Direct Measurement of Neutron-Star Recoil in the Oxygen-Rich Supernova Remnant Puppis A

A sequence of three Chandra X-ray Observatory High Resolution Camera images taken over a span of five years reveals arc-second-scale displacement of RX J0822-4300, the stellar remnant (presumably a neutron star) near the center of the Puppis A supernova remnant. We measure its proper motion to be 0.165+/-0.025 arcsec/yr toward the west-southwest. At a distance of 2 kpc, this corresponds to a transverse space velocity of ~1600 km/s. The space velocity is consistent with the explosion center inferred from proper motions of the oxygen-rich optical filaments, and confirms the idea that Puppis A resulted from an asymmetric explosion accompanied by a kick that imparted roughly 3*10^49 ergs of kinetic energy (some 3 percent of the kinetic energy for a typical supernova) to the stellar remnant. We discuss constraints on core-collapse supernova models that have been proposed to explain neutron star kick velocities

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

Birth Kick Distributions and the Spin-Kick Correlation of Young Pulsars

Evidence from pulsar wind nebula symmetry axes and radio polarization observations suggests that pulsar motions correlate with the spin directions. We assemble this evidence for young isolated pulsars and show how it can be used to quantitatively constrain birth kick scenarios. We illustrate by computing several plausible, but idealized, models where the momentum thrust is proportional to the neutrino cooling luminosity of the proto-neutron star. Our kick simulations include the effects of pulsar acceleration and spin-up and our maximum likelihood comparison with the data constrains the model parameters. The fit to the pulsar spin and velocity measurements suggests that: i) the anisotropic momentum required amounts to ~10% of the neutrino flux, ii) while a pre-kick spin of the star is required, the preferred magnitude is small 10-20rad/s, so that for the best-fit models iii) the bulk of the spin is kick-induced with $\bar \Omega$ ~120rad/s and iv) the models suggest that the anisotropy emerges on a timescale $\tau$ ~1-3s.Comment: 37 pages, 13 figures, ApJ accepte

Bow Shocks from Neutron Stars: Scaling Laws and HST Observations of the Guitar Nebula

The interaction of high-velocity neutron stars with the interstellar medium produces bow shock nebulae, where the relativistic neutron star wind is confined by ram pressure. We present multi-wavelength observations of the Guitar Nebula, including narrow-band H-alpha imaging with HST/WFPC2, which resolves the head of the bow shock. The HST observations are used to fit for the inclination of the pulsar velocity vector to the line of sight, and to determine the combination of spindown energy loss, velocity, and ambient density that sets the scale of the bow shock. We find that the velocity vector is most likely in the plane of the sky. We use the Guitar Nebula and other observed neutron star bow shocks to test scaling laws for their size and H-alpha emission, discuss their prevalence, and present criteria for their detectability in targeted searches. The set of H-alpha bow shocks shows remarkable consistency, in spite of the expected variation in ambient densities and orientations. Together, they support the assumption that a pulsar's spindown energy losses are carried away by a relativistic wind that is indistinguishable from being isotropic. Comparison of H-alpha bow shocks with X-ray and nonthermal, radio-synchrotron bow shocks produced by neutron stars indicates that the overall shape and scaling is consistent with the same physics. It also appears that nonthermal radio emission and H-alpha emission are mutually exclusive in the known objects and perhaps in all objects.Comment: 12 pages, 7 figures (3 degraded), submitted to ApJ; minor revisions and updates in response to referee report. (AASTeX, includes emulateapj5 and onecolfloat5.

Fitting Pulsar Wind Tori

CXO imaging has shown that equatorial tori, often with polar jets, are very common in young pulsar wind nebulae (PWNe). These structures are interesting both for what they reveal about the relativistic wind itself and for the (nearly) model-independent measurement of the neutron star spin orientation they provide. The later is a particularly valuable probe of pulsar emission models and of neutron star physics.We describe here a procedure for fitting simple 3-D torus models to the X-ray data which provides robust estimates of the geometric parameters. An application to 6 PWN tori gives orientations, PWN shock scales and post-shock wind speeds along with statistical errors. We illustrate the use of these data by commenting on the implications for kick physics and for high energy beaming models.Comment: 10 pages, 2 figures; to appear in the Astrophysical Journa

The Duck Redux: An Improved Proper Motion Upper Limit for the Pulsar B1757-24 Near the Supernova Remnant G5.4-1.2

"The Duck" is a complicated non-thermal radio system, consisting of the energetic radio pulsar B1757-24, its surrounding pulsar wind nebula G5.27-0.90 and the adjacent supernova remnant (SNR) G5.4-1.2. PSR B1757-24 was originally claimed to be a young (~15 000 yr) and extreme velocity (>~1500 km/s) pulsar which had penetrated and emerged from the shell of the associated SNR G5.4-1.2, but recent upper limits on the pulsar's motion have raised serious difficulties with this interpretation. We here present 8.5 GHz interferometric observations of the nebula G5.27-0.90 over a 12-year baseline, doubling the time-span of previous measurements. These data correspondingly allow us to halve the previous upper limit on the nebula's westward motion to 14 milliarcseconds/yr (5-sigma), allowing a substantive reevaluation of this puzzling object. We rule out the possibility that the pulsar and SNR were formed from a common supernova explosion ~15 000 yrs ago as implied by the pulsar's characteristic age, but conclude that an old (>~70 000 yr) pulsar / SNR association, or a situation in which the pulsar and SNR are physically unrelated, are both still viable explanations.Comment: 9 pages, including 1 color and 1 B/W figure. Minor changes following referee's report. ApJ, in pres

Proper Motion Measurements of Pulsar B1951+32 in the Supernova Remnant CTB 80

Using the Very Large Array and the Pie Town antenna, we have measured the position of the radio pulsar B1951+32 relative to nearby background radio sources at four epochs between 1989 and 2000. These data show a clear motion for the pulsar of (25 +/- 4) milliarcsec/yr at a position angle (252 +/- 7) degrees (north through east), corresponding to a transverse velocity (240 +/- 40) km/s for a distance to the source of 2 kpc. The measured direction of motion confirms that the pulsar is moving away from the center of its associated supernova remnant, the first time that such a result has been demonstrated. Independent of assumptions made about the pulsar birth-place, we show that the measured proper motion implies an age for the pulsar of (64 +/- 18) kyr, somewhat less than its characteristic age of 107 kyr. This discrepancy can be explained if the initial spin period of the pulsar was (27 +/- 6) ms

Proper-Motion Measurements with the VLA. II. Observations of Twenty-eight Pulsars

Using the Very Large Array, we have measured the proper motions of twenty-eight radio pulsars. On average, the pulsars studied are fainter and more distant than those studied in earlier work, reducing the selection biases inherent in surveys restricted to the Solar neighborhood. The typical measurement precision achieved is a few milliarcseconds per year, corresponding to a few tens of kilometers per second for a pulsar a kiloparsec away. While our results compare well with higher-precision measurements done using very-long baseline interferometry, we find that several earlier proper motion surveys appear to have reported overly optimistic measurement uncertainties, most likely because of a failure to fully account for ionospheric effects. We discuss difficulties inherent in estimating pulsar velocities from proper motions given poorly constrained pulsar distances. Our observations favor a distribution with 20% of pulsars in a low velocity component (sigma_1D = 99 km/s) and 80% in a high velocity component (sigma_1D = 294 km/s). Furthermore, our sample is consistent with a scale height of pulsar birthplaces comparable to the scale height of the massive stars that are their presumed progenitors. No evidence is found in our data for a significant population of young pulsars born far from the plane. We find that estimates of pulsar ages based on kinematics agree well with the canonical spin-down age estimate, but agreement is improved if braking indexes are drawn from a Gaussian distribution centered at n=3 with width 0.8.Comment: 20 pages. Accepted for publication in the Astronomical Journa

PSR B1951+32: A Bow Shock-Confined X-ray Nebula, a Synchrotron Knot and an Optical Counterpart Candidate

The radio pulsar B1951+32 and the supernova remnant CTB 80 provide a rich laboratory for the study of neutron stars and supernova remnants. Here, we present ground-based optical and near-infrared observations of them, along with X-ray observations with Chandra and a re-analysis of archival data obtained with the Hubble Space Telescope. The X-ray observations reveal a cometary pulsar wind nebula which appears to be confined by a bow shock produced by high-velocity motion of the pulsar, making PSR B1951+32 a rare pulsar exhibiting both an H alpha bow shock and a shocked X-ray pulsar wind nebula. The distribution of H alpha and radio continuum emission is indicative of a contact discontinuity of the shocked pulsar winds and shocked ambient medium at \~0.05 pc. On the other hand, the optical synchrotron knot of PSR B1951+32 likely has a flat spectrum in the optical and near-infrared wavebands, and our astrometry is consistent with only one of the two reported optical counterpart candidates for the pulsar.Comment: To appear in ApJ Letter

Birth and Evolution of Isolated Radio Pulsars

We investigate the birth and evolution of Galactic isolated radio pulsars. We begin by estimating their birth space velocity distribution from proper motion measurements of Brisken et al. (2002, 2003). We find no evidence for multimodality of the distribution and favor one in which the absolute one-dimensional velocity components are exponentially distributed and with a three-dimensional mean velocity of 380^{+40}_{-60} km s^-1. We then proceed with a Monte Carlo-based population synthesis, modelling the birth properties of the pulsars, their time evolution, and their detection in the Parkes and Swinburne Multibeam surveys. We present a population model that appears generally consistent with the observations. Our results suggest that pulsars are born in the spiral arms, with a Galactocentric radial distribution that is well described by the functional form proposed by Yusifov & Kucuk (2004), in which the pulsar surface density peaks at radius ~3 kpc. The birth spin period distribution extends to several hundred milliseconds, with no evidence of multimodality. Models which assume the radio luminosities of pulsars to be independent of the spin periods and period derivatives are inadequate, as they lead to the detection of too many old simulated pulsars in our simulations. Dithered radio luminosities proportional to the square root of the spin-down luminosity accommodate the observations well and provide a natural mechanism for the pulsars to dim uniformly as they approach the death line, avoiding an observed pile-up on the latter. There is no evidence for significant torque decay (due to magnetic field decay or otherwise) over the lifetime of the pulsars as radio sources (~100 Myr). Finally, we estimate the pulsar birthrate and total number of pulsars in the Galaxy.Comment: 27 pages, including 15 figures, accepted by Ap