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Identifying Compact Objects in the X-ray Binary Population of Andromeda’s Disc

Presentation #113.06 in the session Spiral Galaxies.

Published onJun 29, 2022
Identifying Compact Objects in the X-ray Binary Population of Andromeda’s Disc

X-ray observations of nearby galaxies provide one of the best laboratories in the universe for studying the populations of two exotic classes of object: black holes and neutron stars. Because they are directly connected to past and current stellar populations through binary synthesis modeling, studies of accreting black hole and neutron star populations can provide insights into the history of star formation and evolution in a galaxy. Using hard X-ray observations attainable with NuSTAR, we are able to differentiate between the accretion states, and thus compact object types, of X-ray binaries (XRBs) in the Andromeda Galaxy, our nearest spiral neighbor. This tells us not only about its own XRB population, but also provides hints about the population and distribution of XRBs in the Milky Way, where this type of population characterization is more difficult due to the ambiguity of the spatial distribution of sources. Using ten ~50ks observations of the disc of Andromeda, we detect 21 sources at 3 σ above the background in the 4-25keV bandpass, and make 1 σ classifications of 12 of those sources with the use of diagnostic color-rate and color-color diagrams, which separate sources into various neutron star and black hole regimes. In addition, we create X-ray luminosity functions for both the full (4-25keV) and hard (12-25keV) band luminosities. We combine this with additional ~500ks coverage to create a luminosity function for the disc, and fit and compare to functions from past softer energy coverage. These classifications will be added to those already included in a deeper M31 survey of the bulge and a more limited coverage of the disc, expanding the knowledge of both the types of sources present in the population as well as their distribution throughout the galaxy.

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