Conal Components Research Articles

On the subpulse modulation, polarization and sub-beam carousel configuration of pulsar B1857−26

New Giant Metre-Wave Radio Telescope (GMRT) observations of the five-component pulsar B1857−26 provide detailed insight into its pulse-sequence modulation phenomena for the first time. The outer conal components exhibit a 7.4-rotation period, longitude-stationary modulation. Several lines of evidence indicate a carousel circulation time ˆ P3 of about 147 stellar rotations, characteristic of a pattern with 20 beamlets. The pulsar nulls some 20 per cent of the time, usually for only a single pulse, and these nulls show no discernible order or periodicity. Finally, the pulsar’s polarization-angle traverse raises interesting issues: if most of its emission comprises a single polarization mode, the full traverse exceeds 180 ◦ ; or if both polarization modes are present, then the leading and the trailing halves of the profiles exhibit two different modes. In either case, the rotating-vector model fails to fit the polarization-angle traverse of the core component.

Radio Phase-Resolved Spectra of the Conal-Double Pulsar B1133+16

Based on dividing the profile into a number of absolute phase intervals, the phase-resolved spectra (PHRS) are derived from published time-aligned average profiles at radio frequencies over two decades for the classic conal-double pulsar B1133+16. The relative spectral index, defined as the difference between the spectral indices of a reference and the given arbitrary phase interval, is obtained by power-law fit at each phase interval. The derived phase-resolved spectra show an ``M-like'' shape, of which the leading part and trailing part are approximately symmetrical. The basic feature of the PHRS is that the spectrum first flattens then steepens as the pulse phase sweeps from the profile center to the profile edges. The PHRS provide a coherent explanation of the major features of profile evolution of B1133+16, namely, the pulse width shrinkage with increasing frequency and the frequency evolution of the relative intensity between the leading and trailing conal components, and the bridge emission. The PHRS may be an indicator for emission spectral variation across the pulsar magnetosphere. Possible mapping from PHRS to emission-location-dependent spectral variation is presented, and some intrinsic mechanisms are discussed.

Core and conal component analysis of pulsar B1237+25

This paper provides a new analysis of this famous five-component (M) pulsar. In addition to the core-active ‘abnormal’ mode of the star, we find two distinct behaviours within its ‘normal’ mode, a ‘quiet-normal’ mode with regular 2.8-period subpulse modulation and little or no core activity, and a ‘flare-normal’ mode, where the core is regularly bright and a nearly four-period modulation is maintained. The flare-normal mode appears to be an intermediate state between the quiet-normal and abnormal behaviours. Short 5–15-pulse flare-normal-mode ‘bursts’ and quiet-normal intervals alternate with each other quasi-periodically, making a cycle some 60–80 pulses in duration. Abnormal-mode intervals are interspersed within this overall cycle, usually persisting for only a few pulses, but occasionally lasting for scores or even many hundreds of pulses. Within subsequences where the core is exceptionally quiet, the pulsar provides a nearly ‘textbook’ example of a central sightline traverse — with a polarization-angle sweep rate measured to be at least some . On this basis it is shown that the sightline impact angle β must be or some 5 per cent of the outer conal beam radius. The core component of the star is found to be incomplete, despite the fact that the full antisymmetric circularly polarized signature of the core is present. The visible core component aligns with the trailing right-hand (RH) portion of the circular signature. Measures either of the circular signature or of the trailing half of the core, however, reiterate the roughly 2.6° angular diameter of the polar cap of the star. We find that the PA traverse of the star is disrupted through the action of orthogonally polarized linear power in the longitude range of the core component. This circumstance can be seen in the ‘hook’ under the core component which usually entails four sense reversals of the PA, with its centre falling at the same PA as the extrema of the overall traverse. This behaviour is modelled to show its effect. Finally, the star provides two well-defined fiducial points from which emission-height estimates can be computed, the respective centres of both the linear PA traverse and the zero-crossing point of the circular polarization signature. We thus find that the outer and inner cones are emitted at heights of some 340 ± 79 and 278 ± 76 km, respectively, such that their field-line ‘feet’ are some 78 ± 7 and 53 ± 5 per cent of the polar cap radius. We also find evidence that the core emission occurs at a height of some 60 km.

Emission Altitude from Relativistic Phase Shift Due to Plasma Corotation in Pulsars

The radio emission from a relativistic plasma in a pulsar magnetosphere experiences a relativistic phase shift due to spin and difference in the emission altitude. Using the single pulse data of PSR B0329 + 54 at 325 MHz and 606 MHz we studied the structure of the pulsar emission beam. The distribution of the pulse components around the central core component indicates that the emission beam consists of four nested cones. The asymmetry in the location of the conal components in the leading versus trailing parts of the profile is interpreted as being due to aberration and retardation in the pulsar magnetosphere. These measurements allow us to determine the precise location of the 4 conal rings of emission. We find that the successive outer cones are emitted at higher altitudes in the magnetosphere. Further, for any given cone, the emission height at the lower frequency is found to be larger than that at the higher frequency. We discuss the implications of our new findings on our understanding of the pulsar emission geometry and its impact on the emission mechanisms. The relativistic plasma moving along the dipolar magnetic field lines emit coherent curvature radiation. The beamed emission occurs in the direction of tangents to the field lines, and to receive it, the sight line must align with the tangent within the beaming angle 1/γ, where γ is the particle Lorentz factor. By solving the viewing geometry in an inclined and rotating dipole magnetic field, we show that, at any given pulse phase, the observer tends to receive radiation only from the specific heights allowed by the geometry.

Pulsar Radio Emission Altitude from Curvature Radiation

We assume that relativistic sources moving along dipolar magnetic field lines emit curvature radiation. The beamed emission occurs in the direction of tangents to the field lines, and to receive it, the sight line must align with the tangent within the beaming angle 1/γ, where γ is the particle Lorentz factor. By solving the viewing geometry in an inclined and rotating dipolar magnetic field, we show that at any given pulse phase, the observer tends to receive radiation only from the specific heights allowed by the geometry. We find that outer conal components are emitted at higher altitudes compared to inner components, including the core. At any pulse phase, low-frequency emission comes from higher altitudes than high-frequency emission. We have modeled the emission heights of pulse components of PSR B0329+54 and estimated field line curvature radii and particle Lorentz factors in the emission regions.

Reception of Radio Waves from Pulsars

The beamed emission by relativistic sources moving along the magnetic dipolar field lines occurs in the direction of tangents to the field lines. To receive such a beamed radiation, the line of sight must align with the tangent within the beaming angle 1/γ, where γ is the particle Lorentz factor. By solving the viewing geometry, in an inclined and rotating dipole magnetic field, we show that at any given pulse phase the observer can receive the radiation only from specific altitudes. We find that the outer conal emission is received from higher altitudes than the inner conal components including the core. At any pulse phase, low frequency emission comes from the higher altitudes than higher-frequency emission. As an application of our model, we have used it to explain the emission heights of conal components in PSR B0329+54.

Centrifugal acceleration of plasma in pulsar magnetosphere

We present a relativistic model for the centrifugal acceleration of plasma bunches and the coherent radio emission in pulsar magnetosphere. We find that rotation broadens the width of leading component compared to the width of trailing component. We explain this difference in the component widths using the nested cone emission geometry. We estimate the effect of pulsar spin on the Stokes parameters, and find that the inclination between the rotation and magnetic axes can introduce an asymmetry in the circular polarization of the conal components. We analyse the single pulse polarization data of PSR B032954 at 606 MHz, and find that in its conal components, one sense of circular polarization dominates in the leading component while the other sense dominates in the trailing component. Our simulation shows that changing the sign of the impact parameter changes the sense of circular polarization as well as the swing of polarization angle

More Emission Cones: Multi-frequency Simulation of the Pulse Profiles of PSR J0437-4715

Pulsar radio emission beams have been studied observationally for a long time, and the suggestion is that they consist of the so-called core and conal components. To reproduce these components is a challenge for any emission model, and that the pulse profile of pulsars changes with frequency presents even a greater challenge. Assuming a local surface magnetic structure (to produce the core or central beam) and a global dipole magnetic field (to produce the conal beams), Gil & Krawczyk (1997) applied curvature radiation to the pulse profile simulation of PSR J0437-4715 (hereafter the GK model). Here we present an alternative multi-frequency simulation of the same profiles within the framework of the Inverse Compton Scattering (ICS) model. It is obtained from our simulation (1) that besides the core, the inner cone and the outer cone, there is an outer-outer cone; (2) that the emission components of the core and cones evolve strongly with frequency. Some important differences between the ICS model and the GK model are discussed, which need to be tested by further observations.

Toward an Empirical Theory of Pulsar Emission. VII. On the Spectral Behavior of Conal Beam Radii and Emission Heights

In this paper we return to the old problem of conal component pair widths and profile dimensions. Observationally, we consider a set of 10 pulsars with prominent conal component pairs for which well-measured profiles exist over the largest frequency range now possible. Apart from some tendency to narrow at high frequency, the conal components exhibit almost constant widths. We use all three profile measures, the component separation as well as the outside half-power and 10% widths, to determine conal beam radii, which are the focus of our subsequent analysis. These radii at different frequencies are well fitted by a relationship introduced by Thorsett, but the resulting parameters are highly correlated. Three different types of behavior are found: one group of stars exhibits a continuous variation of beam radius that can be extrapolated down to the stellar surface along the open field a second group exhibits beam radii that asymptotically approach a minimum high-frequency value that is 3-5 times larger, and a third set shows almost no spectral change in beam radius at all. The first two behaviors are associated with outer-cone component pairs, whereas the constant separation appears to reflect inner-cone emission. The first group, remarkably, can be fitted by a Thorsett relation in which the constant term is constrained to be the field tangent direction at the edge of the polar cap ρpc, but the others cannot. The first group can also be fitted well using an index of - , but the second group cannot. We first compute heights from the conal beam radii, assuming dipolar fields and emission along the last open field lines, which we find are again well fitted by a suitable Thorsett relation. Here we find that the first group can be fitted using a constant term hpc of 10 km and also that the first two groups are remarkably well fitted by an index of - . We then argue that physical emission heights can be estimated using the component separation along an interior annulus of field lines having their feet about halfway out on the polar cap—such values agree well with most existing height values based on physical criteria. Therefore, we find that radius-to-frequency mapping is associated with outer-cone component pairs. The near constant behavior of inner cones is thus arresting. We explore possible interrelationships between the spectral behavior of the component and profile widths produced by both the field line flaring and the changing sight line geometry. We also attempt to understand the physical implications of the parameter values resulting from the Thorsett relation fits.

Polarization of individual pulses of radio pulsars at the low frequencies 40, 60, and 103 MHz

Measurements of the linear polarization of individual pulses at 40, 60, and 103 MHz are presented for ten pulsars. The degree and position angle of a linear polarization were measured with a temporal resolution of 1–7 ms, and the longitudinal distributions of these parameters were constructed for each pulsar at one or more of these frequencies. These are the first such measurements for pulsars B0031-07, B0320 + 39, B0628-28, and B2217 + 47. Apart from B0628-28, all the pulsars are characterized by the simultaneous presence of orthogonal polarization modes in at least one component of the integral profile. The secondary polarization mode increases at frequencies ≤100 MHz for pulsars whose integrated pulses contain pairs of conal components (B0031-07, B0329 + 54, B0834 + 06, B1133 + 16, B2020 + 28). This is manifested both as an expansion of the longitudinal range where the secondary polarization mode is observed and an increase in its contribution to the emission at a given longitude. New data confirming the dependence of the linear polarization of individual pulses on the intensity and mode of the pulsar emission have been obtained.

Towards explanation of pulsar radio spectra

Refraction of radio waves in the ultrarelativistic highly magnetized pulsar plasma is considered. The influence of the eect on the observed pulsar spectra is studied. It is shown that refraction can cause a marked redistribution of the intensity in the emission beam. For a given sight-line trajectory across the beam this aects not only the apparent prole morphology but also the total intensity detected. As refraction is frequency-dependent, it can modify the pulsar spectrum. The spectral changes are peculiar to both the core and conal components. For the typical parameters of the magnetospheric plasma the eect is found to be ecient at the frequencies above 1 GHz. It is demonstrated that refraction can lead to the high-frequency steepening of the core-component spectrum with the subsequent flattening at still higher frequencies. In addition, the eect can account for the inversion of the intensity ratio of the conal components and the consequent break in the spectra of the conal-double pulsars. The high-frequency features of the pulsar spectra are compared with the other observational consequences of refraction. The conclusion is made that at the frequencies above 1 GHz the properties of pulsar radiation are mainly determined by the propagation eects in the magnetospheric plasma.

Influence of Rotation on Pulsar Radiation Characteristics

We present a relativistic model for pulsar radio emission by including the effect of rotation on coherent curvature radiation by bunches. We find that rotation broadens the width of the leading component compared to the width of the trailing component. We estimate the component widths in the average pulse profiles of about 24 pulsars and find that 19 of them have a broader leading component. We explain this difference in component widths by using the nested cone emission geometry. We estimate the effect of pulsar spin on the Stokes parameters and find that the inclination between the rotation and magnetic axes can introduce an asymmetry in the circular polarization of the conal components. We analyze the single-pulse polarization data of PSR B0329+54 at 606 MHz and find that in its conal components, one sign of circular polarization dominates in the leading component, while the other sign dominates in the trailing component. Our simulation shows that changing the sign of the impact parameter changes the sign of the circular polarization as well as the swing of the polarization angle.

Understanding the Radio Emission Geometry of PSR B0329+54

We have analyzed high-quality single pulse data of PSR B0329+54 at 325 MHz and 606 MHz to study the structure of the emission beam. Using the window-threshold technique, which is suitable for detecting weak emission components, we have detected 4 additional emission components in the pulse window. Three of these are new components and the fourth is a confirmation of a recently proposed component. Hence PSR B0329+54 is now known to have 9 emission components - the highest among all known pulsars. The distribution of the pulse components around the central core component indicates that the emission beam consists of four nested cones. The asymmetry in the location of the conal components in the leading versus trailing parts of the profile is interpreted as being due to aberration and retardation in the pulsar magnetosphere. These measurements allow us to determine the precise location of the 4 conal rings of emission. We find that the successive outer cones are emitted at higher altitudes in the magnetosphere. Further, for any given cone, the emission height at the lower frequency is found to be more than that at the higher frequency. The inferred heights range from ~160 km to ~1150 km. The set of ``active'' field lines, from which most of the conal radiation appears to originate, are found to be confined to a region located within ~0.5 to ~0.6 of the polar cap radius. We discuss the implications of our new findings on our understanding of the pulsar emission geometry and its impact on the emission mechanisms.

Spark Model for Pulsar Radiation Modulation Patterns

A nonstationary polar gap model first proposed by Ruderman & Sutherland is modified and applied to spark-associated pulsar emission at radio wavelengths. It is argued that under physical and geometrical conditions prevailing above pulsar polar cap, highly nonstationary spark discharges do not occur at random positions. Instead, sparks should tend to operate in well determined preferred regions. At any instant the polar cap is populated as densely as possible with a number of two-dimensional sparks with a characteristic dimension as well as a typical distance between adjacent sparks being about the polar gap height. Our model differs, however, markedly from its original hollow cone version. The key feature is the quasi-central spark driven by γ-B pair production process and anchored to the local pole of a sunspot-like surface magnetic field. This fixed spark prevents the motion of other sparks toward the pole, restricting it to slow circumferential × drift across the planes of field lines converging at the local pole. We argue that the polar spark constitutes the core pulsar emission and that the annular rings of drifting sparks contribute to conal components of the pulsar beam. We found that the number of nested cones in the beam of typical pulsar should not exceed three; a number also found by Mitra & Deshpande using a completely different analysis. We confront predictions of our model with a variety of pulsar data, including mean profiles morphology and their predicted correlations with properties of the P- pulsar diagram as well as detailed studies of drifting subpulses. We demonstrate that, if the observing geometry is known, the average profile as well as the apparent drift pattern are fully determined by the values of P and . In the accompanying Paper II we develop a self-consistent theory of coherent pulsar radio emission based on the modified polar gap model explored in this paper.

An Inverse Compton Scattering Model of Pulsar Emission. III. Polarization

Qiao and his collaborators recently proposed an inverse Compton scattering model to explain radio emission from pulsars. In this paper, we investigate the polarization properties of pulsar emission in the model. First of all, using the lower frequency approximation, we derived the analytical amplitude of the inverse Compton scattered wave of a single electron in a strong magnetic field. We found that the outgoing radio emission of a single relativistic electron scattering off the "low-frequency waves" produced by gap sparking should be linearly polarized and have no circular polarization at all. However, considering the coherency of the emission from a bunch of electrons, we found that the outgoing radiation from the inner part of the emission beam, i.e., that from the lower emission altitudes, preferentially has to have circular polarization. Computer simulations show that the polarization properties, such as the sense reversal of circular polarization near the pulse center, the S-shape of position angle swing of the linear polarization, and a strong linear polarization in conal components, can be reproduced in the ICS model.

Propagation of radio waves in pulsar magnetospheres

AbstractThe morphological classification of pulsar profiles is interpreted within the frame of the primordial hollow-cone emission model taking into account wave propagation in the magnetosphere. In this context the frequency evolution of the profiles as well as the difference in polarization properties of core and conal components can be explained naturally.

General Relativistic Precession of the Spin Axis of Binary Pulsar B1913+16: First Two Dimensional Maps of the Emission Beam

AbstractRelativistic spin-orbit coupling should cause the spin axis of Binary Pulsar B1913+16 to precess at a rate of about 1 deg / yr. As a result, the pulse profile is expected to exhibit secular evolution. Weisberg, Romani, & Taylor (1989), and Weisberg & Taylor (1992) found that the intensity ratio of the two conal components changed at a one percent per year rate from 1981 to 1989, but that their spacing did not measurably change. They attributed these observations to our line of sight passing across the middle of a patchy cone of emission. Recently Kramer (1998) found that the intensity ratio continued its secular change for another decade, and also detected a narrowing of the conal component separation.We present our analysis of eighteen years of 21 cm pulse profile data. We confirm that the profile is narrowing as precession finally carries our line of sight away from the emission beam axis, and we map the beam in two dimensions. The beam is elongated in the latitude direction, and the degree of elongation grows with radius.

Single Pulse Analysis of the Core-Dominated Pulsar B0611+22

AbstractSingle pulse studies of pulsar radio emission provide a window into the time dependent behavior of the radio loud region. I have analyzed a series of precision polarimetric observations of pulsar B0611+22 to determine the geometry of the emission region. The observations are consistent with a central core emission region, and a periodically present conal component. This identification leads to the surprising result that all emission is from the leading half of the polar cap.

Gaussian Component Decomposition and the Five-Component Profile of Pulsar 1451−68

We have developed a method to explore the detailed structure of the mean pulse profile of pulsars—namely, the Gaussian fit separation of the average profile (GFSAP) method—in which we model the total profile as a superposition of several Gaussian-shaped components. PSR B1451-68 has been observed at six frequencies, 170, 271, 400, 649, 950, and 1612 MHz, providing an excellent basis for a multifrequency analysis. Applying the GFSAP method to PSR 1451-68's ostensibly triple profile, we find strong evidence that it has five components in its profile. Based on this and other evidence, we conclude that PSR 1451-68 must be a member of the five-component (M) species. Benefiting from the separation, we are able to discuss the geometric properties of the three distinct emission zones: the core, the inner cone, and the outer cone. The first implies an inclination angle of 38°, and the 1 GHz widths of the conal radii are compatible with those found in 1993 by Rankin for the pulsar population at large. The radius of the outer cone increases markedly with decreasing frequency. Finally, the spectrum of the core component exhibits a strong break near 800 MHz and, probably, a turnover at about 150 MHz. At low frequency the core component dominates the pulsar's emission, whereas at high frequency the inner conal component pair is the strongest contributor to the emission.

Polarimetric observations of southern pulsars at 800 and 950 MHz

Polarimetric data are presented for a total of 82 (mostly southern) pulsars at observing frequencies of 800 MHz (18 pulsars) and 950 MHz (82 pulsars). 32 of the sources were observed using a high-speed integrator with milliperiod resolution. For many of the pulsars observed, there is no published polarimetry and, for most, these observations constitute the only data between 630 and 1612 MHz. Faraday rotation measurements for 24 pulsars are also presented. Most of the pulsars examined show properties that are readily explicable in terms of core and conal components. Notable results include integrated profile mode-changing identified for the first time in PSR B1358 - 63, confirmation of high inclinations between magnetic and rotation axes in the interpulsars PSR B1055 - 52 and PSR B1702 - 19, and significant rotation measure changes in PSR B1556 - 44 and PSR B1727 - 47. © 1997 RAS.