219 research outputs found
A microlensing measurement of the size of the broad emission line region in the lensed QSO 2237+0305
We present spatially resolved spectroscopic images of the gravitationally
lensed QSO 2237+0305 taken with the GMOS Integral Field Unit (IFU) on the
Gemini North telescope. These observations have the best spatial resolution of
any IFU observations of this object to date and include the redshifted CIII]
and MgII QSO broad lines. Unlike Mediavilla et al. 1998, we find no evidence
for an arc of resolved broad line emission in either the CIII] or MgII lines.
We calculate the image flux ratios of both the integrated emission lines and
the surrounding continua. The flux ratios of the CIII] and MgII emission lines
are consistent with each other but differ substantially from their
corresponding continuum flux ratios and the radio/mid-IR flux ratios previously
published. We argue that the broad emission line region must be microlensed and
the CIII] and MgII emission regions must be approximately the same size and
co-located along the line-of-sight. Assuming a simple model for the broad
emission line region and the continuum region, we show the size of the CIII] /
MgII broad line region is ~0.06 h_{70}^{1/2} pc and the continuum region is <=
0.02 h_{70}^{1/2} pc.Comment: 7 pages, 7 figures. Accepted for publication in MNRA
A VLBA search for binary black holes in active galactic nuclei with double-peaked optical emission line spectra
We have examined a subset of 11 active galactic nuclei (AGN) drawn from a
sample of 87 objects that possess double-peaked optical emission line spectra,
as put forward by Wang et al. (2009a) and are detectable in the FIRST survey at
radio wavelengths. The double-peaked nature of the optical emission line
spectra has been suggested as evidence for the existence of binary black holes
in these AGN, although this interpretation is controversial. We make a simple
suggestion, that direct evidence of binary black holes in these objects could
be searched for in the form of dual sources of compact radio emission
associated with the AGN. To explore this idea, we have used the Very Long
Baseline Array to observe these 11 objects from the Wang et al. (2009a) sample.
Of the 11 objects, we detect compact radio emission from two, SDSS
J151709+335324 and SDSS J160024+264035. Both objects show single components of
compact radio emission. The morphology of SDSS J151709+335324 is consistent
with a recent comprehensive multi-wavelength study of this object by Rosario et
al. (2010). Assuming that the entire sample consists of binary black holes, we
would expect of order one double radio core to be detected, based on radio
wavelength detection rates from FIRST and VLBI surveys. We have not detected
any double cores, thus this work does not substantially support the idea that
AGN with double-peaked optical emission lines contain binary black holes.
However, the study of larger samples should be undertaken to provide a more
secure statistical result, given the estimated detection rates.Comment: 14 pages, 3 figures. To appear in A
Spectroscopy with the Engineering Development Array: cold H at 63 MHz towards the Galactic Centre
The Engineering Development Array (EDA) is a single test station for Square
Kilometre Array (SKA) precursor technology. We have used the EDA to detect
low-frequency radio recombination lines (RRLs) from the Galactic Centre region.
Low-frequency RRLs are an area of interest for future low-frequency SKA work as
these lines provide important information on the physical properties of the
cold neutral medium. In this project we investigate the EDA, its bandpass and
the radio frequency interference environment for low-frequency spectroscopy. We
present line spectra from 30 to 325 MHz for the Galactic Centre region. The
decrease in sensitivity for the EDA at the low end of the receiver prevents
carbon and hydrogen RRLs to be detected below 40 and 60 MHz respectively. RFI
strongly affects frequencies in the range 276-292, 234-270, 131-138, 95-102 and
below 33 MHz. Cn RRLs were detected in absorption for quantum levels n
= 378 to 550 (39-121 MHz) and in emission for n = 272 to 306 (228-325 MHz).
Cn lines were detected in absorption for n = 387 to 696 (39-225 MHz).
Hn RRLs were detected in emission for n = 272 to 480 (59-325 MHz).
Hn lines were detected for n = 387 to 453 (141-225 MHz). The stacked
Hn detection at 63 MHz is the lowest frequency detection made for
hydrogen RRLs and shows that a cold (partially) ionized medium exists along the
line of sight to the Galactic Centre region. The size and velocity of this cold
H gas indicates that it is likely associated with the nearby
Riegel-Crutcher cloud.Comment: 18 pages, 6 figures and 5 table
Subtraction of Bright Point Sources from Synthesis Images of the Epoch of Reionization
Bright point sources associated with extragalactic AGN and radio galaxies are
an important foreground for low frequency radio experiments aimed at detecting
the redshifted 21cm emission from neutral hydrogen during the epoch of
reionization. The frequency dependence of the synthesized beam implies that the
sidelobes of these sources will move across the field of view as a function of
observing frequency, hence frustrating line-of-sight foreground subtraction
techniques. We describe a method for subtracting these point sources from dirty
maps produced by an instrument such as the MWA. This technique combines matched
filters with an iterative centroiding scheme to locate and characterize point
sources in the presence of a diffuse background. Simulations show that this
technique can improve the dynamic range of EOR maps by 2-3 orders of magnitude.Comment: 11 pages, 8 figures, 1 table, submitted to PAS
The emission and scintillation properties of RRAT J2325-0530 at 154 MHz and 1.4 GHz
Rotating Radio Transients (RRATs) represent a relatively new class of pulsar,
primarily characterised by their sporadic bursting emission of single pulses on
time scales of minutes to hours. In addition to the difficulty involved in
detecting these objects, low-frequency (300 MHz) observations of RRATs are
sparse, which makes understanding their broadband emission properties in the
context of the normal pulsar population problematic. Here, we present the
simultaneous detection of RRAT J2325-0530 using the Murchison Widefield Array
(154 MHz) and Parkes radio telescope (1.4 GHz). On a single-pulse basis, we
produce the first polarimetric profile of this pulsar, measure the spectral
index (), pulse energy distributions, and present the pulse
rates in the context of detections in previous epochs. We find that the
distribution of time between subsequent pulses is consistent with a Poisson
process and find no evidence of clustering over the 1.5 hr observations.
Finally, we are able to quantify the scintillation properties of RRAT
J2325-0530 at 1.4 GHz, where the single pulses are modulated substantially
across the observing bandwidth, and show that this characterisation is feasible
even with irregular time sampling as a consequence of the sporadic emission
behaviour.Comment: 18 pages, 8 figures, 5 tables, accepted for publication in PAS
A machine learning classifier for fast radio burst detection at the VLBA
Time domain radio astronomy observing campaigns frequently generate large volumes of data. Our goal is to develop automated methods that can identify events of interest buried within the larger data stream. The V-FASTR fast transient system was designed to detect rare fast radio bursts within data collected by the Very Long Baseline Array. The resulting event candidates constitute a significant burden in terms of subsequent human reviewing time. We have trained and deployed a machine learning classifier that marks each candidate detection as a pulse from a known pulsar, an artifact due to radio frequency interference, or a potential new discovery. The classifier maintains high reliability by restricting its predictions to those with at least 90% confidence. We have also implemented several efficiency and usability improvements to the V-FASTR web-based candidate review system. Overall, we found that time spent reviewing decreased and the fraction of interesting candidates increased. The classifier now classifies (and therefore filters) 80%–90% of the candidates, with an accuracy greater than 98%, leaving only the 10%–20% most promising candidates to be reviewed by humans
Limits on fast radio bursts from four years of the V-FASTR experiment
The V-FASTR experiment on the Very Long Baseline Array was designed to detect
dispersed pulses of milliseconds duration, such as fast radio bursts (FRBs). We
use all V-FASTR data through February 2015 to report V-FASTR's upper limits on
the rates of FRBs, and compare these with re-derived rates from Parkes FRB
detection experiments. V-FASTR's operation at lambda=20 cm allows direct
comparison with the 20 cm Parkes rate, and we derive a power-law limit of
\gamma<-0.4 (95% confidence limit) on the index of FRB source counts,
N(>S)\propto S^\gamma. Using the previously measured FRB rate and the
unprecedented amount of survey time spent searching for FRBs at a large range
of wavelengths (0.3 cm > \lambda > 90 cm), we also place frequency-dependent
limits on the spectral distribution of FRBs. The most constraining frequencies
place two-point spectral index limits of \alpha_{20cm}^{4cm} < 5.8 and
\alpha_{90cm}^{20cm} > -7.6, where fluence F \propto f^\alpha if we assume true
the burst rate reported by Champion et al. (2016) of R(F~0.6 Jy ms) = 7 x 10^3
sky^{-1} day^{-1} (for bursts of ~3 ms duration). This upper limit on \alpha
suggests that if FRBs are extragalactic but non-cosmological, that on average
they are not experiencing excessive free-free absorption due to a medium with
high optical depth (assuming temperature ~8,000 K), which excessively invert
their low-frequency spectrum. This in turn implies that the dispersion of FRBs
arises in either or both of the intergalactic medium or the host galaxy, rather
than from the source itself.Comment: Accepted for publication in Ap
Analysing Astronomy Algorithms for GPUs and Beyond
Astronomy depends on ever increasing computing power. Processor clock-rates
have plateaued, and increased performance is now appearing in the form of
additional processor cores on a single chip. This poses significant challenges
to the astronomy software community. Graphics Processing Units (GPUs), now
capable of general-purpose computation, exemplify both the difficult
learning-curve and the significant speedups exhibited by massively-parallel
hardware architectures. We present a generalised approach to tackling this
paradigm shift, based on the analysis of algorithms. We describe a small
collection of foundation algorithms relevant to astronomy and explain how they
may be used to ease the transition to massively-parallel computing
architectures. We demonstrate the effectiveness of our approach by applying it
to four well-known astronomy problems: Hogbom CLEAN, inverse ray-shooting for
gravitational lensing, pulsar dedispersion and volume rendering. Algorithms
with well-defined memory access patterns and high arithmetic intensity stand to
receive the greatest performance boost from massively-parallel architectures,
while those that involve a significant amount of decision-making may struggle
to take advantage of the available processing power.Comment: 10 pages, 3 figures, accepted for publication in MNRA
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