News

Evidence from IXPE polarimetry - when Sgr A* was (almost) a quasar (a million times brighter than today)

June 21, 2023


X-ray polarimetry provides strong confirmation of a quasar-like outburst, that occurred about 200 years ago, from Sagittarius A*, the supermassive black hole (4x106 Msun) in our Galactic Center (for background see Max Planck Highlight from 2017).  The Nature article X-ray polarization evidence for a 200-year-old flare of Sgr A* "presents the missing piece of evidence that X-rays from the giant molecular clouds are due to reflection of an intense, yet short-lived flare produced at or nearby Sgr A*."  The flare luminosity, in the 1–100 keV band, ranged from a few 1039  to about 1044  erg s−1, depending on the flare duration (a year long burst would corresponds to the lower luminosity while the highest luminosity corresponds to an hour-long burst).  Sgr A* was observed by the Event Horizon Telescope in 2017 and the resulting images and results were published in 2022.                                     

IXPE (left) and Chandra (right) images of the Galactic Center region in the 4-8 keV band.  The region used to derive the polarization is the large green circle. The resulting polarization degree is 31% and the polarization angle (electric field vector) is -48 \pm 11 degrees (counterclockwise from north).  The observed angle is consistent with that expected (-42 degrees) if Sgr A* is the source of continuum radiation reflected from giant molecular clouds.  The polarization degree implies that the outburst occurred about 205 (-30, +50) years ago.