Intra-day Variability of Sagittarius A* at 3 Millimeters

Jon C. Mauerhan(1), Mark Morris(1), Fabian Walter(2), and Frederick K. Baganoff(3)

(1) Department of Physics & Astronomy, University of California, Los Angeles, Los Angeles, CA 90095
(2) Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
(3) Center for Space Research, Massachusetts Institute of Technology, Cambridge MA 02139

Paper: ApJL, Mar 2005, in press

EPrint Server: astro-ph/0503124


We report observations and analysis of flux monitoring of Sagittarius A* at 3-mm wavelength using the OVRO millimeter interferometer over a period of eight 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 uv-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 intra-day variability of 20% and in some cases up to 40% on few-hour time scales emerge from the differenced data on SgrA*. Power spectra of the residuals indicate the presence of hourly variations on all but two of the eight 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) proportional to f-1. Of particular interest is a 2.5 hour variation seen prominently on two consecutive days. An average power spectrum from all eight days of data reveals noteworthy power on this time scale. 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 SgrA* photosphere at 3 mm. Finally, these data have enabled us to produce a high sensitivity 3-mm map of the extended thermal emission surrounding SgrA*.

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