17,001 research outputs found
Catalog of quasars from the Kilo-Degree Survey Data Release 3
We present a catalog of quasars selected from broad-band photometric ugri
data of the Kilo-Degree Survey Data Release 3 (KiDS DR3). The QSOs are
identified by the random forest (RF) supervised machine learning model, trained
on SDSS DR14 spectroscopic data. We first cleaned the input KiDS data from
entries with excessively noisy, missing or otherwise problematic measurements.
Applying a feature importance analysis, we then tune the algorithm and identify
in the KiDS multiband catalog the 17 most useful features for the
classification, namely magnitudes, colors, magnitude ratios, and the stellarity
index. We used the t-SNE algorithm to map the multi-dimensional photometric
data onto 2D planes and compare the coverage of the training and inference
sets. We limited the inference set to r<22 to avoid extrapolation beyond the
feature space covered by training, as the SDSS spectroscopic sample is
considerably shallower than KiDS. This gives 3.4 million objects in the final
inference sample, from which the random forest identified 190,000 quasar
candidates. Accuracy of 97%, purity of 91%, and completeness of 87%, as derived
from a test set extracted from SDSS and not used in the training, are confirmed
by comparison with external spectroscopic and photometric QSO catalogs
overlapping with the KiDS footprint. The robustness of our results is
strengthened by number counts of the quasar candidates in the r band, as well
as by their mid-infrared colors available from WISE. An analysis of parallaxes
and proper motions of our QSO candidates found also in Gaia DR2 suggests that a
probability cut of p(QSO)>0.8 is optimal for purity, whereas p(QSO)>0.7 is
preferable for better completeness. Our study presents the first comprehensive
quasar selection from deep high-quality KiDS data and will serve as the basis
for versatile studies of the QSO population detected by this survey.Comment: Data available from the KiDS website at
http://kids.strw.leidenuniv.nl/DR3/quasarcatalog.php and the source code from
https://github.com/snakoneczny/kids-quasar
F-GAMMA: Multi-frequency radio monitoring of Fermi blazars. The 2.64 to 43 GHz Effelsberg light curves from 2007-2015
The advent of the Fermi-GST with its unprecedented capability to monitor the
entire 4 pi sky within less than 2-3 hours, introduced new standard in time
domain gamma-ray astronomy. To explore this new avenue of extragalactic physics
the F-GAMMA programme undertook the task of conducting nearly monthly,
broadband radio monitoring of selected blazars from January 2007 to January
2015. In this work we release all the light curves at 2.64, 4.85, 8.35, 10.45,
14.6, 23.05, 32, and 43 GHz and present first order derivative data products
after all necessary post-measurement corrections and quality checks; that is
flux density moments and spectral indices. The release includes 155 sources.
The effective cadence after the quality flagging is around one radio SED every
1.3 months. The coherence of each radio SED is around 40 minutes. The released
dataset includes more than measurements. The median fractional
error at the lowest frequencies (2.64-10.45 GHz) is below 2%. At the highest
frequencies (14.6-43 GHz) with limiting factor of the atmospheric conditions,
the errors range from 3% to 9%, respectively.Comment: Accepted for publication in Section: Catalogs and data of Astronomy &
Astrophysic
Gaia astrometry for stars with too few observations - a Bayesian approach
Gaia's astrometric solution aims to determine at least five parameters for
each star, together with appropriate estimates of their uncertainties and
correlations. This requires at least five distinct observations per star. In
the early data reductions the number of observations may be insufficient for a
five-parameter solution, and even after the full mission many stars will remain
under-observed, including faint stars at the detection limit and transient
objects. In such cases it is reasonable to determine only the two position
parameters. Their formal uncertainties would however grossly underestimate the
actual errors, due to the neglected parallax and proper motion. We aim to
develop a recipe to calculate sensible formal uncertainties that can be used in
all cases of under-observed stars. Prior information about the typical ranges
of stellar parallaxes and proper motions is incorporated in the astrometric
solution by means of Bayes' rule. Numerical simulations based on the Gaia
Universe Model Snapshot (GUMS) are used to investigate how the prior influences
the actual errors and formal uncertainties when different amounts of Gaia
observations are available. We develop a criterion for the optimum choice of
priors, apply it to a wide range of cases, and derive a global approximation of
the optimum prior as a function of magnitude and galactic coordinates. The
feasibility of the Bayesian approach is demonstrated through global astrometric
solutions of simulated Gaia observations. With an appropriate prior it is
possible to derive sensible positions with realistic error estimates for any
number of available observations. Even though this recipe works also for
well-observed stars it should not be used where a good five-parameter
astrometric solution can be obtained without a prior. Parallaxes and proper
motions from a solution using priors are always biased and should not be used.Comment: Revised version, accepted 21st of August 2015 for publication in A&
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