Intra-day Variation of Sagittarius A at Short Millimeter Wavelengths

We have performed the monitoring observations of flux density of Sagittarius A at short millimeter wavelengths (100 and 140 GHz bands) on seven years in the period from 1996 to 2003 using the Nobeyama Millimeter Array (NMA). We found intra-day variation of Sgr A in March 2000 flare. The flux density at the peak of the flares increases 100–200% at 100 GHz and 200–400% at 140 GHz (∆S/S), respectively. The two-fold increase timescale of the flare is estimated to be about 1.5 hours at 140 GHz. The intra-day variation at mm-wavelengths has similar increase timescale as those in the X-ray and infrared flares but has smaller amplitude. This short timescale variability suggests that the physical size of the emitting region is smaller than 12 AU (≈ 150 Rs). The decay timescale of the flare was at most 24 hours. Such a light curve with rapid increase and slow decay is similar to that often observed in outburst phenomena with ejections.