X-ray polarization evidence for a 200-yearold flare of Sgr A

The centre of the Milky Way Galaxy hosts a black hole with a solar mass of about 4million (Sagittarius A· (Sgr A)) that is very quiescent at present with a luminosity many orders of magnitude below those of active galactic nuclei1. Reflection of X-rays from Sgr A· by dense gas in the Galactic Centre region offers a means to study its past flaring activity on timescales of hundreds and thousands of years2. The shape ofthe X-ray continuum and the strong fluorescent iron line observed from giant molecular clouds in the vicinity of Sgr A· are consistent with the reflection scenario3-5. If this interpretation is correct, the reflected continuum emission should be polarized6. Here we report observations of polarized X-ray emission in the direction of the molecular clouds in the Galactic Centre using the Imaging X-ray Polarimetry Explorer. We measure a polarization degree of 31% ± 11%, and a polarization angle of -48° ± 11°. The polarization angle is consistent with Sgr A· being the primary source of the emission, and the polarization degree implies that some 200 years ago, the X-ray luminosity of Sgr A· was briefly comparable to that of a Seyfert galaxy.