198 research outputs found
Software correlators as testbeds for RFI algorithms
In-correlator techniques offer the possibility of identifying and/or excising
radio frequency interference (RFI) from interferometric observations at much
higher time and/or frequency resolution than is generally possible with the
final visibility dataset. Due to the considerable computational requirements of
the correlation procedure, cross-correlators have most commonly been
implemented using high-speed digital signal processing boards, which typically
require long development times and are difficult to alter once complete.
"Software" correlators, on the other hand, make use of commodity server
machines and a correlation algorithm coded in a high-level language. They are
inherently much more flexible and can be developed - and modified - much more
rapidly than purpose-built "hardware" correlators. Software correlators are
thus a natural choice for testing new RFI detection and mitigation techniques
for interferometers. The ease with which software correlators can be adapted to
test RFI detection algorithms is demonstrated by the addition of kurtosis
detection and plotting to the widely used DiFX software correlator, which
highlights previously unknown short -duration RFI at the Hancock VLBA station.Comment: 6 pages, 1 figure, accepted for publication in Proceedings of Science
[PoS(RFI2010)035]. Presented at RFI2010, the Third Workshop on RFI Mitigation
in Radio Astronomy, 29-31 March 2010, Groningen, The Netherland
VLBI detection of an Infrared-Faint Radio Source
Infrared-Faint Radio Sources represent a new and unexpected class of object
which is bright at radio wavelengths but unusually faint at infrared
wavelengths. If, like most mJy radio sources, they were either conventional
active or star-forming galaxies in the local Universe, we would expect them to
be detectable at infrared wavelengths, and so their non-detection by the
Spitzer Space Telescope is surprising. Here we report the detection of one of
these sources using Very Long Baseline Interferometry, from which we conclude
that the sources are driven by Active Galactic Nuclei. We suggest that these
sources are either normal radio-loud quasars at high redshift or abnormally
obscured radio galaxies.Comment: accepted by MNRA
Very long baseline astrometry of PSR J1012+5307 and its implications on alternative theories of gravity
PSR J1012+5307, a millisecond pulsar in orbit with a helium white dwarf (WD),
has been timed with high precision for about 25 years. One of the main
objectives of this long-term timing is to use the large asymmetry in
gravitational binding energy between the neutron star and the WD to test
gravitational theories. Such tests, however, will be eventually limited by the
accuracy of the distance to the pulsar. Here, we present VLBI (very long
baseline interferometry) astrometry results spanning approximately 2.5 years
for PSR J1012+5307, obtained with the Very Long Baseline Array as part of the
MSPSRPI project. These provide the first proper motion and absolute position
for PSR J1012+5307 measured in a quasi-inertial reference frame. From the VLBI
results, we measure a distance of kpc (all the estimates
presented in the abstract are at 68% confidence) for PSR J1012+5307, which is
the most precise obtained to date. Using the new distance, we improve the
uncertainty of measurements of the unmodeled contributions to orbital period
decay, which, combined with three other pulsars, places new constraints on the
coupling constant for dipole gravitational radiation
and the fractional time derivative of
Newton's gravitational constant in the local universe. As the uncertainties of the
observed decays of orbital period for the four leading pulsar-WD systems become
negligible in years, the uncertainties for and
will be improved to and
, respectively, predominantly limited by the distance
uncertainties.Comment: published in ApJ (2020ApJ...896...85D
Reconciling optical and radio observations of the binary millisecond pulsar PSR J1640+2224
Previous optical and radio observations of the binary millisecond pulsar PSR
J1640+2224 have come to inconsistent conclusions about the identity of its
companion, with some observations suggesting the companion is a low-mass
helium-core (He-core) white dwarf (WD), while others indicate it is most likely
a high-mass carbon-oxygen (CO) WD. Binary evolution models predict PSR
J1640+2224 most likely formed in a low-mass X-ray binary (LMXB) based on the
pulsar's short spin period and long-period, low-eccentricity orbit, in which
case its companion should be a He-core WD with mass about , depending on metallicity. If it is instead a CO WD, that would
suggest the system has an unusual formation history. In this paper we present
the first astrometric parallax measurement for this system from observations
made with the Very Long Baseline Array (VLBA), from which we determine the
distance to be . We use this distance and a
reanalysis of archival optical observations originally taken in 1995 with the
Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST) in
order to measure the WD's mass. We also incorporate improvements in
calibration, extinction model, and WD cooling models. We find that the existing
observations are not sufficient to tightly constrain the companion mass, but we
conclude the WD mass is with confidence. The limiting
factor in our analysis is the low signal-to-noise ratio of the original HST
observations.Comment: 6 pages, 5 figure
The triggering of local AGN and their role in regulating star formation
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.We explore the processes that trigger local AGN and the role of these AGN in regulating star formation, using ~350 nearby galaxies observed by the mJy Imaging VLBA Exploration at 20cm (mJIVE) survey. The >10^7 K brightness temperature required for an mJIVE detection cannot be achieved via star formation alone, allowing us to unambiguously detect nearby radio AGN and study their role in galaxy evolution. Radio AGN are an order of magnitude more common in early-type galaxies (ETGs) than in their late-type counterparts. The VLBI-detected ETGs in this study have a similar stellar mass distribution to their undetected counterparts, are typically not the central galaxies of clusters and exhibit merger fractions that are significantly higher than in the average ETG. This suggests that these radio AGN (which have VLBI luminosities >10^22 W Hz^-1) are primarily fuelled by mergers, and not by internal stellar mass loss or cooling flows. Our radio AGN are a factor of ~3 times more likely to reside in the UV-optical red sequence than the average ETG. Furthermore, typical AGN lifetimes (a few 10^7 yr) are much shorter than the transit times from blue cloud to red sequence (~1.5 Gyr). This indicates that the AGN are not triggered promptly and appear several dynamical timescales into the associated star formation episode, implying that they typically couple only to residual gas, at a point where star formation has already declined significantly. While evidence for AGN feedback is strong in systems where the black hole is fed by the cooling of hot gas, AGN triggered by mergers appear not to strongly regulate the associated star formation. The inability of the AGN to rapidly quench merger-driven star formation is likely to make merging the dominant mode of star formation in nearby ETGs, in line with evidence for minor mergers being the primary driver of stellar mass growth in these systems.Peer reviewe
Multi-source self-calibration: Unveiling the microJy population of compact radio sources
Context. Very Long Baseline Interferometry (VLBI) data are extremely
sensitive to the phase stability of the VLBI array. This is especially
important when we reach {\mu}Jy r.m.s. sensitivities. Calibration using
standard phase referencing techniques is often used to improve the phase
stability of VLBI data but the results are often not optimal. This is evident
in blank fields that do not have in-beam calibrators. Aims. We present a
calibration algorithm termed Multi-Source Self-Calibration (MSSC) which can be
used after standard phase referencing on wide-field VLBI observations. This is
tested on a 1.6 GHz wide-field VLBI data set of the Hubble Deep Field-North and
the Hubble Flanking Fields. Methods. MSSC uses multiple target sources detected
in the field via standard phase referencing techniques and modifies the
visibili- ties so that each data set approximates to a point source. These are
combined to increase the signal to noise and permit self-calibration. In
principle, this should allow residual phase changes caused by the troposphere
and ionosphere to be corrected. By means of faceting, the technique can also be
used for direction dependent calibration. Results. Phase corrections, derived
using MSSC, were applied to a wide-field VLBI data set of the HDF-N comprising
of 699 phase centres. MSSC was found to perform considerably better than
standard phase referencing and single source self-calibration. All detected
sources exhibited dramatic improvements in dynamic range. Using MSSC, one
source reached the detection threshold taking the total detected sources to
twenty. 60% of these sources can now be imaged with uniform weighting compared
to just 45% with standard phase referencing. The Parseltongue code which
implements MSSC has been released and made publicly available to the
astronomical community (https://github.com/jradcliffe5/multi_self_cal).Comment: 7 pages, 4 figures, accepted to A&
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