Anisotropic Interstellar Scattering toward the Cygnus Region

Desai, K. M.; Fey, A. L.

doi:10.1086/320349

Cited by 37 publications

(41 citation statements)

References 44 publications

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“…The mean value for the image elongation is ¼ 1:2. This value is comparable to that measured for Cyg X-3 (%1.3, Wilkinson et al 1994;Molnar et al 1995) and NGC 6334B (1.2-1.5; Trotter et al 1998), within the range of anisotropies measured for the extragalactic sources seen through the Cygnus region (1.1-1.8; Spangler & Cordes 1988Desai & Fey 2001), and somewhat smaller than the value measured for the OH masers toward W49N (2-3; Desai et al 1994) and the Galactic center (%2.5; Frail et al 1994). Romani et al (1986) have predicted the anisotropy that would be induced, even if the small-scale (diffractive) density fluctuations responsible for angular broadening are isotropic, by refractive effects from large-scale density fluctuations.…”

Section: Imagg E Anisotropysupporting

confidence: 87%

On the Enhanced Interstellar Scattering toward B1849+005

Lazio

2004

ApJ

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This paper reports new Very Large Array ( VLA) and Very Long Baseline Array ( VLBA) observations of the extragalactic source B1849+005 (Galactic coordinates ' ¼ 33N44, b ¼ þ0N21) at frequencies between 0.33 and 15 GHz and the reanalysis of archival VLA observations at 0.33, 1.5, and 4.9 GHz. The structure of this source is complex, confirming previous suggestions, but interstellar scattering dominates the structure of the central component at least to 15 GHz. An analysis of the phase structure functions of the interferometric visibilities shows the density fluctuations along this line of sight to be anisotropic (axial ratio = 1.3) with a frequency-independent position angle and having an inner scale of roughly a few hundred kilometers. The anisotropies occur on length scales of order 10 15 cm (D/5 kpc), which within the context of certain magnetohydrodynamic turbulence theories indicates the length scale on which the kinetic and magnetic energy densities are comparable. A conservative upper limit on the velocity of the scattering material is 1800 km s À1 , based on the lack of changes in the shapes of the 0.33 GHz images. In the 0.33 GHz field of view, there are a number of other sources that might also be heavily scattered, which would suggest that there are large changes in the scattering strength on lines of sight separated by a degree or less. Both B1849+005 and PSR B1849+00 are highly scattered, and they are separated by only 13 0 . If the lines of sight are affected by the same ''clump'' of scattering material, it must be at least 2.3 kpc distant. However, a detailed attempt to account for the scattering observables toward these sources (angular broadening of the extragalactic source, pulse broadening of the pulsar, and upper limits on the angular broadening of the pulsar) does not produce a self-consistent set of parameters for a clump that can reproduce all three measured scattering observables. A clump of H emission, possibly associated with the H ii region G33.418À0.004, lies between these two lines of sight, but it seems unable to account for all of the required excess scattering.

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Section: Imagg E Anisotropysupporting

confidence: 87%

On the Enhanced Interstellar Scattering toward B1849+005

Lazio

2004

ApJ

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“…This scale is much less than the predicted and measured values of the inner scale, which fall in the range 10 2 to 5.5 km (14,15). The amplitude of turbulence in the Galactic Center scattering screen is ∼ 2 − 3 orders of magnitude greater than what is seen in the next most powerful scattering region, NGC 6334B (16), however, suggesting that the Galactic Center case may be atypical.…”

mentioning

confidence: 78%

“…In the case of an image produced by interstellar electron scattering on baselines longer than the inner scale of turbulence, β is the power-law index of electron density fluctuations (13). The parameter β is related to the exponent α of the scattering law (size ∝ λ α ) as β = α + 2, allowing an independent check of the λ 2 law (13,14,15). For the case of the Galactic Center scattering we expect β = 4, in which case Equation 1 is a Gaussian function and x and y are the FWHM in the two axes.…”

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confidence: 99%

Detection of the Intrinsic Size of Sagittarius A* Through Closure Amplitude Imaging

Bower

Falcke

Herrnstein

et al. 2004

Science

185

249

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We have detected the intrinsic size of Sagittarius A*, the Galactic Center radio source associated with a supermassive black hole, showing that the short-wavelength radio emission arises from very near the event horizon of the black hole. Radio observations with the Very Long Baseline Array show that the source has a size of 24 ± 2 Schwarzschild radii at 7 mm wavelength. In one of eight 7-mm epochs we also detect an increase in the intrinsic size of 60 +25 −17 %. These observations place a lower limit to the mass density of Sgr 1

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“…The inner scale may be a few hundred kilometers in the interstellar medium (Spangler & Gwinn 1990). The measured ≈2:1 anisotropy of the scatter broadening of Sgr A * is typical for heavily scattered lines of sight (Desai & Fey 2001); it may indicate that density fluctuations responsible for scattering are aligned with a large-scale magnetic field (Desai et al 1994;Goldreich & Sridhar 1995).…”

Section: Interstellar Scatteringmentioning

confidence: 93%

Discovery of Substructure in the Scatter-Broadened Image of SGR A*

Gwinn

Kovalev

Johnson

et al. 2014

ApJ

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We have detected substructure within the smooth scattering disk of the celebrated Galactic center radio source Sagittarius A* (Sgr A * ). We observed this structure at 1.3 cm wavelength with the Very Long Baseline Array together with the Green Bank Telescope, on baselines of up to 3000 km, long enough to completely resolve the average scattering disk. Such structure is predicted theoretically as a consequence of refraction by large-scale plasma fluctuations in the interstellar medium. Along with the much-studied θ d ∝ λ 2 scaling of angular broadening θ d with observing wavelength λ, our observations indicate that the spectrum of interstellar turbulence is shallow with an inner scale larger than 300 km. The substructure is consistent with an intrinsic size of about 1 mas at 1.3 cm wavelength, as inferred from deconvolution of the average scattering. Further observations of the substructure can set stronger constraints on the properties of scattering material and on the intrinsic size of Sgr A * . These constraints will guide our understanding of the effects of scatter broadening and the emission physics near the black hole in images with the Event Horizon Telescope at millimeter wavelengths.

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Anisotropic Interstellar Scattering toward the Cygnus Region

Cited by 37 publications

References 44 publications

On the Enhanced Interstellar Scattering toward B1849+005

On the Enhanced Interstellar Scattering toward B1849+005

Detection of the Intrinsic Size of Sagittarius A* Through Closure Amplitude Imaging

Discovery of Substructure in the Scatter-Broadened Image of SGR A*

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