system angles. We searched for pulsations in archival Chandra HRC-S observations of X5 and X7, placing 90\% confidence upper limits on their pulsed fractions below 16\%. We use these pulsation limits to constrain the temperature differential of any hot spots, and to then constrain the effects of possible hot spots on the X-ray spectrum and the inferred radius from spectral fitting. We find that hot spots below our pulsation limit could bias the spectroscopically inferred radius downward by up to 28\%. For Cen X-4 (which has deeper published pulsation searches), an undetected hot spot could bias its inferred radius downward by up to 10\%. Improving constraints on pulsations from quiescent LMXBs may be essential for progress in constraining their radii.