W. F. Brisken

Very Long Baseline Array Measurement of Nine Pulsar Parallaxes

We determined the distances to nine pulsars by parallax measurements using the NRAO Very Long Baseline Array, doubling the number of pulsars with accurate distance measurements. Broadband phase modeling was used to calibrate the varying dispersive effects of the ionosphere and remove the resulting phase errors from the phase-referenced VLBI data. The resulting parallaxes have a typical accuracy of 100 microarcseconds or better, yielding distances measurements as accurate as 2%. We also report new proper motion measurements of these pulsars, accurate to 0.4 mas per year or better.We determined the distances to nine pulsars by parallax measurements using the NRAO Very Long Baseline Array, doubling the number of pulsars with accurate distance measurements. Broadband phase modeling was used to calibrate the varying dispersive effects of the ionosphere and remove the resulting phase errors from the phase-referenced VLBI data. The resulting parallaxes have a typical accuracy of 100 μas or better, yielding distance measurements as accurate as 2%. We also report new proper motion measurements of these pulsars, accurate to 400 μas yr-1 or better.

PERIODIC BURSTS OF COHERENT RADIO EMISSION FROM AN ULTRACOOL DWARF

We report the detection of periodic (p = 1.96 hr) bursts of extremely bright, 100% circularly polarized, coherent radio emission from the M9 dwarf TVLM 513-46546. Simultaneous photometric monitoring observations have established this periodicity to be the rotation period of the dwarf. These bursts, which were not present in previous observations of this target, confirm that ultracool dwarfs can generate persistent levels of broadband, coherent radio emission, associated with the presence of kG magnetic fields in a large-scale, stable configuration. Compact sources located at the magnetic polar regions produce highly beamed emission generated by the electron cyclotron maser instability, the same mechanism known to generate planetary coherent radio emission in our solar system. The narrow beams of radiation pass our line of sight as the dwarf rotates, producing the associated periodic bursts. The resulting radio light curves are analogous to the periodic light curves associated with pulsar radio emission highlighting TVLM 513-46546 as the prototype of a new class of transient radio source.

To Lease or Not to Lease from Storage Clouds

In this paper, storage clouds are online services for leasing disk storage. A new modeling tool, formulated from empirical data spanning many years, lets organizations rationally evaluate the benefit of using storage clouds versus purchasing hard disk drives. The business of selling infrastructure as a service through the Internet is growing. This technology trend, also known as cloud computing, lets individuals and organizations outsource their IT requirements to remote data centers, paying for only what they use.Several online services currently lease storage infrastructure. These storage clouds let anyone with a credit card purchase storage capacity online, paying a monthly fee for the storage they use. With the significant growth of societys storage requirements, and the availability of pay-per-use online storage services, when should a consumer consider using storage clouds? The paper focuses on the problem of resolving this buy-or-lease storage decision.

Precision Astrometry with the Very Long Baseline Array: Parallaxes and Proper Motions for 14 Pulsars

Astrometry can bring powerful constraints to bear on a variety of scientific questions about neutron stars, including their origins, astrophysics, evolution, and environments. Using phase-referenced observations at the Very Long Baseline Array (VLBA), in conjunction with pulsar gating and in-beam calibration, we have measured the parallaxes and proper motions for 14 pulsars. The smallest measured parallax in our sample is 0.13 ? 0.02 mas for PSR B1541+09, which has a most probable distance of 7.2+1.3 ?1.1 kpc. We detail our methods, including initial VLA surveys to select candidates and find in-beam calibrators, VLBA phase-referencing, pulsar gating, calibration, and data reduction. The use of the bootstrap method to estimate astrometric uncertainties in the presence of unmodeled systematic errors is also described. Based on our new model-independent estimates for distance and transverse velocity, we investigate the kinematics and birth sites of the pulsars and revisit models of the Galactic electron density distribution. We find that young pulsars are moving away from the Galactic plane, as expected, and that age estimates from kinematics and pulsar spindown are generally in agreement, with certain notable exceptions. Given its present trajectory, the pulsar B2045 ? 16 was plausibly born in the open cluster NGC 6604. For several high-latitude pulsars, the NE2001 electron density model underestimates the parallax distances by a factor of 2, while in others the estimates agree with or are larger than the parallax distances, suggesting that the interstellar medium is irregular on relevant length scales. The VLBA astrometric results for the recycled pulsar J1713+0747 are consistent with two independent estimates from pulse timing, enabling a consistency check between the different reference frames.

Rotational Modulation of the Radio Emission from the M9 Dwarf TVLM 513?46546: Broadband Coherent Emission at the Substellar Boundary?

The Very Large Array was used to observe the ultracool rapidly rotating M9 dwarf TVLM 513-46546 simultaneously at 4.88 and 8.44 GHz. The radio emission was determined to be persistent, variable, and periodic at both frequencies with a period of ~2 hr. This periodicity is in excellent agreement with the estimated period of rotation of the dwarf based on its v sin i of ~60 km s^(-1). This rotational modulation places strong constraints on the source size of the radio-emitting region and hence the brightness temperature of the associated emission. We find the resulting high brightness temperature, together with the inherent directivity of the rotationally modulated component of the emission, difficult to reconcile with incoherent gyrosynchrotron radiation. We conclude that a more likely source is coherent, electron cyclotron maser emission from the low-density regions above the magnetic poles. This model requires the magnetic field of TVLM 513-46546 to take the form of a large-scale, stable dipole or multipole with surface field strengths up to at least 3 kG. We discuss a mechanism by which broadband, persistent electron cyclotron maser emission can be sustained in the low-density regions of the magnetospheres of ultracool dwarfs. A second nonvarying, unpolarized component of the emission may be due to depolarization of the coherent electron cyclotron maser emission or, alternatively, incoherent gyrosynchrotron or synchrotron radiation from a population of electrons trapped in the large-scale magnetic field.

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 kpc) and transverse velocity (1083 km s-1), the highest velocity directly measured for a neutron star. We trace the pulsar back from its present Galactic latitude of 523 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.

Overview of the coordinated ground-based observations of Titan during the Huygens mission

Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titans atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titans atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission.

A PARALLAX DISTANCE AND MASS ESTIMATE FOR THE TRANSITIONAL MILLISECOND PULSAR SYSTEM J1023+0038

The recently discovered transitional millisecond pulsar system J1023+0038 exposes a crucial evolutionary phase of recycled neutron stars for multiwavelength study. The system, comprising the neutron star itself, its stellar companion, and the surrounding medium, is visible across the electromagnetic spectrum from the radio to X-ray/gamma-ray regimes and offers insight into the recycling phase of millisecond pulsar evolution. Here, we report on multiple-epoch astrometric observations with the Very Long Baseline Array (VLBA) which give a system parallax of 0.731 ± 0.022 milliarcseconds (mas) and a proper motion of 17.98 ± 0.05 mas yr–1. By combining our results with previous optical observations, we are able to use the parallax distance of 1368+42 – 39 pc to estimate the mass of the pulsar to be 1.71 ± 0.16 M ☉, and we are also able to measure the three-dimensional space velocity of the system to be 126 ± 5 km s–1. Despite the precise nature of the VLBA measurements, the remaining ~3% distance uncertainty dominates the 0.16 M ☉ error on our mass estimate.

The Distance and Radius of the Neutron Star PSR B0656+14

We present the result of astrometric observations of the radio pulsar PSR B0656+14, made using the Very Long Baseline Array. The parallax of the pulsar is π = 3.47 ± 0.36 milliarcsec, yielding a distance 288 pc. This independent distance estimate has been used to constrain existing models of thermal X-ray emission from the neutron stars photosphere. Simple blackbody fits to the X-ray data formally yield a neutron star radius R∞ ~ 7-8.5 km. With more realistic fits to a magnetized hydrogen atmosphere, any radius between ~13 and ~20 km is allowed.

Proper-Motion Measurements with the VLA. II. Observations of 28 Pulsars

Using the Very Large Array, we have measured the proper motions of 28 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 (σrm1D = 99 km s-1) and 80% in a high-velocity component (σ1D = 294 km s-1). 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.