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                              G C N E W S
                             * Newsflash *

            - The Newsletter for Galactic Center Research -

gcnews@mpifr-bonn.mpg.de             http://www.mpifr-bonn.mpg.de/gcnews
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Vol. 12, No. 16                                              Nov 9, 2000


Recently submitted papers:
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1) Radio Variability of Sagittarius A* - A 106 Day Cycle 
   (Zhao et al., ApJ)
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Email    : jzhao@buzz.harvard.edu
Title    : Radio Variability of Sagittarius A* - A 106 Day Cycle
Author(s): Jun-Hui Zhao(1), G. C. Bower(2), W. M. Goss(2)
Institute: (1) Harvard-Smithsonian CfA, 60 Garden St, MS 78, Cambridge, 
           MA 02138 (2) NRAO-AOC, P. O. Box 0, Socorro, NM 87801
Paper    : ApJ Lett, 2000 accepted
EPrint   : astro-ph/0011169

Abstract:
We report the presence of a 106-day cycle in the radio variability of 
Sgr A* based on an analysis of data observed with the Very Large Array 
(VLA) over the past 20 years. The pulsed signal is most clearly seen at 
1.3 cm with a ratio of cycle frequency to frequency width f/\Delta f= 
2.2+/-0.3. The periodic signal is also clearly observed at 2 cm. At 3.6 
cm the detection of a periodic signal is marginal. No significant 
periodicity is detected at both 6 and 20 cm. Since the sampling 
function is irregular we performed a number of tests to insure that the 
observed periodicity is not the result of noise. Similar results were 
found for a maximum entropy method and periodogram with CLEAN method. 
The probability of false detection for several different noise 
distributions is less than 5% based on Monte Carlo tests. The radio 
properties of the pulsed component at 1.3 cm are spectral index alpha 
1.0+/- 0.1 (for S\propto nu ^ alpha ), amplitude \Delta S=0.42+/- 0.04 
Jy and characteristic time scale \Delta t_FWHM 25+/-5 days. The lack of 
VLBI detection of a secondary component suggests that the variability 
occurs within Sgr A* on a scale of 5 AU, suggesting an instability of 
the accretion disk.
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