A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy

  • Nature volume 556, pages 7073 (05 April 2018)
  • doi:10.1038/nature25029
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The existence of a ‘density cusp’1,2—a localized increase in number—of stellar-mass black holes near a supermassive black hole is a fundamental prediction of galactic stellar dynamics3. The best place to detect such a cusp is in the Galactic Centre, where the nearest supermassive black hole, Sagittarius A*, resides. As many as 20,000 black holes are predicted to settle into the central parsec of the Galaxy as a result of dynamical friction3,4,5; however, so far no density cusp of black holes has been detected. Low-mass X-ray binary systems that contain a stellar-mass black hole are natural tracers of isolated black holes. Here we report observations of a dozen quiescent X-ray binaries in a density cusp within one parsec of Sagittarius A*. The lower-energy emission spectra that we observed in these binaries is distinct from the higher-energy spectra associated with the population of accreting white dwarfs that dominates the central eight parsecs of the Galaxy6. The properties of these X-ray binaries, in particular their spatial distribution and luminosity function, suggest the existence of hundreds of binary systems in the central parsec of the Galaxy and many more isolated black holes. We cannot rule out a contribution to the observed emission from a population (of up to about one-half the number of X-ray binaries) of rotationally powered, millisecond pulsars. The spatial distribution of the binary systems is a relic of their formation history, either in the stellar disk around Sagittarius A* (ref. 7) or through in-fall from globular clusters, and constrains the number density of sources in the modelling of gravitational waves from massive stellar remnants8,9, such as neutron stars and black holes.

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This work was partially supported by NASA contract no. NNG08FD60C. We thank P. Broos of the Chandra ACIS Instrument Team at Penn State for help with the ACIS Extract Package, and D. P. Huenemoerder and H. M. Guenther at MIT for help with the MARX simulation tool. We acknowledge C. Jin and G. Ponti for providing the dust scattering model. We thank A. Generozov, D. Helfand, L. Hui, S. Mandel, B. Metzger, J. Ostriker and N. Stone of Columbia University and Q. D. Wang at the University of Massachusetts at Amherst for discussions. F.E.B. acknowledges support from CONICYT-Chile (Basal-CATA PFB-06/2007, FONDECYT Regular 1141218), the Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS.

Author information


  1. Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, Room 1027, New York, New York 10027, USA

    • Charles J. Hailey
    • , Kaya Mori
    • , Michael E. Berkowitz
    •  & Benjamin J. Hord
  2. Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, 306, Santiago 22, Chile

    • Franz E. Bauer
  3. Millennium Institute of Astrophysics, Vicuña Mackenna, 4860, 7820436 Macul, Santiago, Chile

    • Franz E. Bauer
  4. Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA

    • Franz E. Bauer
  5. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-83, Cambridge, Massachusetts 02138, USA

    • Jaesub Hong


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C.J.H., statistical and population analysis, interpretation and manuscript preparation; K.M., image and spectral analysis, interpretation and manuscript preparation; F.E.B., source extraction, MARX simulations and review; M.E.B., image, spectral and statistical analysis, MARX simulations and review; J.H., source extraction, MARX simulations and review; and B.J.H., image, spectral and population analysis, XSPEC simulations and review.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Charles J. Hailey.

Reviewer Information Nature thanks F. Baganoff, J. Miralda-Escudé and M. Morris for their contribution to the peer review of this work.

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