Time Variability of Interstellar Scattering and Improvements to Pulsar Timing

Abstract

Delay due to multipath scattering in the interstellar medium is a concern for high-precision pulsar timing, particularly if it is not constant over time. We report on 36 weekly observations of the pulsar PSR B1737ϩ13 with the Arecibo telescope that monitored the time variability of the scattering delay. At a frequency of 1380 MHz, the interstellar delay varied between 0.2 and 2.1 ‫1.0ע(‬ ) over 270 days of observation. The delay ms ms was consistent over four observing bands with center frequencies from 1175 to 1470 MHz and scaled as t ∝ , which differs from the scaling expected for Kolmogorov turbulence. We show that another estimation Ϫ3.6‫2.0ע‬ Ϫ4.4 n n technique is feasible for weaker pulsars or smaller telescopes, although it underestimates the delay during episodes of extra scattering detectable through a full secondary spectrum analysis. An array of pulsars distributed around the sky can be used as a sensitive detector of long-wavelength (∼ several light-years) gravitational radiation, and such pulsar timing array observations have been initiated by several groups worldwide. To reach interesting sensitivity levels it is necessary to reduce the sources of error to below 1 , and 100 ns is a target precision ms level. Correction for interstellar scattering delay will be an important step in achieving long-term, submicrosecond timing precision.

Wide-band simultaneous observations of pulsars: disentangling dispersion measure and profile variations

“…The parameters of the pulsars in our sample are typical of those found in the PTAs, the only difference being their longer pulse periods. This suggests that the ISM may not cause as much of a hindrance to pulsar timing projects as first feared (Hemberger & Stinebring 2008).…”

“…This relation was used as early as 1968 to accurately predict the dispersive time delay to within 1 part in 3000 between 40 MHz and 430 MHz (Tanenbaum et al 1968). However, as ever higher timing precision is required for projects like using pulsars for gravitational wave detection (Jenet et al 2005) it is important that any second-order effects of the ISM, such as refractive delays, DM variations and delays associated with pulse broadening from scattering (Foster & Cordes 1990;Cordes & Shannon 2010), are also studied and understood fully (You et al 2008;Hemberger & Stinebring 2008). Most of these proposed effects have strong frequency dependencies, with scaling indices between ν −3 and ν −4 , and are therefore most prominent at low frequencies.…”

The North American Nanohertz Observatory for Gravitational Waves

“…Both the multipath scattering and dispersive effects are observed to change on timescales as short as one day and as long as decades (e.g. Kaspi, Taylor, & Ryba 1994;Cordes et al 1990;Hemberger & Stinebring 2008). This makes multifrequency 1 Green Bank Ultimate Pulsar Processing Instrument (at least two receivers, each with wide bandwidths, such that at least 4 separate frequency bands are accessible) observations a requirement at each epoch.…”

Scintillation Arcs in Low-Frequency Observations of the Timing-Array Millisecond Pulsar PSR J0437–4715

“…First recognized by Stinebring et al (2001) in Arecibo data, detailed studies of these arcs have revealed a variety and richness in their observational manifestations; e.g., forward and reverse arcs, and a chain of arclets (Hill et al 2005;Putney & Stinebring 2006;Brisken et al 2010), which also stimulated a great deal of theoretical and modeling work (Walker et al 2004;Cordes et al 2006;Brisken et al 2010). In the context of PTAs Hemberger & Stinebring (2008) measured scattering delays from their observations of scintillation arcs in PSR B1737+13 (DM=48.9 pc cm 3 -); the delays varied from ∼0.2 to 2 μs at a frequency of 1.3 GHz.…”