David F. Chernoff

The Velocity Distribution of Isolated Radio Pulsars

We infer the velocity distribution of radio pulsars based on large-scale 0.4 GHz pulsar surveys. We do so by modeling the evolution of the locations, velocities, spins, and radio luminosities of pulsars, calculating pulsed flux according to a beaming model and random orientation angles of spin and beam, applying selection effects of pulsar surveys, and comparing model distributions of measurable pulsar properties with survey data using a likelihood function. The surveys analyzed have well-defined characteristics and cover ~95% of the sky. We maximize the likelihood in a six-dimensional space of observables P, , DM, |b|, μ, and F (period, period derivative, dispersion measure, Galactic latitude, proper motion, and flux density, respectively). The models we test are described by 12 parameters that characterize a populations birth rate, luminosity, shutoff of radio emission, birth locations, and birth velocities. We infer that the radio beam luminosity (1) is comparable to the energy flux of relativistic particles in models for spin-driven magnetospheres, signifying that radio emission losses reach nearly 100% for the oldest pulsars, and (2) scales approximately as 1/2, which in magnetosphere models is proportional to the voltage drop available for acceleration of particles. We find that a two-component velocity distribution with characteristic velocities of 90 and 500 km s-1 is greatly preferred to any one-component distribution; this preference is largely immune to variations in other population parameters, such as the luminosity or distance scale or the assumed spin-down law. We explore some consequences of the preferred birth velocity distribution: (1) roughly 50% of pulsars in the solar neighborhood will escape the Galaxy, while ~15% have velocities greater than 1000 km s-1; (2) observational bias against high-velocity pulsars is relatively unimportant for surveys that reach high Galactic |z| distances but is severe for spatially bounded surveys; (3) an important low-velocity population exists that increases the fraction of neutron stars retained by globular clusters and is consistent with the number of old objects that accrete from the interstellar medium; (4) under standard assumptions for supernova remnant expansion and pulsar spin-down, ~10% of pulsars younger than 20 kyr will appear to lie outside of their host remnants. Finally, we comment on the ramifications of our birth velocity distribution for binary survival and the population of inspiraling binary neutron stars relevant to some GRB models and potential sources for LIGO.

Neutron Star Population Dynamics. II. Three-dimensional Space Velocities of Young Pulsars

We use astrometric, distance, and spindown data on pulsars to (1) estimate three-dimensional velocity components, birth distances from the Galactic plane, and ages of individual objects; (2) determine the distribution of space velocities and the scale height of pulsar progenitors; (3) test spindown laws for pulsars; (4) test for correlations between space velocities and other pulsar parameters; and (5) place empirical requirements on mechanisms than can produce high-velocity neutron stars. Our approach incorporates measurement errors, uncertainties in distances, deceleration in the Galactic potential, and differential Galactic rotation. We focus on a sample of proper motion measurements of young ( ?24+19 km s-1 and 700 -->?132+300 km s-1, representing ~86% and ~14% of the population, respectively. The sample velocities are inconsistent with a single-component Gaussian model and are well described by a two-component Gaussian model but do not require models of additional complexity. From the best-fit distribution, we estimate that about 20% of the known pulsars will escape the Galaxy, assuming an escape speed of 500 km s-1. The best-fit, dual-component model, if augmented by an additional, low-velocity ( 1000 km s-1) may be underrepresented (in the observed sample) by a factor ~2.3 owing to selection effects in pulsar surveys. The estimates of scale height and velocity parameters are insensitive to the explicit relation of chronological and spindown ages. A further analysis starting from our inferred velocity distribution allows us to test spindown laws and age estimates. There exist comparably good descriptions of the data involving different combinations of braking index and torque decay timescale. We find that a braking index of 2.5 is favored if torque decay occurs on a timescale of ~3 Myr, while braking indices ~4.5 ? 0.5 are preferred if there is no torque decay. For the sample as a whole, the most probable chronological ages are typically smaller than conventional spindown ages by factors as large as 2. We have also searched for correlations between three-dimensional speeds of individual pulsars and combinations of spin period and period derivative. None appears to be significant. We argue that correlations identified previously between velocity and (apparent) magnetic moment reflect the different evolutionary paths taken by young, isolated (nonbinary), high-field pulsars and older, low-field pulsars that have undergone accretion-driven spinup. We conclude that any such correlation measures differences in spin and velocity selection in the evolution of the two populations and is not a measure of processes taking place in the core collapse that produces neutron stars in the first place. We assess mechanisms for producing high-velocity neutron stars, including disruption of binary systems by symmetric supernovae and neutrino, baryonic, or electromagnetic rocket effects during or shortly after the supernova. The largest velocities seen (~1600 km s-1), along with the paucity of low-velocity pulsars, suggest that disruption of binaries by symmetric explosions is insufficient. Rocket effects appear to be a necessary and general phenomenon. The required kick amplitudes and the absence of a magnetic field-velocity correlation do not yet rule out any of the rocket models. However, the required amplitudes suggest that the core collapse process in a supernova is highly dynamic and aspherical and that the impulse delivered to the neutron star is larger than existing simulations of core collapse have achieved.

Pulsar Jets: Implications for Neutron Star Kicks and Initial Spins

We study implications for the apparent alignment of the spin axes, proper motion directions, and polarization vectors of the Crab and Vela pulsars. The spin axes are deduced from recent Chandra X-Ray Observatory images that reveal jets and nebular structure having de—nite symmetry axes. The alignments indicate that these pulsars were born either in isolation or with negligible velocity contribu- tions from binary motions. We examine the eUects of rotation and the conditions under which spin-kick alignment is produced for theoretical models of neutron star kicks. If the kick is generated promptly during the formation of the neutron star by asymmetric mass ejection and/or neutrino emission, then the alignment requires that the protoneutron star possess, by virtue of the precollapse stellar cores spin, an original spin with period much less than the kick timescale thus spin averaging the kick forces P s q kick , on the star. The kick timescale ranges from 100 ms to 10 s depending on whether the kick is hydrody- namically driven or neutrinomagnetic —eld driven. For hydrodynamical models, spin-kick alignment further requires the rotation period of an asymmetry pattern at the radius near shock breakout (Z100 km) to be much less than ms; this is difficult to satisfy unless rotation plays a dynamically q kick ( 100 important role in the core collapse and explosion (corresponding to ms). Aligned kick and spin P s ( 1 vectors are inherent to the slow process of asymmetric electromagnetic radiation from an oU-centered magnetic dipole. We reassess the viability of this electromagnetic rocket eUect, correcting a factor of 4 error in Harrison and Tademarus calculation that increases the size of the eUect. To produce a kick velocity of order a few hundred kilometers per second requires that the neutron star be born with P s D 1 ms and that spin-down due to r-modedriven gravitational radiation be inefficient compared to standard magnetic braking. The electromagnetic rocket operates on a timescale of order 0.3(B/1013 G)~2 yr. The apparent spin-kick alignment in the Crab and Vela pulsars places important new constraints on each of the mechanisms of neutron star kicks that we consider. Subject headings: pulsars: individual (PSR B0531)21, PSR B0833(45) ¨ stars: neutron ¨ stars: rotationsupernovae: general

Observing binary inspiral in gravitational radiation: One interferometer

Close binary systems of compact objects with less than ten minutes remaining before coalescence are readily identifiable sources of gravitational radiation for the United States Laser Interferometer Gravitational-Wave Observatory (LIGO) and the French-Italian VIRGO gravitational-wave observatory. As a start toward assessing the full capabilities of the LIGO-VIRGO detector network, we investigate the sensitivity of individual LIGO-VIRGO-like interferometers and the precision with which they can determine the characteristics of an inspiralling binary system. Since the two interferometers of the LIGO detector share nearly the same orientation, their joint sensitivity is similar to that of a single, more sensitive interferometer. We express our results for a single interferometer of both initial and advanced LIGO design, and also for the LIGO detector in the limit that its two interferometers share exactly the same orientation. We approximate the secular evolution of a binary system as driven exclusively by its leading-order quadrupole gravitational radiation. Observations of a binary in a single interferometer are described by four characteristic quantities: an amplitude

Neutron Star Population Dynamics. I. Millisecond Pulsars

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An analysis of the distribution of globular clusters with postcollapse cores in the Galaxy

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Magnetohydrodynamic shocks in molecular clouds

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Globular cluster evolution in the Galaxy - a global view

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Radiative Instabilities in Simulations of Spherically Symmetric Supernova Blast Waves

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Black hole–neutron star mergers in globular clusters

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