X-ray polarization evidence for a 200-year-old flare of Sgr A*

Abstract

The centre of the Milky Way Galaxy hosts a black hole with a solar mass of about 4 million (Sagittarius A<SUP>*</SUP> (Sgr A)) that is very quiescent at present with a luminosity many orders of magnitude below those of active galactic nuclei<SUP>1</SUP>. Reflection of X-rays from Sgr A<SUP>*</SUP> by dense gas in the Galactic Centre region offers a means to study its past flaring activity on timescales of hundreds and thousands of years<SUP>2</SUP>. The shape of the X-ray continuum and the strong fluorescent iron line observed from giant molecular clouds in the vicinity of Sgr A<SUP>*</SUP> are consistent with the reflection scenario<SUP>3-5</SUP>. If this interpretation is correct, the reflected continuum emission should be polarized<SUP>6</SUP>. 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<SUP>*</SUP> being the primary source of the emission, and the polarization degree implies that some 200 years ago, the X-ray luminosity of Sgr A<SUP>*</SUP> was briefly comparable to that of a Seyfert galaxy.