1,498 research outputs found
Witnessing the gradual slow-down of powerful extragalactic jets: The X-ray -- optical -- radio connection
A puzzling feature of the {\it Chandra}--detected quasar jets is that their
X-ray emission decreases faster along the jet than their radio emission,
resulting to an outward increasing radio to X-ray ratio. In some sources this
behavior is so extreme that the radio emission peak is located clearly
downstream of that of the X-rays. This is a rather unanticipated behavior given
that the inverse
Compton nature of the X-rays and the synchrotron radio emission are
attributed to roughly the same electrons of the jet's non-thermal electron
distribution. In this note we show that this morphological behavior can result
from the gradual deceleration of a relativistic flow and that the offsets in
peak emission at different wavelengths carry the imprint of this deceleration.
This notion is consistent with another recent finding, namely that the jets
feeding the terminal hot spots of powerful radio galaxies and quasars are still
relativistic with Lorentz factors . The picture of the
kinematics of powerful jets emerging from these considerations is that they
remain relativistic as they gradually decelerate from Kpc scales to the hot
spots, where, in a final collision with the intergalactic medium, they
slow-down rapidly to the subrelativistic velocities of the hot spot advance
speed.Comment: Submitted in ApJ Letters on Jan. 14, 200
The intergalactic magnetic field constrained by Fermi/LAT observations of the TeV blazar 1ES 0229+200
TeV photons from blazars at relatively large distances, interacting with the
optical-IR cosmic background, are efficiently converted into electron-positron
pairs. The produced pairs are extremely relativistic (Lorentz factors of the
order of 1e6 1e7 and promptly loose their energy through inverse Compton
scatterings with the photons of the microwave cosmic background, producing
emission in the GeV band. The spectrum and the flux level of this reprocessed
emission is critically dependent on the intensity of the intergalactic magnetic
field, B, that can deflect the pairs diluting the intrinsic emission over a
large solid angle. We derive a simple relation for the reprocessed spectrum
expected from a steady source. We apply this treatment to the blazar 1ES
0229+200, whose intrinsic very hard TeV spectrum is expected to be
approximately steady. Comparing the predicted reprocessed emission with the
upper limits measured by the Fermi/Large Area Telescope, we constrain the value
of the intergalactic magnetic field to be larger than Gauss, depending on the model of extragalactic background light.Comment: 5 pages 2 figures, revised version accepted for publication in MNRAS
(Letters
Genetic differentiation among populations of the threatened Bellevalia webbiana (Asparagaceae) and its consequence on conservation
The narrow central Italian endemic and threatened Webbâs hyacinth (Bellevalia webbiana), a perennial herb, is a clear example of a species that has disappeared from several localities due to the development of human settlements. We characterized population genetics of this species to infer possible threats to its viability. We used a dominant DNA fingerprinting approach to infer genetic relationships among the five richest populations known for this species (each with N > 50 individuals). We highlighted phenomena of genetic erosion, with values of intrapopulation-gene-diversity quite similar across all populations (mean value 0.113), but a mean F st value only slightly below the mean found in other plant species using similar approaches. Despite an overall genetic similarity among populations, a population from Faenza (Emilia-Romagna) is clearly separated from all the others on genetic grounds, and may be defined as an Evolutionarily Significant Unit, worth of special conservation attention. Interestingly, this latter population is also behaving differently from all the others in terms of both vegetative and reproductive functional strategies. Our results highlight the relevance of evolutionary approaches to conservation biology for preserving a genetic diversity linked to local adaptations
Genetic diversity in the threatened Bellevalia webbiana (Asparagaceae) parallels functional and reproductive traits
Amongst the numerous Italian narrow endemic plants, the Webbâs hyacinth (Bellevalia webbiana Parl., Asparagaceae) is one of the most evolutionarily relevant, and threatened. The range of this bulbous perennial herb is restricted to an area of pre-Apennines (100â700 m a.s.l.) in Tuscany, and
Emilia-Romagna (Central Italy), with two disjunct population groups. Typical habitats for Webbâs hyacinth are open fields, and meadows, wood margins, olive groves, and vineyards. During the last century, Webbâs hyacinth disappeared from several historical localities due to the development of human settlements. For these reasons, this species is currently listed in The IUCN Red List of Threatened Species as Endangered (EN A2c). Despite contributions providing important information on the speciesâ reproductive and functional traits, no population genetic information was available so far. In this study, a DNA fingerprinting approach was applied to study the five richest populations of this species (each with N > 50 individuals): three in Tuscany (Pratolino, Uccellatoio, Tavarnuzze), and two in Emilia-Romagna (Casola Valsenio and Faenza)
Constraints on the Physical Parameters of TeV Blazars
We consider the constraints on the physical parameters of a homogeneous SSC
model that can be derived from the spectral shape and variability of TeV
blazars. Assuming that the relativistic electron spectrum is a broken power
law, where the break energy is a free parameter, we write the
analytical formulae that allow to connect the physical parameters of the model
to observable quantities. The constraints can be summarized in a plane where
the coordinates are the Doppler factor and the magnetic field. The consistency
between the break energy and the balance between cooling and escape and the
interpretation of the soft photon lags measured in some sources as radiative
cooling times are treated as additional independent constraints.
We apply themethod to the case of three well known blazars, PKS 2155-304, Mrk
421 and Mrk 501.Comment: 36 pages, incl. 6 figures in PS format, AAS LaTeX, to be published in
ApJ, Dec 199
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