Discovery of pulsars is one of the main goals for large radio telescopes. The Five-hundred-meter Aperture Spherical radio Telescope (FAST), that incorporates an L-band 19-beam receiver with a system temperature of about 20 K, is the most sensitive radio telescope utilized for discovering pulsars. We designed the snapshot observation mode for a FAST key science project, the Galactic Plane Pulsar Snapshot (GPPS) survey, in which every four nearby pointings can observe a cover of a sky patch of 0.1575 square degrees through beam-switching of the L-band 19-beam receiver. The integration time for each pointing is 300 seconds so that the GPPS observations for a cover can be made in 21 minutes. The goal of the GPPS survey is to discover pulsars within the Galactic latitude of ± 10° from the Galactic plane, and the highest priority is given to the inner Galaxy within ± 5°. Up to now, the GPPS survey has discovered 201 pulsars, including currently the faintest pulsars which cannot be detected by other telescopes, pulsars with extremely high dispersion measures (DMs) which challenge the currently widely used models for the Galactic electron density distribution, pulsars coincident with supernova remnants, 40 millisecond pulsars, 16 binary pulsars, some nulling and mode-changing pulsars and rotating radio transients (RRATs). The follow-up observations for confirmation of new pulsars have polarization-signals recorded for polarization profiles of the pulsars. Re-detection of previously known pulsars in the survey data also leads to significant improvements in parameters for 64 pulsars. The GPPS survey discoveries are published and will be updated at http://zmtt.bao.ac.cn/GPPS/.

Aims. We intend to study of the nulling and subpulse drifting in PSR J1727−2739 in detail to investigate its radiation properties. Methods. The observations were carried out on 20 March, 2004 using the Parkes 64-m radio telescope with a central frequency of 1518 MHz. A total of 5568 single pulses were analysed.Results. This pulsar shows well-defined nulls with lengths lasting from 6 to 281 pulses and separated by burst phases ranging from 2 to 133 pulses. We estimate a nulling fraction of around 68%. No emission in the average pulse profile integrated over all null pulses is detected with significance above 3σ. Most transitions from nulls to bursts are within a few pulses, whereas the transitions from bursts to nulls exhibit two patterns of decay; these transitions either decrease gradually or rapidly. In the burst phase, we find that there are two distinct subpulse drift modes with vertical spacing between the drift bands of 9.7 ± 1.6 and 5.2 ± 0.9 pulse periods, while sometimes there is a third mode with no subpulse drifting. Some mode transitions occur within a single burst, while others are separated by nulls. Different modes have different average pulse profiles. Possible physical mechanisms are discussed.

Pulsar electrodynamics is reviewed emphasizing the role of the inductive electric field in an oblique rotator and the incomplete screening of its parallel component by charges, leaving 'gaps' with E = 0. The response of the plasma leads to a self-consistent electric field that complements the inductive electric field with a potential field leading to an electric drift and a polarization current associated with the total field. The electrodynamic models determine the charge density, ρ, and the current density, J; charge starvation refers to situations where the plasma cannot supply ρ, resulting in a gap and associated particle acceleration and pair creation. It is pointed out that a form of current starvation also occurs implying a new class of gaps. The properties of gaps are discussed, emphasizing that static models are unstable, the role of large-amplitude longitudinal waves and the azimuthal dependence that arises across a gap in an oblique rotator. Wave dispersion in a pulsar plasma is reviewed briefly, emphasizing its role in radio emission. Pulsar radio emission mechanisms are reviewed, and it is suggested that the most plausible is a form of plasma emission.

Pulsar electrodynamics has been built up by taking ingredients from two models, the vacuum-dipole model, which ignores the magnetosphere but includes the inductive electric field due to the obliquely rotating magnetic dipole, and the corotating-magnetosphere model, which neglects the vacuum inductive electric field and assumes a corotating magnetosphere. We argue that the inductive field can be neglected only if it is screened by a current, J sc , which we calculate for a rigidly rotating magnetosphere. Screening of the parallel component of the inductive field can be effective, but the perpendicular component cannot be screened in a pulsar magnetosphere. The incompletely screened inductive electric field has not been included in any model for a pulsar magnetosphere, and taking it into account has important implications. One effect is that it implies that the magnetosphere cannot be corotating, and we suggest that drift relative to corotation offers a natural explanation for the drifting of subpulses. A second effect is that this screening of the parallel inductive electric field must break down in the outer magnetosphere, and this offers a natural explanation for the acceleration of the electrons that produce pulsed gamma-ray emission.

We report detailed investigation of nulling and drifting behavior of two pulsars PSRs J1741−0840 and J1840−0840 observed from the Giant Meterwave Radio Telescope at 625 MHz. PSR J1741−0840 was found to show nulling fraction (NF) of around 30±5% while PSR J1840−0840 was shown to have NF of around 50±6%. We measured drifting behavior from different profile components in PSR J1840−0840 for the first time with the leading component showing drifting with 13.5±0.7 periods while the weak trailing component showed drifting of around 18±1 periods. Large nulling do hamper accuracy of these quantities derived using standard Fourier techniques. A more accurate comparison was drawn from driftband slopes, measured after sub-pulse modeling. These measurements revealed interesting sporadic and irregular drifting behavior in both pulsars. We conclude that the previously reported different drifting periodicities in the trailing component of PSR J1741−0840 is likely due to the spread in these driftband slopes. We also find that both components of PSR J1840−0840 show similar driftband slopes within the uncertainties. Unique nulling-drifting interaction is identified in PSR J1840−0840 where, in most occasions, the pulsar tends to start nulling after what appears to be an end of a driftband. Similarly, when the pulsar switches back to an emission phase, in most occasions it starts at the beginning of a new driftband in both components. Such behaviors have not been detected in any other pulsars to our knowledge. We also found that PSR J1741−0840 seems to have no memory of its previous burst phase while PSR J1840−0840 clearly exhibits memory of its previous state even after longer nulls for both components. We discuss possible explanations for these intriguing nulling-drifting interactions seen in both pulsars based on various pulsar nulling models.

A standard model for the visibility of pulsar radio emission is based on the assumption that the emission is confined to a narrow cone about the tangent to a dipolar field line. The widely accepted rotating vector model (RVM) is an approximation in which the line of sight is fixed and the field line is not strictly tangent to it. We refer to an exact treatment (Gangadhara, 2004) as the tangent model. In the tangent model (but not in the RVM) the visible point changes as a function of pulsar rotational phase, ψ, defining a trajectory on a sphere of radius r. We solve for the trajectory and for the angular velocity of the visible point around it. We note the recent claim that this motion is observable using interstellar holography (Pen et al., 2014). We estimate the error introduced by use of the RVM and find that it is significant for pulsars with emission over a wide range of ψ. The RVM tends to underestimate the range of ψ over which emission is visible. We suggest that the geometry alone strongly favors the visible pulsar radio being emitted at a heights more than ten percent of the light-cylinder distance, where our neglect of retardation effects becomes significant.

We reconsider pulsar electrodynamics for an obliquely rotating pulsar, and propose a way of synthesizing the vacuum dipole model (VDM) and the rotating magnetosphere model (RMM). We first modify the VDM by assuming that the parallel component of the inductive electric field is screened by charges. We refer to the resulting model as the minimal model. We calculate the screening charge density in the minimal model and compare it with the (Goldreich-Julian) charge density in the RMM. We identify the plasma velocity in the minimal model as the electric drift velocity due to the perpendicular component of the inductive electric field. We define a class of synthesized models as a linear combination of a fraction y times the minimal model and 1−y times the RMM. These models require a gap (with E = 0) between the corotating stellar surface and the non-corotating magnetosphere. We present illustrative plots of the charge density, of the location of nulls (where the charge density is zero) and of the three components of the plasma velocity as a function of the angles (θ, ψ) relative to the rotation axis, for specific values of the obliquity α and the parameter y. We discuss the question "Can any pulsar magnetosphere be corotating?" critically, pointing out difficulties associated with setting up corotation in the polar cap region. We speculate that the corotating plasma may flow across the last closed field line from the closedfield region. We suggest that abrupt changes in the spin-down rate in some pulsars may be due to jumps between the RMM and the minimal model.

We report on a detailed study on the mode switching in pulsar J1326−6700 by analysis the data acquired from the Parkes 64-m radio telescope at 1369 MHz. During the abnormal mode, the emission at the central and trailing components becomes extremely weak. Meanwhile, the leading emission shifts towards earlier by almost 2 degrees, and remains in this position for typically less than a minute. The mean flux density of the normal mode is almost five times that of the abnormal mode. Our data show that 85% of the time for PSR J1326−6700 was in the normal mode and 15% was in the abnormal mode. The intrinsic distributions of mode timescales can be well described by Weibull distributions, which presents a certain amount of memory in mode switching. Furthermore, a quasi-periodicity has been identified in the mode switching in pulsar J1326−6700. The estimated delay emission heights based on the kinematical effects indicate that the abnormal mode may be originated from higher altitude than the normal mode.