arXiv subjects

James M. Cordes

Publications and source records attributed to James M. Cordes.

102 records · Page 6Linked to original sources

arXiv2000

Pulsar Jets: Implications for Neutron Star Kicks and Initial Spins

We study implications for the apparent alignment of the spin axes, proper-motions, 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 definite symmetry axes. The alignments indicate these pulsars were born either in isolation or with negligible velocity contributions from binary motions. We examine the effects of rotation and the conditions under which spin-kick alignment is produced for various models of neutron star kicks. If the kick is generated when the neutron star first forms by asymmetric mass ejection or/and neutrino emission, then the alignment requires that the protoneutron star possesses an original spin with period $P_s$ much less than the kick timescale, thus spin-averaging the kick forces. The kick timescale ranges from 100 ms to 10 s depending on whether the kick is hydrodynamically driven or neutrino-magnetic field driven. For hydrodynamical models, spin-kick alignment further requires the rotation period of an asymmetry pattern at the radius near shock breakout (>100 km) to be much less than ~100 ms; this is difficult to satisfy unless rotation plays a dynamically important role in the core collapse and explosion ($P_s\lo 1$ ms). Aligned kick and spin vectors are inherent to the slow process of asymmetric electromagnetic radiation from an off-centered magnetic dipole. We reassess the viability of this effect, correcting a factor of 4 error in Harrison and Tademaru's calculation that increases the size of the effect. To produce a kick velocity of order a few hundred km/s requires that the neutron star be born with an initial spin close to 1 ms and that spindown due to r-mode driven gravitational radiation be inefficient compared to standard magnetic braking.

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arXiv1999

Vortex-interface interactions and generation of glitches in pulsars

We show that the crust-core interface in neutron stars acts as a potential barrier to the peripheral neutron vortices approaching the interface in the model in which these are coupled to the proton vortex clusters. This elementary barrier because of the interaction of vortex magnetic flux with the Meissner currents set up by the crustal magnetic field at the interface. The dominant part of the force is derived from to the cluster-interface interaction. As a result of the stopping of the continuous neutron vortex current through the interface, angular momentum is stored in the superfluid layers in the vicinity of the crust-core interface during the interglitch period. Discontinuous annihilation of proton vortices on the boundary restores the neutron vortex current and spins up the observable crust on short time-scales, leading to a glitch in the spin characteristics of a pulsar.

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arXiv1998

G359.87+0.18: An FR II Radio Galaxy 15 Arcminutes from Sgr A*. Implications for the Scattering Region in the Galactic Center

G359.87+0.18 is an enigmatic object located 15' from Sgr A*. It has been variously classified as an extragalactic source, Galactic jet source, and young supernova remnant. We present new observations of G359.87+0.18 between 0.33 and 15 GHz and use these to argue that this source is an Faranoff-Riley II radio galaxy. We are able to place a crude limit on its redshift of z > 0.1. The source has a spectral index α< -1 (S \propto ν^α), suggestive of a radio galaxy with a redshift z >~ 2. The scattering diameters of Sgr A* and several nearby OH masers (~ 1" at 1 GHz) indicate that a region of enhanced scattering is along the line of sight to the Galactic center. If the region covers the Galactic center uniformly, the implied diameter for a background source is at least 600" at 0.33 GHz, in contrast with the observed 20" diameter of G359.87+0.18. Using the scattering diameter of a nearby OH maser OH 359.762+0.120 and the widths of two, nearby, non-thermal threads, G0.08+0.15 and G359.79+0.17, we show that a uniform scattering region should cover G359.87+0.18. We therefore conclude that the Galactic center scattering region is inhomogeneous on a scale of 5' (~ 10 pc at a distance of 8.5 kpc). This scale is comparable to the size scale of molecular clouds in the Galactic center. The close agreement between these two lengths scales is an indication that the scattering region is linked intimately to the Galactic center molecular clouds.

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arXiv1998

Precession of isolated neutron stars I: Effects of imperfect pinning

We consider the precession of isolated neutron stars in which superfluid is not pinned to the stellar crust perfectly. In the case of perfect pinning, Shaham (1977) showed that there are no slowly oscillatory, long-lived modes. When the assumption of perfect pinning is relaxed, new modes are found that can be long-lived, but are expected to be damped rather than oscillatory, unless the drag force on moving superfluid vortex lines has a substantial component perpendicular to the direction of relative motion. The response of a neutron star to external torques, such as the spindown torque, is also treated. We find that when computing the response of a star to perturbations, assuming perfect coupling of superfluid to normal matter from the start can miss some effects.

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arXiv1998

Hyperstrong Radio-Wave Scattering in the Galactic Center. II. A Likelihood Analysis of Free Electrons in the Galactic Center

The scattering diameters of Sgr A* and several nearby OH masers (~ 1" at 1 GHz) indicate that a region of enhanced scattering is along the line of sight to the Galactic center. We combine radio-wave scattering data and free-free emission and absorption measurements in a likelihood analysis that constrains the following parameters of the GC scattering region: The GC-scattering region separation, d; the angular extent of the region, ψ_l; the outer scale on which density fluctuations occur, l_0; and the gas temperature, T. The maximum likelihood estimates of these parameters are d = 133_{-80}^{+200} pc, 0.5 degrees <= ψ_l <~ 1 degrees, and (l_0/1 pc)^{2/3}T^{-1/2} = 10^{-7 +/- 0.8}. As host media for the scattering, we consider the photoionized surface layers of molecular clouds and the interfaces between molecular clouds and the 10^7 K ambient gas. We are unable to make an unambiguous determination, but we favor an interface model in which the scattering medium is hot (T ~ 10^6 K) and dense (n_e ~ 10 cm^{-3}). The GC scattering region produces a 1 GHz scattering diameter for an extragalactic source of 90", if the region is a single screen, or 180", if the region wraps around the GC, as appears probable. We modify the Taylor-Cordes model for the Galactic distribution of free electrons in order to include an explicit GC component. Pulsars seen through this region will have a dispersion measure of approximately 2000 pc cm^{-3}, of which 75% arises from the GC component. We stress the uniqueness of the GC scattering region, probably resulting from the high-pressure environment in the GC.

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arXiv1998

Hyperstrong Radio-Wave Scattering in the Galactic Center. I. A Survey for Extragalactic Sources Seen Through the Galactic Center

The scattering diameters of Sgr A* and several nearby OH masers (~ 1" at 1 GHz) indicate that a region of enhanced scattering is along the line of sight to the Galactic center. The scattering diameter of an extragalactic source seen through this scattering region is larger by the ratio (Sun-GC distance)/(GC-scattering region separation). This ratio could be a factor of a few, if the scattering region is far from the GC and only a random superposition with it, to more than 100, if the scattering region is within the GC. We have used the VLA to survey 10 (11) fields at 20 cm (6 cm) ranging from 7' to 137' from Sgr A*. Our objective was to identify extragalactic sources and measure their scattering diameters so as to constrain this ratio. In order to find sources within these fields, we have employed pdfCLEAN, an algorithm in which sources are identified in an image by comparing the intensity histogram of the image to that expected from a noise-only image. We found over 100 sources, with the faintest sources being 3 mJy. The average number of sources per field is approximately 10, though fields close to Sgr A* tend to contain fewer sources. In a companion paper we use this survey in assessing the likelihood that the scattering region is within the GC. A number of Galactic sources are included in our source catalog. We discuss a double-lobed source, potentially an X-ray quiet version of 1E1740.7-2942, a shell-like structure with a central point source, and a possible radio transient.

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arXiv1998

Spin Evolution of Pulsars with Weakly Coupled Superfluid Interiors

We discuss the spin evolution of pulsars in the case where a superfluid component of the star is coupled to the observable crust on long, spindown timescales. The momentum transfer from the superfluid interior results in an apparent decay of the external torque and, after a dramatic increase, to an asymptotic decrease of the generic value of the braking index, e.g. $n=3$, to values $n\sim 2.5$ if the magnetic field of the star does not decay over its lifetime. In the case where an exponential decay of the magnetic field towards a residual value occurs, the star undergoes a spin-up phase after which it could emerge in the millisecond sector of the $P$-$\dot P$ diagram.

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arXiv1997

The Radial Extent and Warp of the Ionized Galactic Disk. II. A Likelihood Analysis of Radio-Wave Scattering Toward the Anticenter

We use radio-wave scattering data to constrain the distribution of ionized gas in the outer Galaxy. Like previous models, our model for the H II disk includes parameters for the radial scale length and scale height of the H II, but we allow the H II disk to warp and flare. Our model also includes the Perseus arm. We use a likelihood analysis on 11 extragalactic sources and 7 pulsars. Scattering in the Perseus arm is no more than 60% of the level contributed by spiral arms in the inner Galaxy, equivalent to a 1 GHz scattering diameter of 1.5 mas. Our analysis favors an unwarped, nonflaring disk with a 1 kpc scale height, though this may reflect the non-uniform and coarse coverage provided by the available data. The lack of a warp indicates that VLBI observations near 1 GHz with an orbiting station having baseline lengths of a few Earth diameters will not be affected by interstellar scattering at Galactic latitudes |b| ~ 15 degrees. The radial scale length is 15--20 kpc, but the data cannot distinguish between a gradual decrease in the electron density and a truncated distribution. We favor a truncated one, because we associate the scattering with massive star formation, which is also truncated near 20 kpc. The distribution of electron density turbulence decreases more rapidly with Galactocentric distance than does the hydrogen distribution. Alternate ionizing and turbulent agents---the intergalactic ionizing flux and satellite galaxies passing through the disk---do not contribute significantly to scattering. We cannot exclude the possibility that a largely ionized, but quiescent disk extends to >~ 100 kpc, similar to that for some Ly-alpha absorbers.

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arXiv1997

The Radial Extent and Warp of the Ionized Galactic Disk. I. A VLBA Survey of Extragalactic Sources Toward the Anticenter

We report multifrequency Very Long Baseline Array observations of twelve active galactic nuclei seen toward the Galactic anticenter. All of the sources are at |b| < 10 degrees and seven have |b| < 0.5 degrees. Our VLBA observations can detect an enhancement in the angular broadening of these sources due to an extended H II disk, if the orientation of the H II disk in the outer Galaxy is similar to that of the H I disk. Such an extended H II disk is suggested by the C IV absorption in a quasar's spectrum, the appearance of H I disks of nearby spiral galaxies, and models of Ly-alpha cloud absorbers and the Galactic fountain. We detect eleven of the twelve sources at one or more frequencies; nine of the sources are compact and suitable for an angular broadening analysis. A preliminary analysis of the observed angular diameters suggests that the H II disk does not display considerable warping or flaring and does not extend to large Galactocentric distances (R >~ 100 kpc). A companion paper (Lazio & Cordes 1997) combines these observations with those in the literature and presents a more comprehensive analysis.

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arXiv1997

Scintillation-Induced Intermittency in SETI

We consider interstellar scintillations as a cause of intermittency in radio signals from extraterrestrial intelligence (ETI). We demonstrate that scintillations are very likely to allow initial detections of narrowband signals from distant sources (> 100 pc), while making redetections improbable. We consider three models in order to assess the non-repeating, narrowband events found in recent SETI and to analyze large surveys in general: (I) Radiometer noise; (II) A population of constant Galactic sources undergoing interstellar scintillation,; and (III) Real, transient signals (or hardware errors) of either terrestrial or ET origin. We apply likelihood and Bayesian tests of the models to The Planetary Society/Harvard META data. We find that Models II and III are both highly preferred to Model I, but that Models II and III are about equally likely. Ruling out Model II in favor of Model III requires many more reobservations than were conducted in META *or* the reobservation threshold must be much lower than was used in META. *We cannot rule out the possibility that META events are real, intrinsically steady ETI signals.* We recommend that future surveys use thresholds far below the typical false-alarm threshold to lessen the effects of intermittency. The threshold level is best defined in terms of the recording and computational technology that is available at a cost commensurate with other survey costs.

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arXiv1997

X-ray Emission from the Guitar Nebula

We have detected weak soft X-ray emission from the Pulsar Wind Nebula trailing the high velocity star PSR 2224+65 (the `Guitar Nebula'). This X-ray flux gives evidence of γ~10^7 eV particles in the pulsar wind and constrains the properties of the post-shock flow. The X-ray emission is most easily understood if the shocked pulsar wind is partly confined in the nebula and if magnetic fields in this zone can grow to near equipartition values.

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arXiv1996

Finding Radio Pulsars in and Beyond the Galactic Center

Radio-wave scattering is enhanced dramatically for Galactic center sources in a region with radius >~ 15 arc min. Using scattering from Sgr A* and other sources, we show that pulse broadening for pulsars in the Galactic center is {\em at least} 6.3 ν^{-4} seconds (ν= radio frequency in GHz) and is most likely 50--200 times larger because the relevant scattering screen appears to be within the Galactic center region itself. Pulsars beyond---but viewed through---the Galactic center suffer even greater pulse broadening and are angularly broadened by <~ 2 {\em arc min}. Periodicity searches at radio frequencies are likely to find only long period pulsars and, then, only if optimized by using frequencies >~ 7 GHz and by testing for small numbers of harmonics in the power spectrum. The optimal frequency is $ν~ 7.3 GHz (Δ_{0.1}P\sqrtα)^{-1/4}$ where Δ_{0.1} is the distance of the scattering region from Sgr A* in units of 0.1 kpc, P is the period (seconds), and αis the spectral index. A search for compact sources using aperture synthesis may be far more successful than searches for periodicities because the angular broadening is not so large as to desensitize the survey. We estimate that the number of {\em detectable} pulsars in the Galactic center may range from <= 1 to 100, with the larger values resulting from recent, vigorous starbursts. Such pulsars provide unique opportunities for probing the ionized gas, gravitational potential, and stellar population near Sgr A*.

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