The NANOGrav Nine-year Data Set: Measurement and Analysis of Variations in Dispersion Measures

Abstract

We analyze dispersion measure (DM) variations of 37 millisecond pulsars in the nine-year North American Nanohertz Observatory for Gravitational Waves (NANOGrav) data release and constrain the sources of these variations. DM variations can result from a changing distance between Earth and the pulsar, inhomogeneities in the interstellar medium, and solar effects. Variations are significant for nearly all pulsars, with characteristic timescales comparable to or even shorter than the average spacing between observations. Five pulsars have periodic annual variations, 14 pulsars have monotonically increasing or decreasing trends, and 14 pulsars show both effects. Of the four pulsars with linear trends that have line-of-sight velocity measurements, three are consistent with a changing distance and require an overdensity of free electrons local to the pulsar. Several pulsars show correlations between DM excesses and lines of sight that pass close to the Sun. Mapping of the DM variations as a function of the pulsar trajectory can identify localized interstellar medium features and, in one case, an upper limit to the size of the dispersing region of 4 au. Four pulsars show roughly Kolmogorov structure functions (SFs), and another four show SFs less steep than Kolmogorov. One pulsar has too large an uncertainty to allow comparisons. We discuss explanations for apparent departures from a Kolmogorov-like spectrum, and we show that the presence of other trends and localized features or gradients in the interstellar medium is the most likely cause.

A Second Chromatic Timing Event of Interstellar Origin toward PSR J1713+0747

“…We used an F-test described by Arzoumanian et al (2015) to determine if new parameters should be added but none were found to be significant. Figure 2 shows the Dt 2,1400 ∝ν −2 timeseries modeled by our step-wise DM model and the trajectory of the pulsar across the sky, with decreasing values shown by darker colors, similar to the depiction in Jones et al (2017).…”

“…The DDMtimeseries can show dips as seen in Section 7.2 of Lam et al (2016b;see Figure 9 of that paper) if the pulsar moves through both local high-and low-density structures oriented in specific ways along the LOS. Looking at the timescale t for the events to recover by DDM≈ 6×10 −4 pc cm −3 to their initial values after approximately 100-200 days, the local~D~-( ) -n v t DM 10 20 cm e p 3 , a large value for the ISM (Draine 2011) but one that is marginally consistent with the value estimated for PSRJ1909−3744 and possibly PSRJ1738+0333 (Lam et al 2016b;Jones et al 2017). Since the pulsar's 3D velocity v p is not known, we first assumed above that the pulsar moves purely radially toward the Earth (to produce a negative DDM) with a fiducial velocity v p,P = 100 km s −1 and ignored the contribution of the Sun's motion through its own local environment; for reference, note that the pulsar's transverse velocity is v p,⊥ =36.4 km s −1 , estimated from the parallax and proper motion.…”

“…Previously, a chromatic timing event-that is, a relatively sudden, frequencydependent change in timing properties-was seen in the TOAs starting at approximately MJD54750, interpreted as a DM drop of ≈6×10 −4 pc cm −3 and lasting for ∼100-200 days before returning to the previous DM value; the event was seen in other data sets as well (Demorest et al 2013;Keith et al 2013;Desvignes et al 2016;Jones et al 2017). We report on a second chromatic timing event occurring 7.6 years after the first event.…”

“…A standard component of precision timing models is a frequencydependent dispersive delay proportional to the dispersion measure ( ò = n ds DM e ), the integral of the electron density n e along the line of sight (LOS; Lorimer & Kramer 2012). Temporal variations in the measured dispersive delay have been observed and interpreted as being caused by LOS changes in n e , Earth-pulsar distance and direction changes, solar wind fluctuations, ionospheric electron content variations, contamination from refraction, and more (e.g., Foster & Cordes 1990;Ramachandran et al 2006;Keith et al 2013;Lam et al 2016b;Jones et al 2017). High-precision observations of pulse time-of-arrival (TOA) timeseries over a wide range of frequencies allow for the measurement of a number of propagation effects.…”

Review of pulsar timing array for gravitational wave research

“…Similarly, we could obtain a better estimate of the galaxy merger rate and the population of supermassive black hole binaries in the Universe [8]. PTAs also provide an opportunity to test the theory of gravitation in nanohertz regime [9] Besides gravitational waves detection, other applications of PTAs include providing time standard for long time scales and measurement of solar system ephemerides [10] and a better understanding of the properties of the interstellar medium [11]. The theory of detection of the gravitational wave using PTA is discussed in Sec.II, possible sources of gravitational wave detectable by PTAs are discussed in Sec.III and Sec.IV is the concluding remark with the present status of PTAs and their future targets.…”