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