An Arecibo Search for Pulsars and Transient Sources in M33
  We report on a systematic and sensitive search for pulsars and transient
sources in the nearby spiral galaxy M33, conducted at 1.4 GHz with the Arecibo
telescope's seven-beam receiver system, ALFA. Data were searched for both
periodic and aperiodic sources, up to 1000 pc cm^{-3} in dispersion measure and
on timescales from $\sim$50 \us to several seconds. The galaxy was sampled with
12 ALFA pointings, or 84 pixels in total, each of which was searched for 2-3
hr. We describe the observations, search methodologies and analysis strategies
applicable to multibeam systems, and comment on the data quality and statistics
of spurious events that arise due to radio frequency interference. While these
searches have not led to any conclusive signals of periodic or transient nature
that originate in the galaxy, they illustrate some of the underlying challenges
and difficulties in such searches and the efficacy of simultaneous multiple
beams in the analysis of search output. The implied limits are $\la$5 uJy Mpc^2
in luminosity (at 1400 MHz) for periodic sources in M33 with duty cycles
$\la$5%. For short-duration transient signals (with pulse widths $\la$100 \us),
the limiting peak flux density is 100 mJy, which would correspond to a
5-$\sigma$ detection of bright giant pulses ($\sim$20 kJy) from Crab-like
pulsars if located at the distance of M33. We discuss the implications of our
null results for various source populations within the galaxy and comment on
the future prospects to conduct even more sensitive searches with the upcoming
next-generation instruments including the Square Kilometer Array and its
pathfinders.
