EXTREMELY RAPID VARIATIONS OF WATER MASER EMISSION FROM THE CIRCINUS GALAXY

The water maser lines in the Seyfert nucleus of the Circinus galaxy va ry on timescales as short as a few minutes. The amplitude of one line more than doubled in approximate to 10 minutes, reaching approximate t o 37 Jy, which corresponded to an increase of approximate to 6 L., ass uming isotropic emission, in a single maser feature on a size scale of about 1 AU, based on light-travel time. Other lines vary by up to abo ut 30% on similar timescales. The variability is at least 2 orders of magnitude more rapid than any observed for other Galactic or extragala ctic water masers. The intensity changes cannot be attributed easily t o a mechanism of intrinsic fluctuations. The variability may be the re sult of strong interstellar diffractive scintillation along the line o f sight within our Galaxy. This would be the first example of diffract ive scintillation for any source at 22 GHz and for any source other th an a pulsar. However, only the very shortest timescales for interstell ar scintillation, obtained from pulsar observations and scaled to 22 G Hz, correspond to the observed maser variability. Alternatively, the i ntensity changes may be a reaction to fluctuations in compact backgrou nd or radiative pump sources and thereby may be related to variability of the central engine. The maser spectral features symmetrically brac ket the systemic velocity of the galaxy, with components red- and blue shifted by about +/-(100-200) km s(-1). The spectrum of the Circinus m aser is similar in some respects to that of the maser in NGC 4258, whi ch probably traces a molecular disk rotating around a supermassive obj ect. VLBI observations could reveal whether the maser source in the he art of the Circinus galaxy is part of a similar dynamical system.