Degree Year

2004

Document Type

Thesis

Degree Name

Bachelor of Arts

Department

Physics and Astronomy

Advisor(s)

Daniel R. Stinebring

Keywords

Interstellar medium, Pulsar scintillation

Abstract

We have used pulsar scintillation observations to probe the ionized component of the interstellar medium on AU size scales. Previous work had shown that the presence of scintillation arcs in pulsar secondary spectra requires that the scattering along the line of sight to the pulsar is dominated by a thin screen of scattering material. An isotropic image gives rise to a sharply delineated arc, while an anisotropic image with refractive "hot spots" elongated along the pulsar velocity vector gives rise to detailed substructure and arclets in the secondary spectrum. Twenty-five years of archival scintillation data from the Arecibo Observatory show that arclets are present in ~ 25% of low radio frequency observations of PSR B0834+06 and PSR B1133+16 and that the decorrelation time scale of substructure is ~ 6 months.

Observations of the pulsar PSR B0834+06 at Arecibo identified four isolated arclets at high delays. These arclets were present throughout a month of observations, and their angular separation from the pulsar changed over the course of the month in a linear fashion. This transverse motion is dominated by the velocity of the pulsar and implies an approximate upper limit to the screen velocity of 7 km s-1.

We applied a plasma lens model to these observations assuming that the high delay arclets are caused by refracting plasma lenses in the scattering screen. We place an upper bound of a ~ 0.1 AU on the lens size and estimate an electron density within the lens of ne ~ 200 cm-3. The ionized component of the lens thus has a mass of Ml ~ 10-18 Msun. These parameters are very similar to the predicted parameters for the plasma lenses thought to cause Extreme Scattering Events in quasars.

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