Fe Ka line emission from the Arches cluster region - evidence for ongoing particle bombardment?

We present the results of eight years of XMM-Newton observations of the region surrounding the Arches cluster in the Galactic Center. We study the spatial distribution and temporal behaviour of the Fe-Ka line emission with the objective of identifying the likely source of the excitation. We investigate the variability of the 6.4-keV line emission of four clouds through spectral fitting of the EPIC MOS data with the use of a modelled background, which avoids many of the systematics inherent in local background subtraction. We also employ spectral stacking of both EPIC PN and MOS data to search for evidence of an Fe-K edge feature imprinted on the underlying X-ray continuum. The lightcurves of the Fe-Ka line from three bright molecular knots close to the Arches cluster are found to be constant over the 8-year observation window. West of the cluster, however, we found a bright cloud exhibiting the fastest Fe-Ka variability yet seen in a molecular cloud in the Galactic Center region. The time-averaged spectra of the molecular clouds reveal no convincing evidence of the 7.1-keV edge feature. The EW of the 6.4-keV line emitted by the clouds near the cluster is found to be ~1.0 keV. The observed Fe-Ka line flux and the high EW suggest the fluorescence has a photoionization origin, although excitation by cosmic-ray particles is not specifically excluded. For the three clouds nearest to the cluster, an identification of the source of photo-ionizing photons with an earlier outburst of Sgr A* is however at best tentative. The hardness of the nonthermal component associated with the 6.4-keV line emission might be best explained in terms of bombardment by cosmic-ray particles from the Arches cluster itself. The relatively short-timescale variability seen in the 6.4-keV line emission from the cloud to the West of the cluster is most likely the result of illumination by a nearby transient X-ray source.Comment: 13 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

Determinants of the population growth of the West Nile virus mosquito vector Culex pipiens in a repeatedly affected area in Italy

Background The recent spread of West Nile Virus in temperate countries has raised concern. Predicting the likelihood of transmission is crucial to ascertain the threat to Public and Veterinary Health. However, accurate models of West Nile Virus (WNV) expansion in Europe may be hampered by limited understanding of the population dynamics of their primary mosquito vectors and their response to environmental changes.<p></p> Methods We used data collected in north-eastern Italy (2009–2011) to analyze the determinants of the population growth rate of the primary WNV vector Culex pipiens. A series of alternative growth models were fitted to longitudinal data on mosquito abundance to evaluate the strength of evidence for regulation by intrinsic density-dependent and/or extrinsic environmental factors. Model-averaging algorithms were then used to estimate the relative importance of intrinsic and extrinsic variables in describing the variations of per-capita growth rates.<p></p> Results Results indicate a much greater contribution of density-dependence in regulating vector population growth rates than of any environmental factor on its own. Analysis of an average model of Cx. pipiens growth revealed that the most significant predictors of their population dynamics was the length of daylight, estimated population size and temperature conditions in the 15 day period prior to sampling. Other extrinsic variables (including measures of precipitation, number of rainy days, and humidity) had only a minor influence on Cx. pipiens growth rates.<p></p> Conclusions These results indicate the need to incorporate density dependence in combination with key environmental factors for robust prediction of Cx. pipiens population expansion and WNV transmission risk. We hypothesize that detailed analysis of the determinants of mosquito vector growth rate as conducted here can help identify when and where an increase in vector population size and associated WNV transmission risk should be expected.<p></p&gt

A neutron scattering study of the interplay between structure and magnetism in Ba(Fe1−x_{1-x}Cox_{x})2_2As2_2

Single crystal neutron diffraction is used to investigate the magnetic and structural phase diagram of the electron doped superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. Heat capacity and resistivity measurements have demonstrated that Co doping this system splits the combined antiferromagnetic and structural transition present in BaFe$_2$As$_2$ into two distinct transitions. For $x$=0.025, we find that the upper transition is between the high-temperature tetragonal and low-temperature orthorhombic structures with ($T_{\mathrm{TO}}=99 \pm 0.5$ K) and the antiferromagnetic transition occurs at $T_{\mathrm{AF}}=93 \pm 0.5$ K. We find that doping rapidly suppresses the antiferromagnetism, with antiferromagnetic order disappearing at $x \approx 0.055$. However, there is a region of co-existence of antiferromagnetism and superconductivity. The effect of the antiferromagnetic transition can be seen in the temperature dependence of the structural Bragg peaks from both neutron scattering and x-ray diffraction. We infer from this that there is strong coupling between the antiferromagnetism and the crystal lattice