Multiwavelength Observations of Sgr A*. I. 2019 July 18

We present and analyze ALMA submillimeter observations from a multiwavelength campaign of Sgr A* during 2019 July 18. In addition to the submillimeter, we utilize concurrent mid-infrared (mid-IR; Spitzer) and X-ray (Chandra) observations. The submillimeter emission lags less than delta t approximate to 30 minutes behind the mid-IR data. However, the entire submillimeter flare was not observed, raising the possibility that the time delay is a consequence of incomplete sampling of the light curve. The decay of the submillimeter emission is not consistent with synchrotron cooling. Therefore, we analyze these data adopting an adiabatically expanding synchrotron source that is initially optically thick or thin in the submillimeter, yielding time-delayed or synchronous flaring with the IR, respectively. The time-delayed model is consistent with a plasma blob of radius 0.8 R-S (Schwarzschild radius), electron power-law index p = 3.5 (N(E) proportional to E-p ), equipartition magnetic field of B-eq approximate to 90 Gauss, and expansion velocity v(exp) approximate to 0.004c R-S, p = 2.5, B-eq approximate to 27 Gauss, and v(exp) approximate to 0.014c. Since the submillimeter time delay is not completely unambiguous, we cannot definitively conclude which model better represents the data. This observation presents the best evidence for a unified flaring mechanism between submillimeter and X-ray wavelengths and places significant constraints on the source size and magnetic field strength. We show that concurrent observations at lower frequencies would be able to determine if the flaring emission is initially optically thick or thin in the submillimeter.

Automated summary

The study presents and analyzes ALMA submillimeter observations of Sgr A* during a multiwavelength campaign in July 2019. The submillimeter emission lags approximately 30 minutes behind the mid-IR data, and the decay of the submillimeter emission is not consistent with synchrotron cooling. Two models, one with synchronous flaring and another with time-delayed flaring, are suggested to explain the observations.