The distance and radius of the neutron star PSR B0656+14

We present the result of astrometric observations of the radio pulsar PSR B0656+14, made using the Very Long Baseline Array. The parallax of the pulsar is pi = 3.47 +- 0.36 mas, yielding a distance of 288 +33 -27 pc. This independent distance estimate has been used to constrain existing models of thermal x-ray emission from the neutron star's photosphere. Simple blackbody fits to the x-ray data formally yield a neutron star radius R_inf ~ 7-8.5 km. With more realistic fits to a magnetized hydrogen atmosphere, any radius between ~13 and ~20 km is allowed.Comment: 7 pages including 1 figure. Submitted to ApJL. AASte

Proper Motion of Pulsar B1800-21

We report high angular resolution, multi-epoch radio observations of the young pulsar PSR B1800-21. Using two pairs of data sets, each pair spanning approximately a ten year period, we calculate the proper motion of the pulsar. We obtain a proper motion of mu_alpha=11.6 +- 1.8 mas/yr, mu_delta=14.8 +- 2.3 mas/yr, which clearly indicates a birth position at the extreme edge of the W30 supernova remnant. Although this does not definitively rule out an association of W30 and PSR B1800-21, it does not support an association.Comment: 13 pages, 1 color figure. Replaced with version accepted for publication in Astrophysical Journa

A MERLIN Observation of PSR B1951+32 and its associated Plerion

In an investigative 16 hour L band observation using the MERLIN radio interferometric array, we have resolved both the pulsar PSR B1951+32 and structure within the flat spectral radio continuum region, believed to be the synchrotron nebula associated with the interaction of the pulsar and its `host' supernova remnant CTB 80. The extended structure we see, significant at $\sim$ 4.5 $\sigma$, is of dimensions 2.5" $\times$ 0.75", and suggests a sharp bow shaped arc of shocked emission, which is correlated with similar structure observed in lower resolution radio maps and X-ray images. Using this MERLIN data as a new astrometric reference for other multiwavelength data we can place the pulsar at one edge of the HST reported optical synchrotron knot, ruling out previous suggested optical counterparts, and allowing an elementary analysis of the optical synchrotron emission which appears to trail the pulsar. The latter is possibly a consequence of pulsar wind replenishment, and we suggest that the knot is a result of magnetohydrodynamic (MHD) instabilities. These being so, it suggests a dynamical nature to the optical knot, which will require high resolution optical observations to confirm.Comment: 12 pages, 2 figures. Accepted for publication in ApJ

Pulsar kicks by anisotropic neutrino emission from quark matter

We discuss an acceleration mechanism for pulsars out of their supernova remnants based on asymmetric neutrino emission from quark matter in the presence of a strong magnetic field. The polarized electron spin fixes the neutrino emission from the direct quark Urca process in one direction along the magnetic field. We calculate the magnetic field strength which is required to polarize the electron spin as well as the required initial proto-neutron star temperature for a successfull acceleration mechanism. In addition we discuss the neutrino mean free paths in quark as well as in neutron matter which turn out to be very small. Consequently, the high neutrino interaction rates will wash out the asymmetry in neutrino emission. As a possible solution to this problem we take into account effects from colour superconductivity.Comment: 6 pages, 3 figures, poster contribution at the conference "Nuclear Physics in Astrophysics III",Dresden,March 26-31,200

A wider audience: Turning VLBI into a survey instrument

Radio observations using the Very Long Baseline Interferometry (VLBI) technique typically have fields of view of only a few arcseconds, due to the computational problems inherent in imaging larger fields. Furthermore, sensitivity limitations restrict observations to very compact and bright objects, which are few and far between on the sky. Thus, while most branches of observational astronomy can carry out sensitive, wide-field surveys, VLBI observations are limited to targeted observations of carefully selected objects. However, recent advances in technology have made it possible to carry out the computations required to target hundreds of sources simultaneously. Furthermore, sensitivity upgrades have dramatically increased the number of objects accessible to VLBI observations. The combination of these two developments have enhanced the survey capabilities of VLBI observations such that it is now possible to observe (almost) any point in the sky with milli-arcsecond resolution. In this talk I review the development of wide-field VLBI, which has made significant progress over the last three years.Comment: Invited review at the General Assembly of the Astronomische Gesellschaf

Proper Motions of PSRs B1757-24 and B1951+32: Implications for Ages and Associations

Over the last decade, considerable effort has been made to measure the proper motions of the pulsars B1757-24 and B1951+32 in order to establish or refute associations with nearby supernova remnants and to understand better the complicated geometries of their surrounding nebulae. We present proper motion measurements of both pulsars with the Very Large Array, increasing the time baselines of the measurements from 3.9 yr to 6.5 yr and from 12.0 yr to 14.5 yr, respectively, compared to previous observations. We confirm the non-detection of proper motion of PSR B1757-24, and our measurement of (mu_a, mu_d) = (-11 +/- 9, -1 +/- 15) mas yr^{-1} confirms that the association of PSR B1757-24 with SNR G5.4-1.2 is unlikely for the pulsar characteristic age of 15.5 kyr, although an association can not be excluded for a significantly larger age. For PSR B1951+32, we measure a proper motion of (mu_a, mu_d) = (-28.8 +/- 0.9, -14.7 +/- 0.9) mas yr^{-1}, reducing the uncertainty in the proper motion by a factor of two compared to previous results. After correcting to the local standard of rest, the proper motion indicates a kinetic age of ~51 kyr for the pulsar, assuming it was born near the geometric center of the supernova remnant. The radio-bright arc of emission along the pulsar proper motion vector shows time-variable structure, but moves with the pulsar at an approximately constant separation ~2.5", lending weight to its interpretation as a shock structure driven by the pulsar.Comment: LaTeX file uses emulateapj.cls; 7 pages, 4 figures, to be published ApJ February 10, 2008, v674 p271-278. Revision reflects journal formatting; there are no substantial revision

Precision Astrometry with the Very Long Baseline Array: Parallaxes and Proper Motions for 14 Pulsars

Astrometry can bring powerful constraints to bear on a variety of scientific questions about neutron stars, including their origins, astrophysics, evolution, and environments. Using phase-referenced observations at the VLBA, in conjunction with pulsar gating and in-beam calibration, we have measured the parallaxes and proper motions for 14 pulsars. The smallest measured parallax in our sample is 0.13+-0.02 mas for PSR B1541+09, which has a most probable distance of 7.2+1.3-1.1 kpc. We detail our methods, including initial VLA surveys to select candidates and find in-beam calibrators, VLBA phase-referencing, pulsar gating, calibration, and data reduction. The use of the bootstrap method to estimate astrometric uncertainties in the presence of unmodeled systematic errors is also described. Based on our new model-independent estimates for distance and transverse velocity, we investigate the kinematics and birth sites of the pulsars and revisit models of the Galactic electron density distribution. We find that young pulsars are moving away from the Galactic plane, as expected, and that age estimates from kinematics and pulsar spindown are generally in agreement, with certain notable exceptions. Given its present trajectory, the pulsar B2045-16 was plausibly born in the open cluster NGC 6604. For several high-latitude pulsars, the NE2001 electron density model underestimates the parallax distances by a factor of two, while in others the estimates agree with or are larger than the parallax distances, suggesting that the interstellar medium is irregular on relevant length scales. The VLBA astrometric results for the recycled pulsar J1713+0747 are consistent with two independent estimates from pulse timing, enabling a consistency check between the different reference frames.Comment: 16 pages, 9 figures, 4 tables; results unchanged; revised version accepted by Ap