Time Lag in Sgr A* Intra-Day Variability between the light curves at 90 and 102 GHz

TL;DR

This study observed Sgr A* at 90 and 102 GHz to detect intra-day time lag in variability, finding no significant delay, challenging expectations from the expanding plasma model.

Contribution

First simultaneous high-frequency observations of Sgr A* at 90 and 102 GHz to measure intra-day time lag in variability.

Findings

No significant time lag detected between 90 and 102 GHz.

Results challenge the expanding plasma model predictions.

Intra-day variability shows a zero or negligible delay.

Abstract

We performed the observation of the flux densities of Sgr A* at 90 and 102 GHz on 6 April 2005 using the Nobeyama Millimeter Array in order to detect the time lag between these frequencies. We constructed light curves covering a few hour with 1 min bin, and the Intra-Day Variability, which had a rising phase and intensity peak, of Sgr A* is clearly seen at both frequencies. We calculated the z-transformed discrete correlation function between the light curves of Sgr A* at 90 and 102 GHz. The derived time lag of the flares at these frequencies was approximately zero, contrary to our expectations based on the previously reported time lag at lower frequencies. If the radio flares of Sgr A* are explained by the expanding plasma model, the light curve at 90 GHz would be delayed with respect to the one at 102 GHz. However, we could not find such a delay with statistical significance in our data.