Abstract
ABSTRACT Using data from the Large European Array for Pulsars, and the Effelsberg telescope, we study the scintillation parameters of the millisecond pulsar PSR J0613−0200 over a 7 yr timespan. The ‘secondary spectrum’ – the 2D power spectrum of scintillation – presents the scattered power as a function of time delay, and contains the relative velocities of the pulsar, observer, and scattering material. We detect a persistent parabolic scintillation arc, suggesting scattering is dominated by a thin, anisotropic region. The scattering is poorly described by a simple exponential tail, with excess power at high delays; we measure significant, detectable scattered power at times out to ${\sim}5 \, \mu {\rm s}$, and measure the bulk scattering delay to be between 50 to 200 ns with particularly strong scattering throughout 2013. These delays are too small to detect a change of the pulse profile shape, yet they would change the times of arrival as measured through pulsar timing. The arc curvature varies annually, and is well fitted by a one-dimensional scattering screen ${\sim}40{{\ \rm per\ cent}}$ of the way towards the pulsar, with a changing orientation during the increased scattering in 2013. Effects of uncorrected scattering will introduce time delays correlated over time in individual pulsars, and may need to be considered in gravitational wave analyses. Pulsar timing programmes would benefit from simultaneously recording in a way that scintillation can be resolved, in order to monitor the variable time delays caused by multipath propagation.
Highlights
Radio emission from pulsars experiences several propagation effects from the ionised interstellar medium (ISM), as the index of refraction varies with electron density and frequency
The most stable pulsars are observed on weekly to monthly cadence over many years, and ∼nHz gravitational waves could be observed in timing residuals correlated in time and position on the sky (Hellings & Downs 1983)
The theory of scattering in thin screens is outlined in detail in Walker et al (2004) and Cordes et al (2006), and we summarise some of the pertinent relations here
Summary
Radio emission from pulsars experiences several propagation effects from the ionised interstellar medium (ISM), as the index of refraction varies with electron density and frequency. The most stable pulsars are observed on weekly to monthly cadence over many years, and ∼nHz gravitational waves could be observed in timing residuals correlated in time and position on the sky (Hellings & Downs 1983). Levin et al 2016; Shapiro-Albert et al 2020, see Verbiest & Shaifullah 2018 for a review of how these effects limit precision pulsar timing). Dispersion and scattering both scale strongly with frequency, and are often covariant. Each image has a geometric time delay τi and a fringe rate (or Doppler rate) fD,i, with a magnification μi and intrinsic phase φi In this approximation, the contribution of all of the images is ∑ gE (τ, fD) = √ μi e−iφi δ ( fD − fD,i)δ (τ τi).
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