Young black hole and neutron star systems in the nearby star-forming galaxy M33: the NuSTAR view

We can learn a lot about the formation of compact objects, such as neutron stars and black holes, by studying the X-ray emission from accreting systems in nearby star-forming galaxies. The harder (E>10 keV) X-ray emission in particular allows strong discrimination among the accretion states and compact object types.A NuSTAR survey of M33 was conducted to study the distribution of X-ray binary (XRB) accretion states in an actively star-forming environment. The 6 NuSTAR observations of M33 allow us to construct diagnostic diagrams, which is used to infer XRB accretion states.We have characterized XRB accretion states for ≈32 sources. The XRBs are classified by their compact object types using NuSTAR color-intensity and color-color diagrams. We further characterize the black holes by their accretion states (soft, intermediate, and hard) and the neutron stars by their weak or strong (accreting pulsar) magnetic field. In contrast to a similar NuSTAR survey of M31 (with a low-mass XRB-dominant population), the source population is dominated by high-mass XRBs, allowing the study of a very different population with similar sensitivity. These results provide a significant improvement in our knowledge of high-mass XRB accretion states that proves valuable for theoretical XRB population synthesis studies.