Detection of time-variable structure in Sgr A*

Observations of Sgr A* with a 3-station 1.3 mm wavelength VLBI array were conducted over 3 consecutive days. During that time, a quasar calibrator, 1924-292, exhibited no changes in its small scale structure, but Sgr A* was observed to increase in flux density on all VLBI baselines sometime between the second and third days. This indicates that Sgr A* shifted to a new steady state due to some change in conditions on Schwarzschild-radius scales, possibly in the accretion rate or magnetic field structure. Two individual antennas from the CARMA array were used to increase precision of the amplitude calibration of the VLBI amplitudes.

Fish, Doeleman, et al; ApJL, v 727, L36 (2011), "1.3 mm Wavelength VLBI of Sagittarius A*: Detection of Time-variable Emission on Event Horizon Scales"

Evidence for a flare in Sgr A*. Top panel: plots of correlated flux density vs. baseline length for the quasar 1924-292 from observations with the 1.3 mm VLBI array. Blue: CARMA-SMT, Magenta and Yellow: CARMA-JCMT baselines, Cyan: SMT-JCMT baselines. Bottom panel: same plots but for Sgr A*. Note increase of flux density on all baselines on day 97 (April 7, 2009), indicating brightening of event-horizon-scale structure. Solid line shows the best fit circular Gaussian model, which is constant over all three days. Dotted lines show annular ring models, which differ among the three days, disfavoring the ring model.