Orbital motion near Sagittarius A∗. Constraints from polarimetric ALMA observations

Vol. 665
1. Letters

Orbital motion near Sagittarius A∗. Constraints from polarimetric ALMA observations

Following the epochal observation of the black hole shadow in the elliptical galaxy M87, the Event Horizon Telescope (EHT) imaged the black hole of 4 million solar masses at the center of our Galaxy, Sagittarius A* (Sgr A*). Sgr A* is well known to show intense flaring activity across the electromagnetic spectrum, from X-ray to infrared wavelengths. The ESO Atacama Large Millimeter/submillimeter Array (ALMA) took observations of Sgr A* over two periods: one during a low variability period and the other just after a bright X-ray flare. ALMA observations were part of the EHT effort and registered the phase and the polarization of the signal. The polarimetric light curve after the flare showed a characteristic loop, which can be interpreted and modeled as an orbiting hot spot around the central black hole. Assuming Keplerian motions, the hot spot turns out to be just out of the black hole event horizon, orbiting in a clockwise direction in the sky.