ABSTRACT We report on the subpulse modulation properties of 1198 pulsars using the Thousand-Pulsar-Array programme on MeerKAT. About 35 per cent of the analysed pulsars exhibit drifting subpulses that are more pronounced towards the death line, consistent with previous studies. We estimate that this common phenomenon is detectable in 60 per cent of the overall pulsar population if high-quality data were available for all. This large study reveals the evolution of drifting subpulses across the pulsar population in unprecedented detail. In particular, we find that the modulation period P3 follows a V-shaped evolution with respect to the characteristic age τc, such that the smallest P3 values, corresponding to the Nyquist period P3 ≃ 2, are found at τc ≃ 107.5 yr. The V-shaped evolution can be interpreted and reproduced if young pulsars possess aliased fast intrinsic P3, which monotonically increase, ultimately achieving a slow unaliased P3. Enhancement of irregularities in intrinsic subpulse modulation by aliasing in small-τc pulsars would explain their observed less well defined P3’s and weaker spectral features. Modelling these results as rotating subbeams, their circulation must slow down as the pulsar evolves. This is the opposite to that expected if circulation is driven by $\boldsymbol{E}\times \boldsymbol{B}$ drift. This can be resolved if the observed P3 periodicity is due to a beat between an $\boldsymbol{E}\times \boldsymbol{B}$ system and the pulsar period. As a by-product, we identified the correct periods and spin-down rates for 12 pulsars, for which harmonically related values were reported in the literature.

The recently discovered 100X weaker quiescent (Q) mode in pulsar B0823+26 is X-ray quiet, unlike its usual bright (B) mode. Arecibo polarimetric observations were conducted to confirm the pulsar's orthogonal geometry and investigate the emission associated with its main pulse (MP), interpulse (IP), and postcursor (PC) components. Main results: (1) the pulsar's MP, PC, and IP are present in both modes and exhibit a two-pole orthogonal geometry. (2) The B-mode MP is dominated by core emission with weak conal outriders, whereas, the Q-mode double profile shows mainly residual conal emission with little core. The IP is conal in both modes. (3) Sporadic intrapulse emission trailing the PC is detected in the Q mode. (4) B0823+26 falls close to an (E)over dot boundary of 10(32.5) erg s(-1) (or B-12/P-2 similar to 2.5) between core- and conal-dominated profiles-which also represents a boundary between pairplasma source configurations above the polar cap. For larger energies, the pair-formation front is central, flat, and generates backflow heating, whereas for smaller energies it is peripheral, lower, and produces little heating. (5) Apparently, the pulsar is able to assume both core- and conal-dominated "states" corresponding to its bright and weak modes. These circumstances appear to explain B0823+26's B-mode X-ray bright/core-dominated radio emission or Q-mode X-ray faint/conal radio emission-and why the IP is X-ray quiet in both modes. (6) These same considerations applied to B0943+10 may explain why its brighter radio mode was conal and X-ray quiet, while the weaker one was X-ray bright-because its peripheral sightline would miss most core radiation.

Simultaneous observations of PSR B0823+26 with ESA's XMM-Newton, the Giant Metrewave Radio Telescope and international stations of the Low Frequency Array revealed synchronous X-ray/radio switching between a radio-bright (B) mode and a radio-quiet (Q) mode. During the B mode we detected PSR B0823+26 in 0.2 - 2 keV X-rays and discovered pulsed emission with a broad sinusoidal pulse, lagging the radio main pulse by 0.208 +/- 0.012 in phase, with high pulsed fraction of 70 - 80 per cent. During the Q mode PSR B0823+26 was not detected in X-rays (2 sigma upper limit a factor similar to 9 below the B-mode flux). The total X-ray spectrum, pulse profile and pulsed fraction can globally be reproduced with a magnetized partially ionized hydrogen atmosphere model with three emission components: a primary small hotspot (T similar to 3.6 x 10(6) K, R(-1)7 m), a larger cooler concentric ring (T similar to 1.1 x 10(6) K, R similar to 280 m) and an antipodal hotspot (T similar to 1.1 x 10(6) K, R similar to 100 m), for the angle between the rotation axis and line of sight direction similar to 66 degrees. The latter is in conflict with the radio derived value of (84 +/- 0.7)degrees. The average X-ray flux within hours-long B-mode intervals varied by a factor +/- 20 per cent, possibly correlated with variations in the frequency and lengths of short radio nulls or short durations of weak emission. The correlated X-ray/radio moding of PSR B0823+26 is compared with the anti-correlated moding of PSR B0943+ 10, and the lack of X-ray moding of PSR B1822-09. We speculate that the X-ray/radio switches of PSR B0823+26 are due to variations in the rate of accretion of material from the interstellar medium through which it is passing.

AbstractWe present simultaneous multi-frequency observations of PSR J1822–2256 for the first time, utilizing the unique capabilities of upgraded Giant Meterwave Radio Telescope (uGMRT). No emission is detected in about 10 % of pulses. At least two drift modes and a possibly third rare mode, occur for 66, 21 and 2 % pulses respectively (P3 ~ 17, 7.5 and 5 P0 respectively). The three drift modes and the nulls occur concurrently from 250 to 1500 MHz. Modal average profiles are distinct with their widths increasing with drift rate. These sub-pulse drift related profile mode-changes can provide independent probes of beam geometry and polar gap physics.

We report on simultaneous X-ray and radio observations of the radio-mode-switching pulsar PSR B1822-09 with ESA's XMM-Newton and the Westerbork Synthesis Radio Telescope, Giant Metrewave Radio Telescope and Lovell radio telescopes. PSR B1822-09 switches between a radio-bright and radio-quiet mode, and we discovered a relationship between the durations of its modes and a known underlying radio-modulation time-scale within the modes. We discovered X-ray (energies 0.2-1.4 keV) pulsations with a broad sinusoidal pulse, slightly lagging the radio main pulse in phase by 0.094 ± 0.017, with an energy-dependent pulsed fraction varying from ∼0.15 at 0.3 keV to ∼0.6 at 1 keV. No evidence is found for simultaneous X-ray and radio mode switching. The total X-ray spectrum consists of a cool component (T ∼0.96 × 106 K, hotspot radius R ∼2.0 km) and a hot component (T ∼2.2 × 106 K, R ∼100 m). The hot component can be ascribed to the pulsed emission and the cool component to the unpulsed emission. The high-energy characteristics of PSR B1822-09 resemble those of middle-aged pulsars such as PSR B0656+14, PSR B1055-52 and Geminga, including an indication for pulsed high-energy gamma-ray emission in Fermi Large Area Telescope data. Explanations for the high pulsed fraction seem to require different temperatures at the two poles of this orthogonal rotator, or magnetic anisotropic beaming effects in its strong magnetic field. In our X-ray skymap, we found a harder source at only 5.1 ± 0.5 arcsec from PSR B1822-09, which might be a pulsar wind nebula.

The SKA will discover tens of thousands of pulsars and provide unprecedented data quality on these, as well as the currently known population, due to its unrivalled sensitivity. Here, we outline the state of the art of our understanding of magnetospheric radio emission from pulsars and how we will use the SKA to solve the open problems in pulsar magnetospheric physics.

Radio pulsar J0631+1036 presents a remarkably clear example of a rare four-component profile, and with apparently large aberration/retardation indicated by its linear polarization-angle traverse, but on closer study its profiles are somewhat difficult to understand and interpret. The pulsar's four components do appear to represent inner and outer conal beam pairs with the expected spacing and spectral evolution with frequency. At metre wavelengths, the leading and trailing component pairs are often conflated into an unresolved double form by what seems to be varying amounts of scattering. We assess whether the core/double-cone geometric model, widely used to describe the profiles of slower pulsars, is appropriate for J0631+1036. We find that it is largely compatible apart from difficulties with the emission height and resolved double form of the inner conal features. An aberration/retardation analysis provides 600-km physical emission height values, which are compatible with geometric estimates for the outer conal emission. We also explore several other models and conclude that none are as successful as the core/double-cone model despite its several difficulties.

In this paper, we compare and contrast the emissions of two high nulling fraction pulsars, PSR J1738-2330 and PSR J1752+2359. In both pulsars, the emission bursts appear in a quasi-periodic fashion with typical separations of several hundred pulses. In J1738-2330, there is evidence of two underlying periodicities with memory persisting for at least 11 bursts. In contrast, in J1752+2359, the pattern coherence is rapidly lost and the burst/null lengths appear to be selected randomly from their respective quasi-normal distributions. The typical emission bursts of J1738-2330 exhibit a steady exponential decay of on-pulse energy accompanied by a flickering emission characterized by short frequent nulls towards their end. In the bursts of J1752+2359, the flickering is absent, the decay more pronounced and the energy released during each bright phase is approximately constant. Unlike J1738-2330, the average profiles for the first and last pulses of the bursts of J1752+2359 differ slightly from the pulsar's overall profile, hinting at differences between the two pulsars in their transitions from null to burst state (and vice versa). During its long null phases, J1752+2359 is found to emit random weak inter-burst pulses whose profile peak is somewhat offset with respect to the overall average profile. Such pulses have no equivalent in J1738-2330, or in any known pulsar hitherto. These can pervade the entire emission of this pulsar and have a separate physical origin to normal pulses. On the basis of our comparison, we conclude that a pulsar's nulling fraction, even when high, remains a poor guide to its detailed subpulse behaviour, as previously found for pulsars with small nulling fractions.

Pulsars emit from low-frequency radio waves up to high-energy gamma-rays, generated anywhere from the stellar surface out to the edge of the magnetosphere. Detecting correlated mode changes across the electromagnetic spectrum is therefore key to understanding the physical relationship among the emission sites. Through simultaneous observations, we detected synchronous switching in the radio and x-ray emission properties of PSR B0943+10. When the pulsar is in a sustained radio-"bright" mode, the x-rays show only an unpulsed, nonthermal component. Conversely, when the pulsar is in a radio-"quiet" mode, the x-ray luminosity more than doubles and a 100% pulsed thermal component is observed along with the nonthermal component. This indicates rapid, global changes to the conditions in the magnetosphere, which challenge all proposed pulsar emission theories.

Arecibo observations of the conal triple pulsar B1918+19 at 0.327 and 1.4 GHz are used to analyse its subpulse behaviour in detail. We confirm the presence of three distinct drift modes (A, B, C) plus a disordered mode (N) and show that they follow one another in specific cycles. Interpreting the pulsar's profile as resulting from a sightline traverse which cuts across an outer cone and tangentially grazes an inner cone, we demonstrate that the phase modulation of the inner cone is locked to the amplitude modulation of the outer cone in all the drift modes. The 9 per cent nulls are found to be largely confined to the dominant B and N modes, and, in the N mode, create alternating bunches of nulls and emission in a quasi-periodic manner with an averaged fluctuation rate of about 12 rotation periods (P-1). We explore the assumption that the apparent drift is the first-order alias of a faster drift of subbeams equally spaced around the cones. This is shown to imply that the drift modes A, B and C have a common circulation time of 12P(1) and differ only in the number of subbeams. This time-scale is on the same order as predicted by the classic E x B drift model of Ruderman & Sutherland and also coincides with the N-mode modulation. We therefore arrive at a picture where the circulation speed remains roughly invariant while the subbeams progressively diminish in number from modes A to B to C, and are then re-established during the N mode. We suggest that aliasing combined with subbeam loss may be responsible for the apparently dramatic changes in drift rates in other pulsars.

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We report a discovery of a phase‐related subpulse drift pattern in the relatively less studied pulsar, B1039−19, using high sensitivity GMRT observations at 325 MHz. We observe simultaneous occurrence of four/three drift regions—evident subpulse drifting under the leading and the trailing peaks, less evident subpulse drifting under the small peak near to the leading peak and under the inner saddle region. Subpulse drifting in all the four regions appear to be locked in phase.

We analyse the influence of rotation on shapes of pulse profiles of fast-rotating (millisecond) pulsars. Corotation has two opposing effects: (1) the caustic enhancement of the trailing side (TS) by aberration and retardation (AR), which squeezes the emission into a narrower phase interval; (2) the weakening of the TS caused by the asymmetry of curvature radiation about the dipole axis. Analysis of the radii of curvature of electron trajectories in the inertial observer's frame (IOF) enables these two effects to be considered together. We demonstrate that for dipolar magnetic field lines on the TS there exists a 'caustic phase' beyond which no emission can be observed. This phase corresponds to the zero (or minimum) curvature of the IOF trajectories and maximum bunching of the emission. The maximum gradient of polarization angle (PA) in the S-shaped PA curve is also associated with the curvature minimum and occurs at exactly the same phase. The asymmetry of trajectory curvature with respect to the dipole axis affects the curvature emissivity and the efficiency of pair production, suggesting a minimum at the caustic phase. Emission over a fixed range of altitudes, as expected in millisecond pulsars, leads to broad leading profiles and sharp peaks with a cut-off phase on the TS. We apply our results to the main pulse of the 5 ms pulsar J1012+5307.

We present a single-pulse study of the four-component pulsar J1819+1305, whose 'null' pulses bunch at periodic intervals of around 57 times the rotation period. The emission bursts between the null bunches exhibit characteristic modulations at two shorter periodicities of approximately 6.2 and 3 times the rotation period, the former found largely in the two outer components, and the latter only in the first component. Many bursts commence with bright emission in second component, exhibit positive six-period drift across the full profile width, and end with three-period modulation in the leading component. The 57-period cycle can be modelled geometrically as a sparsely filled subbeam carousel with nulls appearing whenever our line of sight intersects a circulating empty region. This interpretation is compatible with other recent evidence for periodic, carousel-related nulling and appears to support the physics of a polar-gap emission model for 'drifting' subpulses, but the subtle structure of the emission bursts defies an easy explanation.

We present a detailed study of the single pulses of the bright radio pulsar B0834+06, and offer evidence that the dominant periodic modulation in this pulsar's emission governs the occurrence of nulls. The nulls of B0834+06 constitute approximately 9% of the total pulses and we demonstrate that they do not occur at random in the pulse sequence. On the contrary, they are found to occur preferentially close to the minimum of the pulsar's emission cycle, whose period jitters around a central value of P-3 approximate to 2.17 rotation periods. It is likely that the intrinsic duration of the nulls averages about 0.2 times the pulsar rotation period. Surprisingly, the clearly distinct population of nulls and partial nulls of B0834+06 exhibit a 2-peak profile slightly broader than that of the normal emission. This is in contrast to the profile of extremely weak normal pulses, which is narrower than the overall profile. A flow/counterflow model for the pulsar's two components can reproduce the essential observed features of the emission in its dominant mode, with nulls occurring at the point where the minima of the two systems are aligned. This suggests that the observed nulling rate is determined by the chance positioning of our sightline with respect to the system. If the flow is interpreted as part of a circulating carousel after Deshpande & Rankin (1999, 2001 and Asgekar & Deshpande (2005), a fit yields a best estimate of 14 "sparks".

We present a detailed study of the single pulses of the bright radio pulsar B0834+06, and offer evidence that the dominant periodic modulation in this pulsar's emission governs the occurrence of nulls. The nulls of B0834+06 constitute approximately 9 per cent of the total pulses and we demonstrate that they do not occur at random in the pulse sequence. On the contrary, they are found to occur preferentially close to the minimum of the pulsar's emission cycle, whose period jitters around a central value of P-3 approximate to 2.17 rotation periods. It is likely that the intrinsic duration of the nulls averages about 0.2 times the pulsar rotation period. Surprisingly, the clearly distinct population of nulls and partial nulls of B0834+06 exhibit a two-peak profile slightly broader than that of the normal emission. This is in contrast to the profile of extremely weak normal pulses, which is narrower than the overall profile. A flow/counterflow model for the pulsar's two components can reproduce the essential observed features of the emission in its dominant mode, with nulls occurring at the point where the minima of the two systems are aligned. This suggests that the observed nulling rate is determined by the chance positioning of our sightline with respect to the system. If the flow is interpreted as part of a circulating carousel, a fit yields a best estimate of 14 'sparks'.

The emission of PSR B0656+14 can be characterized by two separate populations of highly linearly polarized pulses: bright pulses have a narrow spiky appearance consisting of short quasi-periodic bursts of emission with microstructure, in contrast to the underlying weaker broad pulses. The spiky pulses tend to appear in clusters which arise and dissipate over about 10 periods. We demonstrate that the spiky emission builds a narrow and peaked profile, whereas the weak emission produces a broad hump, which is largely responsible for the shoulders in the total emission profiles at both high and low frequencies. Simultaneous Arecibo Observatory and Westerbork observations further assist us in understanding the emission of this important pulsar. The recently discovered rotating-radio-transient (RRAT) sources are characterized by very bright radio bursts that occur periodically but very infrequently. We find bursts with the same characteristics in B0656+14. These bursts represent pulses from the bright end of an extended smooth pulse-energy distribution and are shown to be unlike giant pulses, giant micropulses or the pulses of normal pulsars. The extreme peak-fluxes of the brightest of these pulses indicates that PSR B0656+14, were it not so near, could only have been discovered as an RRAT source. Longer observations of some of the RRATs have revealed that they, like PSR B0656+14, emit weaker emission in addition to the bursts.

We present a detailed study of the single radio pulses of PSR B0656+14. The emission can be characterized by two separate populations of pulses: bright pulses have a narrow ``spiky'' appearance in contrast to the underlying weaker broad pulses. The shape of the pulse profile requires an unusually long timescale to achieve stability (over 25,000 pulses at 327 MHz) caused by spiky emission. The extreme peak-fluxes of the brightest of these pulses indicates that PSR B0656+14, were it not so near, could only have been discovered as an RRAT source. The strongest bursts represent pulses from the bright end of an extended smooth pulse-energy distribution, which is unlike giant pulses, giant micropulses or the pulses of normal pulsars. Longer observations of the RRATs may reveal that they, like PSR B0656+14, emit weaker emission in addition to the bursts.

Aims. This paper reports on single-pulse radio observations of PSR B1702-19 and their implications for pulsar emission theories. Methods: These observations were made with the Westerbork Synthesis Radio Telescope at 1380 and 328 MHz. The PA-swing is used to constrain possible geometries of the pulsar and the single-pulse data is analysed for subpulse modulation correlations between the main pulse and interpulse. Results: We confirm earlier conclusions that the dipole axis of this pulsar is almost perpendicular to its rotation axis, and report that both its main pulse and interpulse are modulated with a periodicity around 10.4 times the pulsar's rotation. Allowing for the half-period delay between main pulse and interpulse the modulation is found to be precisely in phase. Despite small secular variations in the periodicity, the phase-locking continues over all timescales ranging up to several years. Conclusions: The precision of the phase locking is difficult for current emission theories to explain if the main pulse and interpulse originate from opposing magnetic poles. We therefore also explore the possibility of a bidirectional model, in which all the modulated emission comes from one pole, but is seen from two sides and slightly displaced by aberration and time-delay. In this model the unmodulated emission is directed to us from the opposite pole, requiring the emission of the main pulse to originate from two different poles. This is difficult to reconcile with the observed smooth PA-swing. Whichever model turns out to be correct, the answer will have important implications for emission theories.

Unusual single-pulse behaviour has been identified in two pulsars, B0919+06 and B1859+07. Both stars normally emit bright subpulses in a region near the trailing edge of their profile. However, they occasionally undergo 'events', whereby the emission longitude gradually decreases by about their profile width, remains in this position for typically several tens of pulses, and then gradually returns over a few pulses to the usual longitude. The effect bears some resemblance to a profile 'mode change', but here the effect is gradual and episodic. On close inspection, the separate profiles of the normal and 'event' in each pulsar emission reveal a broad and complex structure - but one which may be understood in terms of the geometry of a conical beam. Possibly the effect entails an extreme example of variable 'absorption' within the magnetosphere, as suspected in other pulsars. Alternatively, it may be caused by intrinsic changes in the emission within the pulsar's beam.