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Recent spectroscopic and VLBI-imaging observations of bright extragalactic maser sources have H 2 O revealed that the megamaser emission often originates in thin circumnuclear disks near the centers of active galactic nuclei (AGNs). Using general radiative and kinematic considerations and taking account of the observed Ñux variability, we argue that the maser emission regions are clumpy, a conclusion that is independent of the detailed mechanism (X-ray heating, shocks, etc.) driving the collisionally pumped masers. We examine scenarios in which the clumps represent discrete gas condensations (i.e., clouds) and do not merely correspond to velocity irregularities in the disk. We show that even two clouds that overlap within the velocity-coherence length along the line of sight could account (through selfampliÐcation) for the entire maser Ñux of a high-velocity "" satellite ÏÏ feature in sources like NGC 4258 and NGC 1068, and we suggest that cloud self-ampliÐcation likely contributes also to the Ñux of the background-amplifying "" systemic ÏÏ features in these objects. Analogous interpretations have previously been proposed for water maser sources in Galactic star-forming regions. We argue that this picture provides a natural explanation of the time-variability characteristics of extragalactic megamaser sources and of their apparent association with Seyfert 2Èlike galaxies. We also show that the requisite cloud space densities and internal densities are consistent with the typical values of nuclear (broad emission line region type) clouds.We examine two scenarios of clumpy disks in which the maser emission is excited by a central continuum source. This excitation mechanism was Ðrst considered in the context of megamaser disks by Neufeld & Maloney, but our proposed models are clearly distinct from their warped, homogeneous disk interpretation. In our Ðrst scenario we consider an annular disk (or "" ring ÏÏ) whose inner edge corresponds to the innermost radius of the observed maser distribution and whose mass is dominated by the clumped, high-density gas component. The shielding of the high-energy continuum, which is required in order that the gas remain molecular, can be provided in this case by the dusty clouds themselves. We show that even the simplest version of this model, in which the disk is Ñat and the continuum radiation reaches the masing clouds through the plane of the disk, can account for the maser observations in NGC 1068. We point out the striking similarities between the maser ring properties as interpreted with this model and the inferred characteristics of the circumnuclear disk in the Galactic center, and we brieÑy discuss the implications of such rings for the AGN accretion disk paradigm.Our second scenario is motivated by the apparent warps observed in some of the imaged megamaser disks and by our Ðnding that the Ñat-disk version of the irradiated ring scenario could apply to a source like NGC 4258 only if the water abundance in the masing clouds were higher than the value implied by eq...

We report on the ␥-ray variability of Mrk 421 at E ␥ Ͼ 300 GeV during the 1995 season, and concentrate on the results of an intense multiwavelength observing campaign in the period April 20 to May 5, which included Ͼ100 MeV ␥-ray, X-ray, extreme-ultraviolet, optical, and radio observations, some of which show evidence for correlated behavior. Rapid variations in the TeV ␥-ray light curve with doubling and decay times of =1 day require a compact emission region and significant Doppler boosting. The TeV data reveal that the ␥-ray emission is best characterized by a succession of rapid flares with a relatively low baseline level of steady emission.

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