arXiv subjects

J. M. Cordes

Publications and source records attributed to J. M. Cordes.

172 records · Page 10Linked to original sources

arXiv2000

Anomalous Radio-Wave Scattering from Interstellar Plasma Structures

This paper considers scattering screens that have arbitrary spatial variations of scattering strength transverse to the line of sight, including screens that are spatially well confined, such as disks and filaments. We calculate the scattered image of a point source and the observed pulse shape of a scattered impulse. The consequences of screen confinement include: (1) Source image shapes that are determined by the physical extent of the screen rather than by the shapes of much-smaller diffracting microirregularities. These include image elongations and orientations that are frequency dependent. (2) Variation with frequency of angular broadening that is much weaker than the trademark ν^{-2} scaling law (for a cold, unmagnetized plasma), including frequency-independent cases; and (3) Similar departure of the pulse broadening time from the usually expected ν^{-4} scaling law. We briefly discuss applications that include scattering of pulses from the Crab pulsar by filaments in the Crab Nebula; image asymmetries from Galactic scattering of the sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background active galactic nuclei by intervening galaxies. We also address the consequences for inferences about the shape of the wavenumber spectrum of electron density irregularities, which depend on scaling laws for the image size and the pulse broadening. Future low-frequency (< 100 MHz) array observations will also be strongly affected by the Galactic structure of scattering material. Our formalism is derived in the context of radio scattering by plasma density fluctuations. It is also applicable to optical, UV and X-ray scattering by grains in the interstellar medium.

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arXiv2000

Parallax and Kinematics of PSR B0919+06 from VLBA Astrometry and Interstellar Scintillometry

Results are presented from a long-term astrometry program on PSR B0919+06 using the NRAO Very Long Baseline Array. With ten observations (seven epochs) between 1994--2000, we measure a proper motion of 18.35 +/- 0.06 mas/yr in RA, 86.56 +/- 0.12 mas/yr in Dec, and a parallax of 0.83 +/- 0.13 mas (68% confidence intervals). This yields a pulsar distance of 1.21 +/- 0.19 kpc, making PSR B0919+06 the farthest pulsar for which a trigonometric parallax has been obtained, and the implied pulsar transverse speed is 505 +/- 80 km/s. Combining the distance estimate with interstellar scintillation data spanning 20 years, we infer the existence of a patchy or clumpy scattering screen along the line of sight in addition to the distributed electron density predicted by models for the Galaxy, and constrain the location of this scattering region to within about 250 parsecs of the Sun. Comparison with the lines of sight towards other pulsars in the same quadrant of the Galaxy permits refinement of our knowledge of the local interstellar matter in this direction.

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arXiv2000

Interstellar Seeing. I. Superresolution Techniques Using Radio Scintillation

Interstellar scintillation can be used to probe transverse sizes of radio sources on scales inaccessible to the nominal resolution of any terrestrial telescope, e.g. $\lesssim 10^{-6}$ arc sec. Methodology is presented that exploits this superresolution phenomenon for both single aperture and interferometer observations. The treatment applies to the saturated (strong-scattering) regime and holds for both thin screens and extended media. A general signal model for radio sources is presented, {\it scintillated amplitude modulated noise}, which applies to compact, incoherent synchrotron sources such as AGNs and gamma-ray burst sources and also to known, coherent sources such as masers and pulsars. The exact probability density function for measured intensities and interferometric visibilities is obtained by solving a general Fredholm problem. An approximate density function is also obtained by using the equivalent number of degrees of freedom in scintillation modulations. The scintillation modulation variance is presented, which includes the effects of source structure and time-bandwidth averaging in the signal processing. Two Bayesian methods are outlined for inferring the sizes of emission regions that use first order statistics of the intensity and visibility. Intensity cross-correlation methods for inferring source sizes are also given. Intensity interferometry in the the radio context is compared to the optical intensity interferometry of Hanbury-Brown and Twiss.

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arXiv2000

Interstellar Seeing. II. The Case of the Vela Pulsar: Source Unresolved

I use a method based on interstellar scintillations for discerning information about source sizes on scales less than one micro-arc sec. I use a comprehensive model for a pulsar signal, scintillated amplitude modulated noise, that includes source fluctuations and noise statistics. The method takes into account time-frequency averaging in the signal processing as well as effects due to source structure. The method is applied to interferometric visibility data on the Vela pulsar which show slightly less scintillation modulation than expected for a point source in the strong scattering regime. The decreased scintillation modulation is likely to be due exclusively to time-frequency averaging rather than from any source size effects. The implied upper limit on source extent, derived through Bayesian inference, is compared to predictions that take into account beaming from the relativistic plasma flow in neutron star magnetospheres. The upper limit for the transverse source size ($\lesssim 400$ km at 95% confidence for a pulsar distance of 0.5 kpc) is easily consistent with conventional models for radio emission regions in pulsar magnetospheres that place them well inside the light cylinder at only a few neutron-star radii from the star's surface.

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arXiv2000

The Gamma Ray Pulsar Population

We apply a likelihood analysis to pulsar detections, pulsar upper limits, and diffuse background measurements from the OSSE and EGRET instruments on the Compton Gamma Ray Observatory to constrain the luminosity law for gamma-ray pulsars and some properties of the gamma-ray pulsar population. We find that the dependence of luminosity on spin period and dipole magnetic field is much steeper at OSSE than at EGRET energies (50-200 keV and >100 MeV, respectively), suggesting that different emission mechanisms are responsible for low- and high-energy gamma-ray emission. Incorporating a spin-down model and assuming a pulsar spatial distribution, we estimate the fraction of the Galactic gamma-ray background due to unidentified pulsars and find that pulsars may be an important component of the OSSE diffuse flux, but are most likely not important at EGRET energies. Using measurements of the diffuse background flux from these instruments, we are able to place constraints on the braking index, initial spin period, and magnetic field of the Galactic pulsar population. We are also able to constrain the pulsar birthrate to be between 1/(25 yr) and 1/(500 yr). Our results are based on a large gamma-ray beam, but they do not scale in a simple way with beam size. With our assumed beam size, the implied gamma-ray efficiency for the EGRET detections is no more than 20%. We estimate that about 20 of the 169 unidentified EGRET sources are probably gamma-ray pulsars. We use our model to predict the pulsar population that will be seen by future gamma-ray instruments and estimate that GLAST will detect roughly 750 gamma-ray pulsars as steady sources, only 120 of which are currently known radio pulsars.

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arXiv1999

A VLA Search for the Geminga Pulsar: A Bayesian Limit on a Scintillating Source

We derive an upper limit of 3 mJy (95% confidence) for the flux density at 317 MHz of the Geminga pulsar (J0633+1746). Our results are based on 7 hours of fast-sampled VLA data, which we averaged synchronously with the pulse period using a period model based on CGRO/EGRET gamma-ray data. Our limit accounts for the fact that this pulsar is most likely subject to interstellar scintillations on a timescale much shorter than our observing span. Our Bayesian method is quite general and can be applied to calculate the fluxes of other scintillated sources. We also present a Bayesian technique for calculating the flux in a pulsed signal of unknown width and phase. Comparing our upper limit of 3 mJy with the quoted flux density of Geminga at 102 MHz, we calculate a lower limit to its spectral index of 2.7. We discuss some possible reasons for Geminga's weakness at radio wavelengths, and the likelihood that many of the unidentified EGRET sources are also radio-quiet or radio-weak Geminga-like pulsars.

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arXiv1998

Pulsar Spin-up, Kinematics, and the Birthrate of Neutron Star Binaries

From considerations of spin evolution and kinematics in the galactic potential, we argue that the pulsars B1913+16, B1534+12, and B2127+11C may be younger than previously assumed, and we find that a lower bound on the formation and merger rate of close double neutron star binaries is $10^{-6.7} f_b^{-1} f_d^{-1}$ yr$^{-1}$, where $f_b$ is the beaming fraction and $f_d$ accounts for the possibility that the known NS-NS binaries are atypical of the underlying population (e.g., if most such binaries are born with shorter orbital periods). If we assume no prior knowledge of the detectable lifetimes of such systems, the rate could be as large as $\simeq 10^{-5.0} f_b^{-1} f_d^{-1}$ yr$^{-1}$. From both plausible bounds on $f_b$ and $f_d$, and a revision of the independently derived limit proposed by Bailes (1996), we argue that a firm upper bound is $10^{-4}$ yr$^{-1}$. We also present a unifying empirical overview of the spin-up of massive binary pulsars based on their distribution in spin period $P$ and spin-down rate $\dot P$, finding evidence for two distinct spin-up processes, one dominated by disk accretion, the other by wind accretion. We argue that the positions of binary pulsars in the $P$-$\dot P$ diagram can be understood if (1) there exists a Galactic population of pulsars in double neutron star systems with combinations of spin and orbital periods that have prevented their detection in surveys to date; (2) recycled pulsars in wide-orbit binaries are not born near the canonical spin-up line in the $P$-$\dot P$ diagram because they were predominantly spun up through wind accretion; and (3) there exists a disfavored evolutionary endpoint for radio pulsars with spin periods 5--30 ms and $\dot P > 10^{-19}$ s-s$^{-1}$.

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arXiv1997

Neutron Star Population Dynamics.II: 3D 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 find that the scale height of the progenitors is approximately 0.13 kpc, that the 3D velocities are distributed in two components with characteristic speeds of 175(+20,-30) km/s and 700(+200,-150) km/s representing 83% and 17% of the population respectively. These results are insensitive to the explicit relation of chronological and spindown ages. We infer that the most probable chronological ages are typically smaller than conventional spindown ages by factors as large as two. We assess mechanisms for producing high-velocity neutron stars in view of the derived velocity distribution function.

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arXiv1997

Neutron Star Population Dynamics.I: Millisecond Pulsars

We study the field millisecond pulsar (MSP) population to infer its intrinsic distribution in spin period and luminosity and to determine its spatial distribution within the Galaxy. Our likelihood analysis on data from extant surveys (22 pulsars with periods <20 ms) accounts for important selection effects. We infer a minimum period cutoff P(min) > 0.65 ms (99% confidence), a period distribution proportional to P^{-2.0 +- 0.33} and a pseudo-luminosity distribution proportional to L_p^{-2.0 +- 0.2} (where L_p = flux density times distance^2, for L_p >= 1.1 mJy kpc^2). We find a vertical scale height 0.65{+0.16,-0.12} kpc. We use our results to estimate the total number and birthrate of MSPs in the disk of the Galaxy. We limit the density contribution of a diffuse halo-like component to <1% of the midplane value. The MSP velocity dispersion is smaller that that of young, long-period pulsars by about a factor of 5. Our best estimate of the 1D velocity kick that is unique to MSP evolution is approximately 40 km s^-1. We discuss the evolutionary relationship of MSPs and low-mass X-ray binaries and prospects for future searches for MSPs.

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arXiv1994

Anisotropic Scattering of OH/IR Stars Towards the Galactic Center

We have made angular broadening measurements in the satellite line of the OH molecule at 1612 MHz of 8 OH/IR stars located near the Galactic Center with the Very Large Array. Early observations with the Very Long Baseline Array had revealed a region of pronounced scattering approximately centered on SgrA*. These new observations show that the compact line emission from these stars is scattered by an amount that is two to three times larger than had been previously seen. Additionally, we measure non-circular scattering disks for these objects, evidence that the electron density variations are distributed anisotropically. The present data suggest that the enhanced turbulence is located near the Galactic Center. However, we cannot completely rule out that the scattering is caused by an unrelated region along the line of sight.

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