We report results from the initial stage of a long-term pulsar survey of the Galactic plane using the Arecibo L-band Feed Array (ALFA), a seven-beam receiver operating at 1.4 GHz with 0.3 GHz bandwidth, and fastdump digital spectrometers. The search targets low Galactic latitudes, |b| 5 • , in the accessible longitude ranges, 32 • ℓ 77 • and 168 • ℓ 214 • . The instrumentation, data processing, initial survey observations, sensitivity, and database management are described. Data discussed here were collected over a 100 MHz passband centered on 1.42 GHz using a spectrometer that recorded 256 channels every 64 µs. Analysis of the data with their full time and frequency resolutions is ongoing. Here, we report the results of a preliminary, low-resolution analysis for which the data were decimated to speed up the processing. We have detected 29 previously known pulsars and discovered 11 new ones. One of these, PSR J1928+1746, with a period of 69 ms and a relatively low characteristic age of 82 kyr, is a plausible candidate for association with the unidentified EGRET source 3EG J1928+1733. Another, PSR J1906+07, is a non-recycled pulsar in a relativistic binary with orbital period of 3.98 hr. In parallel with the periodicity analysis, we also search the data for isolated dispersed pulses. This technique has resulted in the discovery of PSR J0628+09, an extremely sporadic radio emitter with a spin period of 1.2 s. Simulations we have carried out indicate that ∼ 1000 new pulsars will be found in our ALFA survey. In addition to providing a large sample for use in population analyses and for probing the magnetoionic interstellar medium, the survey maximizes the chances of finding rapidly spinning millisecond pulsars and pulsars in compact binary systems. Our search algorithms will exploit the multiple data streams from ALFA to discriminate between radio frequency interference and celestial signals, including pulsars and possibly new classes of transient radio sources.

Abstract. Both nulling and subpulse drifting are poorly understood phenomena. We probe their mechanisms by investigating how they interact in PSR B0809+74. We find that the subpulse drift is not aliased but directly reflects the actual motion of the subbeams. The carousel-rotation time must then be over 200 s, which is much longer than theoretically predicted. The drift pattern after nulls differs from the normal one, and using the absence of aliasing we determine the underlying changes in the subbeam-carousel geometry. We show that after nulls, the subbeam carousel is smaller, suggesting that we look deeper in the pulsar magnetosphere than we do normally. The many striking similarities with emission at higher frequencies, thought to be emitted lower too, confirm this. The emission-height change as well as the striking increase in carousel-rotation time can be explained by a post-null decrease in the polar gap height. This offers a glimpse of the circumstances needed to make the pulsar turn off so dramatically.

We report the discovery of PSR J1906+0746, a young 144-ms pulsar in a highly relativistic 3.98-hr orbit with an eccentricity of 0.085 and expected gravitational wave coalescence time of ∼ 300 Myr. The new pulsar was found during precursor survey observations with the Arecibo 1.4-GHz feed array system and retrospectively detected in the Parkes Multibeam plane pulsar survey data.

We report here on variable propagation effects in over 20 yr of multifrequency timing analysis of pulsar PSR B1937+21 that determine small-scale properties of the intervening plasma as it drifts through the sight line. The phase structure function derived from the dispersion measure variations is in remarkable agreement with that expected from the Kolmogorov spectrum, with a power-law index of 3:66 AE 0:04, valid over an inferred scale range of 0.2-50 AU. The observed flux variation timescale and the modulation index, along with their frequency dependence, are discrepant with the values expected from a Kolmogorov spectrum with infinitesimally small inner scale cutoff, suggesting a caustic-dominated regime of interstellar optics. This implies an inner scale cutoff to the spectrum of $1:3 ; 10 9 m. Our timing solutions indicate a transverse velocity of 9 km s À1 with respect to the solar system barycenter, and 80 km s À1 with respect to the pulsar's LSR. We interpret the frequency-dependent variations of DM as a result of the apparent angular broadening of the source, which is a sensitive function of frequency (/ À2:2 ). The error introduced by this in timing this pulsar is $2.2 s at 1 GHz. The timing error introduced by ''image wandering'' from the slow, nominally refractive scintillation effects is about 125 ns at 1 GHz. The error accumulated due to positional error (due to image wandering) in solar system barycentric corrections is about 85 ns at 1 GHz.