2,097 research outputs found
Theory of Parabolic Arcs in Interstellar Scintillation Spectra
Our theory relates the secondary spectrum, the 2D power spectrum of the radio
dynamic spectrum, to the scattered pulsar image in a thin scattering screen
geometry. Recently discovered parabolic arcs in secondary spectra are generic
features for media that scatter radiation at angles much larger than the rms
scattering angle. Each point in the secondary spectrum maps particular values
of differential arrival-time delay and fringe rate (or differential Doppler
frequency) between pairs of components in the scattered image. Arcs correspond
to a parabolic relation between these quantities through their common
dependence on the angle of arrival of scattered components. Arcs appear even
without consideration of the dispersive nature of the plasma. Arcs are more
prominent in media with negligible inner scale and with shallow wavenumber
spectra, such as the Kolmogorov spectrum, and when the scattered image is
elongated along the velocity direction. The arc phenomenon can be used,
therefore, to constrain the inner scale and the anisotropy of scattering
irregularities for directions to nearby pulsars. Arcs are truncated by finite
source size and thus provide sub micro arc sec resolution for probing emission
regions in pulsars and compact active galactic nuclei. Multiple arcs sometimes
seen signify two or more discrete scattering screens along the propagation
path, and small arclets oriented oppositely to the main arc persisting for long
durations indicate the occurrence of long-term multiple images from the
scattering screen.Comment: 22 pages, 11 figures, submitted to the Astrophysical Journa
Electric field representation of pulsar intensity spectra
Pulsar dynamic spectra exhibit high visibility fringes arising from
interference between scattered radio waves. These fringes may be random or
highly ordered patterns, depending on the nature of the scattering or
refraction. Here we consider the possibility of decomposing pulsar dynamic
spectra -- which are intensity measurements -- into their constituent scattered
waves, i.e. electric field components. We describe an iterative method of
achieving this decomposition and show how the algorithm performs on data from
the pulsar B0834+06. The match between model and observations is good, although
not formally acceptable as a representation of the data. Scattered wave
components derived in this way are immediately useful for qualitative insights
into the scattering geometry. With some further development this approach can
be put to a variety of uses, including: imaging the scattering and refracting
structures in the interstellar medium; interstellar interferometric imaging of
pulsars at very high angular resolution; and mitigating pulse arrival time
fluctuations due to interstellar scattering.Comment: 7 Pages, 2 Figures, revised version, accepted by MNRA
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