| We report on four years of data from an ongoing program of timing millisecond pulsars (MSPs) at the Arecibo radio observatory. In our monthly observations we studied six MSPs, all but one of them binary. Our work had two thrusts. First, we made high-precision measurements of the rotational, astrometric, and (where applicable) orbital parameters of the objects. Of particular interest in binary systems were relativistic post-Keplerian phenomena such as Shapiro delay and periastron advance. These terms yielded information about the system masses when interpreted within general relativity. The masses, in turn, can be used to constrain theories of stellar evolution. Additionally, the measured decay of the binary period in one MSP put limits on violations of the Strong Equivalence Principle. The other focus of our work was on identifying long-term stochastic signals in the timing data. These pointed to interesting physics in the interstellar medium (ISM) or in the pulsar itself that lie outside the equations of motion of our timing model. Along these lines, we measured fluctuations in the content of free electrons in the direction of one of the MSPs. With these, we described the power spectrum of spatial variations in the ISM. Elsewhere, we found timing noise in one MSP that hitherto had been considered one of the most precise celestial clocks. |
