arXiv subjects

J. M. Cordes

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

At least 145 records · Page 8Linked to original sources

arXiv2006

The Cornell High-order Adaptive Optics Survey for Brown Dwarfs in Stellar Systems-II: Results from Monte Carlo Population Analyses

In this second of a two-paper sequence, we present Monte Carlo population simulation results of brown dwarf companion data collected during the Cornell High-order Adaptive Optics Survey for brown dwarf companions (CHAOS). Making reasonable assumptions of orbital parameters (random inclination, random eccentricity and random longitude of pericentre) and age distributions, and using published mass functions, we find that the brown dwarf companion fraction around main sequence stars is 0.0%-9.3% for the 25-100 AU semi-major axis region. We find a corresponding L-dwarf companion fraction of 0.0%-3.3%. We compare our population analysis methods and results with techniques and results presented by several other groups. In this comparison we discover that systematic errors (most notably resulting from orbital projection effects) occur in the majority of previously published brown dwarf companion population estimates, leading authors to claim results not supported by the observational data.

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arXiv2006

Rocking the Lighthouse: Circumpulsar Asteroids and Radio Intermittency

We propose that neutral, circumpulsar debris entering the light cylinder can account for many time-dependent pulsar phenomena that are otherwise difficult to explain. Neutral material avoids propeller ejection and injects sufficient charges -- after heating, evaporation, and ionization -- to alter current flows and pair-production and thus trigger, detune, or extinguish coherent emission. Relevant phenomena, with time scales from seconds to months, include nulls, rotating radio transients (RRATs), rapid changes in pulse profile (``mode changes''), variable subpulse drift rates, quasi-periodic bursts from B1931+24, and torque variations. Over the 10 Myr lifetime of a canonical pulsar with trillion-gauss surface magnetic field, less than a millionth of an Earth mass of material is needed to modulate the Goldreich-Julian current by 100%. Circumpulsar material originates from metal-rich, supernova fallback gas that aggregates into asteroids. Debris disks can inject sufficient material on time scales of interest, yet be too tenuous to form large planets detectable in pulse timing data. Asteroid migration results from collisions and the radiation-driven Yarkovsky and Poynting-Robertson effects. For B1931+24, an asteroid in a $\sim 40$~day elliptical orbit pollutes the magnetosphere stochastically through collisions with other debris. Injection is less likely for hot, young and highly magnetized pulsars or millisecond pulsars that pre-ionize any debris material well outside their small magnetospheres. Injection effects will therefore be most prominent in long-period, cooler pulsars, consistent with the distribution of relevant objects in perid and period derivative. A pulsar's spin history and its radiation-beam orientation may influence whether it displays nulling, RRATs and other effects.

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arXiv2006

Arecibo and the ALFA Pulsar Survey

The recently started Arecibo L-band Feed Array (ALFA) pulsar survey aims to find ~1000 new pulsars. Due to its high time and frequency resolution the survey is especially sensitive to millisecond pulsars, which have the potential to test gravitational theories, detect gravitational waves and probe the neutron-star equation of state. Here we report the results of our preliminary analysis: in the first months we have discovered 21 new pulsars. One of these, PSR J1906+0746, is a young 144-ms pulsar in a highly relativistic 3.98-hr low-eccentricity orbit. The 2.61 +- 0.02 solar-mass system is expected to coalesce in ~300 Myr and contributes significantly to the computed cosmic inspiral rate of compact binary systems.

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arXiv2005

Transient radio bursts from rotating neutron stars

The `radio sky' is relatively unexplored for transient signals, although the potential of radio-transient searches is high, as demonstrated recently by the discovery of a previously unknown type of source which varies on timescales of minutes to hours. Here we report a new large-scale search for radio sources varying on much shorter timescales. This has revealed 11 objects characterized by single, dispersed bursts having durations between 2 and 30 ms. The average time intervals between bursts range from 4 minutes to 3 hours, with radio emission typically detectable for < 1 s per day. From an analysis of the burst arrival times, we have identified periodicities in the range 0.4 - 7 s for ten of the 11 sources, suggesting a rotating neutron star origin. Despite the small number of sources presently detected, their ephemeral nature implies a total Galactic population which significantly exceeds that of the regularly pulsing radio pulsars. Five of the ten sources have periods greater than 4 s, and period derivatives have been measured for three of the sources, with one having a very high inferred magnetic field of 5e13 G, suggesting that this new population is related to other classes of isolated neutron stars observed at X-ray and gamma-ray wavelengths.

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arXiv2005

The young, highly relativistic binary pulsar J1906+0746

We report the discovery of PSR J1906+0746, a young 144-ms pulsar in a highly relativistic 3.98-hr orbit with an eccentricity of 0.085 and expected gravitational wave coalescence time of 300 Myr. The new pulsar was found during precursor survey observations with the Arecibo 1.4-GHz feed array system and retrospectively detected in the Parkes Multibeam plane pulsar survey data. From radio follow-up observations with Arecibo, Jodrell Bank, Green Bank, and Parkes, we have measured the spin-down and binary parameters of the pulsar and its basic spectral and polarization properties. We also present evidence for pulse profile evolution, which is likely due to geodetic precession, a relativistic effect caused by the misalignment of the pulsar spin and total angular momentum vectors. Our measurements show that PSR J1906+0746 is a young object with a characteristic age of 112 kyr. From the measured rate of orbital periastron advance 7.57+/-0.03 deg/yr, we infer a total system mass of 2.61+/-0.02 Msun. While these parameters suggest that the PSR J1906+0746 binary system might be a younger version of the double pulsar system, intensive searches for radio pulses from the companion have so far been unsuccessful. It is therefore not known whether the companion is another neutron star or a massive white dwarf. Regardless of the nature of the companion, a simple calculation suggests that the Galactic birth rate of binaries similar to PSR J1906+0746 is 60/Myr. This implies that PSR J1906+0746 will make a significant contribution to the computed cosmic inspiral rate of compact binary systems.

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arXiv2005

Arecibo Pulsar Survey Using ALFA. I. Survey Strategy and First Discoveries

We report results from the initial stage of a long-term pulsar survey of the Galactic plane using the Arecibo L-band Feed Array (ALFA), a seven-beam receiver operating at 1.4 GHz with 0.3 GHz bandwidth. The search targets Galactic latitudes |b| < 5 deg in the longitude ranges 32 deg < l < 77 deg and 168 deg < l < 77 deg. Data discussed here were collected over a 100 MHz passband centered on 1.42 GHz using a spectrometer that recorded 256 channels every 64 microsec. In a preliminary, standard period-DM analysis, we have detected 29 previously known pulsars and discovered 11 new ones. One of these, with a period of 69 ms and a low characteristic age of 82 kyr, is a plausible candidate for association with the unidentified EGRET source 3EG J1928+1733. Another is a non-recycled pulsar in a relativistic binary with orbital period of 3.98 hr. We also search the data for isolated dispersed pulses, a technique that yielded discovery of an extremely sporadic radio emitter with a spin period of 1.2 s. Simulations we have carried out indicate that about 1000 new pulsars will be found in the ALFA survey. In addition to providing a large sample for use in population analyses and for probing the magnetoionic interstellar medium, the survey maximizes the chances of finding rapidly spinning millisecond pulsars and pulsars in compact binary systems. Our search algorithms will exploit the multiple data streams from ALFA to discriminate between radio frequency interference and celestial signals, including pulsars and possibly new classes of transient radio sources.

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arXiv2005

Pulsar Astrometry at the Microarcsecond Level

Determination of pulsar parallaxes and proper motions addresses fundamental astrophysical questions. We have recently finished a VLBI astrometry project to determine the proper motions and parallaxes of 27 pulsars, thereby doubling the total number of pulsar parallaxes. Here we summarise our astrometric technique and present the discovery of a pulsar moving in excess of 1000 km/s. As an example of the application of high precision pulsar astrometry we also infer the identification of 2 pulsars originating from a disrupted binary in the Cygnus Superbubble.

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arXiv2005

Getting Its Kicks: A VLBA Parallax for the Hyperfast Pulsar B1508+55

The highest velocity neutron stars establish stringent constraints on natal kicks, asymmetries in supernova core collapse, and the evolution of close binary systems. Here we present the first results of a long-term pulsar astrometry program using the VLBA. We measure a proper motion and parallax for the pulsar B1508+55, leading to model-independent estimates of its distance (2.37+0.23-0.20 kpc) and transverse velocity (1083+103-90 km/s), the highest velocity directly measured for a neutron star. We trace the pulsar back from its present Galactic latitude of 52.3 degrees to a birth site in the Galactic plane near the Cyg OB associations, and find that it will inevitably escape the Galaxy. Binary disruption alone is insufficient to impart the required birth velocity, and a natal kick is indicated. A composite scenario including a large kick along with binary disruption can plausibly account for the high velocity.

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arXiv2005

Arecibo timing and single-pulse observations of 17 pulsars

We report on timing and single-pulse observations of 17 pulsars discovered at the Arecibo observatory. The highlights of our sample are the recycled pulsars J1829+2456, J1944+0907 and the drifting subpulses observed in PSR J0815+0939. For the double neutron star binary J1829+2456, in addition to improving upon our existing measurement of relativistic periastron advance, we have now measured the pulsar's spin period derivative. This new result sets an upper limit on the transverse speed of 120 km/s and a lower limit on the characteristic age of 12.4 Gyr. From our measurement of proper motion of the isolated 5.2-ms pulsar J1944+0907, we infer a transverse speed of 188 +/- 65 km/s. This is higher than that of any other isolated millisecond pulsar. An estimate of the speed, using interstellar scintillation, of 235 +/- 45 km/s indicates that the scattering medium along the line of sight is non-uniform. We discuss the drifting subpulses detected from three pulsars in the sample, in particular the remarkable drifting subpulse properties of the 645-ms pulsar J0815+0939. Drifting is observed in all four components of the pulse profile, with the sense of drift varying among the different components. This unusual `bi-drifting'' behaviour challenges standard explanations of the drifting subpulse phenomenon.

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arXiv2005

Pulsars as Tools for Fundamental Physics and Astrophysics

The sheer number of pulsars discovered by the SKA, in combination with the exceptional timing precision it can provide, will revolutionize the field of pulsar astrophysics. The SKA will provide a complete census of pulsars in both the Galaxy and in Galactic globular clusters that can be used to provide a detailed map of the electron density and magnetic fields, the dynamics of the systems, and their evolutionary histories. This complete census will provide examples of nearly every possible outcome of the evolution of massive stars, including the discovery of very exotic systems such as pulsar black-hole systems and sub-millisecond pulsars, if they exist. These exotic systems will allow unique tests of the strong field limit of relativistic gravity and the equation of state at extreme densities. Masses of pulsars and their binary companions -- planets, white dwarfs, other neutron stars, and black holes -- will be determined to $\sim 1$% for hundreds of objects. With the SKA we can discover and time highly-stable millisecond pulsars that comprise a pulsar-timing array for the detection of low-frequency gravitational waves. The SKA will also provide partial censuses of nearby galaxies through periodicity and single-pulse detections, yielding important information on the intergalactic medium.

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arXiv2005

RFI Identification and Mitigation Using Simultaneous Dual Station Observations

RFI mitigation is a critically important issue in radio astronomy using existing instruments as well as in the development of next-generation radio telescopes, such as the Square Kilometer Array (SKA). Most designs for the SKA involve multiple stations with spacings of up to a few thousands of kilometers and thus can exploit the drastically different RFI environments at different stations. As demonstrator observations and analysis for SKA-like instruments, and to develop RFI mitigation schemes that will be useful in the near term, we recently conducted simultaneous observations with Arecibo Observatory and the Green Bank Telescope (GBT). The observations were aimed at diagnosing RFI and using the mostly uncorrelated RFI between the two sites to excise RFI from several generic kinds of measurements such as giant pulses from Crab-like pulsars and weak HI emission from galaxies in bands heavily contaminated by RFI. This paper presents observations, analysis, and RFI identification and excision procedures that are effective for both time series and spectroscopy applications using multi-station data.

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arXiv2004

The Exploration of the Unknown

The Square Kilometre Array is conceived as a telescope which will both test fundamental physical laws and transform our current picture of the Universe. However, the scientific challenges outlined in this book are today's problems--will they still be the outstanding problems that will confront astronomers in the period 2020 to 2050 and beyond, when the SKA will be in its most productive years? If history is any example, the excitement of the SKA will not be in the old questions which are answered, but the new questions that will be raised by the new types of observations it will permit. The SKA is a tool for as-yet-unborn users and there is an onus on its designers to allow for the exploration of the unknown. We outline a philosophy for the design and operation of the SKA that can lead the radio astronomers in the 21st century to add to the many discoveries of new phenomena made by radio astronomers in the 20th century.

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arXiv2004

The Microarcsecond Sky and Cosmic Turbulence

Radio waves are imprinted with propagation effects from ionized media through which they pass. Owing to electron density fluctuations, compact sources (pulsars, masers, and compact extragalactic sources) can display a wide variety of scattering effects. These scattering effects, particularly interstellar scintillation, can be exploited to provide *superresolution*, with achievable angular resolutions (<~ 1 microarcsecond) far in excess of what can be obtained by very long baseline interferometry on terrestrial baselines. Scattering effects also provide a powerful sub-AU probe of the microphysics of the interstellar medium, potentially to spatial scales smaller than 100 km, as well as a tracer of the Galactic distribution of energy input into the interstellar medium through a variety of integrated measures. Coupled with future gamma-ray observations, SKA observations also may provide a means of detecting fainter compact gamma-ray sources. Though it is not yet clear that propagation effects due to the intergalactic medium are significant, the SKA will either detect or place stringent constraints on intergalactic scattering.

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arXiv2004

Strong-Field Tests of Gravity Using Pulsars and Black Holes

The sensitivity of the SKA enables a number of tests of theories of gravity. A Galactic Census of pulsars will discover most of the active pulsars in the Galaxy beamed toward us. In this census will almost certainly be pulsar-black hole binaries as well as pulsars orbiting the super-massive black hole in the Galactic centre. These systems are unique in their capability to probe the ultra-strong field limit of relativistic gravity. These measurements can be used to test the Cosmic Censorship Conjecture and the No-Hair theorem. The large number of millisecond pulsars discovered with the SKA will also provide a dense array of precision clocks on the sky. These clocks will act as the multiple arms of a huge gravitational wave detector, which can be used to detect and measure the stochastic cosmological gravitational wave background that is expected from a number of sources.

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arXiv2004

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.

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arXiv2004

New Binary and Millisecond Pulsars from Arecibo Drift-Scan Searches

We discuss four recycled pulsars found in Arecibo drift-scan searches. PSR J1944+0907 has a spin period of 5.2 ms and is isolated. The 5.8-ms pulsar J1453+19 may have a low-mass companion. We discuss these pulsars in the context of isolated millisecond pulsar formation and the minimum spin period of neutron stars. The isolated 56-ms pulsar J0609+2130 is possibly the remnant of a disrupted double neutron star binary. The 41-ms pulsar J1829+2456 is in a relativistic orbit. Its companion is most likely another neutron star, making this the eighth known double neutron star binary system.

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arXiv2004

Separated at Birth: The origin of the pulsars B2020+28 and B2021+51 in the Cygnus Superbubble

High precision astrometric data have enabled us to determine the trajectories through the Galactic potential for a growing number of pulsars. This has resulted in the discovery of a pulsar pair (B2020+28 and B2021+51) that has a common origin in the Cygnus Superbubble or in one of its related OB associations at an epoch which is comparable with the spin-down ages of the pulsars. Analysis of the Galactic orbits indicates that the progenitors of the pulsars had similar masses and were in a binary system, which was disrupted after the second supernova explosion. The implied pulsar birth velocities are consistent with the high velocities of neutron stars in general. The initial spin period of the pulsar that was formed in the second supernova explosion was approximately 200 ms. A further increase in astrometric accuracy will allow us to more tightly constrain the birth velocities and the kick velocities that were imparted by the two respective supernova explosions. Two additional pulsars in the sample of 24 with parallax measurements may also have originated in the Cygnus Superbubble.

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arXiv2004

PSR J1829+2456: a relativistic binary pulsar

We report the discovery of a new binary pulsar, PSR J1829+2456, found during a mid-latitude drift-scan survey with the Arecibo telescope. Our initial timing observations show the 41-ms pulsar to be in a 28-hr, slightly eccentric, binary orbit. The advance of periastron, omegadot = 0.28 +/- 0.01 deg/yr is derived from our timing observations spanning 200 days. Assuming that the advance of periastron is purely relativistic and a reasonable range of neutron star masses for PSR J1829+2456 we constrain the companion mass to be between 1.22 Msun and 1.38 Msun, making it likely to be another neutron star. We also place a firm upper limit on the pulsar mass of 1.38 Msun. The expected coalescence time due to gravitational-wave emission is long (~60 Gyr) and this system will not significantly impact upon calculations of merger rates that are relevant to upcoming instruments such as LIGO.

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