Solar Flare X-ray Source Motion as a Response to Electron Spectral Hardening

Context: Solar flare hard X-rays (HXRs) are thought to be produced by nonthermal coronal electrons stopping in the chromosphere, or remaining trapped in the corona. The collisional thick target model (CTTM) predicts that sources produced by harder power-law injection spectra should appear further down the legs or footpoints of a flare loop. Therefore, hardening of the injected power-law electron spectrum during flare onset should be concurrent with a descending hard X-ray source. Aims: To test this implication of the CTTM by comparing its predicted HXR source locations with those derived from observations of a solar flare which exhibits a nonthermally-dominated spectrum before the peak in HXRs, known as an early impulsive event. Methods: HXR images and spectra of an early impulsive C-class flare were obtained using the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). Images were reconstructed to produce HXR source height evolutions for three energy bands. Spatially-integrated spectral analysis was performed to isolate nonthermal emission, and to determine the power-law index of the electron injection spectrum. The observed height-time evolutions were then fit with CTTM-based simulated heights for each energy. Results: A good match between model and observed source heights was reached, requiring a density model that agreed well with previous studies of flare loop densities. Conclusions: The CTTM has been used to produce a descent of model HXR source heights that compares well with observations of this event. Based on this interpretation, downward motion of nonthermal sources should indeed occur in any flare where there is spectral hardening in the electron distribution during a flare. However, this would often be masked by thermal emission associated with flare plasma pre-heating.Comment: 8 pages, 5 figure

X-ray Spectroscopy of QSOs with Broad Ultraviolet Absorption Lines

For the population of QSOs with broad ultraviolet absorption lines, we are just beginning to accumulate X-ray observations with enough counts for spectral analysis at CCD resolution. From a sample of eight QSOs [including four Broad Absorption Line (BAL) QSOs and three mini-BAL QSOs] with ASCA or Chandra spectra with more than 200 counts, general patterns are emerging. Their power-law X-ray continua are typical of normal QSOs with Gamma~2.0, and the signatures of a significant column density [N_H~(0.1-4)x10^{23} cm^{-2}] of intrinsic, absorbing gas are clear. Correcting the X-ray spectra for intrinsic absorption recovers a normal ultraviolet-to-X-ray flux ratio, indicating that the spectral energy distributions of this population are not inherently anomalous. In addition, a large fraction of our sample shows significant evidence for complexity in the absorption. The subset of BAL QSOs with broad MgII absorption apparently suffers from Compton-thick absorption completely obscuring the direct continuum in the 2-10 keV X-ray band, complicating any measurement of their intrinsic X-ray spectral shapes.Comment: 9 pages, 6 figures, uses AASTeX. Accepted to the Astrophysical Journa

Strongly correlated gases of Rydberg-dressed atoms: quantum and classical dynamics

We discuss techniques to generate long-range interactions in a gas of groundstate alkali atoms, by weakly admixing excited Rydberg states with laser light. This provides a tool to engineer strongly correlated phases with reduced decoherence from inelastic collisions and spontaneous emission. As an illustration, we discuss the quantum phases of dressed atoms with dipole-dipole interactions confined in a harmonic potential, as relevant to experiments. We show that residual spontaneous emission from the Rydberg state acts as a heating mechanism, leading to a quantum-classical crossover.Comment: 4 pages, 4 figure

X-raying the Winds of Luminous Active Galaxies

We briefly describe some recent observational results, mainly at X-ray wavelengths, on the winds of luminous active galactic nuclei (AGNs). These winds likely play a significant role in galaxy feedback. Topics covered include (1) Relations between X-ray and UV absorption in Broad Absorption Line (BAL) and mini-BAL quasars; (2) X-ray absorption in radio-loud BAL quasars; and (3) Evidence for relativistic iron K BALs in the X-ray spectra of a few bright quasars. We also mention some key outstanding problems and prospects for future advances; e.g., with the International X-ray Observatory (IXO).Comment: 7 pages, 3 figures, to appear in proceedings of the conference "The Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters", June 2009, Madison, Wisconsi

An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres I. Formation of the G-band in metal-poor dwarf stars

Recent developments in the three-dimensional (3D) spectral synthesis code Linfor3D have meant that, for the first time, large spectral wavelength regions, such as molecular bands, can be synthesised with it in a short amount of time. A detailed spectral analysis of the synthetic G-band for several dwarf turn-off-type 3D atmospheres (5850 <= T_eff [K] <= 6550, 4.0 <= log g <= 4.5, -3.0 <= [Fe/H] <= -1.0) was conducted, under the assumption of local thermodynamic equilibrium. We also examine carbon and oxygen molecule formation at various metallicity regimes and discuss the impact it has on the G-band. Using a qualitative approach, we describe the different behaviours between the 3D atmospheres and the traditional one-dimensional (1D) atmospheres and how the different physics involved inevitably leads to abundance corrections, which differ over varying metallicities. Spectra computed in 1D were fit to every 3D spectrum to determine the 3D abundance correction. Early analysis revealed that the CH molecules that make up the G-band exhibited an oxygen abundance dependency; a higher oxygen abundance leads to weaker CH features. Nitrogen abundances showed zero impact to CH formation. The 3D corrections are also stronger at lower metallicity. Analysis of the 3D corrections to the G-band allows us to assign estimations of the 3D abundance correction to most dwarf stars presented in the literature. The 3D corrections suggest that A(C) in CEMP stars with high A(C) would remain unchanged, but would decrease in CEMP stars with lower A(C). It was found that the C/O ratio is an important parameter to the G-band in 3D. Additional testing confirmed that the C/O ratio is an equally important parameter for OH transitions under 3D. This presents a clear interrelation between the carbon and oxygen abundances in 3D atmospheres through their molecular species, which is not seen in 1D.Comment: 19 pages, 13 figures, 4 tables. Accepted for publication in A&

Chemical composition of a sample of bright solar-metallicity stars

We present a detailed analysis of seven young stars observed with the spectrograph SOPHIE at the Observatoire de Haute-Provence for which the chemical composition was incomplete or absent in the literature. For five stars, we derived the stellar parameters and chemical compositions using our automatic pipeline optimized for F, G, and K stars, while for the other two stars with high rotational velocity, we derived the stellar parameters by using other information (parallax), and performed a line-by-line analysis. Chromospheric emission-line fluxes from CaII are obtained for all targets. The stellar parameters we derive are generally in good agreement with what is available in the literature. We provide a chemical analysis of two of the stars for the first time. The star HIP 80124 shows a strong Li feature at 670.8 nm implying a high lithium abundance. Its chemical pattern is not consistent with it being a solar sibling, as has been suggested.Comment: To be published on A

Cost benefit analysis of space communications technology. Volume 2: Final report

For abstract, see preceding accession

Latitudinal aerosol size distribution variation in the Eastern Atlantic Ocean measured aboard the FS-Polarstern

International audienceAerosol size distribution measurements from 0.03 µm to 25 µm diameter were taken at ambient humidity aboard the German research vessel, FS-Polarstern, during a transect from Bremerhaven in northern Germany, to Cape Town in South Africa across latitudes 53°32' N to 33°55' S, denoted cruise number ANT XXI/1. The data were segregated according to air mass history, wind speed and latitude. Under clean marine conditions, the averaged size distributions were generally in good agreement with those reported previously for diameters less than 0.5 µm and can be approximated by two log-normal modes, with significant variation in the mean modal diameters. Two short periods of tri-modal behaviour were observed. Above 0.5 µm, there is indication of a limit to the mechanical generation of marine aerosol over the range of wind speeds observed (~1.7?14.7 m s?1). A new technique to determine the errors associated with aerosol size distribution measurements using Poisson statistics has been applied to the dataset, providing a tool to determine the necessary sample or averaging times for correct interpretation of such data. Finally, the data were also used to investigate the loss rate of condensing gases with potentially important consequences for heterogeneous marine photochemical cycles