Non‐Gaussian Radio‐Wave Scattering in the Interstellar Medium

Abstract

It was recently suggested by Boldyrev & Gwinn that the characteristics of radio scintillations from distant pulsars are best understood if the interstellar electron density fluctuations that cause the time broadening of the radio pulses obey non-Gaussian statistics. In this picture the density fluctuations are inferred to be strong on very small scales ($10 8 -10 10 cm). We argue that such density structures could correspond to the ionized boundaries of molecular regions (clouds) and demonstrate that the power-law distribution of scattering angles that is required to match the observations arises naturally from the expected intersections of our line of sight with randomly distributed, thin, approximately spherical ionized shells of this type. We show that the observed change in the time-broadening behavior for pulsar dispersion measures P30 pc cm À3 is consistent with the expected effect of the general ISM turbulence, which should dominate the scattering for nearby pulsars. We also point out that if the clouds are ionized by nearby stars, then their boundaries may become turbulent on account of an ionization front instability. This turbulence could be an alternative cause of the inferred density structures. An additional effect that might contribute to the strength of the small-scale fluctuations in this case is the expected flattening of the turbulent density spectrum when the eddy sizes approach the proton gyroscale.

Simulating intergalactic quasar scintillation

“…Such scheme is physically motivated by the interpretation of scintillation as a Levy process (e.g. Boldyrev & Gwinn 2003;Boldyrev & Königl 2006), and represents an extension of the thin screen approximation (i.a. Rickett 1990;Goodman 1997;Ferrara & Perna 2001), suitable to treat both interstellar and intergalactic scintillation.…”

Detection of OH Absorption against PSR B1718−35

“…I also had a detection rate of 0% using detected OH emission that could be between the Sun and the pulsar. Since the PSR B1718À35 OH detection was found in a search of six pulsars based on their strong scattering, and PSR B1718À35 is the seventh most strongly scattered pulsar, some credence can be given to the hypothesis of Boldyrev & Königl (2006), but it by no means provides conclusive proof for their idea that scattering originates in PDRs at the boundaries of H ii regions and MCs.…”

“…This yielded a detection rate of 2 out of 25 (8%), which motivated me to look for better selection criteria for pulsars with OH absorption. Boldyrev & Königl (2006) proposed that the strongest interstellar scattering ( ISS) of pulsar signals was not induced by Kolmogorov-like turbulence in the electrons within the ISM, but by refraction at the random interface between ionized and nonionized gas along the line of sight. In particular, it was predicted that the photodissociation regions ( PDRs) at the edges of molecular clouds ( MCs) should produce the strongest scattering.…”

“…In particular, it was predicted that the photodissociation regions ( PDRs) at the edges of molecular clouds ( MCs) should produce the strongest scattering. Boldyrev & Königl (2006) also suggested that a pulsar's time broadening would scale as density to the fourth power ( / n 4 ), so that only the densest PDR along the line of sight will dominate the ISS. This will give the appearance of a single scattering screen toward the pulsar that is consistent with what is observed (e.g., Hill et al 2005).…”

Intermittency of electron density in interstellar kinetic Alfvén wave turbulence

“…7 However, it is likely that ray trajectories will encounter and traverse with greater probability the turbulence of the warm ionized component of the ISM. The underlying physical mechanism and the spatial fluctuation structure remain to be identified.…”