Pulsar Timing Arrays: learning about interstellar medium delays

Gravitational wave detection with a pulsar timing array (PTA) must account accurately for time-variable delays in the interstellar medium. The NANOGrav PTA currently correct for time-variable dispersion delay, but not for the known effects of scattering. We are developing a technique to determine and correct for the scattering delay at each epoch. Our main observable is high-frequency-resolution dynamic spectra of the pulsars involved. In January 2018 we observed 10 intermediate-DM pulsars with the Arecibo radio telescope. The dispersion measures for these pulsars were between 50 and 100 pc cm^-3. We made observations at 430 MHz and 1450 MHz with a range of bandwidth and channel spacing. We formed dynamic and secondary spectra for all of the data sets, detecting scintillation arcs in a number of the pulsars. We present results for these pulsars, tying them in with our previous results for a lower-DM sample. We discuss how observations such as these can be used as a diagnostic of interstellar medium conditions along a particular line of sight. By comparing lower-DM and higher-DM objects we are able to assess the relative importance of time-variable scattering delays for high-precision pulsar timing. This is of value in mitigating time delay fluctuations in PTA observations.