Oral Presentation
Time Variations of the Sgr A* Flux at 230 GHz
Presenter: Yuhei Iwata (Keio University)
Sgr A*, the nucleus of our Galaxy, is one of the most convincing candidates for a supermassive black hole (SMBH) with a mass of ~4x10^6 Msun. It shows short term variabilities and flaring activities in millimeter-wave, infrared, and X-ray bands. These flux variations are considered to be associated with accretion processes close to the SMBH. The observations in the submillimeter band are particularly important to compare its variations with those in other wavelengths, as well as to image the event horizon directly. Here, we report the detection of time variations of the Sgr A* flux at 230 GHz with the ALMA cycle 5 observations (2017.1.00503.S, PI: M. Tsuboi). We used the phase-only self-calibration technique to reduce residual calibration errors. Measuring the flux density of Sgr A* in the snapshots of a 1-min bin, we obtained seven light curves with a 70-min duration. The light curves clearly show time variations with a time scale of a few tens of minutes, which is as short as the orbital period of the innermost stable circular orbit around a black hole with 4x10^6 Msun. Therefore, the observed millimeter emission is considered to have been occurred very close to the Galactic center black hole. We will also discuss the variations of the spectral index.
asiaa.sinica.edu.tw