Elusive Neutron Star Populations: Galactic Center And Intermittent Pulsars

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We present radio transient search algorithms and results from the ongoing Arecibo Pulsar ALFA (PALFA) survey of the Galactic plane. PALFA has discovered seven objects through a search for isolated dispersed pulses. All of these objects are Galactic and have periods between 0.4 and 4.7 s. One of the new discoveries, PSR J0627+16, has a duty cycle of 0.01%, the smallest among known radio pulsars. We discuss the impact of selection effects on the detectability and classification of intermittent sources, and compare the efficiencies of periodicity and single-pulse searches for various pulsar classes. We find that in some cases the apparent intermittency is likely to be caused by off-axis detection or a short time window that selects only a few bright pulses and favors detection with our single-pulse algorithm. In other cases, the intermittency appears to be intrinsic to the source. Accounting for the on-axis gain of the ALFA system, as well as the low gain but large solid-angle coverage of far-out sidelobes, we use the results of the survey so far to place limits on the amplitudes and event rates of transients of arbitary origin. While intermittent pulsars are hard to find because of their emission properties, pulsars orbiting near the massive black hole in the center of the Galaxy have eluded detection because of severe scattering by dense ionized gas. We report the results from three Galactic center pulsar surveys performed with the Green Bank telescope at 9, 5, and 2 GHz. The latter survey discovered three pul- sars whose large dispersion measures and angular proximity to Sgr A* indicate the existence of a Galactic center population of neutron stars. PSR J1746−2850I has a characteristic spindown age of only 13 kyr along with a high surface magnetic field ∼ 4 × 1013 G. It and a second object found in the same telescope pointing, PSR J1746−2850II, which has the highest known dispersion measure among pulsars, may have originated from recent star formation in the Arches or Quintuplet clusters given their angular locations. Along with a third object, PSR J1745−2910, and two pulsars with similarly high dispersion measures reported by Johnston et al. (2006), the five objects found so far are 10′ − 15′ from Sgr A*, consistent with a large pulsar population in the Galactic center, most of whose members are undetectable in relatively low-frequency surveys because of severe scattering broadening. Based on the properties of the new pulsars and the parameters of our surveys, we present a maximum likelihood estimation of the spatial distribution of the Galactic center pulsar population.

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