VLBI measurement of the secular aberration drift

Aims. While analyzing decades of very long baseline interferometry (VLBI) data, we detected the secular aberration drift of the extragalatic radio source proper motions caused by the rotation of the Solar System barycenter around the Galactic center. Our results agree with the predicted estimate to be 4-6 micro arcseconds per year (mu as/yr) towards alpha = 266 degrees and delta = -29 degrees. In addition, we tried to detect the quadrupole systematics of the velocity field. Methods. The analysis method consisted of three steps. First, we analyzed geodetic and astrometric VLBI data to produce radio source coordinate time series. Second, we fitted proper motions of 555 sources with long observational histories over the period 1990-2010 to their respective coordinate time series. Finally, we fitted vector spherical harmonic components of degrees 1 and 2 to the proper motion field. Results. Within the error bars, the magnitude and the direction of the dipole component agree with predictions. The dipole vector has an amplitude of 6.4 +/- 1.5 mu as/yr and is directed towards equatorial coordinates alpha = 263 degrees and delta = -20 degrees. The quadrupole component has not been detected. The primordial gravitational wave density, integrated over a range of frequencies less than 10(-9) Hz, has a limit of 0.0042 h(-2) where h is the normalized Hubble constant is H-0/(100 kms(-1)).