Characterizing Dispersion Measure Events in the Interstellar Medium

Science Center A254

The NANOGrav (North American Nanohertz Observatory for Gravitational Waves) 9 year data release contains unusual deviations in dispersion measure variations caused by abrupt changes in the electron density of the interstellar medium along the line of sight. We employ various techniques in Bayesian analysis, Gaussian process regression, and Markov Chain Monte Carlo sampling to develop statistical models for dispersion measure in current NANOGrav observations, as well as predict future trends. These models are designed to be integrated into a "Quick Look" program, the output of which can then be used to identify DM "events" in recently collected data. We also analyze the negative DM event which occurred along the line of sight to PSR J1713+0747, which we have determined to be the result of an electron-devoid region in the ISM approximately 1.64 AU in width transverse to the LOS and 10^4 AU along the LOS. We then propose a few simple geometrical structures which could approximate the shape of the electron-deficient region in the ISM. Finally, we discuss a few potential physical sources which might explain the existence of such a region.