Detection of a 20-min time lag observed from Sgr A∗ between 8 and 10 GHz with the VLA

Abstract

We report the detection and analysis of a radio flare observed on 17 April 2014 from Sgr A∗ at 9 GHz using the VLA in its A-array configuration. This is the first reported simultaneous radio observation of Sgr A∗ across 16 frequency windows between 8 and 10 GHz. We cross-correlate the lowest and highest spectral windows centred at 8.0 and 9.9 GHz, respectively, and find the 8.0 GHz light-curve lagging 18.37+2.17-2.18 min behind the 9.9 GHz light curve. This is the first time lag found in Sgr A∗'s light curve across a narrow radio frequency bandwidth. We separate the quiescent and flaring components of Sgr A∗ via flux offsets at each spectral window. The emission is consistent with an adiabatically expanding synchrotron plasma, which we fit to the light curves to characterize the two components. The flaring emission has an equipartition magnetic field strength of 2.2 Gauss, size of 14 Schwarzschild radii, average speed of 12 000 km s-1, and electron energy spectrum index (N(E) ∝ E-p), p = 0.18. The peak flare flux at 10 GHz is approximately 25 per cent of the quiescent emission. This flare is abnormal as the inferred magnetic field strength and size are typically about 10 Gauss and few Schwarzschild radii. The properties of this flare are consistent with a transient warm spot in the accretion flow at a distance of 10-100 Schwarzschild radii from Sgr A∗. Our analysis allows for independent characterization of the variable and quiescent components, which is significant for studying temporal variations in these components.