47 research outputs found
Pulsar Parallaxes at 5 GHz with the Very Long Baseline Array
We present the first pulsar parallaxes measured with phase-referenced pulsar
VLBI observations at 5 GHz. Due to the steep spectra of pulsars, previous
astrometric measurements have been at lower frequencies. However, the strongest
pulsars can be observed at 5 GHz, offering the benefit of lower combined
ionospheric and tropospheric phase errors, which usually limit VLBI astrometric
accuracy. The pulsars B0329+54, B0355+54 and B1929+10 were observed for 7
epochs spread evenly over 2 years. For B0329+54, large systematic errors lead
to only an upper limit on the parallax (pi < 1.5 mas). A new proper motion and
parallax were measured for B0355+54 (pi = 0.91 +- 0.16 mas), implying a
distance of 1.04+0.21-0.16 kpc and a transverse velocity of 61+12-9 km/s. The
parallax and proper motion for B1929+10 were significantly improved (pi = 2.77
+- 0.07 mas), yielding a distance of 361+10-8 pc and a transverse velocity of
177+4-5 km/s. We demonstrate that the astrometric errors are correlated with
the angular separation between the phase reference calibrator and the target
source, with significantly lower errors at 5 GHz compared to 1.6 GHz. Finally,
based on our new distance determinations for B1929+10 and B0355+54, we derive
or constrain the luminosities of each pulsar at high energies. We show that,
for thermal emission models, the emitting area for X-rays from PSR B1929+10 is
roughly consistent with the canonical size for a heated polar cap, and that the
conversion of spin-down power to gamma-ray luminosity in B0355+54 must be low.
The new proper motion for B1929+10 also implies that its progenitor is unlikely
to have been the binary companion of the runaway O-star zeta-Ophiuchi.Comment: 8 pages, including 3 figures and 3 tables; emulateapj; ApJ submitte
Atomic hydrogen in the one-sided "compact double" radio galaxy 2050+364
European VLBI Network spectral imaging of the "compact double" radio source
2050+364 in the UHF band at 1049 MHz has resolved the HI absorbing region, and
has shown a faint continuum component to the North (N), in addition to the
well-known East-West double (E, W). Re-examination of VLBI continuum images at
multiple frequencies suggests that 2050+364 may well be a one-sided core-jet
source, which appears as a double over a limited frequency range. One of the
dominant features, W, would then be the innermost visible portion of the jet,
and could be at or adjacent to the canonical radio core. The other, E, is
probably related to shocks at a sudden bend of the jet, towards extended
steep-spectrum region N. A remarkably deep and narrow HI absorption line
component extends over the entire projected extent of 2050+364. It coincides in
velocity with the [OIII] optical doublet lines to within 10 km/s. This HI
absorption could arise in the atomic cores of NLR clouds, and the motion in the
NLR is then remarkably coherent both along the line-of-sight and across a
projected distance of > 300 pc on the plane of the sky. Broader, shallower HI
absorption at lower velocities covers only the plausible core area W. This
absorption could be due to gas which is either being entrained by the inner jet
or is flowing out from the accretion region; it could be related to the BLR.Comment: 10 pages, 7 figures. Accepted in A&
Multi-frequency VLBA Observations of the Compact Double B2 2050+36: Constraints on Interstellar Scattering Revisited
We present multi-frequency observations with the Very Long Baseline Array of
the compact double radio source B2 2050+36. Our observations are at 0.33, 0.61,
1.67, 2.3, and 8.4 GHz, with the 0.61 GHz observations forming the third epoch
of observation of this source at that frequency. At 0.61 GHz, the structure of
B2 2050+36 is dominated by two components 56 mas apart. Within the
uncertainties of the various measurements, this separation has remained
unchanged for the past 16 years. Any differential image wander caused by
refractive interstellar scattering is less than 4 mas. Both the lack of
differential image wander and the frequency dependence of the angular diameter
of B2 2050+36 below 1 GHz indicate that the electron density power spectrum
along this line of sight has a spectral index near the Kolmogorov value, with a
value of 4 being highly unlikely. We conclude that diffractive scattering
dominates along this line of sight; results in the literature indicate that
this conclusion also holds true for the line of sight to the pulsar PSR
B2020+28 (8.7 deg. from B2 2050+36). Comparison of our 1.67 GHz observations
with those obtained 21 years previously place a limit on the projected linear
separation velocity of the two components of c.Comment: 19 pages LaTeX2e with AASTeX 5, 3 figures in 7 PostScript files;
accepted for publication in the Ap
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 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 two, 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.Comment: 16 pages, 9 figures, 4 tables; results unchanged; revised version
accepted by Ap
Prospects for joint radio telescope and gravitational wave searches for astrophysical transients
The radio skies remain mostly unobserved when it comes to transient
phenomena. The direct detection of gravitational waves will mark a major
milestone of modern astronomy, as an entirely new window will open on the
universe. Two apparently independent phenomena can be brought together in a
coincident effort that has the potential to boost both searches. In this paper
we will outline the scientific case that stands behind these future joint
observations and will describe the methods that might be used to conduct the
searches and analyze the data. The targeted sources are binary systems of
compact objects, known to be strong candidate sources for gravitational waves.
Detection of transients coincident in these two channels would be a significant
smoking gun for first direct detection of gravitational waves, and would open
up a new field for characterization of astrophysical transients involving
massive compact objects.Comment: 12 pages, Amaldi 8 Conference (New York, 2009) proceedings pape
The inner jet of radio galaxy NGC 315 as observed with Chandra and the VLA
We present Chandra X-ray results for the jet, nucleus, and gaseous atmosphere
of NGC 315, a nearby radio galaxy whose jet kinematics are known through deep
radio mapping. Diffuse X-ray synchrotron emission is detected from the jet out
to 30 arcsec from the nucleus, through regions both of fast bulk flow and
deceleration. The X-ray to radio flux ratio drops considerably where the flow
decelerates, but the X-ray and radio emissions show similar transverse extents
throughout, requiring distributed particle acceleration to maintain the supply
of X-ray-emitting electrons. A remarkable knotty filament within the jet is
seen in both the radio and X-ray, contributing roughly 10 per cent of the
diffuse emission along its extent at both wavelengths. No completely
satisfactory explanation for the filament is found, though its oscillatory
appearance, roughly aligned magnetic field, and requirements for particle
acceleration, suggest that it is a magnetic strand within a shear layer between
fast inner and slower outer flow.Comment: Accepted for publication in the MNRAS. 13 pages,14 figures (some in
colour
Next Generation Very Large Array Memo No. 6, Science Working Group 1: The Cradle of Life
This paper discusses compelling science cases for a future long-baseline
interferometer operating at millimeter and centimeter wavelengths, like the
proposed Next Generation Vary Large Array (ngVLA). We report on the activities
of the Cradle of Life science working group, which focused on the formation of
low- and high-mass stars, the formation of planets and evolution of
protoplanetary disks, the physical and compositional study of Solar System
bodies, and the possible detection of radio signals from extraterrestrial
civilizations. We propose 19 scientific projects based on the current
specification of the ngVLA. Five of them are highlighted as possible Key
Science Projects: (1) Resolving the density structure and dynamics of the
youngest HII regions and high-mass protostellar jets, (2) Unveiling
binary/multiple protostars at higher resolution, (3) Mapping planet formation
regions in nearby disks on scales down to 1 AU, (4) Studying the formation of
complex molecules, and (5) Deep atmospheric mapping of giant planets in the
Solar System. For each of these projects, we discuss the scientific importance
and feasibility. The results presented here should be considered as the
beginning of a more in-depth analysis of the science enabled by such a
facility, and are by no means complete or exhaustive.Comment: 51 pages, 12 figures, 1 table. For more information visit
https://science.nrao.edu/futures/ngvl
An automated archival VLA transients survey
In this paper we present the results of a survey for radio transients using
data obtained from the Very Large Array archive. We have reduced, using a
pipeline procedure, 5037 observations of the most common pointings - i.e. the
calibrator fields. These fields typically contain a relatively bright point
source and are used to calibrate `target' observations: they are therefore
rarely imaged themselves. The observations used span a time range ~ 1984 - 2008
and consist of eight different pointings, three different frequencies (8.4, 4.8
and 1.4 GHz) and have a total observing time of 435 hours. We have searched for
transient and variable radio sources within these observations using components
from the prototype LOFAR transient detection system. In this paper we present
the methodology for reducing large volumes of Very Large Array data; and we
also present a brief overview of the prototype LOFAR transient detection
algorithms. No radio transients were detected in this survey, therefore we
place an upper limit on the snapshot rate of GHz frequency transients > 8.0 mJy
to rho less than or equal to 0.032 deg^-2 that have typical timescales 4.3 to
45.3 days. We compare and contrast our upper limit with the snapshot rates -
derived from either detections or non-detections of transient and variable
radio sources - reported in the literature. When compared with the current Log
N - Log S distribution formed from previous surveys, we show that our upper
limit is consistent with the observed population. Current and future radio
transient surveys will hopefully further constrain these statistics, and
potentially discover dominant transient source populations. In this paper we
also briefly explore the current transient commissioning observations with
LOFAR, and the impact they will make on the field.Comment: Accepted for publication in MNRA
A Dual Frequency, Multi-Year Monitoring Program of Compact Radio Sources
We present light curves for 149 sources monitored with the Green Bank
Interferometer. The light curves are at two radio frequencies (approximately
2.5 and 8.2 GHz) and range from 3 to 15 yrs in length, covering the interval
1979--1996, and have a typical sampling of one flux density measurement every 2
days. We have used these light curves to conduct various variability analysis
(rms flux density variations and autoregressive, integrated, moving average
modeling) of these sources. We find suggestive, though not unambiguous
evidence, that these sources have a common, broadband mechanism for intrinsic
variations, in agreement with previous studies of a subset of these source. We
also find that the sources generally display a short-term variability (~ 10 d)
that arises from radio-wave scattering in an extended medium. These conclusions
extend those of Fiedler et al. (1987) who used a sub-sample of these data. The
primary motivation for this monitoring program was the identification of
extreme scattering events. In an effort to identify ESEs in a systematic
manner, we have taken the wavelet transform of the light curves. We find 15
events in the light curves of 12 sources that we classify as probable ESEs.
However, we also find that five ESEs previously identified from these data do
not survive our wavelet selection criteria. Future identification of ESEs will
probably continue to rely on both visual and systematic methods. Instructions
for obtaining the data are also presented.Comment: 34 pages, LaTeX with AASTeX-5, 7 figures in 20 PostScript files;
because of their length, Table 1 and Figures 8 and 9 can be obtained from
http://ese.nrl.navy.mil/GBI/GBI.htm