Observability of HOFNARs with SRG/eROSITA

TL;DR

This paper predicts the detectability of hypothetical neutron stars called HOFNARs by the eROSITA survey, suggesting potential discovery or constraints on their properties based on temperature and observational limits.

Contribution

It provides the first population synthesis modeling of HOFNARs' observability with eROSITA, estimating detectable source numbers and constraining their temperature range.

Findings

Approximately 500 sources detectable at 0.01 cts/s for T~10^6 K

Around 100 sources with >0.1 cts/s are identifiable

Non-detection by ROSAT constrains HOFNAR temperatures to below 1.2×10^6 K

Abstract

Neutron stars can appear as sources of different nature. In this paper we address observability of a hypothetical class of neutron stars -- HOt and Fast Non Accreting Rotators, HOFNARs. These objects are heated due to the r-mode instability. With surface temperatures $\sim 10^6$~K they are expected to be thermal soft X-ray emitters. We perform a population synthesis modeling of HOFNARs to predict the number of potentially detectable sources in the eROSITA all-sky survey. For surface temperatures $\sim 10^6$~K we obtain $\sim 500$ sources above the detection limit 0.01~cts~s$^{-1}$ and $\sim 100$ easier identifiable sources with $>0.1$~cts~s$^{-1}$. Temperatures $\gtrsim 1.2\times 10^6$~K start to be in contradiction with non-detection of HOFNARs by ROSAT. Only for $T\lesssim 5\times 10^5$~K numbers predicted for eROSITA turn out to be so low that identification does not look possible. We conclude that eROSITA has good chances to discover HOFNARs, if they exist. Non-detection will put very stringent limits on the properties of this type of neutron stars.