# The NuSTAR Hard X-ray Survey of the Norma Arm Region

## TL;DR

This paper presents a NuSTAR survey of the Norma arm, cataloging 38 X-ray sources, identifying their nature, and analyzing their properties, including plasma temperatures and distribution, to understand the region's X-ray source population.

## Contribution

First comprehensive NuSTAR survey of the Norma arm region, identifying and classifying X-ray sources, and analyzing their spectral properties and distribution.

## Key findings

- 38 X-ray sources detected, including binaries, pulsar wind nebulae, and CVs.
- CV plasma temperatures are 10-20 keV, lower than Galactic Center CVs.
- NuSTAR logN-logS distribution aligns with Chandra's 2-10 keV data assuming a 15 keV thermal spectrum.

## Abstract

We present a catalog of hard X-ray sources in a square-degree region surveyed by NuSTAR in the direction of the Norma spiral arm. This survey has a total exposure time of 1.7 Ms, and typical and maximum exposure depths of 50 ks and 1 Ms, respectively. In the area of deepest coverage, sensitivity limits of $5\times10^{-14}$ and $4\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$ in the 3-10 and 10-20 keV bands, respectively, are reached. Twenty-eight sources are firmly detected and ten are detected with low significance; eight of the 38 sources are expected to be active galactic nuclei. The three brightest sources were previously identified as a low-mass X-ray binary, high-mass X-ray binary, and pulsar wind nebula. Based on their X-ray properties and multi-wavelength counterparts, we identify the likely nature of the other sources as two colliding wind binaries, three pulsar wind nebulae, a black hole binary, and a plurality of cataclysmic variables (CVs). The CV candidates in the Norma region have plasma temperatures of $\approx$10-20 keV, consistent with the Galactic Ridge X-ray emission spectrum but lower than temperatures of CVs near the Galactic Center. This temperature difference may indicate that the Norma region has a lower fraction of intermediate polars relative to other types of CVs compared to the Galactic Center. The NuSTAR log$N$-log$S$ distribution in the 10-20 keV band is consistent with the distribution measured by Chandra at 2-10 keV if the average source spectrum is assumed to be a thermal model with $kT\approx15$~keV, as observed for the CV candidates.