A Hard X-ray Study of the Normal Star-Forming Galaxy M83 with NuSTAR

TL;DR

This study uses NuSTAR to spatially resolve and analyze the hard X-ray emission of the star-forming galaxy M83, revealing the nature of its X-ray sources and constraining the presence of an active galactic nucleus.

Contribution

First to construct the X-ray binary luminosity function in the NuSTAR band for an extragalactic galaxy, providing new insights into high-energy processes in star-forming galaxies.

Findings

Hard X-ray emission dominated by black hole binaries and neutron star X-ray binaries.

X-ray binary luminosity function is steeper than in NGC 253.

No evidence of an active galactic nucleus above 10 keV, suggesting it is highly obscured or very low luminosity.

Abstract

We present results from sensitive, multi-epoch NuSTAR observations of the late-type star-forming galaxy M83 (d=4.6 Mpc), which is the first investigation to spatially resolve the hard (E>10 keV) X-ray emission of this galaxy. The nuclear region and ~ 20 off-nuclear point sources, including a previously discovered ultraluminous X-ray (ULX) source, are detected in our NuSTAR observations. The X-ray hardnesses and luminosities of the majority of the point sources are consistent with hard X-ray sources resolved in the starburst galaxy NGC 253. We infer that the hard X-ray emission is most likely dominated by intermediate accretion state black hole binaries and neutron star low-mass X-ray binaries (Z-sources). We construct the X-ray binary luminosity function (XLF) in the NuSTAR band for an extragalactic environment for the first time. The M83 XLF has a steeper XLF than the X-ray binary XLF in NGC 253, consistent with previous measurements by Chandra at softer X-ray energies. The NuSTAR integrated galaxy spectrum of M83 drops quickly above 10 keV, which is also seen in the starburst galaxies NGC253, NGC 3310 and NGC 3256. The NuSTAR observations constrain any AGN to be either highly obscured or to have an extremely low luminosity of $_{\sim}^<$10$^{38}$ erg/s (10-30 keV), implying it is emitting at a very low Eddington ratio. An X-ray point source consistent with the location of the nuclear star cluster with an X-ray luminosity of a few times 10$^{38}$ erg/s may be a low-luminosity AGN but is more consistent with being an X-ray binary.