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  1. Abstract

    The Galactic electron density model NE2001 describes the multicomponent ionized structure of the Milky Way interstellar medium. NE2001 forward models the dispersion and scattering of compact radio sources, including pulsars, fast radio bursts, active galactic nuclei, and masers, and the model is routinely used to predict the distances of radio sources lacking independent distance measures. Here we present the open-source package NE2001p, a fully Python implementation of NE2001. The model parameters are identical to NE2001 but the computational architecture is optimized for Python, yielding small (<1%) numerical differences between NE2001p and the Fortran code. NE2001p can be used on the command-line and through Python scripts available on PyPI. Future package releases will include modular extensions aimed at providing short-term improvements to model accuracy, including a modified thick disk scale height and additional clumps and voids. This implementation of NE2001 is a springboard to a next-generation Galactic electron density model now in development.

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    Free, publicly-accessible full text available January 9, 2025

    Most fast radio burst (FRB) models can be divided into two groups based on the distance of the radio emission region from the central engine. The first group of models, the so-called ‘nearby’ or magnetospheric models, invoke FRB emission at distances of 109 cm or less from the central engine, while the second ‘far-away’ models involve emission from distances of 1011 cm or greater. The lateral size for the emission region for the former class of models (≲107 cm) is much smaller than the second class of models (≳109 cm). We propose that an interstellar scattering screen in the host galaxy is well-suited to differentiate between the two classes of models, particularly based on the level of modulations in the observed intensity with frequency, in the regime of strong diffractive scintillation. This is because the diffractive length scale for the host galaxy’s interstellar medium scattering screen is expected to lie between the transverse emission-region sizes for the ‘nearby’ and the ‘far-away’ class of models. Determining the strength of flux modulation caused by scintillation (scintillation modulation index) across the scintillation bandwidth (∼1/2πδts) would provide a strong constraint on the FRB radiation mechanism when the scatter broadening (δts) is shown to be from the FRB host galaxy. The scaling of the scintillation bandwidth as ∼ν4.4 may make it easier to determine the modulation index at ≳ 1 GHz.

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    Observations of pulsar scintillation are among the few astrophysical probes of very small-scale (≲ au) phenomena in the interstellar medium (ISM). In particular, characterization of scintillation arcs, including their curvature and intensity distributions, can be related to interstellar turbulence and potentially overpressurized plasma in local ISM inhomogeneities, such as supernova remnants, H ii regions, and bow shocks. Here we present a survey of eight pulsars conducted at the Five-hundred-metre Aperture Spherical Telescope (FAST), revealing a diverse range of scintillation arc characteristics at high sensitivity. These observations reveal more arcs than measured previously for our sample. At least nine arcs are observed toward B1929+10 at screen distances spanning $\sim 90~{{\ \rm per\ cent}}$ of the pulsar’s 361 pc path length to the observer. Four arcs are observed toward B0355+54, with one arc yielding a screen distance as close as ∼105 au (<1 pc) from either the pulsar or the observer. Several pulsars show highly truncated, low-curvature arcs that may be attributable to scattering near the pulsar. The scattering screen constraints are synthesized with continuum maps of the local ISM and other well-characterized pulsar scintillation arcs, yielding a three-dimensional view of the scattering media in context.

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  4. Abstract

    To date, the search for radio technosignatures has focused on sky location as a primary discriminant between technosignature candidates and anthropogenic radio frequency interference (RFI). In this work, we investigate the possibility of searching for technosignatures by identifying the presence and nature of intensity scintillations arising from the turbulent, ionized plasma of the interstellar medium. Past works have detailed how interstellar scattering can both enhance and diminish the detectability of narrowband radio signals. We use the NE2001 Galactic free electron density model to estimate scintillation timescales to which narrowband signal searches would be sensitive, and discuss ways in which we might practically detect strong intensity scintillations in detected signals. We further analyze the RFI environment of the Robert C. Byrd Green Bank Telescope with the proposed methodology and comment on the feasibility of using scintillation as a filter for technosignature candidates.

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  5. Abstract

    We report two low-frequency measurements of the power-law index for the amplitudes of giant radio pulses from the Crab pulsar. The two observations were taken with the Arecibo and Green Bank radio telescopes at center frequencies of 327 MHz and 350 MHz, respectively. We find best-fit values for the differential power-law indexβ(wheredN/dSSβandSis the pulse amplitude) of −2.63 ± 0.05 and −3.6 ± 0.5 from the Arecibo and Green Bank data sets, respectively. Both values are broadly consistent with other values previously measured for the Crab pulsar at low radio frequencies. These reported values may be useful in future giant pulse studies of the Crab pulsar.

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    Fast radio bursts (FRBs) are millisecond-time-scale radio transients, the origins of which are predominantly extragalactic and likely involve highly magnetized compact objects. FRBs undergo multipath propagation, or scattering, from electron density fluctuations on sub-parsec scales in ionized gas along the line of sight. Scattering observations have located plasma structures within FRB host galaxies, probed Galactic and extragalactic turbulence, and constrained FRB redshifts. Scattering also inhibits FRB detection and biases the observed FRB population. We report the detection of scattering times from the repeating FRB 20190520B that vary by up to a factor of 2 or more on minutes to days-long time-scales. In one notable case, the scattering time varied from 7.9 ± 0.4 ms to less than 3.1 ms ($95{{\ \rm per\ cent}}$ confidence) over 2.9 min at 1.45 GHz. The scattering times appear to be uncorrelated between bursts or with dispersion and rotation measure variations. Scattering variations are attributable to dynamic, inhomogeneous plasma in the circumsource medium, and analogous variations have been observed from the Crab pulsar. Under such circumstances, the frequency dependence of scattering can deviate from the typical power law used to measure scattering. Similar variations may therefore be detectable from other FRBs, even those with inconspicuous scattering, providing a unique probe of small-scale processes within FRB environments.

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  7. Abstract

    Radio wave scattering can cause severe reductions in detection sensitivity for surveys of Galactic and extragalactic fast (∼ms duration) transients. While Galactic sources like pulsars undergo scattering in the Milky Way interstellar medium (ISM), extragalactic fast radio bursts (FRBs) can also experience scattering in their host galaxies and other galaxies intervening in their lines of sight. We assess Galactic and extragalactic scattering horizons for fast radio transients using a combination of NE2001 to model the dispersion measure and scattering time (τ) contributed by the Galactic disk, and independently constructed electron density models for the Galactic halo and other galaxies’ ISMs and halos that account for different galaxy morphologies, masses, densities, and strengths of turbulence. For source redshifts 0.5 ≤zs≤ 1, an all-sky, isotropic FRB population has simulated values ofτ(1 GHz) ranging from ∼1μs to ∼2 ms (90% confidence, observer frame) that are dominated by host galaxies, althoughτcan be ≫2 ms at low Galactic latitudes. A population atzs= 5 has 0.01 ≲τ≲ 300 ms at 1 GHz (90% confidence), dominated by intervening galaxies. About 20% of these high-redshift FRBs are predicted to haveτ> 5 ms at 1 GHz (observer frame), and ≳40% of FRBs betweenzs∼ 0.5–5 haveτ≳ 1 ms forν≤ 800 MHz. Our scattering predictions may be conservative if scattering from circumsource environments is significant, which is possible under specific conditions. The percentage of FRBs selected against from scattering could also be substantially larger than we predict if circumgalactic turbulence causes more small-scale (≪1 au) density fluctuations than observed from nearby halos.

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  8. Abstract

    The millisecond pulsar J1713+0747 underwent a sudden and significant pulse shape change between 2021 April 16 and 17 (MJDs 59320 and 59321). Subsequently, the pulse shape gradually recovered over the course of several months. We report the results of continued multifrequency radio observations of the pulsar made using the Canadian Hydrogen Intensity Mapping Experiment and the 100 m Green Bank Telescope in a 3 yr period encompassing the shape change event, between 2020 February and 2023 February. As of 2023 February, the pulse shape had returned to a state similar to that seen before the event, but with measurable changes remaining. The amplitude of the shape change and the accompanying time-of-arrival residuals display a strong nonmonotonic dependence on radio frequency, demonstrating that the event is neither a glitch (the effects of which should be independent of radio frequency,ν) nor a change in dispersion measure alone (which would produce a delay proportional toν−2). However, it does bear some resemblance to the two previous “chromatic timing events” observed in J1713+0747, as well as to a similar event observed in PSR J1643−1224 in 2015.

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  9. Abstract Stellar bow shocks are observed in a variety of interstellar environments and shaped by the conditions of gas in the interstellar medium (ISM). In situ measurements of turbulent density fluctuations near stellar bow shocks are only achievable with a few observational probes, including H α -emitting bow shocks and the Voyager Interstellar Mission (VIM). In this paper, we examine density variations around the Guitar Nebula, an H α bow shock associated with PSR B2224+65, in tandem with density variations probed by VIM near the boundary of the solar wind and ISM. High-resolution Hubble Space Telescope observations of the Guitar Nebula taken between 1994 and 2006 trace density variations over scales from hundreds to thousands of au, while VIM density measurements made with the Voyager 1 Plasma Wave System constrain variations from thousands of meters to tens of au. The power spectrum of density fluctuations constrains the amplitude of the turbulence wavenumber spectrum near the Guitar Nebula to log 10 C n 2 = − 0.8 ± 0.2 m −20/3 and for the very local ISM probed by Voyager to log 10 C n 2 = − 1.57 ± 0.02 m −20/3 . Spectral amplitudes obtained from multiepoch observations of four other H α bow shocks also show significant enhancements from values that are considered typical for the diffuse, warm ionized medium, suggesting that density fluctuations near these bow shocks may be amplified by shock interactions with the surrounding medium or selection effects that favor H α emission from bow shocks embedded in denser media. 
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  10. Abstract

    The interstellar medium hosts a population of scattering screens, most of unknown origin. Scintillation studies of pulsars provide a sensitive tool for resolving these scattering screens and a means of measuring their properties. In this paper, we report our analysis of 34 yr of Arecibo observations of PSR B1133 + 16, from which we have obtained high-quality dynamic spectra and their associated scintillation arcs, arising from the scattering screens located along the line of sight to the pulsar. We have identified six individual scattering screens that are responsible for the observed scintillation arcs, which persist for decades. Using the assumption that the scattering screens have not changed significantly in this time, we have modeled the variations in arc curvature throughout the Earth’s orbit and extracted information about the placement, orientation, and velocity of five of the six screens, with the highest-precision distance measurement placing a screen at just5.460.59+0.54pc from the Earth. We associate the more distant of these screens with an underdense region of the Local Bubble.

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