Extremely Rapid Variations of Water Maser Emission from the Circinus Galaxy

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Abstract
The water maser lines in the Seyfert nucleus of the Circinus galaxy vary on timescales as short as a few minutes. The amplitude of one line more than doubled in ≈ 10 minutes, reaching ≈ 37 Jy, which corresponded to an increase of ≈ 6 L☉, assuming 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 about 30% on similar timescales. The variability is at least 2 orders of magnitude more rapid than any observed for other Galactic or extragalactic water masers. The intensity changes cannot be attributed easily to a mechanism of intrinsic fluctuations. The variability may be the result of strong interstellar diffractive scintillation along the line of sight within our Galaxy. This would be the first example of diffractive scintillation for any source at 22 GHz and for any source other than a pulsar. However, only the very shortest timescales for interstellar scintillation, obtained from pulsar observations and scaled to 22 GHz, correspond to the observed maser variability. Alternatively, the intensity changes may be a reaction to fluctuations in compact background or radiative pump sources and thereby may be related to variability of the central engine. The maser spectral features symmetrically bracket the systemic velocity of the galaxy, with components red- and blueshifted by about ±(100-200) km s-1. The spectrum of the Circinus maser is similar in some respects to that of the maser in NGC 4258, which probably traces a molecular disk rotating around a supermassive object. VLBI observations could reveal whether the maser source in the heart of the Circinus galaxy is part of a similar dynamical system.
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