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Published July 20, 2018 | Submitted
Journal Article Open

Young Accreting Compact Objects in M31: The Combined Power of NuSTAR, Chandra, and Hubble

Abstract

We present 15 high-mass X-ray binary (HMXB) candidates in the disk of M31 for which we are able to infer compact object type, spectral type of the donor star, and age using multiwavelength observations from NuSTAR, Chandra, and the Hubble Space Telescope. The hard X-ray colors and luminosities from NuSTAR permit the tentative classification of accreting X-ray binary systems by compact object type, distinguishing black hole from neutron star systems. We find hard-state black holes, pulsars, and non-magnetized neutron stars associated with optical point-source counterparts with similar frequency. We also find nine non-magnetized neutron stars coincident with globular clusters and an equal number of pulsars with and without point-source optical counterparts. We perform spectral energy distribution (SED) fitting for the most likely optical counterparts to the HMXB candidates, finding seven likely high-mass stars and one possible red helium-burning star. The remaining seven HMXB optical counterparts have poor SED fits, so their companion stars remain unclassified. Using published star formation histories, we find that the majority of HMXB candidates—X-ray sources with UV-bright point-source optical counterpart candidates—are found in regions with star formation bursts less than 50 Myr ago, and three are associated with young stellar ages (<10 Myr). This is consistent with similar studies of HMXB populations in the Magellanic Clouds, M33, NGC 300, and NGC 2403.

Additional Information

© 2018 The American Astronomical Society. Received 2018 March 19; revised 2018 May 31; accepted 2018 June 5; published 2018 July 19. We thank Antara Basu-Zych for useful discussions that led to an improvement of the paper. We acknowledge funding through Chandra program award GO5-17077Z (PI Hornschemeier). Support for this work was provided in part by Chandra Award Number GO5-16085X issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics and Space Administration under contract NAS8-03060.

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