PSR B0329 Research Articles

Intermittent nulls in PSR B0818-13, and the subpulse-drift alias mode

We show that all long nulls in PSR B0818-13 are trains of rapidly alternating nulls and pulses (each shorter than one pulse period). Sometimes only the nulls coincide with our pulse window, resulting in one of the apparently long nulls seen occasionally. We show these are seen as often as expected if during such a train the probability for nulls is 1.2 times less than for pulses. During nulls, the subpulse drift-speed appears to increase. We assume that the carousel of sparks that possibly underlies the subpulses actually slows down, as it does in similar pulsars like PSR B0809+74, and conclude that the subpulse-drift in this pulsar must be aliased. The carousel must then rotate in 30 seconds or less, making it the fastest found to date.

Spectral and timing properties of the X-ray emission from the millisecond pulsar PSR B1821-24

We report results on the timing and spectral analysis of observations of the millisecond pulsar PSR B1821−24 with RXTE, BeppoSAX and Chandra. The X-ray light curve is characterized by two narrow peaks at a phase distance of 0.452 ± 0.002. The average pulsed emission, over the range 1.6-20 keV, is well represented by a single power law with a photon index α = 1.30 −0.02 +0.05 and unabsorbed (2-10 keV) pulsed X-ray flux of 3.9 × 10 −13 erg cm −2 s −1 . We searched for a possible bunching of X-ray photons to verify if the X ray emission has a time structure similar to that of giant pulses and found a negative result.

Properties and Interpretations of Giant Micropulses and Giant Pulses from Pulsars

Giant pulses and giant micropulses from pulsars are distinguished from normal pulsed emission by their large fluxes, rarity, approximately power-law distribution of fluxes and, typically, occurrence in restricted phase windows. Here existing observations of flux distributions are manipulated into a common format and interpreted in terms of theories for wave growth in inhomogeneous media, with the aim of constraining the emission mechanism and source physics for giant pulses and micropulses. Giant micropulses near 2 GHz (PSRs B8033-45 and B1706-44) and 0.4 GHz (PSR B0950+08) have indices $\alpha = 6.5 \pm 0.7$ for the probability distribution $P(E)$ of the electric field $E$, with $P(E) \propto E^{-\alpha}$. Giant pulses (PSRs B0531+24, B1937+214, and B1821-24) have $\alpha$ ranging from $4.6 \pm 0.2$ to $9 \pm 2$, possibly increasing with frequency. These are similar enough to regard giant micropulses and pulses as a single phenomenon with a common physical explanation. The power-law functional form and values of $\alpha$ observed are consistent with predictions for nonlinear wave collapse, but inconsistent with known self-organized critical systems, nonlinear decay processes, and elementary burst theory. While relativistic beaming may be important, its statistics are yet to be predicted theoretically and collapse is currently the favored interpretation. Other possibilities remain, including stochastic growth theory (consistent with normal pulse emission) and, less plausibly, refractive lensing. Unresolved issues remain for all four interpretations and suggestions for further work are given. The differences between normal and giant pulse emission suggest they have distinct source regions and emission processes.

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.

Ellipticity and deviations from orthogonality in the polarization modes of PSR B0329+54

We report on an analysis of the polarization of single pulses of PSR B0329+54 at 328 MHz. We find that the distribution of polarization orientations in the central component diverges strongly from the standard picture of orthogonal polarization modes (OPMs), making a remarkable partial annulus on the Poincare sphere. A second, tightly clustered region of density appears in the opposite hemisphere, at a point antipodal to the centre of the annulus. We argue that this can be understood in terms of birefringent alterations in the relative phase of two elliptically polarized propagation modes in the pulsar magnetosphere (i.e. generalised Faraday rotation). The ellipticity of the modes implies a significant charge density in the plasma, while the presence of both senses of circular polarization, and the fact that only one mode shows the effect, supports the view that refracted ordinary-mode rays are involved in the production of the annulus. At other pulse longitudes the polarization (including the circular component) is broadly consistent with an origin in elliptical OPMs, shown here quantitatively for the first time, however considerable non-orthogonal contributions serve to broaden the orientation distribution in an isotropic manner.

Chandra X-Ray Observatory observations of the globular cluster M28 and its millisecond pulsar PSR B1821–24

Abstract We discuss the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). We detect 46 X-ray sources of which 12 lie within one core radius of the center. We measure the radial distribution of the X-ray sources and fit it to a King profile finding a core radius of r c , x ≈11″. We measure for the first time the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821–24 and find it is best described by a power law with photon index Γ≃1.2. We find marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum, which could be interpreted as cyclotron emission from a corona above the pulsar's polar cap if the magnetic field is strongly different from a centered dipole. We present a spectral analyses of the brightest unidentified source and suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core.

Analysis of the scintillation pattern of the pulsar PSR B0329+54 based on VLBI data at 1650 MHz

The velocity of motion of the diffraction pattern of the pulsar PSR B0329+54 at 1.6 GHz has been measured. Using a specialized method for the amplitude calibration of the VLBI data, scintillation curves were obtained for the ten ground antennas that took part in the experiment jointly with the orbiting antenna HALCA. The session lasted 12 hours. Our analysis shows that the pulsar has two timescales, 20 and 1 min. The velocity of the scintillation pattern was determined by measuring the shift of the maximum of the scintillation-curve cross-correlation function relative to zero. It turned out that the scintillation spots move independently from each other: there is no ordered motion of the diffraction pattern as a whole. The measured drift velocities are from 25 to 40 km/s; the average magnitude of the drift velocity is 39±15 km/s, whereas the magnitude precalculated from the pulsar proper motion taking into account the Earth's motion around the Sun and the Sun's motion with respect to the Local Standard of Rest is 122 km/s. A thin-screen model is unable to provide an adequate description of this behavior.

Properties of the linearly polarized radiation from PSR B0950+08

Measurements of average pulse profiles made with a single linear polarization over the range 41-112 MHz are presented for PSR B0950+08. We show that the observed variable structure of the pulse profiles is a result of Faraday sinusoidal modulation of the pulse intensity with frequency. The rotation measure corresponding to this effect, RM ≈ 4r ad m −2 , is about 3 times greater than the published value of RM = 1.35 rad m −2 (Taylor et al. 1993).

Detection of giant pulses from the pulsar PSR B0031-07

Giant pulses have been detected from the pulsar PSR B0031-07. A pulse with an intensity higher than that of the average pulse by a factor of 50 or more is encountered approximately once per 300 observed periods. The peak flux density of the strongest pulse was 530 Jy, which is a factor of 120 higher than the peak flux density of the average pulse. The giant pulses are a factor of 20 narrower than the integrated profile and are clustered about its center.

Multiband optical observations of the old PSR B0950+08

We report on deep UBVRI -imaging of the field of the 17.3 Myr radiopulsar PSR B0950+08 obtained with the ESO/VLT/FORS1. Firm detection of the candidate pulsar optical counterpart in the BVRI bands with the magnitudes $B=27.06$ ± 0.35, $V=27.05$ ± 0.15, $R_{\rm c}=26.49$ ± 0.10 and $I_{\rm c}=26.20$ ± 0.17 enabled us to study, for the first time, the broadband spectrum of the object. The derived ~$0\farcs24$ offset of the object from the radiopulsar position is insignificant within the $ 0\farcs21$ uncertainties of our astrometry and ~$0\farcs75$ seeing value during the observations. The positional coincidence and unusual colors of the candidate ensure us that we likely detect the optical emission from PSR B0950+08. The optical-near-UV spectrum of the pulsar has a negative slope and can be fitted by a power law ${F_\nu \propto \nu^{-\alpha}}$ with ${\rm \alpha=0.65}$ ± 0.40, suggesting nonthermal emission. Within errors the optical flux is consistent with the power law fit of the ROSAT spectrum from the X-ray counterpart. We analyze our results together with the available multiwavelength data on other isolated pulsars of different ages detected in the optical range and find a significant correlation between their optical and 2–10 keV X-ray luminosities. This implies an origin of the nonthermal emission in both spectral domains. These objects show a significantly non-monotonic evolution of the efficiency of the optical-X-ray photon production from the pulsar spindown power, with a pronounced minimum at the beginning of the middle-age epoch and comparably high efficiencies of younger and older pulsars. This suggests different sensitivities of the photon production processes to the Goldreich-Julian current in magnetospheres of different age pulsars.

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.

Outer-magnetospheric model for Vela-like pulsars: formation of sub-GeV spectrum

We investigate the γ-ray emission from an outer-gap accelerator, which is located in the outer magnetosphere of a pulsar. The charge depletion from the Goldreich–Julian density causes a large electric field along the magnetic field lines. The electric field accelerates electrons and positrons to high energies. We solve the formation of the electric field self-consistently with the radiation and the pair creation processes in one-dimensional geometry along the magnetic field lines, and calculate the curvature spectrum for comparison with the observations. We find that, because the particles escape from the accelerating region with Lorentz factors as large as 107.5, curvature radiation from outside of the gap, in which there is no electric field, still makes an important contribution to the spectrum. Including the outside emission, the model spectra are improved between 100 MeV and 1 GeV for the Vela-type pulsars. Since field line curvature close to the light cylinder affects the spectrum at several hundred MeV, there is a possibility to diagnose the field structure by using the spectrum in these bands. The difference found in the γ-ray spectra of the two pulsars, PSR B0833−45 (Vela) and B1706−44, is deduced from the dimensionless electric current running through the gap. The extension to three-dimensional modelling with self-consistent electrodynamics is necessary to understand the difference in the observed flux between the two pulsars.

Phase-resolved Spectroscopy of the Crab Pulsar with XMM-Newton

The Crab pulsar (PSR B0531+21) has been observed with the EPIC-pn camera on board the ESA XMM-Newton satellite. The high time resolution of the EPIC camera (7 μs) in its special Burst mode in combination with the huge effective area and the high quantum efficiency of the pn-CCD allows phase resolved spectroscopy of good quality over the energy range 0.6–8.5 keV. We performed phase resolved spectroscopy of the modulated flux of the Crab pulsar and found a significant hardening of the photon index in the inter-pulse region. Also the second pulse is harder in index by 0.1 than the first pulse.

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.

Flux Monitoring Using the Toruń 32-m Radio Telescope

We present flux density measurements for PSR B0329+54 at 4.8 GHz obtained during one year.

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

The frequency-dependence of drifting subpulse patterns

Drifting subpulse patterns in pulsar signals are frequently interpreted in terms of a model in which a rotating ring of sparks on the polar cap gives rise to emission from regions of the magnetsophere connected to the sparks by dipolar magnetic field lines. The spacing of drift-bands in time depends on the circulation rate of the polar cap pattern, but to first order the longitudinal phase dependence of the subpulse modulation should obey a frequency-independent relation determined by the geometrical configuration in a similar manner to the polarization position angle. We present here observations at 272-1380 MHz of PSR B0320+39 and PSR B0809+74, both of which show nearly linear drift in two longitude regions, separated by a region of reduced modulation and accompanied by a large step in the phase of the subpulse pattern. We show that the observation of Bartel et al. (1981) that the subpulse spacing for PSR B0809+74 was 1.8 times greater at 102.5 MHz than at 1720 MHz is most likely an artifact of the phase step, which is only present at high frequencies. The phase steps can be understood as a consequence of observing overlapping offset images of the polar cap spark pattern. We also detected more complicated, frequency-dependent behaviour that would require that the images do not simply differ by rotation about their centers. Detailed modelling of non-axisymmetric refraction or distorted magnetic fields is suggested as a means of pursuing an explanation for this phenomenon.

High temporal resolution observations of the pulsar microstructure at 102 MHz

We report observations of the microstructure of 7 pulsars at 102 MHz with a time resolution of up to 1 μs. We have detected fine pulse structure with a μs time-scale for all seven pulsars. We observe that interstellar scattering does not broaden the μs time-scale pulsar microstructure, and find structure of a time-scale much shorter than the interstellar scatter broadening. In addition to the previously known structure with time-scale τ μ of tens of μs, we have detected structures with characteristic time-scale of a few μs, nearly equal to the reciprocal of the analysis bandwidth. This suggests that even shorter micropulses may exist. Micropulses usually cluster in groups, each of duration tens to hundreds of μs, which may be identified as microstructure observed previously with smaller time resolution. In addition to the previously known quasi-periodicity of hundreds of milliseconds, we reveal a structures with characteristic period of tens of μs. We observe structures with short quasi-periodicity to have a limited bandwidth. The characteristic period P μ of these structures in PSR B0950+08 does not have a common value for different frequency regions in the same single pulse. Scattering was revealed as a train of quasi-periodic micropulses with an exponentially damped envelope. The scatter-broadening and decorrelation bandwidth were measured. We report two time-scales of decorrelation bandwidth of pulsar PSR B0950+08 and propose two branches of Δv d (v) dependence. The dispersion measure of PSR B0950+08 as DM = 2.9686 ′ 0.0001 pc cm - 3 has been determined.

Simultaneous single-pulse observations of radio pulsars

In this paper we demonstrate that a large, unexplored reservoir of information about pulsar emission exists, that is directly linked to the radiating particles and their radiation process: We present a study of flux density measurements of individual pulses simultaneously observed at four different frequencies. Correcting for effects caused by the interstellar medium, we derive intrinsic flux density spectra of individual radio pulses observed at several frequencies for the first time. Pulsar B0329+54 was observed at 238, 626, 1412 and 4850 MHz, while observations of PSR B1133+16 were made at 341, 626, 1412 and 4850 MHz. We derive intrinsic pulse-to-pulse modulation indices which show a minimum around 1 GHz. Correlations between the flux densities of different frequency pairs worsen as the frequency separation widens and also tend to be worse for outer profile components. The single pulse spectra of PSR B0329+54 resemble the spectra of the integrated profile. However, the spectral index distributions for the single pulses of PSR B1133+16 show significant deviations from a Gaussian. This asymmetry is caused by very strong pulses with flux densities exceeding the mean value by more than a factor of ten. These strong pulses occur preferentially at the trailing edge of the leading component and appear to be broadband in most cases. Their properties are similar to those of so-called giant pulses, suggesting that these phenomena are related.