The impact of solar wind variability on pulsar timing

Abstract

Context. High-precision pulsar timing requires accurate corrections for dispersive delays of radio waves, parametrized by the dispersion measure (DM), particularly if these delays are variable in time. In a previous paper, we studied the solar wind (SW) models used in pulsar timing to mitigate the excess of DM that is annually induced by the SW and found these to be insufficient for high-precision pulsar timing. Here we analyze additional pulsar datasets to further investigate which aspects of the SW models currently used in pulsar timing can be readily improved, and at what levels of timing precision SW mitigation is possible. Aims. Our goals are to verify: (a) whether the data are better described by a spherical model of the SW with a time-variable amplitude, rather than a time-invariant one as suggested in literature, and (b) whether a temporal trend of such a model’s amplitudes can be detected. Methods. We use the pulsar timing technique on low-frequency pulsar observations to estimate the DM and quantify how this value changes as the Earth moves around the Sun. Specifically, we monitor the DM in weekly to monthly observations of 14 pulsars taken with parts of the LOw-Frequency ARray (LOFAR) across time spans of up to 6 years. We develop an informed algorithm to separate the interstellar variations in the DM from those caused by the SW and demonstrate the functionality of this algorithm with extensive simulations. Assuming a spherically symmetric model for the SW density, we derive the amplitude of this model for each year of observations. Results. We show that a spherical model with a time-variable amplitude models the observations better than a spherical model with a constant amplitude, but that both approaches leave significant SW-induced delays uncorrected in a number of pulsars in the sample. The amplitude of the spherical model is found to be variable in time, as opposed to what has been previously suggested.

High precision measurements of interstellar dispersion measure with the upgraded GMRT

“…Moreover, this pulsar's line of sight passes close to the Sun at a solar elongation of ∼5 degrees. It has been reported previously that the DM shows an increase due to the increase in the heliospheric electron density (solar wind) as its line of sight approaches close to the Sun (Kumar et al 2013;Alam et al 2021;Donner et al 2020;Tiburzi et al 2021). Since this pulsar has several scintles in BAND3 data, we collapsed it to 16 channels to reduce the effect of scintillation.…”

“…PSR J2145−0750 has low solar elongations between December and February every year. This implies that the DM for this pulsar has an excess contribution from solar wind every year when it is close to the Sun (Kumar et al 2013;Madison et al 2019;Tiburzi et al 2019Tiburzi et al , 2021Alam et al 2021;Donner et al 2020). The DM can also be enhanced in case of a violent solar event, such as a coronal mass ejection (CME) or a CME-solar wind or CME-CME interaction, where the electron density in the line of sight can get enhanced.…”

A comparative analysis of pulse time-of-arrival creation methods

“…This feature is mostly a consequence of a so-called "event" , which is known to be hard to model with standard red-noise models (Lam et al 2018;Arzoumanian et al 2020 The remaining structure in the TOAs of PSR J1713+0747 results in some non-unity reduced-χ 2 values, although the temporal nature of this feature causes different telescope sub-sets to be affected to different degrees. The significantly elevated reducedχ 2 values in the PSR J2145−0750 EFF and Nançay data are likely due to a few highly precise TOAs that are affected by the solar wind (Tiburzi et al 2021).…”

Coronal Density Measurements Using Giant Radio Pulses of the Crab Pulsar at the Cycle 24/25 Minimum

“…To this end, in situ plasma observations in close proximity to the Sun are being conducted by the Parker Solar Probe (Fox et al, 2016) and Solar Orbiter (Müller et al, 2020); however, the radial distance range within 10 solar radii (R S ) remains inaccessible to the direct method. The plasma density observations for this region have been conducted using remote sensing techniques, such as white-light brightness (Allen, 1947;Newkirk, 1961;Saito, Poland, and Munro, 1977;Mancuso and Garzelli, 2013), solar radio emissions (Leblanc, Dulk, and Bougeret, 1998;Mercier and Chambe, 2015), interplanetary spacecraft beacons (Stelzried et al, 1970;Tyler et al, 1977;Muhleman, Esposito, and Anderson, 1977;Edenhofer et al, 1977;Esposito, Edenhofer, and Lueneburg, 1980;Muhleman and Anderson, 1981;Bird et al, 1994;Wexler et al, 2019a,b), and pulsar dispersion measures Rankin, 1972, 1973;Weisberg et al, 1976;Cognard et al, 1996;Smirnova, Chashei, and Shishov, 2009;Tokumaru et al, 2020;Tiburzi et al, 2021). Remote sensing measurements of the plasma density using the white-light brightness are limited to a range within a few solar radii because the white-light diminishes rapidly with radial distance.…”

“…In this study, accurate assessment of the interstellar background level is a key issue. Recently, Tiburzi et al (2021) developed a new scheme to discriminate between the interstellar and solar contributions to the observed DMs. In this scheme, a cubic polynomial was used to model the DM variations due to the interstellar medium.…”