Measurement and correction of variations in interstellar dispersion in high-precision pulsar timing

By M. Keith, William A. Coles, R. Shannon, G. Hobbs, R. Manchester, M. Bailes, N. D. Ramesh Bhat, S. Burke-Spolaor, David J. Champion, A. Chaudhary and 9 more. 2012

172 citations recorded by the source corpus.

Signals from radio pulsars show a wavelength-dependent delay due to dispersion in the interstellar plasma. At a typical observing wavelength, this delay can vary by tens of microseconds on 5-yr time-scales, far in excess of signals of interest to pulsar timing arrays, such as that induced by a gravitational wave background. Measurement of these delay variations is not only crucial for the detection of such signals, but also provides an unparalleled measurement of the turbulent interstellar plasma at astronomical unit (au) scales. In this paper we demonstrate that without consideration of wavelength-independent red noise, ‘simple’ algorithms to correct for interstellar dispersion can attenuate signals of interest to pulsar timing arrays. We present a robust method for this correction, which we validate through simulations, and apply it to observations from the Parkes Pulsar Timing Array. Correction for dispersion variations comes at a cost of increased band-limited white noise. We discuss scheduling to minimize this additional noise, and factors, such as scintillation, that can exacerbate the problem. Comparison with scintillation measurements confirms previous results that the spectral exponent of electron density variations in the interstellar medium often appears steeper than expected. We also find a discrete change in dispersion measure of PSR J1603−7202 of ∼2 × 10^(−3) cm^(−3) pc for about 250 d. We speculate that this has a similar origin to the ‘extreme scattering events’ seen in other sources. In addition, we find that four pulsars show a wavelength-dependent annual variation, indicating a persistent gradient of electron density on an au spatial scale, which has not been reported previously.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/4e87c1ceb008263241d8d9db18ce67be23aa3785

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

In Atomic and Molecular Physics, and Optics

Citations in the corpus

The International Pulsar Timing Array: First Data Release (2016, methodology): “This is primarily caused by the fact that our power-law model already contains DM variations at the periodicity of a year, while the analysis by Keith et al. (2013) quantified the total power of DM variations on a yearly time-scale, rather than the excess DM variations beyond a power-law model.”
Timing analysis for 20 millisecond pulsars in the Parkes Pulsar Timing Array (2015, background): “You et al. 2007; Keith et al. 2013, hereafter K13), developing pulsar-based time standards (e.g. Hobbs et al. 2012), and precise measurements of properties of the pulsars themselves (e.g. Verbiest et al. 2008, hereafter V08).”
European Pulsar Timing Array Limits On An Isotropic Stochastic Gravitational-Wave Background (2015, methodology): “…between the GW signal of interest, and other sources of noise in the data, such as the intrinsic spin-noise due to rotational irregularities (e.g. Shannon & Cordes 2010), or delays in the pulse arrival time due to propagation through the interstellar medium (ISM) (e.g. Keith et al. 2013).”
THE NANOGRAV NINE-YEAR DATA SET: OBSERVATIONS, ARRIVAL TIME MEASUREMENTS, AND ANALYSIS OF 37 MILLISECOND PULSARS (2015, result): “Using an independent data set, Keith et al. (2013) also detect significant annual DM modulation in several of the same sources where it is apparent in our results.”