VLBA Observations of a Bursting Water Maser in Orion KL

We have performed multiepoch single-telescope and Very Long Baseline Interferometry (VLBI) observations of an anomalously strong outburst of a water vapor maser in the Orion KL region that occurred in 1998. This burst, dubbed a superburst, exhibited a flux density increase that started in 1997 December and reached a maximum of 4.6 * 10^6 Jy in 1998 September; it then decreased to 0.2 * 106 Jy in 1999 February. The spatial and the velocity structure revealed by our observations showed that the bursting feature consisted of at least two components having two different peak velocities. Moreover, these two components exhibited a relative motion over 287 days with the relative position of the two components changing across the period of the outburst maximum. This suggests that an overlapping of two maser clouds caused the superburst. The brightness temperature reached 1016 K, while the line width was as narrow as 0.48 km s-1 at maximum phase. This extremely high brightness and narrow line width can be understood as the radiation from the maser being highly beamed.