  We have searched 75 unidentified radio sources selected from the NRAO VLA Sky
Survey (NVSS) catalog for the presence of rapidly spinning pulsars and short,
dispersed radio bursts. The sources are radio bright, have no identifications
or optical source coincidences, are more than 5% linearly polarized, and are
spatially unresolved in the catalog. If these sources are fast-spinning pulsars
(e.g. sub-millisecond pulsars), previous large-scale pulsar surveys may have
missed detection due to instrumental and computational limitations, eclipsing
effects, or diffractive scintillation. The discovery of a sub-millisecond
pulsar would significantly constrain the neutron star equation of state and
would have implications for models predicting a rapid slowdown of highly
recycled X-ray pulsars to millisecond periods from, e.g., accretion disk
decoupling. These same sources were previously searched unsuccessfully for
pulsations at 610 MHz with the Lovell Telescope at Jodrell Bank. This new
search was conducted at a different epoch with a new 800 MHz backend on the
NRAO 43-meter Telescope at a center frequency of 1200 MHz. Our search was
sensitive to sub-millisecond pulsars in highly accelerated binary systems and
to short transient pulses. No periodic or transient signals were detected from
any of the target sources. We conclude that diffractive scintillation,
dispersive smearing, and binary acceleration are unlikely to have prevented
detection of the large majority of the sources if they are pulsars, though we
cannot rule out eclipsing, nulling or intermittent emission, or radio
interference as possible factors for some non-detections. Other (speculative)
possibilities for what these sources might be include radio-emitting magnetic
cataclysmic variables or older pulsars with aligned magnetic and spin axes.
