36 research outputs found
Redshift determination through weighted phase correlation: a linearithmic implementation
peer reviewedWe present a new algorithm having a time complexity of O(N log N) and designed to retrieve the phase at which an input signal and a set of not necessarily orthogonal templates match at best in a weighted chi-squared sense. The proposed implementation is based on an orthogonalization algorithm and thus also benefits from a high numerical stability. We successfully apply this method to the redshift determination of quasars from the twelfth Sloan Digital Sky Survey (SDSS) quasar catalog and derive the proper spectral reduction and redshift selection methods. Also provided are the derivations of the redshift uncertainty and of the associated confidence. Results of this application are comparable to the performances of the SDSS pipeline while not having a quadratic time dependency
Weighted principal component analysis: a weighted covariance eigendecomposition approach
We present a new straightforward principal component analysis (PCA) method
based on the diagonalization of the weighted variance-covariance matrix through
two spectral decomposition methods: power iteration and Rayleigh quotient
iteration. This method allows one to retrieve a given number of orthogonal
principal components amongst the most meaningful ones for the case of problems
with weighted and/or missing data. Principal coefficients are then retrieved by
fitting principal components to the data while providing the final
decomposition. Tests performed on real and simulated cases show that our method
is optimal in the identification of the most significant patterns within data
sets. We illustrate the usefulness of this method by assessing its quality on
the extrapolation of Sloan Digital Sky Survey quasar spectra from measured
wavelengths to shorter and longer wavelengths. Our new algorithm also benefits
from a fast and flexible implementation.Comment: 12 pages, 9 figure
Spectroscopic binaries as observed by the future Gaia space mission
peer reviewedThe future Gaia satellite will observe a large number of stars through its three main channels: astrometric, photometric and, for the brightest stars, spectroscopic. The satellite is equipped with the RVS spectrograph, which will provide medium-resolution spectra over a small wavelength range. These spectra should allow us to identify stars exhibiting a composite spectrum, either because of a chance alignment or a true binarity. We discuss the various aspects related to the data treatment of the binary candidates and describe the algorithms that are intended to be included in the processing pipeline
Searching for strong gravitational lenses
Strong gravitational lenses provide unique laboratories for cosmological and
astrophysical investigations, but they must first be discovered - a task that
can be met with significant contamination by other astrophysical objects and
asterisms. Here we review strong lens searches, covering various sources
(quasars, galaxies, supernovae, FRBs, GRBs, and GWs), lenses (early- and
late-type galaxies, groups, and clusters), datasets (imaging, spectra, and
lightcurves), and wavelengths. We first present the physical characteristics of
the lens and source populations, highlighting relevant details for constructing
targeted searches. Search techniques are described based on the main lensing
feature that is required for the technique to work, namely one of: (i) an
associated magnification, (ii) multiple spatially-resolved images, (iii)
multiple redshifts, or (iv) a non-zero time delay between images. To use the
current lens samples for science, and for the design of future searches, we
list several selection biases that exist due to these discovery techniques. We
conclude by discussing the future of lens searches in upcoming surveys and the
new population of lenses that will be discovered.Comment: 54 pages, 15 figures, submitted to Space Science Reviews, Topical
Collection "Strong Gravitational Lensing", eds. J. Wambsganss et a
Gaia Data Release 3: The first Gaia catalogue of variable AGN
One of the novelties of the Gaia-DR3 with respect to the previous data
releases is the publication of the multiband light curves of about 1 million
AGN. The goal of this work was the creation of a catalogue of variable AGN,
whose selection was based on Gaia data only. We first present the
implementation of the methods to estimate the variability parameters into a
specific object study module for AGN. Then we describe the selection procedure
that led to the definition of the high-purity variable AGN sample and analyse
the properties of the selected sources. We started from a sample of millions of
sources, which were identified as AGN candidates by 11 different classifiers
based on variability processing. Because the focus was on the variability
properties, we first defined some pre-requisites in terms of number of data
points and mandatory variability parameters. Then a series of filters was
applied using only Gaia data and the Gaia Celestial Reference Frame 3
(Gaia-CRF3) sample as a reference.The resulting Gaia AGN variable sample, named
GLEAN, contains about 872000 objects, more than 21000 of which are new
identifications. We checked the presence of contaminants by cross-matching the
selected sources with a variety of galaxies and stellar catalogues. The
completeness of GLEAN with respect to the variable AGN in the last Sloan
Digital Sky Survey quasar catalogue is about 47%, while that based on the
variable AGN of the Gaia-CRF3 sample is around 51%. From both a comparison with
other AGN catalogues and an investigation of possible contaminants, we conclude
that purity can be expected to be above 95%. Multiwavelength properties of
these sources are investigated. In particular, we estimate that about 4% of
them are radio-loud. We finally explore the possibility to evaluate the time
lags between the flux variations of the multiple images of strongly lensed
quasars, and show one case.Comment: 19 pages, 31 figures, 2 table. This paper is part of Gaia Data
Release 3 (DR3). In press for A&
Gaia GraL: Gaia DR2 Gravitational Lens Systems. VII. XMM-Newton Observations of Lensed Quasars
© 2022. The Author(s). Published by the American Astronomical Society. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.3847/1538-4357/ac4476We present XMM-Newton X-ray observations of nine confirmed lensed quasars at 1 ≲ z ≲ 3 identified by the Gaia Gravitational Lens program. Eight systems are strongly detected, with 0.3-8.0 keV fluxes F 0.3-8.0 ≳ 5 ×10-14 erg cm-2 s-1. Modeling the X-ray spectra with an absorbed power law, we derive power-law photon indices and 2-10 keV luminosities for the eight detected quasars. In addition to presenting sample properties for larger quasar population studies and for use in planning for future caustic-crossing events, we also identify three quasars of interest: a quasar that shows evidence of flux variability from previous ROSAT observations, the most closely separated individual lensed sources resolved by XMM-Newton, and one of the X-ray brightest quasars known at z > 3. These sources represent the tip of the discoveries that will be enabled by SRG/eROSITA.Peer reviewe
Gaia GraL: Gaia DR2 gravitational lens systems – VIII. A radio census of lensed systems
© 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/We present radio observations of 24 confirmed and candidate strongly lensed quasars identified by the Gaia Gravitational Lenses working group. We detect radio emission from eight systems in 5.5 and 9 GHz observations with the Australia Telescope Compact Array (ATCA), and 12 systems in 6 GHz observations with the Karl G. Jansky Very Large Array (VLA). The resolution of our ATCA observations is insufficient to resolve the radio emission into multiple lensed images, but we do detect multiple images from 11 VLA targets. We have analysed these systems using our observations in conjunction with existing optical measurements, including measuring offsets between the radio and optical positions for each image and building updated lens models. These observations significantly expand the existing sample of lensed radio quasars, suggest that most lensed systems are detectable at radio wavelengths with targeted observations, and demonstrate the feasibility of population studies with high-resolution radio imaging.Peer reviewe
Gaia GraL: Gaia DR2 Gravitational Lens Systems. VIII. A radio census of lensed systems
We present radio observations of 24 confirmed and candidate strongly lensed
quasars identified by the Gaia Gravitational Lenses (GraL) working group. We
detect radio emission from 8 systems in 5.5 and 9 GHz observations with the
Australia Telescope Compact Array (ATCA), and 12 systems in 6 GHz observations
with the Karl G. Jansky Very Large Array (VLA). The resolution of our ATCA
observations is insufficient to resolve the radio emission into multiple lensed
images, but we do detect multiple images from 11 VLA targets. We have analysed
these systems using our observations in conjunction with existing optical
measurements, including measuring offsets between the radio and optical
positions, for each image and building updated lens models. These observations
significantly expand the existing sample of lensed radio quasars, suggest that
most lensed systems are detectable at radio wavelengths with targeted
observations, and demonstrate the feasibility of population studies with high
resolution radio imaging
Gaia GraL: Gaia DR2 Gravitational Lens Systems. VII. XMM-Newton Observations of Lensed Quasars
Abstract
We present XMM-Newton X-ray observations of nine confirmed lensed quasars at 1 ≲ z ≲ 3 identified by the Gaia Gravitational Lens program. Eight systems are strongly detected, with 0.3–8.0 keV fluxes F
0.3−8.0 ≳ 5 ×10−14 erg cm−2 s−1. Modeling the X-ray spectra with an absorbed power law, we derive power-law photon indices and 2–10 keV luminosities for the eight detected quasars. In addition to presenting sample properties for larger quasar population studies and for use in planning for future caustic-crossing events, we also identify three quasars of interest: a quasar that shows evidence of flux variability from previous ROSAT observations, the most closely separated individual lensed sources resolved by XMM-Newton, and one of the X-ray brightest quasars known at z > 3. These sources represent the tip of the discoveries that will be enabled by SRG/eROSITA.</jats:p