16 research outputs found
The Water Maser in MG 0414+0534: The Influence of Gravitational Microlensing
Water masers have been observed in several high redshift active galactic
nuclei, including the gravitationally lensed quasar MG 0414+0534. This quasar
is lensed into four images, and the water maser is detected in two of them. The
broadening of the maser emission line and its velocity offset are consistent
with a group of masers associated with a quasar jet. If the maser group is
microlensed we can probe its structure and size by observing its microlensing
behaviour over time. We present results of a high resolution numerical analysis
of microlensing of the maser in MG 0414+0534, using several physically
motivated maser models covering a range of sizes and emission profiles.
Time-varying spectra of the microlensed maser are generated, displayed, and
analysed, and the behaviour of the different models compared. The observed
maser line in MG 0414+0534 is consistent with maser spots as in other quasar
jets, provided substructure is de-magnified or currently lost in noise;
otherwise smooth extended maser models are also candidates to generate the
observed spectrum. Using measures of spectral variability we find that if the
maser has small substructure of ~ 0.002 pc then a variation of 0.12 mag in flux
and 2.0 km/s in velocity centroid of the maser line could be observed within 2
decades. For the smallest maser model in this study a magnification of > 35 is
possible 22% of the time, which is of significance in the search for other
lensed masers.Comment: 13 pages, 7 low-quality figures, 1 table, accepted for MNRA
Spectral index of the Galactic foreground emission in the 50-87 MHz range
Radiometry using individual dipole antennas is a potentially effective way to
study the cosmological epoch referred to as Cosmic Dawn (z~20) through
measurement of sky brightness arising from the 21~cm transition of neutral
hydrogen, provided this can be disentangled from much stronger Galactic and
extragalactic foregrounds. In the process, measured spectra of integrated sky
brightness temperature can be used to quantify properties of the foreground
emission. In this work we analyze data from the Large-aperture Experiment to
Detect the Dark Age (LEDA) in the range 50-87 MHz to constrain the spectral
index of foreground emission in the northern sky. We focus on two
zenith-directed LEDA radiometers and study how estimates of vary with
local sidereal time (LST). We correct for the effect of gain pattern
chromaticity and compare estimated absolute temperatures with simulations.
During times with the best observing conditions, for a "reference" radiometer,
we estimate that varies from -2.55 to -2.58, consistent with previous
measurements of the southern sky and simulated sky models. Using data from the
second, experimental, radiometer, we observe a similar trend vs. LST although
with slightly smaller , in the range. We infer
good instrument stability from consistency in computed spectral indices at a
level of 1-2 for LST=9-12.5h, using data distributed between mid-2018
to mid-2019. Evidence for spectral curvature is weak owing to residual
systematic errors, other than when the Galactic Center is in the sky, at which
time we find evidence for negative curvature, ~-0.4.Comment: 13 pages, 19 figure
A 21-cm power spectrum at 48 MHz, using the Owens Valley Long Wavelength Array
The Large-aperture Experiment to detect the Dark Age (LEDA) was designed to
measure the 21-cm signal from neutral hydrogen at Cosmic Dawn, 15-30. Using observations made with the 200 m diameter core of the
Owens Valley Long Wavelength Array (OVRO-LWA), we present a 2-D cylindrical
spatial power spectrum for data at 43.1-53.5 MHz ()
incoherently integrated for 4 hours, and an analysis of the array sensitivity.
Power from foregrounds is localized to a "wedge" within
space. After calibration of visibilities using 5 bright compact sources
including VirA, we measure $\Delta^2(k) \approx 2 \times 10^{12}\
\mathrm{mK}^2\Delta^2(k)z = \Delta^2(k) \approx
100^2^{-1}\alpha$ and X-ray sources, as
predicted for a range of theoretical model parameters.Comment: 16 pages, 11 figures. Accepted for MNRAS; replaced with accepted
versio
The Radio Sky at Meter Wavelengths: m-Mode Analysis Imaging with the Owens Valley Long Wavelength Array
A host of new low-frequency radio telescopes seek to measure the 21-cm
transition of neutral hydrogen from the early universe. These telescopes have
the potential to directly probe star and galaxy formation at redshifts , but are limited by the dynamic range they can achieve
against foreground sources of low-frequency radio emission. Consequently, there
is a growing demand for modern, high-fidelity maps of the sky at frequencies
below 200 MHz for use in foreground modeling and removal. We describe a new
widefield imaging technique for drift-scanning interferometers,
Tikhonov-regularized -mode analysis imaging. This technique constructs
images of the entire sky in a single synthesis imaging step with exact
treatment of widefield effects. We describe how the CLEAN algorithm can be
adapted to deconvolve maps generated by -mode analysis imaging. We
demonstrate Tikhonov-regularized -mode analysis imaging using the Owens
Valley Long Wavelength Array (OVRO-LWA) by generating 8 new maps of the sky
north of with 15 arcmin angular resolution, at frequencies
evenly spaced between 36.528 MHz and 73.152 MHz, and 800 mJy/beam thermal
noise. These maps are a 10-fold improvement in angular resolution over existing
full-sky maps at comparable frequencies, which have angular resolutions . Each map is constructed exclusively from interferometric observations
and does not represent the globally averaged sky brightness. Future
improvements will incorporate total power radiometry, improved thermal noise,
and improved angular resolution -- due to the planned expansion of the OVRO-LWA
to 2.6 km baselines. These maps serve as a first step on the path to the use of
more sophisticated foreground filters in 21-cm cosmology incorporating the
measured angular and frequency structure of all foreground contaminants.Comment: 27 pages, 18 figure
The LOFAR Transients Pipeline
Current and future astronomical survey facilities provide a remarkably rich
opportunity for transient astronomy, combining unprecedented fields of view
with high sensitivity and the ability to access previously unexplored
wavelength regimes. This is particularly true of LOFAR, a
recently-commissioned, low-frequency radio interferometer, based in the
Netherlands and with stations across Europe. The identification of and response
to transients is one of LOFAR's key science goals. However, the large data
volumes which LOFAR produces, combined with the scientific requirement for
rapid response, make automation essential. To support this, we have developed
the LOFAR Transients Pipeline, or TraP. The TraP ingests multi-frequency image
data from LOFAR or other instruments and searches it for transients and
variables, providing automatic alerts of significant detections and populating
a lightcurve database for further analysis by astronomers. Here, we discuss the
scientific goals of the TraP and how it has been designed to meet them. We
describe its implementation, including both the algorithms adopted to maximize
performance as well as the development methodology used to ensure it is robust
and reliable, particularly in the presence of artefacts typical of radio
astronomy imaging. Finally, we report on a series of tests of the pipeline
carried out using simulated LOFAR observations with a known population of
transients.Comment: 30 pages, 11 figures; Accepted for publication in Astronomy &
Computing; Code at https://github.com/transientskp/tk
The 21 cm Power Spectrum from the Cosmic Dawn: First Results from the OVRO-LWA
The 21 cm transition of neutral hydrogen is opening an observational window into the Cosmic Dawn of the universe—the epoch of first star formation. We use 28 hr of data from the Owens Valley Radio Observatory Long Wavelength Array to place upper limits on the spatial power spectrum of 21 cm emission at z ≈ 18.4 (Δ_(21) ≾ 10^4 mK), and within the absorption feature reported by the EDGES experiment. In the process we demonstrate the first application of the double Karhunen–Loève transform for foreground filtering, and diagnose the systematic errors that are currently limiting the measurement. We also provide an updated model for the angular power spectrum of low-frequency foreground emission measured from the northern hemisphere, which can be used to refine sensitivity forecasts for next-generation experiments