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

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.

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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.

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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.

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Authors

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.

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Correspondence to Kerstin Perez.

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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
Extended Data Table 2 Fits with hard emission modelled as power law
Extended Data Table 3 Fits with hard emission modelled as thermal bremsstrahlung

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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

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