2006ApJ...653.1314R

We have studied the OH masers in the star-forming region W3(OH), with data obtained from the Very Long Baseline Array (VLBA). The data provide an angular resolution of ∼5 mas and a velocity resolution of 106 m/s. A novel analytical procedure allows us to differentiate between broadband temporal intensity fluctuations introduced by instrumental gain variations plus interstellar diffractive scintillation and intrinsic narrowband variations. Drawn from this 12.5 hr observation, our data are sensitive to variations with timescales of minutes to hours. We find statistically significant intrinsic variations with timescales of ∼15-20 minutes or slower, based on the velocity-resolved fluctuation spectra. These variations are seen predominantly toward the line shoulders. The peak of the line profiles shows little variation, suggesting that they perhaps exhibit saturated emission. The associated modulation index of the observed fluctuation varies from statistically insignificant values at the line center to about unity away from the line center. Based on light-travel-time considerations, the 20 minute timescale of intrinsic fluctuations translates to a spatial dimension of ∼2-3 AU along the sight lines. On the other hand, the transverse dimension of the sources, estimated from their observed angular sizes of about ∼3 mas, is about 6 AU. We argue that these source sizes are intrinsic and are not affected by interstellar scatter broadening. The implied peak brightness temperature of the 1612/1720 maser sources is about ∼2x10¹³ K and a factor of about 5 higher for the 1665 line.