We present the results of PSRπ, a large astrometric project targeting radio pulsars using the Very Long Baseline Array (VLBA). From our astrometric database of 60 pulsars, we have obtained ...parallax-based distance measurements for all but 3, with a parallax precision that is typically ∼45 as and approaches 10 as in the best cases. Our full sample doubles the number of radio pulsars with a reliable ( 5 ) model-independent distance constraint. Importantly, many of the newly measured pulsars are well outside the solar neighborhood, and so PSRπ brings a near-tenfold increase in the number of pulsars with a reliable model-independent distance at d > 2 kpc. Our results show that both widely used Galactic electron density distribution models contain significant shortcomings, particularly at high Galactic latitudes. When comparing our results to pulsar timing, two of the four millisecond pulsars in our sample exhibit significant discrepancies in their proper motion estimates. With additional VLBI observations that extend our sample and improve the absolute positional accuracy of our reference sources, we will be able to additionally compare pulsar absolute reference positions between VLBI and timing, which will provide a much more sensitive test of the correctness of the solar system ephemerides used for pulsar timing. Finally, we use our large sample to estimate the typical accuracy attainable for differential VLBA astrometry of pulsars, showing that for sufficiently bright targets observed eight times over 18 months, a parallax uncertainty of 4 as per arcminute of separation between the pulsar and calibrator can be expected.
Context. Quantifying the fraction of active galactic nuclei (AGN) in the faint radio population and understanding their relation with star-forming activity are fundamental to studies of galaxy ...evolution. Very long baseline interferometry (VLBI) observations are able to identify AGN above relatively low redshifts (z> 0.1) since they provide milli-arcsecond resolution. Aims. We have created an AGN catalogue from 2865 known radio sources observed in the Cosmic Evolution Survey (COSMOS) field, which has exceptional multi-wavelength coverage. With this catalogue we intend to study the faint radio sky with statistically relevant numbers and to analyse the AGN – host galaxy co-evolution, making use of the large amount of ancillary data available in the field. Methods. Wide-field VLBI observations were made of all known radio sources in the COSMOS field at 1.4 GHz to measure the AGN fraction, in particular in the faint radio population. We describe in detail the observations, data calibration, source detection and flux density measurements, parts of which we have developed for this survey. The combination of number of sources, sensitivity, and area covered with this project are unprecedented. Results. We have detected 468 radio sources, expected to be AGN, with the Very Long Baseline Array (VLBA). This is, to date, the largest sample assembled of VLBI detected sources in the sub-mJy regime. The input sample was taken from previous observations with the Very Large Array (VLA). We present the catalogue with additional optical, infrared and X-ray information. Conclusions. We find a detection fraction of 20 ± 1%, considering only those sources from the input catalogue which were in principle detectable with the VLBA (2361). As a function of the VLA flux density, the detection fraction is higher for higher flux densities, since at high flux densities a source could be detected even if the VLBI core accounts for a small percentage of the total flux density. As a function of redshift, we see no evolution of the detection fraction over the redshift range 0.5 <z< 3. In addition, we find that faint radio sources typically have a greater fraction of their radio luminosity in a compact core – ~70% of the sub-mJy sources detected with the VLBA have more than half of their total radio luminosity in a VLBI-scale component, whereas this is true for only ~30% of the sources that are brighter than 10 mJy. This suggests that fainter radio sources differ intrinsically from brighter ones. Across our entire sample, we find the predominant morphological classification of the host galaxies of the VLBA detected sources to be early type (57%), although this varies with redshift and at z> 1.5 we find that spiral galaxies become the most prevalent (48%). The number of detections is high enough to study the faint radio population with statistically significant numbers. We demonstrate that wide-field VLBI observations, together with new calibration methods such as multi-source self-calibration and mosaicing, result in information which is difficult or impossible to obtain otherwise.
Abstract
Current methods of measuring distances to pulsar scattering screens rely on a single screen dominating the scintillation pattern. We present a novel technique to reconstruct the scattered ...flux of a pulsar and solve for the scattering geometry in cases where the scattering environment along the line of sight to the pulsar is complex and may be composed of multiple scattering screens. This technique combines interferometric visibilities with cross-correlations of single-station intensities. It takes advantage of the fact that if one considers the interference of radiation from two points in the scattered image in delay–delay rate space, the visibilities are sensitive to the mean angular position of the points, while the cross-correlated intensities are sensitive to their angular separation. By combining the visibilities and the cross-correlated intensities, it is possible to measure the angular locations of both points in the pair. We show that this technique is able to reconstruct the published scattering geometry of PSR B0834+06. We then apply this technique to one-dimensional simulations of more complicated scattering systems, where we find that it can distinguish features from different scattering screens. This technique holds promise for studies of the interstellar medium and pulsars themselves: It will allow the application of scintillometry techniques, such as resolving pulsar emission regions using interstellar scattering, to sources for which a current lack of understanding of the scattering environment has precluded their use.
Using astrometric VLBI observations, we have determined the parallax of the black hole X-ray binary V404 Cyg to be 0.418 +- 0.024 mas, corresponding to a distance of 2.39 +- 0.14 kpc, significantly ...lower than the previously accepted value. This model-independent estimate is the most accurate distance to a Galactic stellar-mass black hole measured to date. With this new distance, we confirm that the source was not super-Eddington during its 1989 outburst. The fitted distance and proper motion imply that the black hole in this system likely formed in a supernova, with the peculiar velocity being consistent with a recoil (Blaauw) kick. The size of the quiescent jets inferred to exist in this system is <1.4 AU at 22 GHz. Astrometric observations of a larger sample of such systems would provide useful insights into the formation and properties of accreting stellar-mass black holes.
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.
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 62 hr. This periodicity is in excellent agreement with the estimated period of rotation of the dwarf based on its v sin i of 660 km s super(-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.
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.
Abstract
Investigations of small-scale structure in the interstellar medium through pulsar scintillation and the use of scattering screens as interferometers to resolve pulsar emission regions rely ...on reconstructions of the spatial distributions of scattered pulsar flux and the distances to scattering screens. In many cases, a single, highly anisotropic scattering screen is responsible for pulsar scintillation. This leads to a sparsity of information in the secondary spectrum which can be leveraged to measure the scattering geometry and map the distribution of scattered images of the pulsar using Very Long Baseline Interferometry. It is also possible to make these same measurements using only the autocorrelations from simultaneous observations at multiple stations. We compare the results from these two methods applied to PSR B0834+06, and find that the two main measurables, the effective distance and velocity, obtained from these two analyses agree to within ∼10 per cent. The relative ease of using autocorrelations from simultaneous single-dish measurements, where the data rates are much lower, only millisecond clock precision is required and the results are less sensitive to the calibration of the data, means that this method opens up a more observationally accessible route for mapping pulsar scattering screens. Since this method does not require the recording and storage of baseband data, it can be applied to ultrawide-band observations, which typically have unmanageable baseband data rates but are vital for testing theoretical models of pulsar scintillation.
The effects of the antenna far-field power pattern limits the imaging performance of modern wide-bandwidth, high-sensitivity interferometric radio telescopes. Given a model for the aperture ...illumination pattern (AIP) of the antenna, referred to as the A-term, the wide-band (WB) A-Projection algorithm corrects for the effects of its time, frequency, and polarization structure. The level to which this correction is possible depends on how accurately the A-term represents the true AIP. In this paper, we describe the A-Solver methodology that combines physical modeling with optimization to holographic measurements to build an accurate model for the AIP. Using a parametrized ray-tracing code as the predictor, we solve for the frequency dependence of the antenna optics and show that the resulting low-order model for the Karl G. Jansky Very Large Array antenna captures the dominant frequency-dependent terms. The A-Solver methodology described here is generic and can be adapted for other types of antennas as well. The parameterization is based on the physical characteristics of the antenna structure and optics and is therefore arguably a compact representation (minimized degrees of freedom) of the frequency-dependent structure of the antenna A-term. In this paper, we also show that the parameters derived from A-Solver methodology are expected to improve sensitivity and imaging performance out to the first side-lobe of the antenna.
PDF
