1,336 research outputs found
Probing dissipation mechanisms in BL Lac jets through X-ray polarimetry
The dissipation of energy flux in blazar jets plays a key role in the
acceleration of relativistic particles. Two possibilities are commonly
considered for the dissipation processes, magnetic reconnection -- possibly
triggered by instabilities in magnetically-dominated jets -- , or shocks -- for
weakly magnetized flows. We consider the polarimetric features expected for the
two scenarios analyzing the results of state-of-the-art simulations. For the
magnetic reconnection scenario we conclude, using results from global
relativistic MHD simulations, that the emission likely occurs in turbulent
regions with unstructured magnetic fields, although the simulations do not
allow us to draw firm conclusions. On the other hand, with local
particle-in-cell simulations we show that, for shocks with a magnetic field
geometry suitable for particle acceleration, the self-generated magnetic field
at the shock front is predominantly orthogonal to the shock normal and becomes
quasi-parallel downstream. Based on this result we develop a simplified model
to calculate the frequency-dependent degree of polarization, assuming that
high-energy particles are injected at the shock and cool downstream. We apply
our results to HBLs, blazars with the maximum of their synchrotron output at
UV-soft X-ray energies. While in the optical band the predicted degree of
polarization is low, in the X-ray emission it can ideally reach 50\%,
especially during active/flaring states. The comparison between measurements in
the optical and in the X-ray band made during active states (feasible with the
planned {\it IXPE} satellite) are expected to provide valuable constraints on
the dissipation and acceleration processes.Comment: 9 pages, 6 figures, accepted for publication by MNRA
Post-natal erythromycin exposure and risk of infantile hypertrophic pyloric stenosis: a systematic review and meta-analysis
PURPOSE: Macrolide antibiotics, erythromycin, in particular, have been linked to the development of infantile hypertrophic pyloric stenosis (IHPS). Our aim was to conduct a systematic review of the evidence of whether post-natal erythromycin exposure is associated with subsequent development of IHPS. METHODS: A systematic review of postnatal erythromycin administration and IHPS was performed. Papers were included if data were available on development (yes/no) of IHPS in infants exposed/unexposed to erythromycin. Data were meta-analysed using Review Manager 5.3. A random effects model was decided on a priori due to heterogeneity of study design; data are odds ratio (OR) with 95 % CI. RESULTS: Nine papers reported data suitable for analysis; two randomised controlled trials and seven retrospective studies. Overall, erythromycin exposure was significantly associated with development of IHPS [OR 2.45 (1.12-5.35), p = 0.02]. However, significant heterogeneity existed between the studies (I (2) = 84 %, p < 0.0001). Data on erythromycin exposure in the first 14 days of life was extracted from 4/9 studies and identified a strong association between erythromycin exposure and subsequent development IHPS [OR 12.89 (7.67-2167), p < 0.00001]. CONCLUSION: This study demonstrates a significant association between post-natal erythromycin exposure and development of IHPS, which seems stronger when exposure occurs in the first 2 weeks of life
Electron Positron Annihilation Radiation from SgrA East at the Galactic Center
Maps of the Galactic electron-positron annihilation radiation show evidence
for three distinct and significant features: (1) a central bulge source, (2)
emission in the Galactic plane, and (3) an enhancement of emission at positive
latitudes above the Galactic Center. In this paper, we explore the possibility
that Sgr A East, a very prominent radio structure surrounding the Galactic
nucleus, may be a significant contributer to the central bulge feature. The
motivation for doing so stems from a recently proposed link between this radio
object and the EGRET gamma-ray source 2EG J1746-2852. If this association is
correct, then Sgr A East is also expected to be a source of copious positron
production. The results presented here show that indeed Sgr A East must have
produced a numerically significant population of positrons, but also that most
of them have not yet had sufficient time to thermalize and annihilate. As such,
Sgr A East by itself does not appear to be the dominant current source of
annihilation radiation, but it will be when the positrons have cooled
sufficiently and they have become thermalized. This raises the interesting
possibility that the bulge component may be due to the relics of earlier
explosive events like the one that produced Sgr A East.Comment: This manuscript was prepared with the AAS Latex macros v4.0 It is 37
pages long and has 16 figure
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
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)
Probing shock acceleration in BL Lac jets through X-ray polarimetry: the time-dependent view
Polarimetric measurements, especially if extended at high energy, are
expected to provide important insights into the mechanisms underlying the
acceleration of relativistic particles in jets. In a previous work we have
shown that the polarization of the synchrotron X-ray emission produced by
highly energetic electrons accelerated by a mildly relativistic shock carries
essential imprints of the geometry and the structure of the magnetic fields in
the downstream region. Here we present the extension of our analysis to the
non-stationary case, especially suitable to model the highly variable emission
of high-energy emitting BL Lacs. We anticipate a large (),
almost time-independent degree of polarization in the hard/medium X-ray band, a
prediction soon testable with the upcoming mission {\it IXPE}. The situation in
other bands, in particular in the optical, is more complex. A monotonic
decrease of the optical degree of polarization is observed during the
development of a flare. At later stages reaches zero and then it starts
to increase, recovering large values at late times. The instant at which
is marked by a rotation of the polarization angle by degrees.
However, at optical frequencies it is likely that more than one region
contributes to the observed emission, potentially making it difficult to detect
the predicted behavior.Comment: 11 pages, 9 figures, accepted for publication by MNRA
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