Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

Extended hard-X-ray emission in the inner few parsecs of the Galaxy

Nature volume 520, pages 646–649 (2015)Cite this article

Subjects

Abstract

The Galactic Centre hosts a puzzling stellar population in its inner few parsecs, with a high abundance of surprisingly young, relatively massive stars bound within the deep potential well of the central supermassive black hole, Sagittarius A* (ref. 1). Previous studies suggest that the population of objects emitting soft X-rays (less than 10 kiloelectronvolts) within the surrounding hundreds of parsecs, as well as the population responsible for unresolved X-ray emission extending along the Galactic plane, is dominated by accreting white dwarf systems2,3,4,5. Observations of diffuse hard-X-ray (more than 10 kiloelectronvolts) emission in the inner 10 parsecs, however, have been hampered by the limited spatial resolution of previous instruments. Here we report the presence of a distinct hard-X-ray component within the central 4 × 8 parsecs, as revealed by subarcminute-resolution images in the 20–40 kiloelectronvolt range. This emission is more sharply peaked towards the Galactic Centre than is the surface brightness of the soft-X-ray population5. This could indicate a significantly more massive population of accreting white dwarfs, large populations of low-mass X-ray binaries or millisecond pulsars, or particle outflows interacting with the surrounding radiation field, dense molecular material or magnetic fields. However, all these interpretations pose significant challenges to our understanding of stellar evolution, binary formation, and cosmic-ray production in the Galactic Centre.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Figure 1: The 20–40 keV image of the inner 5′ × 5′ (12 pc × 12 pc) of the Galaxy.
Figure 2: Profiles of the 20–40 keV data and spatial model along Galactic longitude and latitude.
Figure 3: Unfolded broadband X-ray spectrum of the southwest region.

Similar content being viewed by others

References

  1. Genzel, R., Eisenhauer, F. & Gillessen, S. The Galactic Center massive black hole and nuclear star cluster. Rev. Mod. Phys. 82, 3121–3195 (2010).

    Article  ADS  CAS  Google Scholar 

  2. Muno, M. P. et al. The spectra and variability of X-ray sources in a deep Chandra observation of the Galactic Center. Astrophys. J. 613, 1179–1201 (2004).

    Article  ADS  CAS  Google Scholar 

  3. Krivonos, R. et al. Hard X-ray emission from the Galactic ridge. Astron. Astrophys. 463, 957–967 (2007).

    Article  ADS  Google Scholar 

  4. Revnivtsev, M. et al. Discrete sources as the origin of the Galactic X-ray ridge emission. Nature 458, 1142–1144 (2009).

    Article  ADS  CAS  PubMed  Google Scholar 

  5. Heard, V. & Warwick, R. XMM-Newton observations of the Galactic Centre region — I: The distribution of low-luminosity X-ray sources. Mon. Not. R. Astron. Soc. 428, 3462–3477 (2013).

    Article  ADS  Google Scholar 

  6. Goldwurm, A. in The Galactic Center: a Window to the Nuclear Environment of Disk Galaxies (eds Morris, M. R., Wang, Q. D. & Yuan, F.) Sect. 3 394–401 (Astron. Soc. Pacif. Conf. Ser. Vol. 439, 2011).

    Google Scholar 

  7. Muno, M. P. et al. A catalog of X-ray point sources from two megaseconds of Chandra observations of the Galactic Center. Astrophys. J. Suppl. Ser. 181, 110–128 (2009).

    Article  ADS  CAS  Google Scholar 

  8. Yuasa, T., Makishima, K. & Nakazawa, K. Broadband spectral analysis of the Galactic ridge X-ray emission. Astrophys. J. 753, 129 (2012).

    Article  ADS  CAS  Google Scholar 

  9. Bélanger, G. et al. A persistent high-energy flux from the heart of the Milky Way: INTEGRAL’s view of the Galactic Center. Astrophys. J. 636, 275–289 (2006).

    Article  ADS  Google Scholar 

  10. Harrison, F. A. et al. The Nuclear Spectroscopic Telescope Array (NuSTAR) high-energy X-ray mission. Astrophys. J. 770, 103 (2013).

    Article  ADS  CAS  Google Scholar 

  11. Christopher, M. H., Scoville, N. Z., Stolovy, S. R. & Yun, M. S. HCN and HCO+ observations of the Galactic circumnuclear disk. Astrophys. J. 622, 346–365 (2005).

    Article  ADS  CAS  Google Scholar 

  12. Zhao, J.-H., Morris, M. R. & Goss, W. M. in The Galactic Center: Feeding and Feedback in a Normal Galactic Nucleus (eds Sjouwerman, L. O., Lang, C. C. & Ott, J.) 364–368 (IAU Symp. Vol. 303, 2014).

    Google Scholar 

  13. Wang, Q. D., Lu, F. J. & Gotthelf, E. V. G359.95–0.04: an energetic pulsar candidate near Sgr A*. Mon. Not. R. Astron. Soc. 367, 937–944 (2006).

    Article  ADS  CAS  Google Scholar 

  14. Nynka, M. et al. G359.97–0.038: a hard X-ray filament associated with a supernova shell-molecular cloud interaction. Astrophys. J. 800, 119 (2015).

    Article  ADS  CAS  Google Scholar 

  15. Nynka, M. et al. High-energy X-rays from J174545.5–285829, the Cannonball: a candidate pulsar wind nebula associated with Sgr A East. Astrophys. J. 778, L31 (2013).

    Article  ADS  Google Scholar 

  16. Menten, K. M., Reid, M. J., Eckart, A. & Genzel, R. The position of Sagittarius A*: accurate alignment of the radio and infrared reference frames at the Galactic Center. Astrophys. J. 475, L111–L114 (1997).

    Article  ADS  CAS  Google Scholar 

  17. Smith, R., Brickhouse, N., Liedahl, D. & Raymond, J. Collisional plasma models with APEC/APED: emission-line diagnostics of hydrogen-like and helium-like ions. Astrophys. J. 556, L91–L95 (2001).

    Article  ADS  CAS  Google Scholar 

  18. Muno, M. et al. Diffuse X-ray emission in a deep Chandra image of the Galactic Center. Astrophys. J. 613, 326–342 (2004).

    Article  ADS  CAS  Google Scholar 

  19. Park, S. et al. A candidate neutron star associated with Galactic Center supernova remnant Sagittarius A East. Astrophys. J. 631, 964–975 (2005).

    Article  ADS  CAS  Google Scholar 

  20. Sakano, M., Warwick, R. S., Decourchelle, A. & Predehl, P. XMM-Newton observations of Sagittarius A East. Mon. Not. R. Astron. Soc. 350, 129–139 (2004).

    Article  ADS  CAS  Google Scholar 

  21. Zhang, S. et al. High-energy X-ray detection of G359.89-0.08 (Sgr A-E): magnetic flux tube emission powered by cosmic rays? Astrophys. J. 784, 6 (2014).

    Article  ADS  Google Scholar 

  22. Johnson, S. P., Dong, H. & Wang, Q. D. A large scale survey of X-ray filaments in the Galactic Center. Mon. Not. R. Astron. Soc. 399, 1429–1440 (2009).

    Article  ADS  CAS  Google Scholar 

  23. Davidson, J. et al. The luminosity of the Galactic Center. Astrophys. J. 387, 189–211 (1992).

    Article  ADS  Google Scholar 

  24. Suleimanov, V., Revnivtsev, M. & Ritter, H. RXTE broadband X-ray spectra of intermediate polars and white dwarf mass estimates. Astron. Astrophys. 435, 191–199 (2005).

    Article  ADS  Google Scholar 

  25. Merrit D., ed. Dynamics and Evolution of Galactic Nuclei Ch. 5 276 (Princeton Series in Astrophysics, Princeton Univ. Press, 2013).

  26. Degenaar, N. et al. A four-year XMM-Newton/Chandra monitoring campaign of the Galactic centre: analysing the X-ray transients. Astron. Astrophys. 545, A49–A73 (2012).

    Article  Google Scholar 

  27. Muno, M. P. et al. An overabundance of transient X-ray binaries within 1 pc of the Galactic Center. Astrophys. J. 622, L113–L116 (2005).

    Article  ADS  CAS  Google Scholar 

  28. Menou, K., Narayan, R. & Lasota, J.-P. A population of faint nontransient low-mass black hole binaries. Astrophys. J. 513, 811–826 (1999).

    Article  ADS  Google Scholar 

  29. Heinke, C. et al. Discovery of a second transient low-mass X-ray binary in the globular cluster NGC 6440. Astrophys. J. 714, 894–903 (2010).

    Article  ADS  Google Scholar 

  30. Hooper, D. & Linden, T. On the origin of the gamma rays from the Galactic Center. Phys. Rev. D 84, 123005 (2011).

    Article  ADS  CAS  Google Scholar 

  31. Barrière, N. M. et al. NuSTAR detection of high-energy X-ray emission and rapid variability from Sagittarius A* flares. Astrophys. J. 786, 46 (2014).

    Article  ADS  Google Scholar 

  32. An, H. et al. In-flight PSF calibration of the NuSTAR hard X-ray optics. Proc. SPIE 9144, 1–10 (2014).

    Google Scholar 

  33. Fruscione, A. A. et al. CIAO: Chandra’s data analysis system. Proc. SPIE 6270, 1–12 (2006).

    Google Scholar 

  34. Arnaud, K. A. in Astronomical Data Analysis Software and Systems V (eds Jacoby, J. H. & Barnes, J.) 17–20 (ASP Conf. Ser. Vol. 101, 1996).

    Google Scholar 

  35. Verner, D. A., Ferland, G. J., Korista, K. T. & Yakovlev, D. G. Atomic data for astrophysics II. New analytic FITS for photoionization cross sections of atoms and ions. Astrophys. J. 465, 487–510 (1996).

    Article  ADS  CAS  Google Scholar 

  36. Wilms, J., Allen, A. & McCray, R. On the absorption of X-rays in the interstellar medium. Astrophys. J. 542, 914–924 (2000).

    Article  ADS  CAS  Google Scholar 

  37. Lu, F. J., Yuan, T. T. & Lou, Y.-Q. An imaging and spectral study of 10 X-ray filaments around the Galactic Center. Astrophys. J. 673, 915–927 (2008).

    Article  ADS  CAS  Google Scholar 

  38. Bamba, A. et al. X-ray evolution of pulsar wind nebulae. Astrophys. J. 719, L116 (2010).

    Article  ADS  Google Scholar 

  39. Quataert, E. & Loeb, A. Nonthermal THz to TeV emission from stellar wind shocks in the Galactic Center. Astrophys. J. 635, L45–L48 (2005).

    Article  ADS  CAS  Google Scholar 

  40. Cirelli, M., Kadastik, M., Raidal, M. & Strumia, A. Model-independent implications of the e±, cosmic ray spectra on properties of Dark Matter. Nucl. Phys. B 813, 1–21 (2009).

    Article  ADS  CAS  MATH  Google Scholar 

  41. Fan, J., Katz, A., Randall, L. & Reece, M. Double-disk dark matter. Phys. Dark Univ. 2, 139–156 (2013).

    Article  CAS  Google Scholar 

  42. Homan, J. et al. The X-ray properties of the black hole transient MAXI J1659-152 in quiescence. Astrophys. J. 775, 9 (2013).

    Article  ADS  CAS  Google Scholar 

  43. Degenaar, N. & Wijnands, R. A four-year baseline Swift study of enigmatic X-ray transients located near the Galactic Center. Astron. Astrophys. 524, A69 (2010).

    Article  ADS  Google Scholar 

  44. Zavlin, V. E. XMM-Newton observations of four millisecond pulsars. Astrophys. J. 638, 951–962 (2006).

    Article  ADS  CAS  Google Scholar 

  45. Manchester, R. N., Hobbs, G. B., Teoh, A. & Hobbs, M. The Australia Telescope National Facility pulsar catalogue. Astron. J. 129, 1993–2006 (2005).

    Article  ADS  Google Scholar 

  46. Takata, J., Cheng, K. S. & Taam, R. E. X-ray and gamma-ray emissions from rotation powered millisecond pulsars. Astrophys. J. 745, 100 (2012).

    Article  ADS  Google Scholar 

Download references

Acknowledgements

This work was supported by NASA contract no. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). We also thank A. Canipe, J. Dodaro, D. Hong and T. V. T. Luu for assistance with data preparation and analysis. F.E.B. acknowledges support from Basal-CATA PFB-06/2007, CONICYT-Chile (FONDECYT 1141218 and EMBIGGEN Anillo ACT1101), and Project IC120009 “Millennium Institute of Astrophysics (MAS)” funded by the Iniciativa Científica Milenio del Ministerio de Economía, Fomento y Turismo.

Author information

Authors and Affiliations

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

    Kerstin Perez, Charles J. Hailey, Kaya Mori, Melania Nynka & Shuo Zhang

  2. Haverford College, 370 Lancaster Avenue, KINSC L109, Haverford, 19041, Pennsylvania, USA

    Kerstin Perez

  3. Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Santiago 22, 306, Chile

    Franz E. Bauer

  4. Millennium Institute of Astrophysics, Vicuña Mackenna 4860, Macul, 7820436, Santiago, Chile

    Franz E. Bauer

  5. Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, 80301, Colorado, USA

    Franz E. Bauer

  6. Space Science Laboratory, UC Berkeley, 7 Gauss Way, Berkeley, 94720, California, USA

    Roman A. Krivonos, Nicolas M. Barrière, Steven E. Boggs, William W. Craig, John A. Tomsick & Andreas Zoglauer

  7. Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 37-555, Cambridge, 02139, Massachusetts, USA

    Frederick K. Baganoff

  8. DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800, Lyngby, Denmark

    Finn E. Christensen

  9. Lawrence Livermore National Laboratory, PO Box 808, Livermore, 94551-0808, California, USA

    William W. Craig

  10. Cahill Center for Astronomy and Astrophysics, 1200 East California Boulevard, MC 290-17, California Institute of Technology, Pasadena, 91125, California, USA

    Brian W. Grefenstette, Fiona A. Harrison & Kristin K. Madsen

  11. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-83, Cambridge, 02138, Massachusetts, USA

    Jonathan E. Grindlay & Jaesub Hong

  12. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, 4800 Oak Grove Drive, MS 169-221, 91109, California, USA

    Daniel Stern

  13. NASA Goddard Space Flight Center, 8800 Greenbelt Road, Code 662, Greenbelt, 20771, Maryland, USA

    Daniel R. Wik & William W. Zhang

Authors
  1. Kerstin Perez
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Charles J. Hailey
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Franz E. Bauer
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Roman A. Krivonos
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. Kaya Mori
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. Frederick K. Baganoff
    View author publications

    You can also search for this author in PubMed Google Scholar

  7. Nicolas M. Barrière
    View author publications

    You can also search for this author in PubMed Google Scholar

  8. Steven E. Boggs
    View author publications

    You can also search for this author in PubMed Google Scholar

  9. Finn E. Christensen
    View author publications

    You can also search for this author in PubMed Google Scholar

  10. William W. Craig
    View author publications

    You can also search for this author in PubMed Google Scholar

  11. Brian W. Grefenstette
    View author publications

    You can also search for this author in PubMed Google Scholar

  12. Jonathan E. Grindlay
    View author publications

    You can also search for this author in PubMed Google Scholar

  13. Fiona A. Harrison
    View author publications

    You can also search for this author in PubMed Google Scholar

  14. Jaesub Hong
    View author publications

    You can also search for this author in PubMed Google Scholar

  15. Kristin K. Madsen
    View author publications

    You can also search for this author in PubMed Google Scholar

  16. Melania Nynka
    View author publications

    You can also search for this author in PubMed Google Scholar

  17. Daniel Stern
    View author publications

    You can also search for this author in PubMed Google Scholar

  18. John A. Tomsick
    View author publications

    You can also search for this author in PubMed Google Scholar

  19. Daniel R. Wik
    View author publications

    You can also search for this author in PubMed Google Scholar

  20. Shuo Zhang
    View author publications

    You can also search for this author in PubMed Google Scholar

  21. William W. Zhang
    View author publications

    You can also search for this author in PubMed Google Scholar

  22. Andreas Zoglauer
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

K.P.: image analysis, spectral analysis, modelling of the Sagittarius A* region, interpretation and manuscript preparation. C.J.H.: interpretation, manuscript preparation and review. F.E.B.: interpretation and manuscript review. R.K.: image analysis and interpretation. K.M.: interpretation, manuscript preparation and review. F.K.B., N.M.B., S.E.B., J.E.G., J.H. and J.A.T.: interpretation and manuscript review. F.E.C., W.W.Z.: optics production and calibration. W.W.C.: optics and instrumentation production and response, and observation planning. B.W.G., K.K.M., D.R.W. and A.Z.: software for data analysis, background modelling, and calibration. F.A.H.: NuSTAR principal investigator, observation planning, interpretation and manuscript review. M.N., S.Z.: data preparation and interpretation. D.S.: observation planning and manuscript review.

Corresponding author

Correspondence to Kerstin Perez.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 The best-fit 20–40 keV spatial model and residual image.

a, The model, consisting of a point-like Gaussian, an extended Gaussian, and a background model as described in the main text. The radio position of Sgr A16 and the Chandra positions of the Cannonball19, G359.95−0.0413 and G359.97−0.03822 are overlaid. The FWHM extent of the two-dimensional Gaussian fit to the extended emission is indicated by the solid ellipse. The image is shown with a linear grey-scale distribution with the range chosen to highlight the extended emission, in units of total counts per pixel (bar at bottom). b, The residual image (observed minus model). Insets in a and b show a magnified view of the central 40″ × 40″ of the main panels, with 1σ, 2σ and 3σ error contours on the centroid positions of the extended (magenta) and point-source (green) two-dimensional Gaussian models, as well as the position of Sgr A

Extended Data Figure 2 Profiles of the 20–40 keV data and exponential spatial model along Galactic longitude and latitude.

a, Galactic latitude; b, Galactic longitude. Top panels, the data (red, 1σ errors) are fitted to a two-dimensional model consisting of the surface brightness model used in ref. 5 and described in the main text (dashed line) and a point-like Gaussian source (thin solid line) that describes the emission from G359.95–0.04, both convolved with the on-axis NuSTAR PSF, as well as a flat background (dash-dot line) that varies between detector chips. Profiles are obtained by integrating from 25″ on either side of each axis, with the origin defined at the position of Sgr A

Extended Data Figure 3 Regions used for extraction of background spectra for NuSTAR observations 30001002001 (red), 30001002003 (white), and 30001002004 (green).

The regions used for spectral analysis of the diffuse emission are indicated by the dashed lines. These regions are overlaid on the NuSTAR 20–40 keV image. The colour scale shows units of total counts per pixel. The x and y axes indicate coordinates of right ascension and declination, respectively.

Extended Data Figure 4 Unfolded 2–40 keV energy spectrum from the southwest region indicated in Fig. 1.

Top panel, the 2–10 keV spectrum is constructed from XMM-Newton data from the PN (black), MOS1 (red), and MOS2 (green) instruments. The 10–40 keV spectrum is constructed from NuSTAR focal plane A (dark blue) and focal plane B (cyan) data. The fit to the data comprises two absorbed thermal plasmas plus an absorbed power-law, multiplied by a separate normalization factor for each data set to account for small calibration differences between instruments. The dashed lines indicate the separate model components. Fit residuals are shown in the lower panel. Full parameters of the spectral model are given in Extended Data Table 2. Error bars, 1σ statistical errors.

Extended Data Figure 5 Unfolded 2–40 keV energy spectrum from the northeast region indicated in Fig. 1.

Top panel, the spectrum is composed as in Extended Data Fig. 4. The model fitted to the data comprises an absorbed two-temperature thermal plasma plus a power-law, multiplied by a separate normalization factor for each data set. Fit residuals are shown in the lower panel. Full parameters of the spectral model are given in Extended Data Table 2. Error bars, 1σ statistical errors.

Extended Data Figure 6 Unfolded 2–40 keV energy spectrum from the northeast region indicated in Fig. 1.

The spectrum is composed as in Extended Data Fig. 4. The model fitted to the data comprises an absorbed two-temperature thermal plasma plus a thermal bremsstrahlung, multiplied by a separate normalization factor for each data set. Fit residuals are shown in the lower panel. Full parameters of the spectral model are given in Extended Data Table 3. Error bars, 1σ statistical errors.

Extended Data Table 1 Best fit 2D Gaussian models
Full size table
Extended Data Table 2 Fits with hard emission modelled as power law
Full size table
Extended Data Table 3 Fits with hard emission modelled as thermal bremsstrahlung
Full size table

PowerPoint slides

Rights and permissions

Reprints and permissions

About this article

Cite this article

Perez, K., Hailey, C., Bauer, F. et al. Extended hard-X-ray emission in the inner few parsecs of the Galaxy. Nature 520, 646–649 (2015). https://doi.org/10.1038/nature14353

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nature14353

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing