Extended hard-X-ray emission in the inner few parsecs of the Galaxy (original) (raw)

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 systems 2-5. 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 3 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 population 5. This could indicate a significantly more massive population of accreting white dwarfs, large populations of lowmass 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. The Galactic Centre region is dense with X-ray-emitting objects 6 ; it contains the supernova remnant Sagittarius (Sgr) A East, the colliding stellar winds surrounding Sgr A*, the hot plasma of the Sgr A East plume, dozens of magnetic X-ray filaments, and thousands of resolved 7 and unresolved point sources that constitute the Galactic ridge X-ray emisison 3-5,8. In hard X-rays, the INTEGRAL satellite has detected emission centred within 19 of the Galactic Centre 9. However, the spatial resolution of INTEGRAL's IBIS coded aperture mask (129) has motivated speculation that the emission results not from a single object, but from a collection of the many surrounding X-ray sources 3. The NuSTAR X-ray observatory 10 , which has an effective area extending from 3 to 79 keV and an angular resolution of 180 (equivalent to 0.7 pc at the Galactic Centre), viewed the Galactic Centre for a total of 281 ks in July, August and October of 2012. The image of the central 12 pc 3 12 pc of the Galaxy (Fig. 1) in the 20-40 keV energy band reveals for the first time a faint diffuse emission that is peaked at the Galactic Centre and extends along the Galactic plane. The image is dominated by this feature, whose spectrum and localization within several parsecs of Sgr A* distinguish it from other unresolved X-ray emission in the Galaxy, and which has no obvious correlation with radio images of the dense molecular gas of the circumnuclear disk 11 or the dust and gas of Sgr A West 12. The features prominent in soft-X-ray images no longer visibly contribute, with the exception of bright pointlike emission from the pulsar wind nebula G359.9520.04 13 and fainter emission from the X-ray filament G359.9720.038 14 and the Cannonball 15 neutron star.