174 research outputs found
Multi-band optical variability of a newly discovered twelve blazars sample from 2013-2019
Here we present the first optical photometric monitoring results of a sample
of twelve newly discovered blazars from the ICRF - Gaia CRF astrometric link.
The observations were performed from April 2013 until August 2019 using eight
telescopes located in Europe. For a robust test for the brightness and colour
variability, we use Abbe criterion and F-test. Moreover, linear fittings are
performed to investigate the relation in the colour-magnitude variations of the
blazars. Variability was confirmed in the case of 10 sources; two sources,
1429+249 and 1556+335 seem to be possibly variable. Three sources (1034+574,
1722+119, and 1741+597) have displayed large amplitude brightness change of
more than one magnitude. We found that the seven sources displayed
bluer-when-brighter variations, and one source showed redder-when-brighter
variations. We briefly explain the various AGN emission models which can
explain our results.Comment: 14 pages, 8 figures, 11 tables, 6 supplement figures can be provided
on request, MNRAS in pres
Multifrequency Photo-polarimetric WEBT Observation Campaign on the Blazar S5 0716+714: Source Microvariability and Search for Characteristic Timescales
Here we report on the results of the WEBT photo-polarimetric campaign
targeting the blazar S5~0716+71, organized in March 2014 to monitor the source
simultaneously in BVRI and near IR filters. The campaign resulted in an
unprecedented dataset spanning \,h of nearly continuous, multi-band
observations, including two sets of densely sampled polarimetric data mainly in
R filter. During the campaign, the source displayed pronounced variability with
peak-to-peak variations of about and "bluer-when-brighter" spectral
evolution, consisting of a day-timescale modulation with superimposed hourlong
microflares characterized by \,mag flux changes. We performed an
in-depth search for quasi-periodicities in the source light curve; hints for
the presence of oscillations on timescales of \,h and \,h do
not represent highly significant departures from a pure red-noise power
spectrum. We observed that, at a certain configuration of the optical
polarization angle relative to the positional angle of the innermost radio jet
in the source, changes in the polarization degree led the total flux
variability by about 2\,h; meanwhile, when the relative configuration of the
polarization and jet angles altered, no such lag could be noted. The
microflaring events, when analyzed as separate pulse emission components, were
found to be characterized by a very high polarization degree () and
polarization angles which differed substantially from the polarization angle of
the underlying background component, or from the radio jet positional angle. We
discuss the results in the general context of blazar emission and energy
dissipation models.Comment: 16 pages, 17 Figures; ApJ accepte
Mind the Cultural Gap: Bridging Language-Specific DBpedia Chapters for Question Answering
International audienceIn order to publish information extracted from language specific pages of Wikipedia in a structured way, the Semantic Web community has started an effort of internationalization of DBpedia. Language specific DBpedia chapters can contain very different information from one language to another, in particular they provide more details on certain topics, or fill information gaps. Language specific DBpedia chapters are well connected through instance interlinking, extracted from Wikipedia. An alignment between properties is also carried out by DBpedia contributors as a mapping from the terms in Wikipedia to a common ontology, enabling the exploitation of information coming from language specific DBpedia chapters. However, the mapping process is currently incomplete, it is time-consuming as it is performed manually, and it may lead to the introduction of redundant terms in the ontology. In this chapter we first propose an approach to automatically extend the existing alignments, and we then present an extension of QAKiS, a system for Question Answering over Linked Data that allows to query language specific DB-pedia chapters relying on the above mentioned property alignment. In the current version of QAKiS, English, French and German DBpedia chapters are queried using a natural language interface
QueDI: From Knowledge Graph Querying to Data Visualization
Abstract
While Open Data (OD) publishers are spur in providing data as Linked Open Data (LOD) to boost innovation and knowledge creation, the complexity of RDF querying languages, such as SPARQL, threatens their exploitation. We aim to help lay users (by focusing on experts in table manipulation, such as OD experts) in querying and exploiting LOD by taking advantage of our target users' expertise in table manipulation and chart creation.
We propose QueDI (Query Data of Interest), a question-answering and visualization tool that implements a scaffold transitional approach to 1) query LOD without being aware of SPARQL and representing results by data tables; 2) once reached our target user comfort zone, users can manipulate and 3) visually represent data by exportable and dynamic visualizations. The main novelty of our approach is the split of the querying phase in SPARQL query building and data table manipulation.
In this article, we present the QueDI operating mechanism, its interface supported by a guided use-case over DBpedia, and the evaluation of its accuracy and usability level
Dissecting the long-term emission behaviour of the BL Lac object Mrk 421
We report on long-term multiwavelengthmonitoring of blazar Mrk 421 by the GLAST-AGILE
Support Program of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration and
Steward Observatory, and by the Swift and Fermi satellites. We study the source behaviour in
the period 2007–2015, characterized by several extreme flares. The ratio between the optical,
X-ray and γ -ray fluxes is very variable. The γ -ray flux variations show a fair correlation with
the optical ones starting from 2012.We analyse spectropolarimetric data and find wavelengthdependence
of the polarization degree (P), which is compatible with the presence of the
host galaxy, and no wavelength dependence of the electric vector polarization angle (EVPA).
Optical polarimetry shows a lack of simple correlation between P and flux and wide rotations of
the EVPA.We build broad-band spectral energy distributions with simultaneous near-infrared
and optical data from the GASP-WEBT and ultraviolet and X-ray data from the Swift satellite.
They show strong variability in both flux and X-ray spectral shape and suggest a shift of
the synchrotron peak up to a factor of ∼50 in frequency. The interpretation of the flux and
spectral variability is compatible with jet models including at least two emitting regions that
can change their orientation with respect to the line of sight.http://10.0.4.69/mnras/stx2185Accepted manuscrip
Multi-band Optical Variability of the TeV Blazar PG 1553+113 in 2019
We report the flux and spectral variability of PG 1553+113 on intra-night
(IDV) to short-term timescales using BVRI data collected over 91 nights from 28
February to 8 November 2019 employing ten optical telescopes: three in
Bulgaria, two each in India and Serbia, and one each in Greece, Georgia, and
Latvia. We monitored the blazar quasi-simultaneously for 16 nights in the V and
R bands and 8 nights in the V, R, I bands and examined the light curves (LCs)
for intra-day flux and colour variations using two powerful tests: the
power-enhanced F-test and the nested ANOVA test. The source was found to be
significantly (> 99%) variable in 4 nights out of 27 in R-band, 1 out of 16 in
V-band, and 1 out of 6 nights in I-band. No temporal variations in the colours
were observed on IDV timescale. During the course of these observations the
total variation in R-band was 0.89 mag observed. We also investigated the
spectral energy distribution (SED) using B, V, R, and I band data. We found
optical spectral indices in the range of 0.878+-0.029 to 1.106+-0.065 by
fitting a power law to these SEDs of PG 1553+113. We found that the source
follows a bluer-when-brighter trend on IDV timescales. We discuss possible
physical causes of the observed spectral variability.Comment: 13 pages,8 figures, 7 tables, Accepted for publication in MNRA
Extreme photometric and polarimetric variability of blazar S4 0954+65 at its maximum optical and γ-ray brightness levels
In 2022 the BL Lac object S4 0954+65 underwent a major variability phase, reaching its historical maximum brightness in the
optical and γ -ray bands. We present optical photometric and polarimetric data acquired by the Whole Earth Blazar Telescope
(WEBT) Collaboration from 2022 April 6 to July 6. Many episodes of unprecedented fast variability were detected, implying
an upper limit to the size of the emitting region as low as 10−4 parsec. The WEBT data show rapid variability in both the degree
and angle of polarization. We analyse different models to explain the polarization behaviour in the framework of a twisting
jet model, which assumes that the long-term trend of the flux is produced by variations in the emitting region viewing angle.
All the models can reproduce the average trend of the polarization degree, and can account for its general anticorrelation with
the flux, but the dispersion of the data requires the presence of intrinsic mechanisms, such as turbulence, shocks, or magnetic
reconnection. The WEBT optical data are compared to γ -ray data from the Fermi satellite. These are analysed with both fixed
and adaptive binning procedures. We show that the strong correlation between optical and γ -ray data without measurable delay
assumes different slopes in faint and high brightness states, and this is compatible with a scenario where in faint states we mainly
see the imprint of the geometrical effects, while in bright states the synchrotron self-Compton process dominates
- …