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tokens: We thank Alberto Krone-Martins and Lennart Lindegren for expert advice on Gaia , Floor van Leeuwen for advice about astrometry, and Fred Adams, Michael Grudić, and Anushree Sengupta for useful feedback on the article. We would also like to thank the referee for helpful comments. E.D.F. and K.V.G. are supported by NASA grant NNX15AF42G and Chandra grants AR7-18002X, GO7-18001X, and G06-17015x, and the Chandra ACIS Team contract SV474018 (G. Garmire L. Townsley, principal investigators) issued by the Chandra X-ray Center under contract NAS8-03060. Facility: Gaia. Software: astro (Kelvin 2014 ), celestial (Robotham 2016 ), MVN (Korkmaz et al. 2014 ), R (R Core Team 2018 ), relaimpo (Grömping 2006 ), spatstat (Baddeley Turner 2005 ), TOPCAT (Taylor 2005 ).