The Frequency Dependence of Scintillation Arc Thickness in Pulsar B1133+16. (arXiv:1811.04519v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Stinebring_D/0/1/0/all/0/1">Dan R. Stinebring</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rickett_B/0/1/0/all/0/1">Barney J. Rickett</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ocker_S/0/1/0/all/0/1">Stella Koch Ocker</a>
Scintillation arcs have become a powerful tool for exploring scattering in
the ionized interstellar medium. There is accumulating evidence that the
scattering from many pulsars is extremely anisotropic resulting in highly
elongated, linear brightness functions. We present a three-frequency (327~MHz,
432~MHz, 1450~MHz) Arecibo study of scintillation arcs from one nearby, bright,
high-velocity pulsar, PSR~B1133+16. We show that a one-dimensional (1D), linear
brightness function is in good agreement with the data at all three observing
frequencies. We use two methods to explore the broadening of the 1D brightness
function $B(theta)$ as a function of frequency: 1) crosscuts of the forward
arc at constant delay and 2) a 1D modeling of $B(theta)$ using a comparison
between model and observed secondary spectrum as a goodness-of-fit metric. Both
methods show that the half-power width of $B(theta)$ deviates from the
expected dependence $propto nu^{-a}$, where $nu$ is the observing frequency
. Our estimates of $a$ have moderately large uncertainties but imply $a
lesssim1.8$, and so are inconsistent with the expected $a = 2.0$ for plasma
refraction or $a = 2.2$ for Kolmogorov turbulence. In addition the shape of
$B(theta)$ cuts off more steeply than predicted for Kolmogorov turbulence.
Ultimately, we conclude that the underlying physics of the broadening mechanism
remains unexplained. Our results place the scattering screen at a distance that
is broadly consistent with an origin at the boundary of the Local Bubble.
Scintillation arcs have become a powerful tool for exploring scattering in
the ionized interstellar medium. There is accumulating evidence that the
scattering from many pulsars is extremely anisotropic resulting in highly
elongated, linear brightness functions. We present a three-frequency (327~MHz,
432~MHz, 1450~MHz) Arecibo study of scintillation arcs from one nearby, bright,
high-velocity pulsar, PSR~B1133+16. We show that a one-dimensional (1D), linear
brightness function is in good agreement with the data at all three observing
frequencies. We use two methods to explore the broadening of the 1D brightness
function $B(theta)$ as a function of frequency: 1) crosscuts of the forward
arc at constant delay and 2) a 1D modeling of $B(theta)$ using a comparison
between model and observed secondary spectrum as a goodness-of-fit metric. Both
methods show that the half-power width of $B(theta)$ deviates from the
expected dependence $propto nu^{-a}$, where $nu$ is the observing frequency
. Our estimates of $a$ have moderately large uncertainties but imply $a
lesssim1.8$, and so are inconsistent with the expected $a = 2.0$ for plasma
refraction or $a = 2.2$ for Kolmogorov turbulence. In addition the shape of
$B(theta)$ cuts off more steeply than predicted for Kolmogorov turbulence.
Ultimately, we conclude that the underlying physics of the broadening mechanism
remains unexplained. Our results place the scattering screen at a distance that
is broadly consistent with an origin at the boundary of the Local Bubble.
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