Epoch-dependent Interstellar Scintillations and Timing Variations for the Millisecond Pulsar B1937+21

PSR B1937+21 is the brightest millisecond pulsar in the northern sky and serves as a laboratory for studying uncertainties and systematic changes in pulse times-of-arrival. Its high flux, its relatively high degree of scattering along the line-of-sight, and its giant pulses interact in a dynamic way to affect individual times-of-arrival on different timescales. Results are relevant to understanding the timing of other millisecond pulsars, and thus to the larger effort to detect gravitational waves with pulsar timing arrays. We present data from an Arecibo Observatory campaign on PSR B1937+21 with baseband data at 1.4 GHz (200 MHz bandwidth), where standard NANOGrav timing measurements are taken. By utilizing the rich dynamic spectrum data from these campaigns, we analyze the noise budget of this pulsar on very short and very long timescales due to the changing intervening interstellar medium. We quantify changes in the diffractive timescale of B1937+21 over months of observations through an autocorrelation function analysis.