Overexpression of a NAC transcription factor delays leaf senescence and increases grain nitrogen concentration in wheat

Increasing the duration of leaf photosynthesis during grain filling using slow-senescing functional stay-green phenotypes is a possible route for increasing grain yields in wheat (Triticum aestivum L.). However, delayed senescence may negatively affect nutrient remobilisation and hence reduce grain protein concentrations and grain quality. A novel NAC1-type transcription factor (hereafter TaNAC-S) was identified in wheat, with gene expression located primarily in leaf/sheath tissues, which decreased during post-anthesis leaf senescence. Expression of TaNAC-S in the second leaf correlated with delayed senescence in two doubled-haploid lines of an Avalon 3 Cadenza population (lines 112 and 181), which were distinct for leaf senescence. Transgenic wheat plants overexpressing TaNAC-S resulted in delayed leaf senescence (stay-green phenotype). Grain yield, aboveground biomass, harvest index and total grain N content were unaffected, but NAC over-expressing lines had higher grain N concentrations at similar grain yields compared to non-transgenic controls. These results indicate that TaNAC-S is a negative regulator of leaf senescence, and that delayed leaf senescence may lead not only to increased grain yields but also to increased grain protein concentrations.D. Zhao, A. P. Derkx, D.-C. Liu, P. Buchner, M. J. Hawkesfor

X-ray bolometric corrections for Compton-thick active galactic nuclei

We present X-ray bolometric correction factors, $\kappa_{Bol}$ ($\equiv L_{Bol}/L_X$), for Compton-thick (CT) active galactic nuclei (AGN) with the aim of testing AGN torus models, probing orientation effects, and estimating the bolometric output of the most obscured AGN. We adopt bolometric luminosities, $L_{Bol}$, from literature infrared (IR) torus modeling and compile published intrinsic 2--10 keV X-ray luminosities, $L_{X}$, from X-ray torus modeling of NuSTAR data. Our sample consists of 10 local CT AGN where both of these estimates are available. We test for systematic differences in $\kappa_{Bol}$ values produced when using two widely used IR torus models and two widely used X-ray torus models, finding consistency within the uncertainties. We find that the mean $\kappa_{Bol}$ of our sample in the range $L_{Bol}\approx10^{42}-10^{45}$ erg/s is log$_{10}\kappa_{Bol}=1.44\pm0.12$ with an intrinsic scatter of $\sim0.2$ dex, and that our derived $\kappa_{Bol}$ values are consistent with previously established relationships between $\kappa_{Bol}$ and $L_{Bol}$ and $\kappa_{Bol}$ and Eddington ratio. We investigate if $\kappa_{Bol}$ is dependent on $N_H$ by comparing our results on CT AGN to published results on less-obscured AGN, finding no significant dependence. Since many of our sample are megamaser AGN, known to be viewed edge-on, and furthermore under the assumptions of AGN unification whereby unobscured AGN are viewed face-on, our result implies that the X-ray emitting corona is not strongly anisotropic. Finally, we present $\kappa_{Bol}$ values for CT AGN identified in X-ray surveys as a function of their observed $L_X$, where an estimate of their intrinsic $L_{X}$ is not available, and redshift, useful for estimating the bolometric output of the most obscured AGN across cosmic time.Comment: Accepted for publication in Ap

X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue

AGN are known to have complex X-ray spectra that depend on both the properties of the accreting SMBH (e.g. mass, accretion rate) and the distribution of obscuring material in its vicinity ("torus"). Often however, simple and even unphysical models are adopted to represent the X-ray spectra of AGN. In the case of blank field surveys in particular, this should have an impact on e.g. the determination of the AGN luminosity function, the inferred accretion history of the Universe and also on our understanding of the relation between AGN and their host galaxies. We develop a Bayesian framework for model comparison and parameter estimation of X-ray spectra. We take into account uncertainties associated with X-ray data and photometric redshifts. We also demonstrate how Bayesian model comparison can be used to select among ten different physically motivated X-ray spectral models the one that provides a better representation of the observations. Despite the use of low-count spectra, our methodology is able to draw strong inferences on the geometry of the torus. For a sample of 350 AGN in the 4 Ms Chandra Deep Field South field, our analysis identifies four components needed to represent the diversity of the observed X-ray spectra: (abridged). Simpler models are ruled out with decisive evidence in favour of a geometrically extended structure with significant Compton scattering. Regarding the geometry of the obscurer, there is strong evidence against both a completely closed or entirely open toroidal geometry, in favour of an intermediate case. The additional Compton reflection required by data over that predicted by toroidal geometry models, may be a sign of a density gradient in the torus or reflection off the accretion disk. Finally, we release a catalogue with estimated parameters such as the accretion luminosity in the 2-10 keV band and the column density, $N_{H}$, of the obscurer.Comment: 28 pages, 18 figures, catalogue available from https://www.mpe.mpg.de/~jbuchner/agn_torus/analysis/cdfs4Ms_cat/, software available from https://github.com/JohannesBuchner/BX

Three-Dimensional MHD Simulation of FTEs Produced by Merging at an Isolated Point in a Sheared Magnetic Field Configuration

We present predictions for the evolution of FTEs generated by localized bursts of reconnection on a planar magnetopause that separates a magnetosheath region of high densities and weak magnetic field from a magnetospheric region of low densities and strong magnetic field. The magnetic fields present a shear angle of 105 degrees. Reconnection forms a pair of FTEs each crossing the magnetopause in the field reversal region and bulging into the magnetosphere and magnetosheath. At their initial stage they can be characterized as flux tubes since the newly reconnected magnetic field lines are not twisted. Reconnection launches Alfvenic perturbations that propagate along the FTEs generating high-speed jets, which move the pair of FTEs in opposite directions. As the FTE moves, it displaces the ambient magnetic field and plasma producing bipolar magnetic field and plasma velocity signatures normal to the nominal magnetopause in the regions surrounding the FTE. The combination of the ambient plasma with the FTE flows generates a vortical velocity pattern around the reconnected field lines. During its evolution the FTE evolves to a flux rope configuration due to the twist of the magnetic field lines. The alfvenic perturbations propagate faster along the part of the FTE bulging into the magnetosphere than in the magnetosheath, and due to the differences between the plasma and magnetic field properties the perturbations have slightly different signatures in the two regions. As a consequence, the FTEs have different signatures depending on whether the satellite encounters the part bulging into the magnetosphere or into the magnetosheath

The X-ray luminosity function of Active Galactic Nuclei in the redshift interval z=3-5

We combine deep X-ray survey data from the Chandra observatory and the wide-area/shallow XMM-XXL field to estimate the AGN X-ray luminosity function in the redshift range z=3-5. The sample consists of nearly 340 sources with either photometric (212) or spectroscopic (128) redshift in the above range. The combination of deep and shallow survey fields provides a luminosity baseline of three orders of magnitude, Lx(2-10keV)~1e43-1e46erg/s at z>3. We follow a Bayesian approach to determine the binned AGN space density and explore their evolution in a model-independent way. Our methodology accounts for Poisson errors in the determination of X-ray fluxes and uncertainties in photometric redshift estimates. We demonstrate that the latter is essential for unbiased measurement of space densities. We find that the AGN X-ray luminosity function evolves strongly between the redshift intervals z=3-4 and z=4-5. There is also suggestive evidence that the amplitude of this evolution is luminosity dependent. The space density of AGN with Lx<1e45erg/s drops by a factor of 5 between the redshift intervals above, while the evolution of brighter AGN appears to be milder. Comparison of our X-ray luminosity function with that of UV/optical selected QSOs at similar redshifts shows broad agreement at bright luminosities, Lx>1e45erg/s. The faint-end slope of UV/optical luminosity functions however, is steeper than for X-ray selected AGN. This implies that the type-I AGN fraction increases with decreasing luminosity at z>3, opposite to trends established at lower redshift. We also assess the significance of AGN in keeping the hydrogen ionised at high redshift. Our X-ray luminosity function yields ionising photon rate densities that are insufficient to keep the Universe ionised at redshift z>4. A source of uncertainty in this calculation is the escape fraction of UV photons for X-ray selected AGN.Comment: MNRAS accepte

Instantaneous Normal Mode analysis of liquid HF

We present an Instantaneous Normal Modes analysis of liquid HF aimed to clarify the origin of peculiar dynamical properties which are supposed to stem from the arrangement of molecules in linear hydrogen-bonded network. The present study shows that this approach is an unique tool for the understanding of the spectral features revealed in the analysis of both single molecule and collective quantities. For the system under investigation we demonstrate the relevance of hydrogen-bonding ``stretching'' and fast librational motion in the interpretation of these features.Comment: REVTeX, 7 pages, 5 eps figures included. Minor changes in the text and in a figure. Accepted for publication in Phys. Rev. Let

SPIDERS: Selection of spectroscopic targets using AGN candidates detected in all-sky X-ray surveys

SPIDERS (SPectroscopic IDentification of eROSITA Sources) is an SDSS-IV survey running in parallel to the eBOSS cosmology project. SPIDERS will obtain optical spectroscopy for large numbers of X-ray-selected AGN and galaxy cluster members detected in wide area eROSITA, XMM-Newton and ROSAT surveys. We describe the methods used to choose spectroscopic targets for two sub-programmes of SPIDERS: X-ray selected AGN candidates detected in the ROSAT All Sky and the XMM-Newton Slew surveys. We have exploited a Bayesian cross-matching algorithm, guided by priors based on mid-IR colour-magnitude information from the WISE survey, to select the most probable optical counterpart to each X-ray detection. We empirically demonstrate the high fidelity of our counterpart selection method using a reference sample of bright well-localised X-ray sources collated from XMM-Newton, Chandra and Swift-XRT serendipitous catalogues, and also by examining blank-sky locations. We describe the down-selection steps which resulted in the final set of SPIDERS-AGN targets put forward for spectroscopy within the eBOSS/TDSS/SPIDERS survey, and present catalogues of these targets. We also present catalogues of ~12000 ROSAT and ~1500 XMM-Newton Slew survey sources which have existing optical spectroscopy from SDSS-DR12, including the results of our visual inspections. On completion of the SPIDERS program, we expect to have collected homogeneous spectroscopic redshift information over a footprint of ~7500 deg$^2$ for >85 percent of the ROSAT and XMM-Newton Slew survey sources having optical counterparts in the magnitude range 17<r<22.5, producing a large and highly complete sample of bright X-ray-selected AGN suitable for statistical studies of AGN evolution and clustering.Comment: MNRAS, accepte