46 research outputs found
Preliminary Sunyaev Zel'dovich Observations of Galaxy Clusters with OCRA-p
We present 30 GHz Sunyaev Zel'dovich (SZ) observations of a sample of four
galaxy clusters with a prototype of the One Centimetre Receiver Array (OCRA-p)
which is mounted on the Torun 32-m telescope. The clusters (Cl0016+16,
MS0451.6-0305, MS1054.4-0321 and Abell 2218) are popular SZ targets and serve
as commissioning observations. All four are detected with clear significance
(4-6 sigma) and values for the central temperature decrements are in good
agreement with measurements reported in the literature. We believe that
systematic effects are successfully suppressed by our observing strategy. The
relatively short integration times required to obtain these results demonstrate
the power of OCRA-p and its successors for future SZ studies.Comment: 9 pages, 2 figures. Accepted by MNRAS, online earl
A VSA search for the extended Sunyaev-Zel'dovich Effect in the Corona Borealis Supercluster
We present interferometric imaging at 33 GHz of the Corona Borealis
supercluster, using the extended configuration of the Very Small Array. A total
area of 24 deg^2 has been imaged, with an angular resolution of 11 arcmin and a
sensitivity of 12 mJy/beam. The aim of these observations is to search for
Sunyaev-Zel'dovich (SZ) detections from known clusters of galaxies in this
supercluster and for a possible extended SZ decrement due to diffuse warm/hot
gas in the intercluster medium. We measure negative flux values in the
positions of the ten richest clusters in the region. Collectively, this implies
a 3.0-sigma detection of the SZ effect. In the clusters A2061 and A2065 we find
decrements of approximately 2-sigma. Our main result is the detection of two
strong and resolved negative features at -70+-12 mJy/beam (-157+-27 microK) and
-103+-10 mJy/beam (-230+-23 microK), respectively, located in a region with no
known clusters, near the centre of the supercluster. We discuss their possible
origins in terms of primordial CMB anisotropies and/or SZ signals related to
either unknown clusters or to a diffuse extended warm/hot gas distribution. Our
analyses have revealed that a primordial CMB fluctuation is a plausible
explanation for the weaker feature (probability of 37.82%). For the stronger
one, neither primordial CMB (probability of 0.33%) nor SZ can account alone for
its size and total intensity. The most reasonable explanation, then, is a
combination of both primordial CMB and SZ signal. Finally, we explore what
characteristics would be required for a filamentary structure consisting of
warm/hot diffuse gas in order to produce a significant contribution to such a
spot taking into account the constraints set by X-ray data.Comment: 16 pages, 10 figures. Accepted in MNRA
The HIV Care Continuum among Female Sex Workers: A Key Population in Lilongwe, Malawi
ObjectiveThe HIV care continuum among female sex workers (FSW), a key population, has not been well characterized, especially within the generalized epidemics of sub-Saharan Africa. This was the first study to characterize the HIV care continuum among FSW in Lilongwe, Malawi.MethodsFrom July through September 2014, we used venue-based sampling to enroll 200 adult FSW in Lilongwe, Malawi into a cross-sectional evaluation assessing HIV care continuum outcomes. Seropositive FSW, identified using HIV rapid testing, received rapid CD4 counts in addition to viral loads using dried blood spots. We calculated proportions of HIV-infected FSW who had history of care, were on ART, and had suppressed viral load and we used Poisson regression to estimate the associations of demographic characteristics and transmission risk behaviors with each outcome.ResultsHIV seroprevalence was 69% (n = 138). Among all FSW the median age was 24 years (IQR: 22–28). Among the 20% who were newly diagnosed and reported previously testing negative, the median time since last HIV test was 11 months (interquartile range: 3–17). The majority (69%) of HIV-infected FSW had a history of HIV care, 52% reported current ART use, and 45% were virally suppressed. Of the FSW who reported current ART use, 86% were virally suppressed. Transmission risk behaviors were not associated with continuum outcomes.ConclusionsFSW in Lilongwe were predominately young and have a high HIV prevalence. Only half of HIV-infected FSW reported current ART use, but the majority of those on ART were virally suppressed. To reduce ongoing transmission and improve health outcomes, increased HIV testing, care engagement, and ART coverage is urgently needed among FSW. Universal testing and treatment strategies for all FSW in Malawi must be strongly considered
Substance use and HIV infection awareness among HIV-infected female sex workers in Lilongwe, Malawi
HIV diagnosis, the first step in HIV care and treatment engagement, may be inhibited by substance use among female sex workers (FSW). We assessed the relationship between alcohol and marijuana use and lack of HIV infection awareness among HIV-infected FSW in Lilongwe, Malawi
Very Small Array observations of the Sunyaev-Zel'dovich effect in nearby galaxy clusters
We present VSA observations (~34GHz) on scales ~20 arcmin towards a complete,
X-ray-flux-limited sample of seven clusters at redshift z<0.1. Four have
significant SZ detections in the presence of CMB primordial anisotropy. We use
a bayesian MCMC method for inference from the VSA data, with X-ray priors on
cluster positions and temperatures, and radio priors on sources. We make
assumptions of beta-model gas distributions and of hydrostatic equilibrium, to
evaluate probability densities for the gas mass and total mass out to r_200.
Our combined estimate of the gas fraction is 0.08^{+0.06}_{-0.04}h^{-1} The
random errors are poor (note that the errors are higher than would have been
obtained with the usual chi-squared method) but the control of bias is good. We
have described the MCMC analysis method specifically in terms of SZ but hope
the description will be of more general use. We find that the effects of
primordial CMB contamination tend to be similar in the estimates of both the
gas mass and total mass over our narrow range of angular scales, so that there
is little effect of primordials on the gas fraction determination. Using our
total mass estimates we find a normalisation of the mass-temperature relation
based on the profiles from the VSA cluster pressure maps that is in good
agreement with recent M-T determinations from X-ray cluster measurements.Comment: Replaces earlier version. 16 pages, 5 figures, LaTeX. Minor revisions
to content, accepted by MNRAS for publicatio
Blazars in the Fermi Era: The OVRO 40-m Telescope Monitoring Program
The Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope
provides an unprecedented opportunity to study gamma-ray blazars. To capitalize
on this opportunity, beginning in late 2007, about a year before the start of
LAT science operations, we began a large-scale, fast-cadence 15 GHz radio
monitoring program with the 40-m telescope at the Owens Valley Radio
Observatory (OVRO). This program began with the 1158 northern (declination>-20
deg) sources from the Candidate Gamma-ray Blazar Survey (CGRaBS) and now
encompasses over 1500 sources, each observed twice per week with a ~4 mJy
(minimum) and 3% (typical) uncertainty. Here, we describe this monitoring
program and our methods, and present radio light curves from the first two
years (2008 and 2009). As a first application, we combine these data with a
novel measure of light curve variability amplitude, the intrinsic modulation
index, through a likelihood analysis to examine the variability properties of
subpopulations of our sample. We demonstrate that, with high significance
(7-sigma), gamma-ray-loud blazars detected by the LAT during its first 11
months of operation vary with about a factor of two greater amplitude than do
the gamma-ray-quiet blazars in our sample. We also find a significant (3-sigma)
difference between variability amplitude in BL Lacertae objects and
flat-spectrum radio quasars (FSRQs), with the former exhibiting larger
variability amplitudes. Finally, low-redshift (z<1) FSRQs are found to vary
more strongly than high-redshift FSRQs, with 3-sigma significance. These
findings represent an important step toward understanding why some blazars emit
gamma-rays while others, with apparently similar properties, remain silent.Comment: 23 pages, 24 figures. Submitted to ApJ
Sunyaev Zel'dovich observations of a statistically complete sample of galaxy clusters with OCRA-p
We present 30 GHz Sunyaev Zel'dovich observations of a statistically complete
sample of galaxy clusters with OCRA-p. The clusters are the 18 most X-ray
luminous clusters at z > 0.2 in the ROSAT Brightest Cluster Sample. We correct
for contaminant radio sources via supplementary observations with the Green
Bank Telescope, also at 30 GHz, and remove a cluster that is contaminated by an
unresolved X-ray source. All 17 remaining clusters have central SZ effects with
Comptonisation parameter y_0 exceeding 1.9x10^-4, and 13 are detected at
significance > 3 sigma. We use our data to examine scalings between y_0 and
X-ray temperature, X-ray luminosity, and the X-ray mass proxy Y_X, and find
good agreement with predictions from self-similar models of cluster formation,
with an intrinsic scatter in y_0 of about 25%. We also comment on the success
of the observations in the face of the contaminant source population, and the
implications for upcoming cm-wave surveys.Comment: 14 pages, 2 figures, accepted by MNRA
High sensitivity measurements of the CMB power spectrum with the extended Very Small Array
We present deep Ka-band ( GHz) observations of the CMB made
with the extended Very Small Array (VSA). This configuration produces a
naturally weighted synthesized FWHM beamwidth of arcmin which covers
an -range of 300 to 1500. On these scales, foreground extragalactic
sources can be a major source of contamination to the CMB anisotropy. This
problem has been alleviated by identifying sources at 15 GHz with the Ryle
Telescope and then monitoring these sources at 33 GHz using a single baseline
interferometer co-located with the VSA. Sources with flux densities \gtsim 20
mJy at 33 GHz are subtracted from the data. In addition, we calculate a
statistical correction for the small residual contribution from weaker sources
that are below the detection limit of the survey.
The CMB power spectrum corrected for Galactic foregrounds and extragalactic
point sources is presented. A total -range of 150-1500 is achieved by
combining the complete extended array data with earlier VSA data in a compact
configuration. Our resolution of allows the first 3
acoustic peaks to be clearly delineated. The is achieved by using mosaiced
observations in 7 regions covering a total area of 82 sq. degrees. There is
good agreement with WMAP data up to where WMAP data run out of
resolution. For higher -values out to , the agreement in
power spectrum amplitudes with other experiments is also very good despite
differences in frequency and observing technique.Comment: 16 pages. Accepted in MNRAS (minor revisions
Searching for non-Gaussianity in the VSA data
We have tested Very Small Array (VSA) observations of three regions of sky
for the presence of non-Gaussianity, using high-order cumulants, Minkowski
functionals, a wavelet-based test and a Bayesian joint power
spectrum/non-Gaussianity analysis. We find the data from two regions to be
consistent with Gaussianity. In the third region, we obtain a 96.7% detection
of non-Gaussianity using the wavelet test. We perform simulations to
characterise the tests, and conclude that this is consistent with expected
residual point source contamination. There is therefore no evidence that this
detection is of cosmological origin. Our simulations show that the tests would
be sensitive to any residual point sources above the data's source subtraction
level of 20 mJy. The tests are also sensitive to cosmic string networks at an
rms fluctuation level of (i.e. equivalent to the best-fit observed
value). They are not sensitive to string-induced fluctuations if an equal rms
of Gaussian CDM fluctuations is added, thereby reducing the fluctuations due to
the strings network to rms . We especially highlight the usefulness
of non-Gaussianity testing in eliminating systematic effects from our data.Comment: Minor corrections; accepted for publication to MNRA
Radio source calibration for the VSA and other CMB instruments at around 30 GHz
Accurate calibration of data is essential for the current generation of CMB
experiments. Using data from the Very Small Array (VSA), we describe procedures
which will lead to an accuracy of 1 percent or better for experiments such as
the VSA and CBI. Particular attention is paid to the stability of the receiver
systems, the quality of the site and frequent observations of reference
sources. At 30 GHz the careful correction for atmospheric emission and
absorption is shown to be essential for achieving 1 percent precision. The
sources for which a 1 percent relative flux density calibration was achieved
included Cas A, Cyg A, Tau A and NGC7027 and the planets Venus, Jupiter and
Saturn. A flux density, or brightness temperature in the case of the planets,
was derived at 33 GHz relative to Jupiter which was adopted as the fundamental
calibrator. A spectral index at ~30 GHz is given for each. Cas A,Tau A, NGC7027
and Venus were examined for variability. Cas A was found to be decreasing at
percent per year over the period March 2001 to August 2004.
In the same period Tau A was decreasing at percent per year. A
survey of the published data showed that the planetary nebula NGC7027 decreased
at percent per year over the period 1967 to 2003. Venus showed
an insignificant ( percent) variation with Venusian illumination.
The integrated polarization of Tau A at 33 GHz was found to be
percent at pa .}Comment: 13 pages, 15 figures, submitted to MNRA