Near-infrared Polarimetry of flares from Sgr A* with Subaru/CIAO

TL;DR

This study observed near-infrared flares from Sgr A* using Subaru, confirming highly polarized synchrotron emission and detecting polarization variations that suggest orbiting hotspots near the black hole.

Contribution

First near-infrared polarization monitoring of Sgr A* flares with Subaru, revealing polarization variability and constraining hotspot orbital inclination.

Findings

Flares are highly polarized, confirming synchrotron emission.

Detected polarization degree increase and angle swing during flares.

Constraints on hotspot orbital inclination: 45 to 90 degrees.

Abstract

We have performed near-infrared monitoring observations of Sgr A*, the Galactic center radio source associated with a supermassive black hole, with the near-infrared camera CIAO and the 36-element adaptive optics system on the Subaru telescope. We observed three flares in the Ks band (2.15micron) during 220 min monitoring on 2008 May 28, and confirmed the flare emission is highly polarized, supporting the synchrotron radiation nature of the near-infrared emission. Clear variations in the degree and position angle of polarization were also detected: an increase of the degree of polarization of about 20 %, and a swing of the position angle of about 60 - 70 degrees in the declining phase of the flares. The correlation between the flux and the degree of polarization can be well explained by the flare emission coming from hotspot(s) orbiting Sgr A*. Comparison with calculations in the literature gives a constraint to the inclination angle i of the orbit of the hotspot around Sgr A*, as 45 < i < 90 degrees (close to edge-on).