625 research outputs found
The nature of optical and near-infrared variability of BL Lacertae
Since 1997, BL Lacertae has undergone a phase of high optical activity, with
the occurrence of several prominent outbursts. Starting from 1999, the Whole
Earth Blazar Telescope (WEBT) consortium has organized various multifrequency
campaigns on this blazar, collecting tens of thousands of data points. One of
the main issues in the analysis of this huge dataset has been the study of
colour variability. The massive amount of optical and near-infrared data
collected during the campaigns enables us to perform a deep analysis of
multiband data, with the aim of understanding the flux variability mechanisms.
We use a new approach for the analysis of these data, focusing on the source
spectral evolution. We show that the overall behaviour of the BL Lacertae light
and colour curves can be explained in terms of changing viewing angle of a
moving, discrete emitting region, which causes variable Doppler boosting of the
corresponding radiation. A fractal helical structure is suggested to be at the
origin of the different time scales of variability.Comment: 6 pages, 8 figures, accepted by A&
The long-term optical spectral variability of BL Lacertae
We present the results from a study of the long-term optical spectral
variations of BL Lacertae, using the long and well-sampled B and R-band light
curves of the Whole Earth Blazar Telescope (WEBT) collaboration, binned on time
intervals of 1 day. The relation between spectral slope and flux (the spectrum
gets bluer as the source flux increases) is well described by a power-law
model, although there is significant scatter around the best-fitting model
line. To some extent, this is due to the spectral evolution of the source
(along well-defined loop-like structures) during low-amplitude events, which
are superimposed on the major optical flares, and evolve on time scales of a
few days. The "bluer-when-brighter" mild chromatism of the long-term variations
of the source can be explained if the flux increases/decreases faster in the B
than in the R band. The B and R-band variations are well correlated, with no
significant, measurable delays larger than a few days. On the other hand, we
find that the spectral variations lead those in the flux light curves by ~ 4
days. Our results can be explained in terms of Doppler factor variations due to
changes in the viewing angle of a curved and inhomogeneous emitting jet.Comment: 7 pages, 5 figures, accepted for publication in A&
Optical variability of the BL Lacertae object GC 0109+224. Multiband behaviour and time scales from a 7-years monitoring campaign
We present the most continuous data base of optical
observations ever published on the BL Lacertae object GC 0109+224, collected
mainly by the robotic telescope of the Perugia University Observatory in the
period November 1994-February 2002. These observations have been complemented
by data from the Torino Observatory, collected in the period July 1995-January
1999, and Mt. Maidanak Observatory (December 2000). GC 0109+224 showed rapid
optical variations and six major outbursts were observed at the beginning and
end of 1996, in fall 1998, at the beginning and at the end of 2000, and at the
beginning of 2002. Fast and large-amplitude drops characterized its flux
behaviour. The magnitude ranged from 13.3 (16.16 mJy) to 16.46 (0.8 mJy),
with a mean value of 14.9 (3.38 mJy). In the periods where we collected
multi-filter observations, we analyzed colour and spectral indexes, and the
variability patterns during some flares. The long-term behaviour seems
approximatively achromatic, but during some isolated outbursts we found
evidence of the typical loop-like hysteresis behaviour, suggesting that rapid
optical variability is dominated by non-thermal cooling of a single emitting
particle population. We performed also a statistical analysis of the data,
through the discrete correlation function (DCF), the structure function (SF),
and the Lomb-Scargle periodogram, to identify characteristic times scales, from
days to months, in the light curves, and to quantify the mode of variability.
We also include the reconstruction of the historical light curve and a
photometric calibration of comparison stars, to favour further extensive
optical monitoring of this interesting blazar.Comment: 13 pages, 11 PS figures, 1 EPS figure, 3 tables, accepted by
Astronomy and Astrophysics. Uses A&A documentclass aa.cls, and the package
graphicx.st
Polarimetric Properties of Blazars Caught by the WEBT
Active galactic nuclei come in many varieties. A minority of them are
radio-loud, and exhibit two opposite prominent plasma jets extending from the
proximity of the supermassive black hole up to megaparsec distances. When one
of the relativistic jets is oriented closely to the line of sight, its emission
is Doppler beamed and these objects show extreme variability properties at all
wavelengths. These are called "blazars". The unpredictable blazar variability,
occurring on a continuous range of time-scales, from minutes to years, is most
effectively investigated in a multi-wavelength context. Ground-based and space
observations together contribute to give us a comprehensive picture of the
blazar emission properties from the radio to the gamma-ray band. Moreover, in
recent years, a lot of effort has been devoted to the observation and analysis
of the blazar polarimetric radio and optical behaviour, showing strong
variability of both the polarisation degree and angle. The Whole Earth Blazar
Telescope (WEBT) Collaboration, involving many tens of astronomers all around
the globe, has been monitoring several blazars since 1997. The results of the
corresponding data analysis have contributed to the understanding of the blazar
phenomenon, particularly stressing the viability of a geometrical
interpretation of the blazar variability. We review here the most significant
polarimetric results achieved in the WEBT studies.Comment: Review published in "Galaxies" as part of the Special Issue
"Polarimetry as a Probe of Magnetic Fields in AGN Jets", Academic Editors:
Margo Aller, Jose L. G\'omez and Eric Perlma
Evs and bioengineering: From cellular products to engineered nanomachines
Extracellular vesicles (EVs) are natural carriers produced by many different cell types that have a plethora of functions and roles that are still under discovery. This review aims to be a compendium on the current advancement in terms of EV modifications and re-engineering, as well as their potential use in nanomedicine. In particular, the latest advancements on artificial EVs are discussed, with these being the frontier of nanomedicine-based therapeutics. The first part of this review gives an overview of the EVs naturally produced by cells and their extraction methods, focusing on the possibility to use them to carry desired cargo. The main issues for the production of the EV-based carriers are addressed, and several examples of the techniques used to upload the cargo are provided. The second part focuses on the engineered EVs, obtained through surface modification, both using direct and indirect methods, i.e., engineering of the parental cells. Several examples of the current literature are proposed to show the broad variety of engineered EVs produced thus far. In particular, we also report the possibility to engineer the parental cells to produce cargo-loaded EVs or EVs displaying specific surface markers. The third and last part focuses on the most recent advancements based on synthetic and chimeric EVs and the methods for their production. Both top-down or bottom-up techniques are analyzed, with many examples of applications
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