Intraday Variability of Sagittarius A* at 3 Millimeters

Abstract

We report observations and analysis of flux monitoring of Sagittarius A* at 3 mm wavelength using the Owens Valley Radio Observatory millimeter interferometer over a period of 8 days (2002 May 23-30). Frequent phase and flux referencing (every 5 minutes) with the nearby calibrator source J1744-312 was employed to control for instrumental and atmospheric effects. Time variations are sought by computing and subtracting, from each visibility in the database, an average visibility obtained from all the data acquired in our monitoring program having similar u-v spacings. This removes the confusing effects of baseline-dependent, correlated flux interference caused by the static, thermal emission from the extended source Sgr A West. Few-day variations up to ~20% and intraday variability of ~20% and in some cases up to ~40% on few-hour timescales emerge from the differenced data on Sgr A*. Power spectra of the residuals indicate the presence of hourly variations on all but 2 of the 8 days. Monte Carlo simulation of red-noise light curves indicates that the hourly variations are well described by a red-noise power spectrum with P(f) ∝ f-1. Of particular interest is an ~2.5 hr variation seen prominently on 2 consecutive days. An average power spectrum from all 8 days of data reveals noteworthy power on this timescale. There is some indication that few-hour variations are more pronounced on days when the average daily flux is highest. We briefly discuss the possibility that these few-hour variations are due to the dynamical modulation of accreting gas around the central supermassive black hole as well as the implications for the structure of the Sgr A* photosphere at 3 mm. Finally, these data have enabled us to produce a high-sensitivity 3 mm map of the extended thermal emission surrounding Sgr A*.