NuSTAR observations of the unidentified INTEGRAL sources: constraints on the Galactic population of HMXBs

TL;DR

This study uses NuSTAR and Swift/XRT observations to identify faint hard X-ray sources from INTEGRAL, providing insights into the population of high-mass X-ray binaries in the Galaxy and highlighting a potential deficit of persistent HMXBs below a certain flux threshold.

Contribution

It presents new spectral and variability analyses that confirm classifications of several sources and discusses implications for the Galactic HMXB population based on these identifications.

Findings

Confirmed classifications of five sources as AGN and intermediate polar.

Identified a possible deficit of persistent HMXBs below a specific flux.

Highlighted the need for further classification of faint hard X-ray sources.

Abstract

The NuSTAR Legacy program 'Unidentified INTEGRAL sources' targeted faint hard X-ray sources revealed by INTEGRAL in the Galactic plane in order to provide conclusive identification of their nature and insights on the population of faint hard X-ray sources. The NuSTAR and Swift/XRT observations obtained in 2015-2017 contributed to the successful identification of five persistent sources. Here, we report on the spectral and variability analyses which helped to consolidate the classifications of IGR J10447-6027, IGR J16181-5407 and IGR J20569+4940 as active galactic nuclei, and IGR J17402-3656 as an intermediate polar. An optical spectrum of the blazar IGR J20569+4940 is also presented. Combining these results with successful identifications of other such faint and persistent INTEGRAL sources reported in the literature, we investigate possible implications for the population of persistent high mass X-ray binaries (HMXBs) below the identification completion limit of the INTEGRAL survey. The current trend hints at a deficit of persistent HMXBs below $F_{\rm 17-60keV} = 10^{-11}$ erg cm$^{-2}$ s$^{-1}$, but additional efforts dedicated to classifying faint hard X-ray sources are needed before we can draw solid conclusions.