129 research outputs found
Confirmation of two extended objects along the line of sight to PKS1830-211 with ESO-VLT adaptive optics imaging
We report on new high-resolution near-infrared images of the gravitationally
lensed radio source PKS1830-211, a quasar at z=2.507. These adaptive optics
observations, taken with the Very Large Telescope (VLT), are further improved
through image deconvolution. They confirm the presence of a second object along
the line of sight to the quasar, in addition to the previously known spiral
galaxy. This additional object is clearly extended in our images. However, its
faint luminosity does not allow to infer any photometric redshift. If this
galaxy is located in the foreground of PKS1830-211, it complicates the modeling
of this system and decreases the interest in using PKS1830-211 as a means to
determine H0 via the time delay between the two lensed images of the quasar.Comment: Accepted in A&A Letter
Interferometric Phase Calibration Sources in the Declination Range 0deg to -30deg
We present a catalog of 321 compact radio sources in the declination range
0deg > delta > -30deg. The positions of these sources have been measured with a
two-dimensional rms accuracy of 35 milliarcseconds using the NRAO Very Large
Array. Each source has a peak flux density >50 mJy at 8.4 GHz. We intend for
this catalog to be used mainly for selection of phase calibration sources for
radio interferometers, although compact radio sources have other scientific
uses.Comment: 9 pages. To appear in ApJS. Catalog (Table 3) is abbreviated in
printed version. Complete catalog available at
ftp://ftp.aoc.nrao.edu/pub/staff/jwrobel/WPW2003_ApJS.tx
Why Do Compact Active Galactic Nuclei at High Redshift Scintillate Less?
The fraction of compact active galactic nuclei (AGNs) that exhibit
interstellar scintillation (ISS) at radio wavelengths, as well as their
scintillation amplitudes, have been found to decrease significantly for sources
at redshifts z > 2. This can be attributed to an increase in the angular sizes
of the \muas-scale cores or a decrease in the flux densities of the compact
\muas cores relative to that of the mas-scale components with increasing
redshift, possibly arising from (1) the space-time curvature of an expanding
Universe, (2) AGN evolution, (3) source selection biases, (4) scatter
broadening in the ionized intergalactic medium (IGM) and intervening galaxies,
or (5) gravitational lensing. We examine the frequency scaling of this redshift
dependence of ISS to determine its origin, using data from a dual-frequency
survey of ISS of 128 sources at 0 < z < 4. We present a novel method of
analysis which accounts for selection effects in the source sample. We
determine that the redshift dependence of ISS is partially linked to the
steepening of source spectral indices ({\alpha}^8.4_4.9) with redshift, caused
either by selection biases or AGN evolution, coupled with weaker ISS in the
{\alpha}^8.4_4.9 < -0.4 sources. Selecting only the -0.4 < {\alpha}^8.4_4.9 <
0.4 sources, we find that the redshift dependence of ISS is still significant,
but is not significantly steeper than the expected (1+z)^0.5 scaling of source
angular sizes due to cosmological expansion for a brightness temperature and
flux-limited sample of sources. We find no significant evidence for scatter
broadening in the IGM, ruling it out as the main cause of the redshift
dependence of ISS. We obtain an upper limit to IGM scatter broadening of <
110\muas at 4.9 GHz with 99% confidence for all lines of sight, and as low as <
8\muas for sight-lines to the most compact, \sim 10\muas sources.Comment: 38 pages, 13 figures, accepted for publication in The Astrophysical
Journa
Dual-Frequency Observations of 140 Compact, Flat-Spectrum Active Galactic Nuclei for Scintillation-Induced Variability
The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey
detected a drop in Interstellar Scintillation (ISS) for sources at redshifts z
> 2, indicating an apparent increase in angular diameter or a decrease in flux
density of the most compact components of these sources, relative to their
extended emission. This can result from intrinsic source size effects or
scatter broadening in the Intergalactic Medium (IGM), in excess of the expected
(1+z)^0.5 angular diameter scaling of brightness temperature limited sources
due to cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations
and data analysis for a sample of 140 compact, flat-spectrum sources which may
allow us to determine the origin of this angular diameter-redshift relation by
exploiting their different wavelength dependences. In addition to using ISS as
a cosmological probe, the observations provide additional insight into source
morphologies and the characteristics of ISS. As in the MASIV Survey, the
variability of the sources is found to be significantly correlated with
line-of-sight H-alpha intensities, confirming its link with ISS. For 25
sources, time delays of about 0.15 to 3 days are observed between the
scintillation patterns at both frequencies, interpreted as being caused by a
shift in core positions when probed at different optical depths. Significant
correlation is found between ISS amplitudes and source spectral index; in
particular, a large drop in ISS amplitudes is observed at spectral indices of <
-0.4 confirming that steep spectrum sources scintillate less. We detect a
weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the
mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2
sources relative to the z < 2 sources, as opposed to the factor of 3 decrease
observed at 4.9 GHz. This suggests scatter broadening in the IGM.Comment: 30 pages, 14 figures, accepted for publication in the Astronomical
Journa
A Compact Extreme Scattering Event Cloud Towards AO 0235+164
We present observations of a rare, rapid, high amplitude Extreme Scattering
Event toward the compact BL-Lac AO 0235+164 at 6.65 GHz. The ESE cloud is
compact; we estimate its diameter between 0.09 and 0.9 AU, and is at a distance
of less than 3.6 kpc. Limits on the angular extent of the ESE cloud imply a
minimum cloud electron density of ~ 4 x 10^3 cm^-3. Based on the amplitude and
timescale of the ESE observed here, we suggest that at least one of the
transients reported by Bower et al. (2007) may be attributed to ESEs.Comment: 11 pages, 2 figure
The Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey II: The First Four Epochs
We report on the variability of 443 flat spectrum, compact radio sources
monitored using the VLA for 3 days in 4 epochs at ~ 4 month intervals at 5 GHz
as part of the Micro-Arcsecond Scintillation-Induced Variability (MASIV)
survey. Over half of these sources exhibited 2-10% rms variations on timescales
over 2 days. We analyzed the variations by two independent methods, and find
that the rms variability amplitudes of the sources correlate with the emission
measure in the ionized Interstellar Medium along their respective lines of
sight. We thus link the variations with interstellar scintillation of
components of these sources, with some (unknown) fraction of the total flux
density contained within a compact region of angular diameter in the range
10-50 micro-arcseconds. We also find that the variations decrease for high mean
flux density sources and, most importantly, for high redshift sources. The
decrease in variability is probably due either to an increase in the apparent
diameter of the source, or a decrease in the flux density of the compact
fraction beyond z ~ 2. Here we present a statistical analysis of these results,
and a future paper will the discuss the cosmological implications in detail.Comment: 62 pages, 13 figures. Accepted for publication in the Astrophysical
Journa
Using bottleneck adapters to identify cancer in clinical notes under low-resource constraints
Processing information locked within clinical health records is a challenging task that remains an active area of research in biomedical NLP. In this work, we evaluate a broad set of machine learning techniques ranging from simple RNNs to specialised transformers such as BioBERT on a dataset containing clinical notes along with a set of annotations indicating whether a sample is cancer-related or not. Furthermore, we specifically employ efficient fine-tuning methods from NLP, namely, bottleneck adapters and prompt tuning, to adapt the models to our specialised task. Our evaluations suggest that fine-tuning a frozen BERT model pre-trained on natural language and with bottleneck adapters outperforms all other strategies, including full fine-tuning of the specialised BioBERT model. Based on our findings, we suggest that using bottleneck adapters in low-resource situations with limited access to labelled data or processing capacity could be a viable strategy in biomedical text mining
Interstellar Scintillation Observations of 146 Extragalactic Radio Sources
From 1979--1996 the Green Bank Interferometer was used by the Naval Research
Laboratory to monitor the flux density from 146 compact radio sources at
frequencies near 2 and 8 GHz. We filter the ``light curves'' to separate
intrinsic variations on times of a year or more from more rapid interstellar
scintilation (ISS) on times of 5--50 d. Whereas the intrinsic variation at 2
GHz is similar to that at 8 GHz (though diminished in amplitude), the ISS
variation is much stronger at 2 than at 8 GHz. We characterize the ISS
variation by an rms amplitude and a timescale and examine the statistics of
these parameters for the 121 sources with significant ISS at 2 GHz. We model
the scintillations using the NE2001 Galactic electron model assuming the
sources are brightness-limited.
We find the observed rms amplitude to be in general agreement with the model,
provided that the compact components of the sources have about 50% of their
flux density in a component with maximum brightness temperatures
--K. Thus our results are consistent with cm-wavelength VLBI
studies of compact AGNs, in that the maximum brightness temperatures found are
consistent with the inverse synchrotron limit at K, boosted
in jet configurations by Doppler factors up to about 20. The average of the
observed 2 GHz ISS timescales is in reasonable agreement with the model at
Galactic latitudes above about 10\de. At lower latitudes the observed
timescales are too fast, suggesting that the transverse plasma velocity
increases more than expected beyond about 1 kpc.Comment: 32 pages, 16 figures. Submitted to Ap
Red Companions to a z=2.15 Radio Loud Quasar
We have conducted observations of the environment around the z=2.15 radio
loud quasar 1550-269 in search of distant galaxies associated either with it or
the z=2.09 CIV absorber along its line of sight. Such objects will be
distinguished by their red colours, R-K>4.5. We find five such objects in a 1.5
arcmin^2 field around the quasar, with typical K magnitudes of ~20.4 and no
detected R band emission. We also find a sixth object with K=19.6+/-0.3, and
undetected at R, just two arcseconds from the quasar. The nature of all these
objects is currently unclear, and will remain so until we have determined their
redshifts. We suggest that it is likely that they are associated with either
the quasar or the CIV absorber, in which case their properties might be similar
to those of the z=2.38 red Ly-alpha emitting galaxies discovered by Francis et
al. (1997). The small separation between the quasar and the brightest of our
objects suggests that it may be the galaxy responsible for the CIV metal line
absorption system. The closeness to the quasar and the red colour might have
precluded similar objects from being uncovered in previous searches for
emission from CIV and eg. damped absorbers.Comment: To appear in "Photometric Redshifts and High Redshift Galaxies", eds.
R. Weymann, L. Storrie-Lombardi, M. Sawicki & R. Brunne
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