The Case for PSR J1614-2230 as a NICER Target

TL;DR

This paper argues that PSR J1614-2230 is a promising target for NICER to measure neutron star radii, which can constrain dense matter physics despite its low X-ray count rate.

Contribution

It demonstrates that combining NICER data with existing mass measurements can yield a robust lower limit on the neutron star's radius, informing equations of state.

Findings

Potential to determine a lower radius limit with ~0.5-0.7 km uncertainty

Good fits can exclude significant biases from unmodeled components

Lower radius limits can rule out some dense matter equations of state

Abstract

The Neutron star Interior Composition ExploreR (NICER) is expected to launch in early 2017 and will gather X-ray data on neutron stars and other high-energy sources from a berth on the International Space Station. Its prime scientific goal is to measure the masses and radii of non-accreting neutron stars via fits to the energy-dependent waveforms produced by the rotation of hot spots on their surfaces. These measurements will provide valuable input to theoretical models of cold matter beyond nuclear density. Here we propose that PSR J1614$-$2230, despite its low count rate, is a promising source to observe with NICER. The reason is that XMM-Newton observations suggest that the fractional oscillation amplitude from PSR J1614$-$2230 could be high enough that this star cannot be very compact. We show that if we analyze 0.5 Ms of NICER data and 0.1 Ms of nearby off-source data and combine that analysis with the known mass of this star, we would find a robust lower limit to the radius with a statistical uncertainty of only $\sim 0.5-0.7$ km. We also show that even if there is an unmodeled nonthermal component modulated at the pulsation frequency, good statistical fits could rule out significant biases. The low count rate will make reliable upper limits on the radius difficult, but the lower limit could rule out some equations of state that are currently being discussed. This analysis would require a good estimate of the non-source background, so Chandra observations of the vicinity of PSR J1614$-$2230 would be helpful.