X-ray polarization evidence for a 200 years-old flare of Sgr A${}^{\ast }$

The center of the Milky Way Galaxy hosts a $\sim$4 million solar mass black hole (Sgr A${}^{\ast }$) that is currently very quiescent with a luminosity many orders of magnitude below those of active galactic nuclei. Reflection of X-rays from Sgr A${}^{\ast }$ by dense gas in the Galactic Center region offers a means to study its past flaring activity on times scales of hundreds and thousands of years. The shape of the X-ray continuum and the strong fluorescent iron line observed from giant molecular clouds in the vicinity of Sgr A${}^{\ast }$ are consistent with the reflection scenario. If this interpretation is correct, the reflected continuum emission should be polarized. Here we report observations of polarized X-ray emission in the direction of the Galactic center molecular clouds using the Imaging X-ray Polarimetry Explorer (IXPE). We measure a polarization degree of 31% $\pm$ 11%, and a polarization angle of $-$48${}^{\circ }$ $\pm$ 11${}^{\circ }$. The polarization angle is consistent with Sgr A${}^{\ast }$ being the primary source of the emission, while the polarization degree implies that some 200 years ago the X-ray luminosity of Sgr A${}^{\ast }$ was briefly comparable to a Seyfert galaxy.