Fast radio bursts (FRBs) are millisecond-duration events thought to originate beyond the Milky Way galaxy. Uncertainty surrounding the burst sources, and their propagation through intervening plasma, has limited their use as cosmological probes. We report on a mildly dispersed (dispersion measure 266.5 ± 0.1 parsecs per cubic centimeter), exceptionally intense (120 ± 30 janskys), linearly polarized, scintillating burst (FRB 150807) that we directly localize to 9 square arc minutes. On the basis of a low Faraday rotation (12.0 ± 0.7 radians per square meter), we infer negligible magnetization in the circum-burst plasma and constrain the net magnetization of the cosmic web along this sightline to <21 nanogauss, parallel to the line-of-sight. The burst scintillation suggests weak turbulence in the ionized intergalactic medium.
Extreme scattering events (ESEs) are distinctive fluctuations in the brightness of astronomical radio sources caused by occulting plasma lenses in the interstellar medium. The inferred plasma pressures of the lenses are ∼ 10 3 times the ambient pressure, challenging our understanding of gas conditions in the Milky Way. Using a new survey technique, we have discovered an ESE while it was in progress. We report radio and optical follow-up observations. Modelling of the radio data demonstrates that the lensing structure is a density enhancement and that the lens is diverging, ruling out one of two competing physical models. Our technique will uncover many more ESEs, addressing a long-standing mystery of the small-scale gas structure of the Galaxy.Distinctive variations in the radio light curves of quasars were first identified serendipitously while using the Green Bank Interferometer in support of routine astrometric observations (1), and were named 'extreme scattering events' (ESEs
Intensity scintillations of radio pulsars are known to originate from interference between waves scattered by the electron density irregularities of interstellar plasma, often leading to parabolic arcs in the two-dimensional power spectrum of the recorded dynamic spectrum. The degree of arc curvature depends on the distance to the scattering plasma and its transverse velocity with respect to the line of sight. We report the observation of annual and orbital variations in the curvature of scintillation arcs over a period of 16 yr for the bright millisecond pulsar, PSR J0437−4715. These variations are the signature of the relative transverse motions of Earth, the pulsar, and the scattering medium, which we model to obtain precise measurements of parameters of the pulsar’s binary orbit and the scattering medium itself. We observe two clear scintillation arcs in most of our >5000 observations, and we show that they originate from scattering by thin screens located at distances D 1 = 89.8 ± 0.4 pc and D 2 = 124 ± 3 pc from Earth. The best-fit scattering model we derive for the brightest arc yields the pulsar’s orbital inclination angle, i = 137.°1 ± 0.°3, and longitude of ascending node, Ω = 206.°3 ± 0.°4. Using scintillation arcs for precise astrometry and orbital dynamics can be superior to modeling variations in the diffractive scintillation timescale, because the arc curvature is independent of variations in the level of turbulence of interstellar plasma. This technique can be used in combination with pulsar timing to determine the full three-dimensional orbital geometries of binary pulsars and provides parameters essential for testing theories of gravity and constraining neutron star masses.
We use data on extreme radio scintillation to demonstrate that this phenomenon is associated with hot stars in the solar neighborhood. The ionized gas responsible for the scattering is found at distances up to 1.75 pc from the host star, and on average must comprise ∼10 5 distinct structures per star. We detect azimuthal velocities of the plasma, relative to the host star, up to 9.7 km s 1 -, consistent with warm gas expanding at the sound speed. The circumstellar plasma structures that we infer are similar in several respects to the cometary knots seen in the Helix and in other planetary nebulae. There the ionized gas appears as a skin around tiny molecular clumps. Our analysis suggests that molecular clumps are ubiquitous circumstellar features, unrelated to the evolutionary state of the star. The total mass in such clumps is comparable to the stellar mass.
We have measured the scintillation properties of 151 young, energetic pulsars with the Parkes radio telescope and have identified two extreme scattering events (ESEs). Towards PSR J1057−5226 we discovered a three-year span of strengthened scattering during which the variability in flux density and the scintillation bandwidth decreased markedly. The transverse size of the scattering region is ∼23 au, and strong flux density enhancement before and after the ESE may arise from refractive focusing. Long observations reveal scintillation arcs characteristic of interference between rays scattered at large angles, and the clearest arcs appear during the ESE. The arcs suggest scattering by a screen 100-200 pc from the earth, perhaps ionized filamentary structure associated with the boundary of the local bubble(s).Towards PSR J1740−3015 we observed a "double dip" in the measured flux density similar to ESEs observed towards compact extragalactic radio sources. The observed shape is consistent with that produced by a many-au scale diverging plasma lens with electron density ∼500 cm −3 . The continuing ESE is at least 1500 d long, making it the longest detected event to date. These detections, with materially different observational signatures, indicate that well-calibrated pulsar monitoring is a keen tool for ESE detection and ISM diagnostics. They illustrate the strong rôle au-scale non-Kolmogorov density fluctuations and the local ISM structure play in such events and are key to understanding both their intrinsic physics and their impact on other phenomena, particularly fast radio bursts.
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