255 research outputs found
Unveiling the monster heart: unbeamed properties of blazar 4C 71.07
4C 71.07 is a high-redshift blazar whose optical radiation is dominated by
quasar-like nuclear emission. We here present the results of a spectroscopic
monitoring of the source to study its unbeamed properties. We obtained 24
optical spectra at the Nordic Optical Telescope (NOT) and William Herschel
Telescope (WHT) and 3 near-infrared spectra at the Telescopio Nazionale Galileo
(TNG). They show no evidence of narrow emission lines. The estimate of the
systemic redshift from the H and H broad emission lines leads to
. Notwithstanding the nearly face-on orientation
of the accretion disc, the high-ionization emission lines present large
broadening as well as noticeable blueshifts, which increase with the ionizing
energy of the corresponding species. This is a clear indication of strong
ionized outflows. Line broadening and blueshift appear correlated. We applied
scaling relationships to estimate the mass of the supermassive black hole from
the Balmer and C IV lines, taking into account the prescriptions to correct for
outflow. They give . We derived an
Eddington luminosity
, and a broad line region luminosity . The line fluxes do not show significant
variability in time. In particular, there is no line reaction to the jet
flaring activity detected in 2015 October and November. This implies that the
jet gives no contribution to the photoionization of the broad line region in
the considered period.Comment: 13 pages, 13 figures, in press for MNRA
Exploring the circumstellar environment of the young eruptive star V2492 Cyg
Context. V2492 Cyg is a young eruptive star that went into outburst in 2010.
The near-infrared color changes observed since the outburst peak suggest that
the source belongs to a newly defined sub-class of young eruptive stars, where
time-dependent accretion and variable line-of-sight extinction play a combined
role in the flux changes.
Aims. In order to learn about the origin of the light variations and to
explore the circumstellar and interstellar environment of V2492 Cyg, we
monitored the source at ten different wavelengths, between 0.55 \mu m and 2.2
\mu m from the ground and between 3.6 \mu m and 160 \mu m from space.
Methods. We analyze the light curves and study the color-color diagrams via
comparison with the standard reddening path. We examine the structure of the
molecular cloud hosting V2492 Cyg by computing temperature and optical depth
maps from the far-infrared data.
Results. We find that the shapes of the light curves at different wavelengths
are strictly self-similar and that the observed variability is related to a
single physical process, most likely variable extinction. We suggest that the
central source is episodically occulted by a dense dust cloud in the inner
disk, and, based on the invariability of the far-infrared fluxes, we propose
that it is a long-lived rather than a transient structure. In some respects,
V2492 Cyg can be regarded as a young, embedded analog of UX Orionis-type stars.
Conclusions. The example of V2492 Cyg demonstrates that the light variations
of young eruptive stars are not exclusively related to changing accretion. The
variability provided information on an azimuthally asymmetric structural
element in the inner disk. Such an asymmetric density distribution in the
terrestrial zone may also have consequences for the initial conditions of
planet formation.Comment: 9 pages, 7 figures, 2 online tables, accepted for publication in A&
A New Multi-Wavelength Census of Blazars
Context:Blazars are the rarest and most powerful active galactic nuclei,
playing a crucial and growing role in today multi-frequency and multi-messenger
astrophysics. Current blazar catalogs, however, are incomplete and particularly
depleted at low Galactic latitudes. Aims: We aim at augmenting the current
blazar census to build a catalog of blazar candidates with homogeneous sky
coverage that can provide candidate counterparts to unassociated gamma-ray
sources, sources of high-energy neutrino emission, and ultra-high energy cosmic
rays. Methods: Starting from the ALMA Calibrator Catalog we built a catalog of
1580 blazar candidates (ALMA Blazar Candidates, ABC) for which we collect
multi-wavelength information. We also compared ABC sources with existing blazar
catalogs. Results: The ABC catalogue fills the lack of low Galactic latitude
sources in current blazar catalogues. ABC sources are significantly dimmer than
known blazars in Gaia g band, and they appear bluer in SDSS and WISE colors.
The majority of ABC sources (~ 90%) have optical spectra that classify them as
QSO, while the remaining sources resulted galactic objects. ABC sources are
similar in X-rays to known blazar, while in gamma-rays they are on average
dimmer and softer, indicating a significant contribution of FSRQ sources.
Making use of WISE colours, we classified 715 ABC sources as candidate
gamma-ray blazar of different classes. Conclusions: We built a new catalogue of
1580 candidate blazars with a rich multi-wavelength data-set, filling the lack
of low Galactic latitude sources in current blazar catalogues. This will be
particularly important to identify the source population of high energy
neutrinos or ultra-high energy cosmic rays. The data collected by the upcoming
LSST surveys will provide a key tool to investigate the possible blazar nature
of these sources.Comment: 53 pages, 32 figures, 6 tables. Accepted for publication on A&
Variability and evolution of the optical polarization of a sample of gamma-ray blazars
We present a polarization variability analysis of a sample of 26 -ray
blazars monitored by the Steward Observatory between 2008 and 2018 in the
optical band. We investigate the properties and long-term variability of their
optical polarization, searching for differences between blazar types. We
observe that BL Lac objects are typically less polarized and less variable than
flat spectrum radio quasars (FSRQs). Moreover, BL Lacs display a distribution
of their polarization angle typically oriented in a preferential direction,
contrary to the rather random distribution of FSRQs. For the latter blazar
type, as well as those sources showing a bright stellar emission, we take into
account the depolarizing effect introduced by the broad line region and the
host galaxy on the measured polarization degree. In this sample we also observe
that BL Lacs present an uncorrelated evolution of the flux and the
polarization. Contrary, FSRQs show a correlation before the depolarization
correction, that is lost however after considering this effect. In addition, we
study the behaviour of the polarization angle, searching for angle rotations in
its long-term evolution. We derive that the FSRQs studied here show rotations
more frequently than BL Lac objects by a factor 1.5. During these periods
we also observe a systematic decrease of the polarization fraction, as well as
a marginal flux increase, not significant however to connect rotations with
optical flares. We interpret these results within the extended shock-in-jet
scenario, able to explain the overall features observed here for the
polarization of the blazar sample.Comment: Accepted for publication in MNRA
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
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&
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