Pulsar Radio Emission Research Articles

Maser Emission in Astrophysical Plasmas: 2003 Robert Ellery Lecture

AbstractThere are three distinct types of ‘coherent emission’ in astrophysical plasmas: plasma emission (e.g. in solar radio bursts), electron cyclotron maser emission (e.g. in Jupiter's radio bursts), and pulsar radio emission. The development and current status of our understanding of coherent emission is reviewed, concentrating on plasma emission and electron cyclotron maser emission for which there is direct information on the distributions of electrons that produce the radiation. A generic model for a coherent emission process involves a maser generating radiation in a natural mode of the ambient plasma, and operating near marginal stability. A specific coherent emission mechanism involves the form of free energy to drive the maser, a pump that provides the free energy, and the plasma instability that leads to wave growth. The nature of coherence and its measurement through higher order intensity correlations are discussed.

A Model for the Challenging "Bi-drifting" Phenomenon in PSR J0815+09

A new drifting pulsar, PSR J0815+09, was discovered in the Arecibo drift-scan searches. An intriguing feature of this source is that within the four pulse components in the integrated pulse profile, the subpulse drifting direction in the two leading components is opposite from that in the two trailing components. In the view that the leading theoretical model for pulsar subpulse drifting can only interpret one-direction subpulse drifting, the observed bi-drifting phenomenon from PSR J0815+09 poses a great challenge to the pulsar theory. The inner annular gap (IAG), a new type of inner particle accelerator, was recently proposed to explain both γ-ray and radio emission from pulsars. Here we show that the coexistence of the IAG and the conventional inner core gap offers a natural interpretation to the bi-drifting phenomenon. In particular, the peculiar drifting behavior in PSR J0815+09 can be reproduced within the inverse Compton scattering model for pulsar radio emission.

Unraveling the drift behaviour of the remarkable pulsar PSR B0826–34

Pulsars with drifting subpulses are thought to be an important key to unlocking the mystery of how radio pulsars work. We present new results from high sensitivity GMRT observations of – a wide profile pulsar that exhibits an interesting but complicated drifting pattern. We provide a model to explain the observed subpulse drift properties of this pulsar, including the apparent reversals of the drift direction. In this model, is close to being an aligned rotator. Using information about the polarization and frequency evolution of the pulse profile, we solve for the emission geometry of this pulsar and show that the angle between the rotation axis and the dipole magnetic axis is less than 5°. As a result, our line of sight samples a circular path that is entirely within the emission beam. We see evidence for as many as 6 to 7 drifting bands in the main pulse at 318 MHz, which are all part of a circulating system of about 15 spark-associated subpulse emission beams that form, upon averaging, one conal ring of the mean emission. We also see evidence for a second ring of emission, which becomes dominant at higher frequencies (above 1 GHz) due to the nature of the emission geometry. We model the subpulse drift behaviour of this pulsar in detail, providing quantitative treatments of the aliasing problem and various effects of geometry which play an important role. The observed drift rate is an aliased version of the true drift rate which is such that a subpulse drifts to the location of the adjacent subpulse (or a multiple thereof) in about one pulsar period. We show that small variations, of the order of 3–8%, in the mean drift rate are then enough to explain the apparent reversals of drift direction seen in the data. We find the mean circulation time of the drift pattern to be significantly longer than the predictions of the original [CITE] model and propose an explanation for this, which relates to modified models with temperature regulated partial ion flow in the polar vacuum gap. The small variations in drift rate are then explained by very small heating and cooling effects – less than 3500 K change in the ~2.5 106 K surface temperature of the neutron star polar cap. From a detailed consideration of the variation of the mean subpulse separation across the main pulse window, we show that the circulating spark pattern is not centred around the dipole axis, but around a point much closer (within a degree or so) to the rotation axis. This is an indicator of the presence of a “local pole” corresponding to the non-dipolar magnetic fields that are expected to be present close to the neutron star surface. thus provides a very rich and powerful system in which to explore important aspects of the physics of pulsar radio emission and neutron star magnetospheres.

On the origin of giant pulses in radio pulsars

Induced Compton scattering of radio emission off the particles of the ultrarelativistic highly magnetized plasma of pulsar magnetosphere is considered. The scattering between the photon beams of substantially differing frequencies and orientations appears to result in photon transfer to the higher frequency. With the decreasing spectrum of pulsar radio emission, this may imply a significant amplification of the higher-frequency intensity. A detailed analysis shows that the rays of widely spaced frequencies can have different orientations, at least at certain locations in the pulsar magnetosphere. The numerical estimates confirm the possibility of an efficient beam-to-beam scattering in the pulsars known as giant pulse sources. Thus, the intensity amplification on account of induced scattering of the lower-frequency radiation is suggested to underlie the giant pulse phenomenon. A huge pulse-to-pulse scatter in the observed intensities of the giant pulse sources (over more than three orders of magnitude) testifies to significant variations of the efficiency of amplification. If this variability is attributed to the fluctuations of both the particle number density and the incident intensity and the latter are considered as the partially correlated Gaussian random quantities, one arrives at the power-law distribution of strong pulses in intensity. The formation of the giant pulse substructure, the broad-band spectral behaviour of the scattering efficiency and the link of giant pulses to the high-energy emission are discussed as well.

Circular polarization in pulsar radio emission due to intrinsically relativistic effects

Observations of single pulses imply that the circular polarization (CP) at a fixed pulsar phase, in some pulsars, can be large and variable from pulse to pulse. In many cases the variations can include changes in the handedness at a fixed phase. Possible explanations for the variable CP are sought in terms of elliptical polarization of the natural wave modes. The pulsar plasma is assumed to be a highly relativistically streaming, one-dimensional, strongly magnetized, electron–positron gas with a net charge density and intrinsically relativistic random motions in its rest frame. It is shown that in such a plasma the polarization of the natural modes includes two regimes where intrinsically relativistic effects determine the CP. One regime arises from aberration causing a small cone of CP about the backward (anti-streaming) direction in the rest frame to expand into the forward hemisphere in the pulsar frame. This mechanism can lead to significant CP at relatively high frequencies. The other mechanism arises from the intrinsically relativistic random motions, which allow a new regime of CP that has no counterpart in a cold plasma; however, for typical parameters this mechanism is effective only at frequencies higher than is consistent with our neglect of the cyclotron resonance. It is suggested that the explanation for the observed CP requires that the CP arise as a propagation effect, and be determined by the properties of the wave modes at a polarization limiting region.

A free-electron laser in the pulsar magnetosphere

We have studied systematically the free-electron laser in the context of high brightness pulsar radio emission. In this paper, we have numerically examined the case where a transverse electromagnetic wave is distorting the motion of a relativistic electron beam while travelling over one stellar radius (≈). For different sets of parameters, coherent emission is generated by bunches of beam electrons in the radio domain, with bandwidths of 3 GHz. Pulse power often reached , which corresponds with brightness temperature of . The duration of these pulses is of the order of nanoseconds. In the context of pulsar radio emission, our results indicate that the laser can produce elementary bursts of radiation which build up the observed microstructures of a few tens of microseconds duration. The process is sensitive mostly to the beam particles energy, number density and the background magnetic field, but much less so to the transverse wave parameters. We demonstrate that the operation of a free-electron laser with a transverse electromagnetic wiggler in the pulsar magnetosphere occurs preferably at moderate Lorentz factors , high beam density where is the Goldreich-Julian density at a stellar radius , and finally, at large altitude where the background magnetic field is low .

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.

Single pulses as emission from multiple subsources

Single pulses of pulsar radio emission are modelled as the superposition of radiation originating from many small subsources that are randomly distributed in the emission region. The individual subsources are given an intrinsic finite angular beaming. This model can produce fluctuations in intensity and pulse profiles that are similar to the microstructure observed in some pulsars. The statistics of the phase-resolved flux density of a simulated single pulse can be approximated by a lognormal distribution, which is in good agreement with observations.

The geometry of the double-pulsar system J0737-3039 from systematic intensity variations.

Two pulsars (PSR J0737-3039A and B) were recently discovered in highly relativistic orbits around one another. The system contains a rapidly rotating pulsar with a spin period of 22.7 ms and a slow companion with a spin period of 2.77 s, referred to here as 'A' and 'B', respectively. A unique property of the system is that the pulsed radio flux from B increases systematically by almost two orders of magnitude during two short portions of its orbit. Here we report a geometrical model of the system that simultaneously explains the intensity variations of B and provides constraints on the spin axis orientation and emission geometry of A. Our model assumes that B's pulsed radio flux increases when illuminated by emission from A. We predict that A's pulse profile will evolve considerably over the next several years owing to geodetic precession until it disappears entirely in 15-20 years.

Toward explanation of microstructure in pulsar radio emission

Propagation of relativistically beamed radio photons through the ultrarelativistic highly magnetized plasma of pulsar magnetosphere is considered. The photons are found to be subject to an additional strong focusing inside the beam because of induced Compton scattering off the plasma particles. Incorporating this focusing effect is shown to improve on the angular beaming model of pulsar microstructure. For the typical conditions in the scattering region, the photon beam appears up to a few hundred times squeezed and its size becomes compatible with the observed micropulse widths. The induced Compton scattering can also account for the polarization properties of micropulses. Furthermore, given a favourable observational geometry, the focusing effect can explain the giant micropulses observed in the Vela and some other pulsars.

The effect of differential refraction on wave propagation in rotating pulsar magnetospheres

Refraction of wave propagation in a corotating pulsar magnetospheric plasma is considered as a possible interpretation for observed asymmetric pulse profiles with multiple components. The pulsar radio emission produced inside the magnetosphere propagates outwards through the rotating magnetosphere, subject to refraction by the intervening plasma that is spatially inhomogeneous. Both effects of a relativistic distribution of the plasma and rotation on wave propagation are considered. It is shown that refraction coupled with rotation can produce asymmetric conal structures of the profile. The differential refraction due to the rotation can cause the conal structures to skew towards the rotation direction and leads to asymmetry in relative intensities between the leading and trailing components. Both of these features are potentially observable.

Rotational Sweepback of Magnetic Field Lines in Geometric Models of Pulsar Radio Emission

We study the rotational distortions of the vacuum dipole magnetic field in the context of geometric models of the radio emission from pulsars. We find that at low altitudes the rotation deflects the local direction of the magnetic field by at most an angle of the order of r, where rn = r/Rlc, r is the radial distance, and Rlc is the light cylinder radius. To the lowest (i.e., second) order in rn, this distortion is symmetric with respect to the plane containing the dipole axis and the rotation axis [ plane]. The lowest order distortion that is asymmetric with respect to the (Ω, μ) plane is third-order in rn. These results confirm the common assumption that the rotational sweepback has negligible effect on the position angle (P.A.) curve. We show, however, that the influence of the sweepback on the outer boundary of the open field line region (open volume) is a much larger effect, of the order of r. The open volume is shifted backward with respect to the rotation direction by an angle δov ~ 0.2 sin αr, where α is the dipole inclination with respect to the rotation axis. The associated phase shift of the pulse profile Δϕov ~ 0.2r can easily exceed the shift caused by combined effects of aberration and propagation time delays (≈2rn). This strongly affects the misalignment of the center of the P.A. curve and the center of the pulse profile, thereby modifying the delay-radius relation. Contrary to intuition, the effect of sweepback dominates over other effects when emission occurs at low altitudes. For rn ≲ 3 × 10-3 the shift becomes negative; i.e., the center of the P.A. curve precedes the profile center. With the sweepback effect included, the modified delay-radius relation predicts larger emission radii and is in much better agreement with the other methods of determining rn.

The Intrinsic Intensity Modulation of PSR B1937+21 at 1410 MHz

The single-pulse properties of the millisecond radio pulsar PSR B1937+21 are studied in the 1410 MHz radio band. Aside from occasional ``giant pulses'' occurring in restricted regions of pulse phase, the emission appears to be remarkably stable, showing no pulse-to-pulse fluctuations other then those induced by propagation through the interstellar medium. This type of behavior has not been seen in any other pulsar although it was seen in previous 430 MHz observations of this source. The stability of PSR B1937+21 can be understood in the context of the sparking gap model of radio pulsar emission. Given the emission properties of this source at 430 MHz, this model predicts that the emission at all higher frequencies will be just as stable. Since the stability depends on the outflow velocity of the emitting plasma, an upper bound may be placed on its Lorentz factor.

Two scales for the frequency dependence of the scattering of pulsar radio emission

We have measured the pulse broadening by scattering at 40, 60, and 111 MHz for the pulsars PSR B0809+74, B0950+08, B1919+21, and B2303+30. The frequency dependence of the scatter-broadening parameter is analyzed based on these measurements and data from the literature. The dependence obtained purely from the literature data is not consistent with the theory, and the scattering magnitudes differs considerably from the data of the catalog of 706 pulsars of Taylor et al. A two-component model for the frequency dependence of the scattering of the pulsar radio emission in the interstellar medium is proposed. Allowing for the presence of two scattering scales removes both inconsistencies between the observational data for these four pulsars and differences between the observed and theoretical frequency dependences for the scattering, as well as the need to invoke anomalous scattering magnitudes. The data of the catalog of Taylor et al. need to be corrected for the difference in the scattering magnitudes in the two branches of the frequency dependence.

On the Excess Dispersion in the Polarization Position Angle of Pulsar Radio Emission

The polarization position angles (P.A.'s) of pulsar radio emission occupy a distribution that can be much wider than what is expected from the average linear polarization and the off-pulse instrumental noise. Contrary to our limited understanding of the emission mechanism, the excess dispersion in P.A. implies that pulsar P.A.'s vary in a random fashion. An eigenvalue analysis of the measured Stokes parameters is developed to determine the origin of the excess P.A. dispersion. The analysis is applied to sensitive, well-calibrated polarization observations of PSR B1929+10 and PSR B2020+28. The analysis clarifies the origin of polarization fluctuations in the emission and reveals that the excess P.A. dispersion is caused by the isotropic inflation of the data-point cluster formed from the measured Stokes parameters. The inflation of the cluster is not consistent with random fluctuations in P.A., as might be expected from random changes in the orientation of the magnetic field lines in the emission region or from stochastic Faraday rotation in either the pulsar magnetosphere or the interstellar medium. The inflation of the cluster, and thus the excess P.A. dispersion, is attributed to randomly polarized radiation in the received pulsar signal. The analysis also indicates that orthogonal polarization modes (OPMs) occur where the radio emission is heavily modulated. In fact, OPMs may occur only where the modulation index exceeds some critical value, βc 0.3.

Curvature Radiation in Pulsar Magnetospheric Plasma

We consider the curvature radiation of a pointlike charge moving relativistically along curved magnetic field lines through a pulsar magnetospheric electron-positron plasma. We demonstrate that the radiation power is largely suppressed compared with the vacuum case, but still at a considerable level, high enough to explain the observed pulsar luminosities. The outgoing waves are polarized perpendicularly to the plane of the dipolar magnetic field lines. Our results strongly support coherent curvature radiation by the spark-associated solitons as a plausible mechanism of pulsar radio emission.

Correlations Between X-ray and Radio Pulses in Vela

Pulse-shape changes correlating with flux have been shown to exist in the Vela pulsar's radio emission (Krishnamohan & Downs 1983). The aim of this work was to look for correlations between radio and X-ray flux in Vela on a pulse-by-pulse basis. We found that Vela emits significantly more X-rays during radio pulses with strong flux than during weak radio flux pulses, which indicates that the mechanisms creating the Vela pulsar's X-ray and radio emission must be related.

Orthogonal Polarization Modes in Pulsar Radio Emission

General features of polarization transfer in the plasma of pulsar magnetospheres are outlined. A technique of plasma density diagnostics based on observed polarization profiles of radio pulses is developed. For the first time, it appears possible to obtain the profiles of the plasma density across the open field line tube from observations. The multiplicities derived are compatible with those predicted by modern theories of pair cascade and show a perfect exponential decrease towards the tube edge. Implications of the results are briefly discussed.

A Model for Non-thermal Optical Emission from Radio Pulsars

Interpretation of the non-thermal optical emission of pulsars as a spontaneous synchrotron re-emission of the secondary particles involved in the process of resonant absorption of pulsar radio emission is suggested. Our model can explain the non-thermal high-energy luminosities and spectra of both young and middle-aged pulsars. Moreover, it implies a physical link between the radio and higher-energy emission of pulsars.

Centrifugal Acceleration in Pulsar Magnetospheres

We present a relativistic model of pulsar radio emission by plasma accelerated along the rotating magnetic field lines projected on to a 2D plane perpendicular to the rotation axis. We have derived the expression for the trajectory of a particle, and estimated the spectrum of radio emission by the plasma bunches. We used the parameters given by Peyman & Gangadhara (2002). The analytical expressions for the Stokes parameters are obtained, and their values compared with the observed profiles. The one sense of circular polarization, observed in many pulsars, can be explained in light of our model.