Coronal--Temporal Correlations in GX339-4: Hysteresis, Possible Reflection Changes, and Implications for ADAFs

We present spectral fits and timing analysis of Rossi X-ray Timing Explorer observations of GX339-4. These observations were carried out over a span of more than two years and encompassed both the soft/high and hard/low states. Hysteresis in the soft state/hard state transition is observed. The hard state exhibits a possible anti-correlation between coronal compactness (i.e., spectral hardness) and the covering fraction of cold, reflecting material. The correlation between `reflection fraction' and soft X-ray flux, however, appears to be more universal. Furthermore, low flux, hard state observations - taken over a decline into quiescence- show that the Fe line, independent of `reflection fraction', remains broad and at a roughly constant equivalent width, counter to expectations from ADAF models. All power spectral densities (PSD) of the hard state X-ray lightcurves are describable as the sum of just a few broad, quasi-periodic features with frequencies that roughly scale as coronal compactness to the -3/2 power. Similar to observations of Cyg X-1, time lags between soft and hard variability anti-correlate with coronal compactness. A stronger correlation is seen between the time lags and the `reflection fraction'.Comment: 29 Pages, 17 Figures, 6 Tables. Accepted for Publication in MNRAS. (Abstract Abridged

Self-Consistent Thermal Accretion Disk Corona Models for Compact Objects: I. Properties of the Corona and the Spectrum of Escaping Radiation

We present the properties of accretion disk corona (ADC) models, where the radiation field, the temperature, and the total opacity of the corona are determined self-consistently. We use a non-linear Monte Carlo code to perform the calculations. As an example, we discuss models where the corona is situated above and below a cold accretion disk with a plane-parallel (slab) geometry, similar to the model of Haardt and Maraschi. By Comptonizing the soft radiation emitted by the accretion disk, the corona is responsible for producing the high-energy component of the escaping radiation. Our models include the reprocessing of radiation in the accretion disk. Here, the photons either are Compton reflected or photo-absorbed, giving rise to fluorescent line emission and thermal emission. The self-consistent coronal temperature is determined by balancing heating (due to viscous energy dissipation) with Compton cooling, determined using the fully relativistic, angle-dependent cross-sections. The total opacity is found by balancing pair productions with annihilations. We find that, for a disk temperature kT_bb \lta 200 eV, these coronae are unable to have a self-consistent temperature higher than \sim 120 keV if the total optical depth is \gta 0.2, regardless of the compactness parameter of the corona and the seed opacity. This limitation corresponds to the angle-averaged spectrum of escaping radiation having a photon index \gta 1.8 within the 5 keV - 30 keV band. Finally, all models that have reprocessing features also predict a large thermal excess at lower energies. These constraints make explaining the X-ray spectra of persistent black hole candidates with ADC models very problematic.Comment: 15 pages, Latex, 9 .eps figures, uses emulateapj.sty (included). To be published in ApJ, October 1, 1997, Vol. 48

Low Luminosity States of the Black Hole Candidate GX 339-4. I. ASCA and Simultaneous Radio/RXTE Observations

We discuss a series of observations of the black hole candidate GX 339-4 in low luminosity, spectrally hard states. We present spectral analysis of three separate archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data sets and eight separate Rossi X-ray Timing Explorer (RXTE) data sets. Three of the RXTE observations were strictly simultaneous with 843 MHz and 8.3-9.1 GHz radio observations. All of these observations have (3-9 keV) flux approximately < 10^{-9} ergs s^{-1} cm^{-2}. The ASCA data show evidence for an 6.4 keV Fe line with equivalent width 40 eV, as well as evidence for a soft excess that is well-modeled by a power law plus a multicolor blackbody spectrum with peak temperature 150-200 eV. The RXTE data sets also show evidence of an Fe line with equivalent widths 20-140 eV. Reflection models show a hardening of the RXTE spectra with decreasing X-ray flux; however, these models do not exhibit evidence of a correlation between the photon index of the incident power law flux and the solid angle subtended by the reflector. `Sphere+disk' Comptonization models and Advection Dominated Accretion Flow (ADAF) models also provide reasonable descriptions of the RXTE data. The former models yield coronal temperatures in the range 20-50 keV and optical depths of \tau ~ 3. The model fits to the X-ray data, however, do not simultaneously explain the observed radio properties. The most likely source of the radio flux is synchrotron emission from an extended outflow of size greater than O(10^7 GM/c^2).Comment: 18 pages in latex emulateapj.sty. Accepted for publication in the Astrophysical Journa

RXTE Observation of Cygnus X-1: Spectral Analysis

We present the results of the analysis of the broad-band spectrum of Cygnus X-1 from 3.0 to 200 keV, using data from a 10 ksec observation by the Rossi X-ray Timing Explorer. The spectrum can be well described phenomenologically by an exponentially cut-off power law with a photon index Gamma = 1.45 +/- 0.02 (a value considerably harder than typically found), e-folding energy E_fold = 162 +/- 9 keV, plus a deviation from a power law that formally can be modeled as a thermal blackbody with temperature kT_bb = 1.2 +/1 0.2 keV. Although the 3 - 30 keV portion of the spectrum can be fit with a reflected power law with Gamma = 1.81 +/- 0.01 and covering fraction f = 0.35 +/- 0.02, the quality of the fit is significantly reduced when the HEXTE data in the 30 - 100 keV range is included, as there is no observed hardening in the power law within this energy range. As a physical description of this system, we apply the accretion disc corona models of Dove, Wilms & Begelman (1997) --- where the temperature of the corona is determined self-consistently. A spherical corona with a total optical depth tau = 1.6 +/- 0.1 and an average temperature kT_c = 87 +/- 5 keV, surrounded by an exterior cold disc, does provide a good description of the data (reduced chi-squared = 1.55). These models deviate from the data by up to 7% in the 5 - 10 keV range, and we discuss possible reasons for these discrepancies. However, considering how successfully the spherical corona reproduces the 10 - 200 keV data, such ``photon-starved'' coronal geometries seem very promising for explaining the accretion processes of Cygnus X-1.Comment: Revised version (added content). 8 pages, 6 figures, 1 table.tex file, latex, uses mn.sty. Accepted for publication in MNRA

RXTE Observation of Cygnus X-1: II. Timing Analysis

We present timing analysis for a Rossi X-ray Timing Explorer observation of Cygnus X-1 in its hard/low state. This was the first RXTE observation of Cyg X-1 taken after it transited back to this state from its soft/high state. RXTE's large effective area, superior timing capabilities, and ability to obtain long, uninterrupted observations have allowed us to obtain measurements of the power spectral density (PSD), coherence function, and Fourier time lags to a decade lower in frequency and half a decade higher in frequency than typically was achieved with previous instruments. Notable aspects of our observations include a weak 0.005 Hz feature in the PSD coincident with a coherence recovery; a `hardening' of the high-frequency PSD with increasing energy; a broad frequency range measurement of the coherence function, revealing rollovers from unity coherence at both low and high frequency; and an accurate determination of the Fourier time lags over two and a half decades in frequency. As has been noted in previous similar observations, the time delay is approximately proportional to f^(-0.7), and at a fixed Fourier frequency the time delay of the hard X-rays compared to the softest energy channel tends to increase logarithmically with energy. Curiously, the 0.01-0.2 Hz coherence between the highest and lowest energy bands is actually slightly greater than the coherence between the second highest and lowest energy bands. We carefully describe all of the analysis techniques used in this paper, and we make comparisons of the data to general theoretical expectations. In a companion paper, we make specific comparisons to a Compton corona model that we have successfully used to describe the energy spectral data from this observation.Comment: To Be Published in the Astrophysical Journal. 18 pages. Uses emulatepaj.st

Integrating biofortified crops into community development programs

Extensive research has demonstrated the effectiveness and acceptability of biofortification. There is now a need to establish effective, sustainable delivery systems for biofortified crops in countries with a high burden of micronutrient malnutrition. This requires country-level partnerships between national governments, non-governmental organizations (NGOs), the private sector and research institutions. World Vision (WV) is an international NGO with experience integrating biofortified crops as an agriculture-for-nutrition intervention within rural development projects, most extensively in Mozambique, Burundi, and Uganda. In WV’s experience, successful integration and uptake of biofortified crops within broader community development programs is reliant on key elements of both the enabling environment and the program design. The optimal mix and intensity of interventions is not yet known; implementation science research is needed to enhance understanding of the critical elements of successful adoption, intensification, scale-up and sustainability of biofortified crops, and the human, technical and financial resources required to achieve this and to contribute to the sustainable reduction of micronutrient malnutrition.Keywords: nutrition, agriculture, biofortification, integration, multi-sectoral, nutrition-sensitive, nutrition policy, micronutrients, diet diversificatio