Brightness temperature constraints from interferometric visibilities
  The brightness temperature is an effective parameter that describes the
physical properties of emitting material in astrophysical objects. It is
commonly determined by imaging and modeling the structure of the emitting
region and estimating its flux density and angular size. Reliable approaches
for visibility-based estimates of brightness temperature are needed for
interferometric experiments in which poor coverage of spatial frequencies
prevents successful imaging of the source structure, for example, in
interferometric measurements made at millimeter wavelengths or with orbiting
antennas. Such approaches can be developed by analyzing the relations between
brightness temperature and visibility amplitude and its r.m.s. error. A method
is introduced for directly calculating the lower and upper limits of the
brightness temperature from visibility measurements. The visibility-based
brightness temperature estimates are shown to agree well with the image-based
estimates obtained in the 2\,cm MOJAVE survey and the 3\,mm CMVA survey, with
good agreement achieved for interferometric measurements at spatial frequencies
exceeding $\approx 2\times 10^8$. The method provides an essential tool for
constraining brightness temperature in all interferometric experiments with
poor imaging capability.
