Pulsar Proper Motion Research Articles

X‐Ray Emission from PSR B1800−21, Its Wind Nebula, and Similar Systems

We detected X-ray emission from PSR B1800-21 and its synchrotron nebula with the Chandra X-ray Observatory. The pulsar's observed flux is (1.4+/-0.2) 10^{-14} ergs cm^{-2} s^{-1} in the 1-6 keV band. The spectrum can be described by a two-component PL+BB model, suggesting a mixture of thermal and magnetospheric emission. For a plausible hydrogen column density n_{H}=1.4 10^{22} cm^{-2}, the PL component has a slope Gamma=1.4+/-0.6 and a luminosity L_{psr}^{nonth}=4 10^{31}(d/4 kpc)^2 ergs s^{-1}. The properties of the thermal component (kT=0.1-0.3 keV, L^{bol}=10^{31}-10^{33} ergs s^{-1}) are very poorly constrained because of the strong interstellar absorption. The compact, 7''\times4'', inner pulsar-wind nebula (PWN), elongated perpendicular to the pulsar's proper motion, is immersed in a fainter asymmetric emission. The observed flux of the PWN is (5.5+/-0.6) 10^{-14} ergs cm^{-2} s^{-1} in the 1-8 keV band. The PWN spectrum fits by a PL model with Gamma=1.6+/-0.3, L=1.6 10^{32} (d/4 kpc})^2 ergs s^{-1}. The shape of the inner PWN suggests that the pulsar moves subsonically and X-ray emission emerges from a torus associated with the termination shock in the equatorial pulsar wind. The inferred PWN-pulsar properties (e.g., the PWN X-ray efficiency, L_{pwn}/\dot{E}~10^{-4}; the luminosity ratio, L_{pwn}/L_{psr}^{nonth}=4; the pulsar wind pressure at the termination shock, p_s=10^{-9} ergs cm^{-3}) are very similar to those of other subsonically moving Vela-like objects detected with Chandra (L_{pwn}/\dot{E}=10^{-4.5}-10^{-3.5}, L_{pwn}/L_{psr}^{nonth}~5, p_s=10^{-10}-10^{-8} ergs cm^{-1}).

X-ray emission properties of the old pulsar PSR B2224+65

Using archival Chandra data we studied the X-ray emission properties of PSR B2224+65 and its environment. Albeit limited by photon statistics the spectral analysis suggests that the bulk of the emission from PSR B2224+65 is non-thermal. Fitting a power-law to the observed energy spectrum yields a photon index of $\Gamma=1.58^{+0.43}_{-0.33}$. The possible origin of the non-thermal pulsar emission is discussed in the context of the outer-gap model. We did not find any evidence for a compact nebula around PSR B2224+65 though the Chandra data reveal the existence of an extended feature which appears to be associated with PSR B2224+65. It extends from the pulsar position about 2 arcmin to the north-west. Its orientation deviates by $\sim118^{\circ}$ from the pulsar's proper motion direction. Investigating its energy spectrum shows that the emission of this extended feature is much harder than that of the pulsar itself and is non-thermal in nature.

The optical polarisation of the Vela pulsar revisited

Aims. In this work we present a revised measurement of the phase-averaged optical polarisation of the Vela pulsar (PSR B0833-45), for which only one value has been published so far (Wagner & Seifert 2000, ASP Conf. Ser., 202, 315). Methods. Our measurement has been obtained through an accurate reanalysis of archival polarisation observations with the FORS1 instrument at the VLT. Results. We measured a phase-averaged linear polarisation degree P L = 9.4% ± 4% and a position angle θ = 146° ±11°, very close to the ones for the axis of symmetry of the X-ray arcs and jets detected by Chandra and of the pulsar proper motion. Conclusions. We compared the measured phase-averaged optical polarisation with the expectations of different pulsars' magnetosphere models. We find that all models consistently predict values of the phase-averaged linear polarisation that are too high with respect to the observed one. This is probably a consequence of present models' limitations that neglect the contributions of various depolarisation effects. Interestingly, for the outer gap model we find that, when assuming synchrotron radiation for the optical emission, the observed polarisation position angle also implies an alignment between the pulsar rotational axis and the axis of symmetry of the X-ray arcs and jets.

Features of the Acoustic Mechanism of Core‐Collapse Supernova Explosions

In the context of 2D, axisymmetric, multigroup, radiation/hydrodynamic simulations of core-collapse supernovae over the full 180° domain, we present an exploration of the progenitor dependence of the acoustic mechanism of explosion. All progenitor models we have tested with our Newtonian code explode. However, some of the cores left behind in our simulations, particularly for the more massive progenitors, have baryon masses that are larger than the canonical ~1.5 M☉ of well-measured pulsars. We investigate the roles of the standing accretion shock instability (SASI), the excitation of core g-modes, the generation of core acoustic power, the ejection of matter with r-process potential, the windlike character of the explosion, and the fundamental anisotropy of the blasts. We find that the breaking of spherical symmetry is central to the supernova phenomenon, the delays to explosion can be long, and the blasts, when top-bottom asymmetric, are self-collimating. We see indications that the initial explosion energies are larger for the more massive progenitors and smaller for the less massive progenitors and that the neutrino contribution to the explosion energy may be an increasing function of progenitor mass. However, the explosion energy is still accumulating by the end of our simulations and has not converged to final values. The degree of explosion asymmetry we obtain is completely consistent with that inferred from the polarization measurements of Type Ic supernovae. Furthermore, we calculate for the first time the magnitude and sign of the net impulse on the core due to anisotropic neutrino emission and suggest that hydrodynamic and neutrino recoils in the context of our asymmetric explosions afford a natural mechanism for observed pulsar proper motions.

Neutron Star Kicks: Mechanisms and Observational Constraints

Observations over the last decade have shown that neutron stars (NSs) receive a kick velocity (of order a hundred to a thousand km s−1) at birth. The physical origin of the kicks and the related supernova asymmetry is an unsolved problem. We study observational constraints on kicks for isolated pulsars and for neutron stars in binary systems. For several young pulsars, X-ray observations of compact nebulae showed that pulsar proper motion is aligned with the spin direction as defined by the symmetry axis of the nebula. Such alignment is also seen from a new analysis of radio polarization data for a large number of pulsars. On the other hand, for various NS binaries (including double NS systems, binaries with massive main-sequence star companion and binaries with massive white-dwarf companion), we find that the kick velocity is misaligned with the the NS spin axis in a number of systems, and the NS spin period (when available) in these systems is generally longer than several hundreds milliseconds. These observational constraints suggest that the kick timescale is hundreds of milliseconds to 1s, so that spin-kick alignment or misalignment can be obtained depending on the initial spin period of the NS. We discuss the implication of our result for various NS kick mechanisms.

Probing the Pulsar Wind Nebula of PSR B0355+54

We present XMM-Newton and Chandra X-ray observations of the middle-aged radio pulsar PSR B0355+54. Our X-ray observations reveal emission not only from the pulsar itself, but also from a compact diffuse component extending ~50'' in the opposite direction to the pulsar's proper motion. There is also evidence for the presence of fainter diffuse emission extending ~5' from the point source. The compact diffuse feature is well-fitted with a power-law, the index of which is consistent with the values found for other pulsar wind nebulae. The morphology of the diffuse component is similar to the ram-pressure confined pulsar wind nebulae detected for other sources. The X-ray emission from the pulsar itself is described well by a thermal plus power-law fit, with the thermal emission most likely originating in a hot polar cap.

Supernova remnant S 147 and its associated neutron star(s)

The supernova remnant S 147 harbors the pulsar PSR J 0538+2817 whose characteristic age is more than an order of magnitude greater than the kinematic age of the system (inferred from the angular offset of the pulsar from the geometric center of the supernova remnant and the pulsar proper motion). To reconcile this discrepancy we propose that PSR J 0538+2817 could be the stellar remnant of the first supernova explosion in a massive binary system and therefore could be as old as its characteristic age. Our proposal implies that S 147 is the diffuse remnant of the second supernova explosion (that disrupted the binary system) and that a much younger second neutron star (not necessarily manifesting itself as a radio pulsar) should be associated with S 147. We use the existing observational data on the system to suggest that the progenitor of the supernova that formed S 147 was a Wolf-Rayet star (so that the supernova explosion occurred within a wind bubble surrounded by a massive shell) and to constrain the parameters of the binary system. We also restrict the magnitude and direction of the kick velocity received by the young neutron star at birth and find that the kick vector should not strongly deviate from the orbital plane of the binary system.

Neutron Star Kicks in Isolated and Binary Pulsars: Observational Constraints and Implications for Kick Mechanisms

We study observational constraints on neutron star (NS) kicks for isolated pulsars and for neutron stars in binary systems. We are particularly interested in the evidence of kick-spin alignment/misalignment and its dependence on the neutron star initial spin period. For several young pulsars, X-ray observations of compact nebulae showed that pulsar proper motion is aligned with the spin direction as defined by the symmetry axis of the nebula. We also critically examine the measurements of the proper motion and the projected spin axis from a large sample of pulsars with well-calibrated polarization data. We find that among the two dozen pulsars for which reliable measurements are available, there is a significant correlation between the spin axis and the proper motion. For various NS binaries, including double NS systems, binaries with massive main-sequence star companion and binaries with massive white-dwarf companion, we obtain constraints on the kick magnitudes and directions from the observed orbital characteristics of the system. We find that the kick velocity is misaligned with the the NS spin axis in a number of systems, and the NS spin period (when available) in these systems is generally longer than several hundreds milliseconds. These constraints, together with spin-kick alignment observed in many isolated pulsars, suggest that the kick timescale is hundreds of milliseconds to 1 s, so that spin-kick alignment or misalignment can be obtained depending on the initial spin period of the NS. We discuss the implication of our result for various NS kick mechanisms.

Proper Motion of the Crab Pulsar Revisited

It has been suggested that the Crab pulsar's proper motion is aligned with the symmetry axis of the pulsar wind nebula. We have revisited this question, examining >6 years of F547M WFPC2 chip 3 images to obtain a best-fit value of μ* = 14.9 ± 0.8 mas yr-1 at P.A. 278° ± 3°. At 26° ± 3° to the nebula axis, this substantially relaxes constraints on the birth kick of this pulsar. Such misalignment allows the momentum to be imparted over ~1 s timescales.

A statistical study of 233 pulsar proper motions

We present and analyse a catalogue of 233 pulsars with proper motion measurements. The sample contains a wide variety of pulsars including recycled objects and those associated with globular clusters or supernova remnants. After taking the most precise proper motions for those pulsars for which multiple measurements are available, the majority of the proper motions (58%) are derived from pulsar timing methods, 41% using interferometers and the remaining 1% using optical telescopes. Many of the 1-D and 2-D speeds (referring to speeds measured in one coordinate only and the magnitudes of the transverse velocities respectively) derived from these measurements are somewhat lower than earlier estimates because of the use of the most recent electron density model in determining pulsar distances. The mean 1-D speeds for the normal and recycled pulsars are 152(10) and 54(6) km/s respectively. The corresponding mean 2-D speeds are 246(22) and 87(13) km/s. PSRs B2011+38 and B2224+64 have the highest inferred 2-D speeds of ~1600 km/s. We study the mean speeds for different subsamples and find that, in general, they agree with previous results. Applying a novel deconvolution technique to the sample of 73 pulsars with characteristic ages less than 3 Myr, we find the mean 3-D pulsar birth velocity to be 400(40) km/s. The distribution of velocities is well described by a Maxwellian distribution with 1-D rms sigma=265 km/s. There is no evidence for a bimodal velocity distribution. The proper motions for PSRs B1830-08 and B2334+61 are consistent with their proposed associations with the supernova remnants W41 and G114.3+0.3 respectively.

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.

Pulsar recoil by large-scale anisotropies in supernova explosions.

Assuming that the neutrino luminosity from the neutron star core is sufficiently high to drive supernova explosions by the neutrino-heating mechanism, we show that low-mode (l=1,2) convection can develop from random seed perturbations behind the shock. A slow onset of the explosion is crucial, requiring the core luminosity to vary slowly with time, in contrast to the burstlike exponential decay assumed in previous work. Gravitational and hydrodynamic forces by the globally asymmetric supernova ejecta were found to accelerate the remnant neutron star on a time scale of more than a second to velocities above 500 km s(-1), in agreement with observed pulsar proper motions.

The Variable Jet of the Vela Pulsar

Observations of the Vela pulsar-wind nebula (PWN) with the Chandra X-ray Observatory have revealed a complex, variable PWN structure, including inner and outer arcs, a jet in the direction of the pulsar's proper motion, and a counter-jet in the opposite direction, embedded in diffuse nebular emission. From the analysis of thirteen Chandra observations spread over about 2.5 years we found that this outer jet shows particularly strong variability, changing its shape and brightness. We observed bright blobs in the outer jet moving away from the pulsar with apparent speeds (0.3-0.6)c and fading on time-scales of days to weeks. The spectrum of the outer jet fits a power-law model with a photon index of 1.3. The X-ray emission of the outer jet can be interpreted as synchrotron radiation of ultrarelativistic electrons/positrons. This interpretation allows one to estimate the magnetic field, ~100 microGauss, maximum energy of X-ray emitting electrons, $2 \times 10^{14}$ eV, and energy injection rate, $8\times 10^{33}$ erg/s, for the outer jet. In the summed PWN image, we see a faint, strongly bent, extension of the outer jet. The more extreme bends closer to the pulsar, as well as the apparent side motions of the outer jet, can be associated with kink instabilities of a magnetically confined, pinched jet flow. Another feature found in the summed image is a dim, 2'-long outer counter-jet, which also shows a power-law spectrum with photon index of 1.2-1.5. Southwest of the jet/counter-jet (i.e., approximately perpendicular to the direction of pulsar's proper motion), an extended region of diffuse emission is seen. Relativistic particles responsible for this radiation are apparently supplied by the outer jet.

Pulsar PSR B0656+14, the Monogem Ring, and the Origin of the "Knee" in the Primary Cosmic-Ray Spectrum

The Monogem ring is a bright, diffuse, 25° diameter supernova remnant easily visible in soft X-ray images of the sky. Projected within the ring is a young radio pulsar, PSR B0656+14. An association between the remnant and pulsar has been considered but was seemingly ruled out by the direction and magnitude of the pulsar proper motion and by a distance estimate that placed the pulsar twice as far from Earth as the remnant. Here we show that in fact the pulsar was born very close to the center of the expanding remnant, both in distance and in projection. The inferred pulsar and remnant ages are in good agreement. The conclusion that the pulsar and remnant were born in the same supernova explosion is nearly inescapable. The remnant distance and age are in remarkable concordance with the predictions of a model for the primary cosmic-ray energy spectrum in which the knee feature is produced by a single dominant source.

Search for the Optical Counterpart of the Nearby Pulsar PSR J0108−1431

We present the results of the first deep optical observations of the field of the old (~108 yr), nearby, isolated pulsar PSR J0108-1431 in an attempt to detect its optical counterpart. The observations were performed using the FORS1 instrument at the focus of the European Southern Observatory Antu Telescope of the Very Large Telescope. Observations with the Australia Telescope Compact Array were made to determine an accurate position for the radio pulsar at the current epoch. The imaging data, obtained in the V, B, and U passbands, reveal no counterpart at the revised radio position down to V 28, B 28.6, and U 26.4. For a distance of 130 pc, estimated from the pulsar's dispersion measure, our constraints on the optical flux put an upper limit of T = 4.5 × 104 K for the surface temperature of the neutron star, assuming a stellar radius R∞ = 13 km. Our new radio position allows us to place an upper limit on the pulsar proper motion of 82 mas yr-1, which, for d = 130 pc, implies a transverse velocity 50 km s-1.

Probing the Masses of the PSR J0621+1002 Binary System through Relativistic Apsidal Motion

Orbital, spin and astrometric parameters of the millisecond pulsar PSR J0621+1002 have been determined through six years of timing observations at three radio telescopes. The chief result is a measurement of the rate of periastron advance, omega_dot = 0.0116 +/- 0.0008 deg/yr. Interpreted as a general relativistic effect, this implies the sum of the pulsar mass, m_1, and the companion mass, m_2, to be M = m_1 + m_2 = 2.81 +/- 0.30 msun. The Keplerian parameters rule out certain combinations of m_1 and m_2, as does the non-detection of Shapiro delay in the pulse arrival times. These constraints, together with the assumption that the companion is a white dwarf, lead to the 68% confidence maximum likelihood values of m_1 = 1.70(+0.32 -0.29) msun and m_2 =0.97(+0.27 - 0.15) msun and to the 95% confidence maximum likelihood values of m_1 = 1.70(+0.59 -0.63) msun and m_2 = 0.97(+0.43 -0.24) msun. The other major finding is that the pulsar experiences dramatic variability in its dispersion measure (DM), with gradients as steep as 0.013 pc cm^{-3} / yr. A structure function analysis of the DM variations uncovers spatial fluctuations in the interstellar electron density that cannot be fit to a single power law, unlike the Kolmogorov turbulent spectrum that has been seen in the direction of other pulsars. Other results from the timing analysis include the first measurements of the pulsar's proper motion, mu = 3.5 +/- 0.3 mas / yr, and of its spin-down rate, dP/dt = 4.7 x 10^{-20}, which, when corrected for kinematic biases and combined with the pulse period, P = 28.8 ms, gives a characteristic age of 1.1 x 10^{10} yr and a surface magnetic field strength of 1.2 x 10^{9} G.

The Distance to the Vela Pulsar Gauged withHubble Space TelescopeParallax Observations

The distance to the Vela pulsar (PSR B0833-45) has been traditionally assumed to be 500 pc. Although affected by a significant uncertainty, this value stuck to both the pulsar and the supernova remnant. In an effort to obtain a model-free distance measurement, we have applied high-resolution astrometry to the pulsar V ~ 23.6 optical counterpart. Using a set of five Hubble Space Telescope Wide Field Planetary Camera 2 observations, we have obtained the first optical measurement of the annual parallax of the Vela pulsar. The parallax turns out to be 3.4 ± 0.7 mas, implying a distance of 294 pc, i.e., a value significantly lower than previously believed. This affects the estimate of the pulsar absolute luminosity and of its emission efficiency at various wavelengths and confirms the exceptionally high value of Ne toward the Vela pulsar. Finally, the complete parallax database allows for a better measurement of the Vela pulsar proper motion [μα cos(δ) = -37.2 ± 1.2 mas yr-1; μδ = 28.2 ± 1.3 mas yr-1 after correcting for the peculiar motion of the Sun], which, at the parallax distance, implies a transverse velocity of ≈65 km s-1. Moreover, the proper-motion position angle appears especially well aligned with the axis of symmetry of the X-ray nebula as seen by Chandra. Such an alignment allows us to assess the space velocity of the Vela pulsar to be 81 km s-1.

Scintillation measurements of the millisecond pulsar PSR J0030+0451 and pulsar space velocities

Scintillation observations of the nearby single millisecond pulsar (MSP) PSR J0030+0451 were carried out with the Parkes 64 m radiotelescope at three different epochs in 1999. From analysis of the dynamic spectrum we obtained the amplitude of the electron density power spectrum and a scintillation velocity km s-1 . This result shows that the Shklovskii effect on the spin-down rate is negligible. We also performed a correlation analysis between pulsar proper motions (V pm ) and scintillation velocities (V iss ) using updated measurements for a sample of 77 objects, 17 of which are MSPs. The full sample shows a correlation coefficient at an extremely high significance level, while for the MSP sub-sample (excluding 2 outliers) we obtain .

Measurement of the Parallax of PSR B0950+08 Using the VLBA

A new technique has been developed to remove the ionosphere's distorting effects from low-frequency VLBI data. By fitting dispersive and nondispersive components to the phases of multifrequency data, the ionosphere can be effectively removed from the data without the use of a priori calibration information. This technique, along with the new gating capability of the VLBA correlator, was used to perform accurate astrometry on pulsar B0950+08, resulting in a much improved measurement of this pulsar's proper motion (μα = -1.6 ± 0.4 mas yr-1, μδ = 29.5 ± 0.5 mas yr-1) and parallax (π = 3.6 ± 0.3 mas). This puts the pulsar at a distance of 280 ± 25 pc, about twice as far as previous estimates, but in good agreement with models of the electron density in the Local Bubble.

No pulsar kicks from deformed neutrinospheres

In a supernova core, magnetic fields cause a directional variation of the neutrino refractive index so that resonant flavor oscillations would lead to a deformation of the ``neutrinosphere'' for, say, \ensuremath{\tau} neutrinos. The associated anisotropic neutrino emission was proposed as a possible origin of the observed pulsar proper motions. We argue that this effect was vastly overestimated because the variation of the temperature over the deformed neutrinosphere is not an adequate measure for the anisotropy of neutrino emission. The neutrino flux is generated inside the neutron star core and is transported through the atmosphere at a constant luminosity, forcing the temperature gradient in the atmosphere to adjust to the inflow of energy from below. Therefore, no emission anisotropy is caused by a deformation of the neutrinosphere to lowest order. An estimate of the higher-order corrections must take into account the modified atmospheric temperature profile in response to the deformation of the neutrinosphere and the corresponding feedback on the core. We go through this exercise in the framework of a simplified model which can be solved analytically.