The Enormous Outburst of the 7.9 km s-1 Water-Maser Feature in Orion KL

We report on a spectacular flare in the water-vapor maser emission from the KL nebula, which we found in 1997 December. The flare, at a velocity of 7.9 km s⁻¹, continued to increase in intensity exponentially and had a flux density greater than 3.5×10⁶ Jy. The central velocity of the flare component was 7.9 km s⁻¹ and the line width was less than 40 kHz. We did not detect a “rapid„ time variation in the intensity. The flare component is currently increasing steadily. A preliminary VLBI observation was made using a Japanese domestic VLBI network, called J-Net, in order to determine the location of the flare. Its position is $\alpha= 05^\textrm h32^\textrm m 46^\textrm s.6, \delta=-5^\circ 24^\prime 27^{\prime \prime}_.{2} (1950.0) \pm 1^{\prime \prime}.$ In this region, strong outbursts were reported during the period from 1979 August to 1985 November (Matveenko et al. 1988, AAA 49.131.147). Both the central velocity and the position of the present flare seems to be the same as that of previous ones. This suggests a common origin of these phenomena.