Pulsar Rotation Research Articles

The long-term rotation dynamics of neutron stars with differentially rotating unmagnetized core

We consider the pulsar long-term rotation dynamics taking into account the non-rigidity of neutron star rotation. We restrict our attention to the models with two essential assumptions: (1) crust–core interaction occurs via the viscosity (magnetic coupling is not important); (2) neutron star shape is symmetrical over the magnetic axis. The neutron star core is described by linearized quasi-stationary Newtonian hydrodynamical equations in one-fluid and two-fluid (neutron superfluidity) approximations. It is shown that in this case the pulsar inclination angle evolves to 0 ◦ or 90 ◦ very quickly. Since such fast evolution seems to contradict the observation

Magnetocentrifugal acceleration of bulk motion of plasma in pulsar magnetosphere

Acceleration of bulk motion of plasma due to magneto centrifugal mechanism is investigated for different shapes of the field lines of 3-D magnetic field. It is shown that this mechanism can be efficient provided that the field line is twisted into direction of rotation. According to the last results of the numerical modelling a fraction of the field lines with such geometrical shape apparently exist in the pulsar magnetosphere close to the last closed field line. In this case the Lorentz factor $\gamma$ increases with the radius $r$ along these field lines as $\gamma\sim (1-(r\Omega/c)^2)^{-1}$, where $\Omega$ - angular velocity of the pulsar rotation. The magneto centrifugal mechanism provides acceleration of the electrons to the energy which is sufficient to explain the observed VHE pulsed $\gamma$-rays from Crab pulsar.

Testing the rotating lighthouse model with the double pulsar system PSR J0737-3039A/B

Each of the two pulsars in the double pulsar PSR J0737-3039A/B system exhibits not only the pulses emanating from itself, but also displays modulations near the pulse period of the other. Freire et al. (2009, MNRAS, 396, 1764) have put forward a technique using the modulation of B by A to determine the sense of rotation of pulsar A relative to its orbital motion, among other quantities. In this paper, we present another technique with the same purpose. While the Freire et al. approach analyzes pulse arrival times, ours instead uses periods or frequencies (their inverses), which can be experimentally determined via power spectral analysis similar to that used in pulsar searches. Our technique is based on the apparent change in spin period of a body when it is measured from an orbiting platform (the other pulsar), and is shown to be entirely analogous to the difference between the sidereal and solar spin period of the Earth (i.e., the sidereal and solar day). Two benefits of this approach are its conceptual and computational simplicity. The direct detection of spin with this technique will observationally validate the rotating lighthouse model of pulsar emission, while the detection of the relative directions of spin and orbital angular momenta has important evolutionary implications. Our technique can be used on other binary systems exhibiting mutually induced phenomena.

TIMING OBSERVATIONS OF 27 PULSARS AT THE PUSHCHINO OBSERVATORY FROM 1978 TO 2012

We present results from timing observations of 27 pulsars made at the Pushchino Observatory over 33.5 yr between 1978 July and 2012 February. We also analyze archival Jet Propulsion Laboratory data of 10 pulsars to extend individual data span to 43.5 yr. We detected a new phenomenon in the timing behavior of two pulsars, B0823+26 and B1929+10, that demonstrates a rapid change of pulsar rotation parameters such that the sign of the second derivative $\ddot\nu$ is reversed. An analysis of the $\ddot\nu$ changes showed that this process can be considered as a modulation process in $\ddot\nu$. We showed that the process of rapidly changing of pulsar rotation parameters represents a new type of rotational irregularity that, together with three other types of rotational irregularities (discrete glitches, slow glitches and quasi-periodic oscillations), forms a large-scale structure of timing noise. These effects are all the cause of the deviation of the timing behavior of most ordinary pulsars from a simple $\nu,\,\dot\nu$ spin-down model. We found that all four types of observed rotational irregularities have evolving nature. Irregularities in pulsar rotation rate pass through three evolutional stages that show that a certain type of rotational irregularity can occur only at a certain stage of pulsar rotation evolution. The age boundaries between different evolutionary stages are indistinct and diffusive. This fact is because different pulsars having similar properties evolve along different paths. The evolutionary scenario of the occurrence of rotational irregularities explains well many of the observed properties of pulsar rotation.

THE ROLE OF ROTATION AND POLAR-CAP CURRENTS ON PULSAR RADIO EMISSION AND POLARIZATION

The perturbations such as rotation and PC--current have been believed to be greatly affecting the pulsar radio emission and polarization. The two effects have not been considered simultaneously in the literature, however, each one of these has been considered separately and deduced the picture by simply superposing them, but such an approach can lead to spurious results. Hence by considering pulsar rotation and PC--current perturbations together instead of one at a time we have developed a single particle curvature radiation model, which is expected to be much more realistic. By simulating a set of typical pulse profiles we have made an attempt to explain most of the observational results on pulsar radio emission and polarization. The model predicts that due to the perturbations leading side component can become either stronger or weaker than the corresponding trailing one in any given cone depending on the passage of sight line and modulation (nonuniform source distribution). Further we find that the phase delay of polarization angle inflection point with respect to the core component greatly depends upon the viewing geometry. The correlation between the sign reversal of circular polarization and the polarization angle swing in the case of core dominated pulsars become obscure once the perturbations and modulation become significant. However the correlation that the negative circular polarization associates with the increasing polarization angle and vice versa shows up very clearly in the case of conal-double pulsars. The `kinky' type distortions in polarization angle swing could be due to the incoherent superposition of modulated emissions in the presence of strong perturbations.

Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes

Faraday rotation measurements using the current and next generation of low-frequency radio telescopes will provide a powerful probe of astronomical magnetic fields. However, achieving the full potential of these measurements requires accurate removal of the time-variable ionospheric Faraday rotation contribution. We present ionFR, a code that calculates the amount of ionospheric Faraday rotation for a specific epoch, geographic location, and line-of-sight. ionFR uses a number of publicly available, GPS-derived total electron content maps and the most recent release of the International Geomagnetic Reference Field. We describe applications of this code for the calibration of radio polarimetric observations, and demonstrate the high accuracy of its modeled ionospheric Faraday rotations using LOFAR pulsar observations. These show that we can accurately determine some of the highest-precision pulsar rotation measures ever achieved. Precision rotation measures can be used to monitor rotation measure variations - either intrinsic or due to the changing line-of-sight through the interstellar medium. This calibration is particularly important for nearby sources, where the ionosphere can contribute a significant fraction of the observed rotation measure. We also discuss planned improvements to ionFR, as well as the importance of ionospheric Faraday rotation calibration for the emerging generation of low-frequency radio telescopes, such as the SKA and its pathfinders.

Ultra High Energy Electrons Powered by Pulsar Rotation

A new mechanism of particle acceleration, driven by the rotational slow down of the Crab pulsar, is explored. The rotation, through the time dependent centrifugal force, can efficiently excite unstable Langmuir waves in the electron-positron (hereafter e±) plasma of the star magnetosphere. These waves, then, Landau damp on electrons accelerating them in the process. The net transfer of energy is optimal when the wave growth and the Landau damping times are comparable and are both very short compared to the star rotation time. We show, by detailed calculations, that these are precisely the conditions for the parameters of the Crab pulsar. This highly efficient route for energy transfer allows the electrons in the primary beam to be catapulted to multiple TeV (~ 100 TeV) and even PeV energy domain. It is expected that the proposed mechanism may, unravel the puzzle of the origin of ultra high energy cosmic ray electrons.

New theory may explain slowing of pulsar rotation

New theory may explain slowing of pulsar rotation

Radio pulsar variability

AbstractPulsars are potentially the most remarkable physical laboratories we will ever use. Although in many senses they are extremely clean systems there are a large number of instabilities and variabilities seen in the emission and rotation of pulsars. These need to be recognised in order to both fully understand the nature of pulsars, and to enable their use as precision tools for astrophysical investigations. Here I describe these effects, discuss the wide range of timescales involved, and consider the implications for precision pulsar timing.

High magnetic field pulsars with magnetar-like activity

AbstractTo study the origin of magnetars, a unique opportunity is provided by detecting an excess of X-ray thermal radiation of the radio pulsars (rotation powered pulsars) with dipolar magnetic fields as high as magnetars. The excess is probably caused by decay of the magnetic field as seen in magnetars. In order to investigate whether the rotation powered pulsars have the excess flux and the hard-tail component similar to magnetars, we observed PSR J0726-2612 which has a 3.44 s period and a 3 × 1013 G inferred dipolar magnetic field, with Suzaku for 44 ks on 2011 November 16-17. We report this observational result. We also compare with other observations and discuss a decaying of the magnetic field for normal radio pulsars.

FRATs: Searching for fast radio transient in real-time with LOFAR

AbstractLOFAR is an innovative new radio interferometer operating at low radio frequencies from 10 to 270 MHz. It combines a large field-of-view, high fractional bandwidth, rapid response, and a wide range of baselines from tens of meters to thousand kilometers. Its use of phased-array technology and its digital nature make LOFAR an extremely versatile instrument to search for transient radio phenomena on all time scales. Here we discuss in particular the search for fast radio transients (FRATs) at sub-second time scales. In fact, at these time scales the radio sky is rather dynamic due to coherent emission processes. Objects like pulsars, flaring stars, or planets like Jupiter are able to produce bright short flares. For pulsars, most previous detection strategies made use of the rotation of pulsars to detect them, using Fourier techniques, but it is also possible to detect pulsars and other objects through their single pulses. Such surveys have, e.g., led in the previous decade to the detection of Rapid Radio Transients (RRATS), but the unprobed search space is still rather large. LOFAR is now conducting a rather unique survey over the entire northern sky, searching for bright dispersed single radio pulses. This FRATs survey makes use of the LOFAR transient buffer boards (TBBs), which had initially been used to detect nanosecond radio pulses from cosmic rays. The TBBs store the radio data from each single receiver element of LOFAR and allow one to look back in time. A trigger system that runs parallel to normal imaging observation allows one to detect single pulses in an incoherent beam of all LOFAR stations, covering several tens to hundred square degrees at once. Once triggered, the data can be used to localize the pulse and to discriminate cosmic sources from terrestrial interference through 3D localization. The system has been successfully tested with known pulsars and first results of the ongoing survey will be presented.

Magnetic fields in our Milky Way Galaxy and nearby galaxies

AbstractMagnetic fields in our Galaxy and nearby galaxies have been revealed by starlight polarization, polarized emission from dust grains and clouds at millimeter and submillimeter wavelength, the Zeeman effect of spectral lines or maser lines from clouds or clumps, diffuse radio synchrotron emission from relativistic electrons in interstellar magnetic fields, and the Faraday rotation of background radio sources as well as pulsars for our Milky Way. It is easy to get a global structure for magnetic fields in nearby galaxies, while we have observed many details of magnetic fields in our Milky Way, especially by using pulsar rotation measure data. In general, magnetic fields in spiral galaxies probably have a large-scale structure. The fields follow the spiral arms with or without the field direction reversals. In the halo of spiral galaxies magnetic fields exist and probably also have a large-scale structure as toroidal and poloidal fields, but seem to be slightly weaker than those in the disk. In the central region of some galaxies, poloidal fields have been detected as vertical components. Magnetic field directions in galaxies seem to have been preserved during cloud formation and star formation, from large-scale diffuse interstellar medium to molecular clouds and then to the cloud cores in star formation regions or clumps for the maser spots. Magnetic fields in galaxies are passive to dynamics.

Polynomial regression calculation of the Earth's position based on millisecond pulsar timing

Prior to achieving high precision navigation of a spacecraft using X-ray observations, a pulsar rotation model must be built and analysis of the precise position of the Earth should be performed using ground pulsar timing observations. We can simulate time-of-arrival ground observation data close to actual observed values before using pulsar timing observation data. Considering the correlation between the Earth's position and its short arc section of an orbit, we use polynomial regression to build the correlation. Regression coefficients can be calculated using the least square method, and a coordinate component series can also be obtained; that is, we can calculate Earth's position in the Barycentric Celestial Reference System according to pulse arrival time data and a precise pulsar rotation model. In order to set appropriate parameters before the actual timing observations for Earth positioning, we can calculate the influence of the spatial distribution of pulsars on errors in the positioning result and the influence of error source variation on positioning by simulation. It is significant that the threshold values of the observation and systematic errors can be established before an actual observation occurs; namely, we can determine the observation mode with small errors and reject the observed data with big errors, thus improving the positioning result.

The Magnetic Field of the Milky Way from Faraday Rotation of Pulsars and Extragalactic Sources

Faraday rotation towards polarised pulsars and extragalactic sources is the best observable for determining the configuration of the magnetic field of the Galaxy in its plane and also at high latitudes. The Galactic magnetic field plays an important role in numerous astrophysical processes, including star formation and propagation of ultrahigh-energy cosmic rays; it is also an important component in measurements of the cosmological microwave background. This review article provides a brief overview of the latest advancements in the field, from an observer’s point of view. The most recent results based on pulsar rotation measures are discussed, which show that we have begun to confidently resolve the main features of the Galactic magnetic field on kiloparsec scales, both in the Solar neighbourhood and at larger distances. As we are currently in great anticipation of polarisation observations with new, state-of-the-art telescopes and hardware, a brief overview of how much this field of research will benefit from the upcoming pulsar surveys is also given.

Modelling the interaction between relativistic and non-relativistic winds in the binary system PSR B1259−63/SS2883★- II. Impact of the magnetization and anisotropy of the pulsar wind

In this paper, we present a numerical study of the properties of the flow produced by the collision of a magnetized anisotropic pulsar wind with its environment in binary system. We compare the impact of both the magnetic field and the wind anisotropy to the benchmark case of a purely hydrodynamical (HD) interaction of isotropic winds, which has been studied in detail by Bogovalov et al. (2008). We consider the interaction in axisymmetric approximation, i.e. the pulsar rotation axis is assumed to be oriented along the line between the pulsar and the optical star and the effects related to the pulsar orbiting are neglected. The impact of the magnetic field is studied for the case of weak magnetization (with magnetization parameter $\sigma<0.1$), which is consistent with conventional models of pulsar winds. The effects related to anisotropy in pulsar winds are modeled assuming that the kinetic energy flux in a non-magnetized pulsar wind is strongly anisotropic, with the minimum at the pulsar rotation axis and the maximum in the perpendicular direction. We show that, although both considered effects change the shape of the region occupied by the terminated pulsar wind, their impact appears to be small. In particular, for the magnetization of the pulsar wind below 0.1, the magnetic field pressure remains well below the plasma pressure in the post-shock region. Thus, in the case of interaction of a pulsar with the stellar wind environment (opposite to the case of plerions, i.e. the pulsar interaction with the interstellar medium, when the magnetic field becomes dynamically important independently on the wind magnetization) the HD approach represents a feasible approximation for numerical modelling.

Spin down of rotating compact magnetized strange stars in general relativity

We find that in general relativity slow down of the pulsar rotation due to the magnetodipolar radiation is more faster for the strange star with comparison to that for the neutron star of the same mass. Comparison with astrophysical observations on pulsars spindown data may provide an evidence for the strange star existence and, thus, serve as a test for distinguishing it from the neutron star.

Optimal filtration and a pulsar time scale

An algorithm is proposed for constructing a group (ensemble) pulsar time based on the application of optimal Wiener filters. This algorithm makes it possible to separate the contributions of variations of the atomic time scale and of the pulsar rotation to barycentric residual deviations of the pulse arrival times. The method is applied to observations of the pulsars PSR B1855+09 and PSR B1937+21, and is used to obtain corrections to UTC relative to the group pulsar time PTens. Direct comparison of the terrestial time TT(BIPM06) and the group pulsar time PTens shows that they disagree by no more than 0.4 ± 0.17 µs. Based on the fractional instability of the time difference TT(BIPM06)-PTens, σ z = (0.5 ± 2) × 10−15, a new limit for the energy density of the gravitational-wave background is established at the level Ω g h 2 ∼ 10−9.

X-RAY PULSATIONS FROM THE RADIO-QUIET GAMMA-RAY PULSAR IN CTA 1

Prompted by the Fermi-LAT discovery of a radio-quiet gamma-ray pulsar inside the CTA 1 supernova remnant, we obtained a 130 ks XMM-Newton observation to assess the timing behavior of this pulsar. Exploiting both the unprecedented photon harvest and the contemporary Fermi-LAT timing measurements, a 4.7σ single-peak pulsation is detected, making PSR J0007+7303 the second example, after Geminga, of a radio-quiet gamma-ray pulsar also seen to pulsate in X-rays. Phase-resolved spectroscopy shows that the off-pulse portion of the light curve is dominated by a power-law, non-thermal spectrum, while the X-ray peak emission appears to be mainly of thermal origin, probably from a polar cap heated by magnetospheric return currents, pointing to a hot spot varying throughout the pulsar rotation.

Searching for gravitational waves from known pulsars using the and statistics

In search for gravitational waves emitted by known isolated pulsars in data collected by a detector, one can assume that the frequency of the wave, its spindown parameters and the position of the source in the sky are known, and so the almost monochromatic gravitational-wave signal we are looking for depends on at the most four parameters: overall amplitude, initial phase, polarization angle and inclination angle of the pulsar's rotation axis with respect to the line of sight. We derive two statistics by means of which one can test whether data contain such gravitational-wave signals: the -statistic for signals which depend on only two unknown parameters (overall amplitude and initial phase), and the -statistic for signals depending on all four parameters. We study, by means of the Fisher matrix, the theoretical accuracy of the maximum-likelihood estimators of the signal's parameters and we present the results of the Monte Carlo simulations we performed to test the accuracy of these estimators.

CYCLICAL CHANGES IN THE TIMING RESIDUALS FROM THE PULSAR B0919+06

We report the detection of a large glitch in the pulsar B0919+06 (J0922+0638). The glitch occurred in 2009 November 5 (MJD 55140) and was characterized by a fractional increase in the rotation frequency of Deltanu/nu=1.3x10^{-6}. A large glitch happens in the pulsar whose rotation has unstable character. We present the results of the analysis of the rotation behavior of this pulsar over the 30-year time span from 1979 to 2009. These results show that the pulsar's rotation frequency underwent continuous, slow oscillations which look like glitch-like events. During the 1991-2009 interval, the pulsar experienced a continuous sequence of 12 slow glitches with a fractional increase in the rotation frequency Deltanu/nu=1.5x10^{-9}. All the slow glitches observed have a similar signature related to a slow increase in the rotation frequency during 200 days and the subsequent relaxation back to the pre-glitch value during 400 days. We show that a continuous sequence of such slow glitches is characterized by practically identical amplitudes equal to Deltanu=3.5x10^{-9} Hz and identical time intervals between glitches of about 600 days and is well described by a periodic sawtooth-like function. The detection of two different phenomena, such as a large glitch and a sequence of slow glitches, indicates the presence of two types of discontinuities in the rotation frequency of the pulsar B0919+06. These discontinuities can be classified as normal and slow glitches.