Millisecond and Binary Pulsars as Nature's Frequency Standards. III.   Fourier Analysis and Spectral Sensitivity of Timing Observations to   Low-Frequency Noise
  Millisecond and binary pulsars are the most stable natural frequency standards which admits to introduce modified versions of universal and ephemeris time scales based correspondingly on the intrinsic rotation of pulsar and on its orbital motion around barycenter of a binary system. Measured stability of these time scales depends on numerous physical phenomena which affect rotational and orbital motion of the pulsar and observer on the Earth, perturb propagation of electromagnetic pulses from pulsar to the observer and bring about random fluctuations in the rate of atomic clock used as a primary time reference in timing observations. On the long time intervals the main reason for the instability of the pulsar time scales is the presence of correlated, low-frequency timing noise in residuals of times of arrivals (TOA) of pulses from the pulsar which has both astrophysical and geophysical origin. Hence, the timing noise can carry out the important physical information about interstellar medium, interior structure of the pulsar, stochastic gravitational waves coming from the early universe, etc. Each specific type of the low-frequency noise can be described in framework of power law spectrum model. Although the data processing of pulsar timing observations in time domain seems to be the most imformative it is significantly important to know to which spectral bands single and binary pulsars, considered as detectors of the low-frequency noise signal, are the most sensitive. The given problem is examined in the present article. 
Sergei M. Kopeikin (FSU of Jena), Vladimir A. Potapov (PRAO, ASC FIAN)