Session 117 - Instrumentation and Techniques.
Oral session, Thursday, January 18
Salon del Rey North, Hilton

## [117.06] A Method of the Precision Doppler Measurements of Transverse Velocities

L. M. Ozernoy (CSI/GMU and GSFC/NASA)

Recent progress in techniques of measurements makes possible to implement the relativisic Doppler effect to measuring transverse velocities. The proposed method that makes it feasible employs two key elements: (i) use of a stabilized gas absorption cell placed before the spectrograph, which serves as a velocity metric and (ii) elimination of a much larger longitudinal (radial) Doppler effect and extraction of the transverse velocity component via modulation the latter by the orbital motion of the Earth. If \Delta\nu=\nu_max-\nu_min is the amplitude of the frequency shift of some line (whose proper frequency is \nu_0) during one revolution of Earth around the Sun, the inferred tangential velocity of the source is given by V_t= c^2øver 2v\sin b\Delta\nuøver \nu_l , where c is the speed of light, v is the Earth mean velocity, b is the source's ecliptic latitude, \nu=\nu_l\left(1-V_t^2/c^2\right)^1/2 is the measured frequency that includes both the longitudinal and transverse Doppler effects, and \nu_l=\nu_0\left(1-V_l^2/c^2\right)^1/2\big /\left(1+V_l/c\right) [1+(v/c)\cos (\widehatv, V_l)] absorbs the entire longitudinal (radial) Doppler effect; \nu_l is assumed to kept constant on the time scale of at least one year.

The routine precision in an individual measuring velocity shifts is currently 7 m/s (iodine absorption cell), which enables one to determine transverse velocities with uncertainty \sigma_V_t\sim35 km/s. Fourier analysis would permit to extract the embedded periodicities with amplitudes of \sim1 m/s, which translates into \sigma_V_t\sim5 km/s.

A telescope placed on a satellite orbiting around the Earth would make possible a much quicker accumulation of the periodic changes in the precision Doppler velocity measurements, although at the expense of a smaller velocity v that modulates the data. At radio wavelengths, the use of a cosmic megamaser source within the object whose transverse velocity is to be measured would also result in a larger uncertainty in V_t. \vspace3mm