Determining electron column density fluctuations in a dominant scattering region using pulsar scintillation

Abstract

Density fluctuations in the ionised interstellar medium have a profound effect on radio pulsar observations, through angular scattering, intensity scintillations, and small changes in time delays from dispersion. Here we show that it is possible to recover the variations in dispersive delays that originate from a dominant scattering region using measurements of the dynamic spectrum of intensity scintillations, provided that the pulsar velocity and scattering region location are known. We provide a theoretical framework for the technique, which involves estimating the phase gradient from the dynamic spectra and integrating that gradient to obtain phase variations. It can be used to search for “extreme scattering events” (ESEs) in pulsars for which precision dispersion delay measurements are not otherwise possible, or to separate true dispersion variations from apparent variability caused by frequency-dependent pulse shape changes. We demonstrate that it works in practice by recovering an ESE in PSR J1603−7202, which is known from precision dispersion delay measurements from pulsar timing. For this pulsar, we find that the phase gradients also track the long-term variations in electron column density observed by pulsar timing, indicating that the column density variations and the scattering are dominated by the same thin scattering screen. We identify a sudden increase in the scintillation strength and magnitude of phase gradients over ∼days in 2010, indicating a compact structure. A decrease in the electron density in 2012 was associated with persistent phase gradients and preceded a period of decreased scintillation strength and an absence of scintillation arcs.

The Gravitational-wave Background Null Hypothesis: Characterizing Noise in Millisecond Pulsar Arrival Times with the Parkes Pulsar Timing Array

“…Plasma intermittency could cause departures from such a process, as could the presence of discrete structures in the IISM. Consequently, in addition to the Gaussian process DM variations, we include a Gaussian-shaped DM event for PSR J1603−7202 (Equation (7) of Goncharov et al 2021a), to describe its extreme scattering event (Coles et al 2015;Reardon & Coles 2023) and annual DM variations (Equation (8) of Goncharov et al 2021a) for PSR J0613−0200 (Keith et al 2013).…”

Tearing-mediated Reconnection in Magnetohydrodynamic Poorly Ionized Plasmas. I. Onset and Linear Evolution

“…We anticipate that similar physics could be at play in poorly ionized turbulent plasmas, the most immediate applications being to protoplanetary disks and the colder phases of the interstellar medium. Concerning the latter, the structures produced by the onset of tearing could affect the transport of cosmic rays and the scintillation of pulsar signals (Hopkins et al 2022;Kempski & Quataert 2022;Stinebring et al 2022;Fielding et al 2023;Reardon & Coles 2023). Finally, our work complements multifluid numerical studies of magnetic reconnection in the low solar atmosphere (e.g., Leake et al 2013;Ni et al 2018), in which the ionization fraction varies from ∼10 −4 in the photosphere to ∼1 at the top of the chromosphere and explosive events such as jets and flares occur that have been attributed to magnetic reconnection.…”