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      The first low-mass black hole X-ray binary identified in quiescence outside of a globular cluster

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          Abstract

          The observed relation between the X-ray and radio properties of low-luminosity accreting black holes has enabled the identification of multiple candidate black hole X-ray binaries (BHXBs) in globular clusters. Here we report an identification of the radio source VLA J213002.08+120904 (aka M15 S2), recently reported in Kirsten et al. 2014, as a BHXB candidate. They showed that the parallax of this flat-spectrum variable radio source indicates a 2.2\(^{+0.5}_{-0.3}\) kpc distance, which identifies it as lying in the foreground of the globular cluster M15. We determine the radio characteristics of this source, and place a deep limit on the X-ray luminosity of \(\sim4\times10^{29}\) erg s\(^{-1}\). Furthermore, we astrometrically identify a faint red stellar counterpart in archival Hubble images, with colors consistent with a foreground star; at 2.2 kpc its inferred mass is 0.1-0.2 \(M_{\odot}\). We rule out that this object is a pulsar, neutron star X-ray binary, cataclysmic variable, or planetary nebula, concluding that VLA J213002.08+120904 is the first accreting black hole X-ray binary candidate discovered in quiescence outside a globular cluster. Given the relatively small area over which parallax studies of radio sources have been performed, this discovery suggests a much larger population of quiescent BHXBs in our Galaxy, \(2.6\times10^4-1.7\times10^8\) BHXBs at \(3\sigma\) confidence, than has been previously estimated (\(\sim10^2-10^4\)) through population synthesis.

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          Author and article information

          Journal
          2016-05-01
          Article
          10.3847/0004-637X/825/1/10
          1605.00270

          http://arxiv.org/licenses/nonexclusive-distrib/1.0/

          Custom metadata
          13 pages, 6 Figures, Accepted to ApJ
          astro-ph.HE

          High energy astrophysical phenomena

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