This paper reports the development of an experiment (TEPEE/GReAT) to test the Equivalence Principle (EP) at a level of accuracy equal to 5 × 10-15, by means of a differential accelerometer free falling in a cryogenic vacuum capsule released from a stratospheric balloon. Such an accuracy requires resolving a very small signal out of the instrument's intrinsic noise and the noise associated with the instrument's motion. Imperfections in the construction of the detector introduce gravity gradient noise that it is possible to separate from the violation signal spinning the detector around an horizontal axis in order to have the EP violation signal and the gravity gradients one modulated at two different frequencies. Experimental results on prototype instruments showing high sensitivity and common mode rejection factor are shown.

The high resolution obtained through the use of VLBI gives an unique opportunity to directly observe the interaction of an expanding radio supernova with its surrounding medium. We present here results from our VLBI observations of the young supernovae SN 1979C, SN 1986J, and SN 2001gd.

We have made 3.6 cm VLBI observations of the RS CVn binary star IM Pegasi (HR 8703) approximately four times per year since 1997 in support of the NASA/Stanford Relativity Gyroscope Experiment (Gravity Probe B). Phase-referenced maps reveal structural changes in the radio emission of the star on hour time scales during several of the sessions. Analyses of the VLBI phase delays with a Kalman-filter estimator reveal submilliarcsecond motions of the radio centroid of the star on hour and even subhour time scales. The observed structural changes and centroid motions often coincide with rapid changes in the star's flux density, as measured with the VLA. We report on our latest results and summarize our findings to date.

We determined with VLBI the coordinates of PSR B1937+21 and compare them with other determinations obtained with interferometry and timing. All the position estimates agree within one combined standard error, after the timing positions have each been rotated to the IERS extragalactic reference frame.

The technique of differential astrometry using the phase-delay VLBI observable promises fractional precisions of ≃2 × 10–9 in the determination of the separation of sources 5° or 6° apart on the sky (Guirado et al. 1995a; Lara et al. 1996). In our present research we seek further improvement in this technique through using triplets of radio sources, which provide a closure constraint in the determination of relative angular positions. This constraint not only eases the resolution of the phase-cycle ambiguities (a major problem in the least-squares approach to astrometry with phase delays), but it also strongly constrains the space of allowable parameter values.

The nearby IrrII galaxy M82 (3C 231, NGC3034) is known to have a complex, very elongated radio brightness distribution in the central region of the galaxy (e.g., Kronberg and Wilkinson 1975). Because of the galaxy’s proximity (distance ~ 3.3 Mpc; Tammann and Sandage 1968), the brightness distribution can be investigated in considerable detail. Recently Unger et al. (1984) and Kronberg, Biermann, and Schwab (1985; see also Kronberg 1986) distinguished about 20 compact components in the central region, most of them unresolved with an upper limit on their angular sizes of ~ 150 mas corresponding to an upper limit on their linear sizes of ~ 2 pc. About half of the components were observed at more than one frequency and at several epochs and were found typically to have steep spectra between 5 and 15 GHz and (Kronberg and Sramek 1985) slowly decreasing flux densities.

From five sets of VLBI observations spaced between 1972 and 1981, we estimated the positions of components of the superluminal quasar 3C 345 relative to the position of the single component of the quasar NRAO 512. The relative proper motion of the easternmost component of 3C 345, believed to be the “core”, was found to be 0.02±0.02 mas/yr. This result is consistent with the “core” being stationary and the “jet” components moving with respect to NRAO 512.