Detection of periodic variations in the vertical velocities of Galactic masers

We have collected literature data on Galactic masers with trigonometric parallaxes measured by means of very-long-baseline interferometry (VLBI). We have obtained series of residual tangential (ΔVcirc) and radial (ΔVR) velocities for 107 masers. Based on these series, we have re-determined the parameters of the Galactic spiral density wave using the method of spectral (periodogram) analysis. The tangential and radial perturbation amplitudes are fθ = 6.0 ± 2.6 km s−1 and fR = 7.2 ± 2.2 km s−1, respectively; the perturbation wavelengths are λθ = 3.2 ± 0.5 kpc and λR = 3.0 ± 0.6 kpc for a four-armed spiral model, m = 4. The phase of the Sun, χ⊙, in the spiral density wave is −79° ± 14° and −199° ± 16° from the residual tangential and radial velocities, respectively. The most interesting result of this work is detecting a wave in vertical spatial velocity (W) versus distance R from the Galactic rotation axis. From spectral analysis, we have found the following characteristics for this wave: perturbation wavelength λW = 3.4 ± 0.7 kpc and amplitude fW = 4.3 ± 1.2 km s−1.