The NANOGrav 12.5-Year Data Set: The Frequency Dependence of Pulse Jitter in Precision Millisecond Pulsars

Low-frequency gravitational-wave experiments require the highest timing precision from an array of the most stable millisecond pulsars. Several known sources of noise on short timescales in single radio- pulsarobservations are well described by a simple model of three components: template-fitting from a finite signal-to-noise ratio, pulse phase/amplitude jitterfrom single-pulse stochasticity, and scintillation errors from short-timescale interstellar scattering variations. Currently template-fitting errors dominate, but as radio telescopespush towards higher signal-to-noise ratios, jitterbecomes the next dominant term for most millisecond pulsars. Understanding the statistics of jitterbecomes crucial for properly characterizing arrival-time uncertainties. We characterize the radio-frequency dependence of jitterusing data on 48 pulsarsin the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) timing program. We detect significant jitterin 43 of the pulsarsand test several functional forms for its frequency dependence; we find significant frequency dependence for 30 pulsars. We find moderate correlations of rms jitterwith pulse width (R = 0.62) and number of profile components (R = 0.40); the single-pulse rms jitteris typically ~1% of pulse phase. The average frequency dependence for all pulsarsusing a power-law model has index -0.42. We investigate the jittervariations for the interpulse of PSR~B1937+21 and find no significant deviations from the main pulse rms jitter. We also test the time-variation of jitterin two pulsarsand find that systematics likely bias the results for high-precision pulsars. Pulsar timing arrayanalyses must properly model jitteras a significant component of the noise within the detector.