European Pulsar Network Research Articles

Topology of Pulsar Profiles (ToPP)

Some of the most important information on a radio pulsar is derived from its average pulse profile. Many early pulsar studies were necessarily based on only a few such profiles. In these studies, discrete profile components were linked to emission mechanism models for individual stars through human interpretation. For the population as a whole, profile morphology must reflect the geometry and overall evolution of the radio emitting regions. The problem, however, is that this population is becoming too large for individual intensive studies of each source. Moreover, connecting profiles from a large collection of pulsars rapidly becomes cumbersome. In this article, we present ToPP, the first-ever unsupervised method to sort pulsars by profile-shape similarity using graph topology. We applied ToPP to the publicly available European Pulsar Network profile database, providing the first organised visual overview of multi-frequency profiles representing 90 individual pulsars. We found discrete evolutionary tracks varying from simple single-component profiles at all frequencies towards diverse mixtures of more complex profiles with frequency evolution. The profile evolution is continuous, extending out to millisecond pulsars, and does not fall into sharp classes. We interpret the profiles as being a mixture of pulsar core-cone emission type, spin-down energetics, and the line-of-sight impact angle towards the magnetic axis. We show how ToPP can systematically classify sources into the Rankin empirical profile scheme. ToPP comprises one of the key unsupervised methods that will be essential to exploring upcoming pulsar census data, such as the data expected from the Square Kilometer Array.

Frequency Evolution Behavior of Pulse Profile of PSR B1737+13 with the Inverse Compton Scattering Model

Abstract The radio radiation mechanism is one of the open questions in pulsar physics. Multiband observations are very important for constraining the pulsar radiation mechanism. In this paper, we investigate the pulse profiles of PSR B1737+13 and its evolution with the frequency. The integrated pulse profiles are obtained from the European Pulsar Network and the Australia Telescope National Facility data, together with recent observations from the largest dish Five-hundred-meter Aperture Spherical radio Telescope. The radiation components are separated with the squared hyperbolic secant functions, and the radiation altitudes of each radiation component at different frequencies are calculated. It is found that the radio radiation at different frequencies comes from different altitudes. The frequency evolutions of separations for the inner and outer cone components are studied. It is found that the separations of the inner and outer cone components have opposite frequency dependence. We simulate the RFM of PSR B1737+13 with the inverse Compton scattering (ICS) model and find that the RFM can be naturally described by the ICS model. Through the simulation, the radio radiation region of PSR B1737+13 is determined, and the result shows that the radio radiation of this pulsar may be generated in the annular gap region.

Synthetic Pulsar (Speculative Sonification)

Synthetic Pulsar (Speculative Sonification) looks at the pulsar as a dazzling multi-dimensional object of scientific and creative focus. While the pulsar tra- verses a wide range of disciplines – astrophysics, radio technique, and sound technology – the workings and nature of such an object can never be fully cap- tured, hence remaining incompletely understood. Proposed work probes this complexity through a process of speculative data sonification: amplifying, mod- ulating and augmenting astrophysical data - sourced from the European Pulsar Network - within an original implementation of pulsar synthesis program (nuPG) designed by the author.

A Systematic Study of the Frequency Evolution Behavior of Pulsar Pulse Profiles

Abstract The frequency evolution behavior of radio pulse profiles is an important clue for the radiation mechanism of a pulsar. Combined with the latest results of the European Pulsar Network and the Australia Telescope National Facility data, we systematically study the evolution of W 50 (the full width of the pulse profile at 50% of the pulsar amplitude) for 74 pulsars with frequencies from tens to thousands of megahertz. We find that 71 pulsars show “absorption” features, which indicates that the absorption is an universal phenomenon in the pulsar population and independent of the type of pulsar profile. It is found that for most pulsars, the absorption features appear in the frequency range of 100–800 MHz, and the evolution trend of W 50 at frequencies less than ∼100 MHz and larger than ∼800 MHz can be described as a power-law function.We quantify the absorption intensity and analyze the correlation between the absorption intensity and the pulsar parameters (such as the spin period, magnetic field, and age of the pulsar). It is found that there is no direct correlation between them. Our results will provide good samples and clues for the study of the physical mechanism of the absorption phenomenon.

A Simulation of Radius-frequency Mapping for PSR J1848-0123 with an Inverse Compton Scattering Model

Abstract The radio radiation mechanism is one of the open questions for pulsars. Multiband observations are very helpful to constrain pulsar radiation models. We observed PSR J1848-0123 at the 5 GHz band with the Shanghai Tianma 65 m radio telescope (TMRT). The 5 GHz observation together with the European Pulsar Network (EPN) and CSIRO ATNF Data Archives databases provides a broadband evolution property of the pulse profiles for this pulsar. We reprocessed these data sets to obtain the multiband pulse profiles. It was found that the pulse profile of PSR J1848-0123 narrows with increasing emission frequency. We separated the emission components of each pulse profile with a multi-Gaussian fitting method, and then calculated the beaming angle and emission radius of each component with the basic radiation geometry of pulsar. With the calculated beam angles and radiation radii, we simulated the radius-frequency mapping of this pulsar with an inverse Compton scatting model (ICS). It was found that the observed radio radiation could be generated in the core gap region above the polar cap of the pulsar. The beaming angle and emission radius versus frequency for PSR J1848-0123 can be explained well with the ICS model.

A LOFAR census of non-recycled pulsars: average profiles, dispersion measures, flux densities, and spectra

We present first results from a LOFAR census of non-recycled pulsars. The census includes almost all such pulsars known (194 sources) at declinations ${\rm Dec}> 8^\circ$ and Galactic latitudes $|{\rm Gb}| > 3^\circ$, regardless of their expected flux densities and scattering times. Each pulsar was observed for $\geq 20$ minutes in the contiguous frequency range of 110--188 MHz. Full-Stokes data were recorded. We present the dispersion measures, flux densities, and calibrated total intensity profiles for the 158 pulsars detected in the sample. The median uncertainty in census dispersion measures ($1.5 \times 10^{-3}$ pc cm$^{-3}$) is ten times smaller, on average, than in the ATNF pulsar catalogue. We combined census flux densities with those in the literature and fitted the resulting broadband spectra with single or broken power-law functions. For 48 census pulsars such fits are being published for the first time. Typically, the choice between single and broken power-laws, as well as the location of the spectral break, were highly influenced by the spectral coverage of the available flux density measurements. In particular, the inclusion of measurements below 100 MHz appears essential for investigating the low-frequency turnover in the spectra for most of the census pulsars. For several pulsars, we compared the spectral indices from different works and found the typical spread of values to be within 0.5--1.5, suggesting a prevailing underestimation of spectral index errors in the literature. The census observations yielded some unexpected individual source results, as we describe in the paper. Lastly, we will provide this unique sample of wide-band, low-frequency pulse profiles via the European Pulsar Network Database.

FREQUENCY DEPENDENCE OF PULSE WIDTH FOR 150 RADIO NORMAL PULSARS

The frequency dependence of pulse width is studied for 150 normal pulsars, mostly selected from the European Pulsar Network, for which the 10\% multifrequency pulse widths can be well fit with the Thorsett relationship W_{10}=A\nu^{\mu}+W_{\rm 10, min}. The relative fraction of pulse width change between 0.4~GHz and 4.85~GHz, \eta=(W_{4.85}-W_{0.4})/W_{0.4}, is calculated in terms of the best-fit relationship for each pulsar. It is found that 81 pulsars (54\%) have $\eta<-10$\% (group A), showing considerable profile narrowing at high frequencies, 40 pulsars (27\%) have $-$10\%$\leq\eta\leq 10$\% (group B), meaning a marginal change in pulse width, and 29 pulsars (19\%) have $\eta>10$\% (group C), showing a remarkable profile broadening at high frequencies. The fractions of the group-A and group-C pulsars suggest that the profile narrowing phenomenon at high frequencies is more common than the profile broadening phenomenon, but a large fraction of the group-B and group-C pulsars (a total of 46\%) is also revealed. The group-C pulsars, together with a portion of group-B pulsars with a slight pulse broadening, can hardly be explained using the conventional radius-to-frequency mapping, which only applies to the profile narrowing phenomenon. Based on a recent version of the fan beam model, a type of broadband emission model, we propose that the diverse frequency dependence of pulse width is a consequence of different types of distribution of emission spectra across the emission region. The geometrical effect predicting a link between the emission beam shrinkage and the spectrum steepening is tested but disfavored.

Feasibility analysis for attitude estimation based on pulsar polarization measurement

One of the important characteristics of pulsar radiation is polarization. It is considered not only as a probe for recognizing the structure of a magnetic field, but also as a lighthouse for estimating spacecraft attitude via orientation information between the pulsar and the detector. Although polarization of a pulsar has been studied for decades, until recently applications to determination of spacecraft attitude have been seldom reported. This paper deals with analysis of the feasibility of applying polarization information to attitude estimation. The stability factor (SFR) and observation fluctuation factor (OFR) are introduced to analyze the stability of a pulsar’s polarized position angle. Based on European Pulsar Network (EPN) data, several simulated instances are used to demonstrate that the accuracy requirement of attitude determination can be met via polarization measurement. The SFR of a pulsar is evaluated using simulated polarization data, and the OFR is used to analyze the relationship between fluctuation extent and observation time. Simulation results show that the polarized measurement of candidate pulsars PSR B0470-28 and PSR B2319+60 reaches the specification for attitude determination.

Stokes tomography of radio pulsar magnetospheres - I. Linear polarization

Polarimetric studies of pulsar radio emission traditionally concentrate on how the Stokes vector (I, Q, U, V) varies with pulse longitude, with special emphasis on the position angle (PA) swing of the linearly polarized component. The interpretation of the PA swing in terms of the rotating vector model is limited by the assumption of an axisymmetric magnetic field and the degeneracy of the output with respect to the orientation and magnetic geometry of the pulsar; different combinations of the latter two properties can produce similar PA swings. This paper introduces Stokes phase portraits as a supplementary diagnostic tool with which the orientation and magnetic geometry can be inferred more accurately. The Stokes phase portraits feature unique patterns in the I-Q, I-U, and Q-U planes, whose shapes depend sensitively on the magnetic geometry, inclination angle, beam and polarization patterns, and emission altitude. We construct look-up tables of Stokes phase portraits and PA swings for pure and current-modified dipole fields, filled core and hollow cone beams, and two empirical linear polarization models, L/I = \cos \theta_0 and L/I = \sin \theta_0, where \theta_0 is the colatitude of the emission point. We compare our look-up tables to the measured phase portraits of 24 pulsars in the European Pulsar Network online database. We find evidence in 60% of the objects that the radio emission region may depart significantly from low altitudes, even when the PA swing is S-shaped and/or the pulse-width-period relation is well satisfied. On the other hand, the data are explained adequately if the emission altitude exceeds ~10% of the light cylinder radius. We conclude that Stokes phase portraits should be analysed concurrently with the PA swing and pulse profiles in future when interpreting radio pulsar polarization data.

On the aberration-retardation effects in pulsars

The magnetospheric locations of pulsar radio emission region are not well known. The actual form of the so--called radius--to--frequency mapping should be reflected in the aberration--retardation (A/R) effects that shift and/or delay the photons depending on the emission height in the magnetosphere. Recent studies suggest that in a handful of pulsars the A/R effect can be discerned w.r.t the peak of the central core emission region. To verify these effects in an ensemble of pulsars we launched a project analysing multi--frequency total intensity pulsar profiles obtained from the new observations from the Giant Meterwave Radio Telescope (GMRT), Arecibo Observatory (AO) and archival European Pulsar Network (EPN) data. For all these profiles we measure the shift of the outer cone components with respect to the core component which is necessary for establishing the A/R effect. Within our sample of 23 pulsars 7 show the A/R effects, 12 of them (doubtful cases) show a tendency towards this effect, while the remaining 4 are obvious counter examples. The counter--examples and doubtful cases may arise from uncertainties in determination of the location of the meridional plane and/or the core emission component. It hence appears that the A/R effects are likely to operate in most pulsars from our sample. We conclude that in cases where those effects are present the core emission has to originate below the conal emission region.

Radio spectrum of the AXP J1810−197 and of its profile components

As part of a European Pulsar Network (EPN) multi-telescope observing campaign, we performed simultaneous multi-frequency observations at 1.4, 4.9 and 8.4 GHz during July 2006 and quasi-simultaneous multi-frequency observations from Decem- ber 2006 until July 2007 at 2.7, 4.9, 8.4, 14.6 and 32 GHz, in order to obtain flux density measurements and spectral features of the 5.5-sec radio-emitting magnetar AXP J1810-197. We monitored the spectral evolution of its pulse shape which consists of a main pulse (MP) and an interpulse (IP). We present the flux density spectrum of the average profile and of the separate pulse components of this first-known radio-emitting transient anomalous X-ray pulsar. We observe a decrease of the flux density by a factor of 10 within 8 months and follow the disappearance of one of the two main components. Although the spectrum is generally flat, we observe large fluctuations of the spectral index with time. For that reason we have made some measurements of modulation indices for individual pulses in order to also investigate the origin of these fluctuations.

Frequency dependence of orthogonal polarisation modes in pulsars

We have carried out a study of the orthogonal polarisation mode behaviour as a function of frequency of 18 pulsars, using average pulsar data from the European Pulsar Network (EPN). Assuming that the radiation consists of two 100% polarised completely orthogonal superposed modes we separated these modes, resulting in average pulse profiles of each mode at multiple frequencies for each pulsar. Furthermore, we studied the frequency dependence of the relative intensity of these modes. We found in many pulsars that the average pulse profiles of the two modes differ in their dependence on frequency. In particular, we found that pulse components that are dominated by one mode tend to increase in intensity with increasing frequency with respect to the rest of the profile.

Simultaneous single-pulse observations of radio pulsars

We present the first results from a programme of multi-frequency simultaneous single pulse observations carried out as part of the European Pulsar Network. We detail the main data analysis methods and apply them to simultaneous observations of the strong pulsar B0329+54 at 1.4 and 2.7 GHz using the Jodrell Bank and Effelsberg radio telescopes respectively. The pulses at different frequencies are highly correlated in their total intensity, as seen in previous experiments, and generally show consistent position angles of the linearly polarized component. In contrast, the circularly polarized emission sometimes shows clear differences between pulses received at different frequencies. These results are unexpected and warrant further follow-up studies to interpret them in the context of the intrinsic bandwidth of pulsar radiation.