Current Challenges Can Help Fuel Future Opportunities

Native juniper trees and invasive plants pose an expanding threat to the survival of the sagebrush ecosystem in arid western rangelands. As the trees mature, they compete with other native plants that are valuable food sources for wildlife and cattle. After these plants die back, the bare patches of soil that remain are vulnerable to erosion and encroachment from other species like cheatgrass, an invasive nonnative annual that fuels wildfires. Wildfires threaten livestock ranches, are suppressed in large part by efforts funded with taxpayer dollars, and increase the probability that invasive plants will spread and thrive in the postfire landscape. So while western juniper, pinyon pine, and other similar native trees have a role to play in this ecosystem, the thickets that are rapidly spreading across the rangelands need to be brought under control. Fortunately, the challenge these trees present to healthy rangelands also creates an opportunity for using them as a feedstock for renewable jet fuel production. And this new opportunity means that western states could significantly contribute to meeting national goals for the production of renewable bioenergy in the United States—and help the U.S. military develop homegrown and sustainable sources of alternative biofuels for powering its assortment of vehicles, planes, and ships. Agricultural Research Service researchers already have a framework in place for tackling the many issues that need to be resolved before turning a tree into a gallon of renewable fuel. In 2010, U.S. Department of Agriculture Secretary Tom Vilsack created five Regional Biomass Research Centers to maximize existing USDA research resources for bioenergy development. The goal of this effort is to develop sustainable, regional approaches for producing feedstocks so that rural communities across the country have opportunities to participate in the emerging biofuels and biobased-products economy. (See “ARS and the Regional Biomass Research Centers,” Agricultural Research, September 2012.

On Estimating the High-Energy Cutoff in the X-ray Spectra of Black Holes via Reflection Spectroscopy

The fundamental parameters describing the coronal spectrum of an accreting black hole are the slope $\Gamma$ of the power-law continuum and the energy $E_{cut}$ at which it rolls over. Remarkably, this parameter can be accurately measured for values as high as 1 MeV by modeling the spectrum of X-rays reflected from a black hole accretion disk at energies below 100 keV. This is possible because the details in the reflection spectrum, rich in fluorescent lines and other atomic features, are very sensitive to the spectral shape of the hardest coronal radiation illuminating the disk. We show that fitting simultaneous NuSTAR (3-79 keV) and low-energy (e.g., Suzaku) data with the most recent version of our reflection model RELXILL, one can obtain reasonable constraints on $E_{cut}$ at energies from tens of keV up to 1 MeV, for a source as faint as 1 mCrab in a 100 ks observation.Comment: Accepted for publication in ApJL, 6 pages, 5 figure

X-ray Reflection Spectroscopy of the Black Hole GX 339-4: Exploring the Hard State with Unprecedented Sensitivity

We analyze {\it simultaneously} six composite {\it RXTE} spectra of GX 339--4 in the hard state comprising 77 million counts collected over 196 ks. The source spectra are ordered by luminosity and spanthe range 1.6\% to 17\% of the Eddington luminosity. Crucially, using our new tool {\tt pcacorr}, we re-calibrate the data to a precision of 0.1\%, an order of magnitude improvement over all earlier work. Using our advanced reflection model {\tt relxill}, we target the strong features in the component of emission reflected from the disk, namely, the relativistically-broadened Fe K emission line, the Fe K edge and the Compton hump. We report results for two joint fits to the six spectra: For the first fit, we fix the spin parameter to its maximal value ($a_*=0.998$) and allow the inner disk radius $R_{\rm in}$ to vary. Results include (i) precise measurements of $R_{\rm in}$, with evidence that the disk becomes slightly truncated at a few percent of Eddington; and (ii) an order-of-magnitude swing with luminosity in the high energy cutoff, which reaches $>890$ keV at our lowest luminosity. For the second fit, we make the standard assumption in estimating spin that the inner edge of the accretion disk is located at the innermost stable circular orbit ($R_\mathrm{in} = R_\mathrm{ISCO}$) and find $a_* = 0.95^{+0.03}_{-0.05}$ (90\% confidence, statistical). For both fits, and at the same level of statistical confidence, we estimate that the disk inclination is $i = 48\pm 1$ deg and that the Fe abundance is super-solar, $A_\mathrm{Fe} = 5\pm1$.Comment: Accepted for publication in ApJ, 20 pages, 13 figure

Fabrication and characterization of locally resonant acoustic metamaterials made with resonators generated from core-shell drops

Acoustic metamaterials promise the remarkable ability to control, direct, and manipulate sound waves. Within this infant field, a promising approach to fabricate locally resonant acoustic metamaterials is the use of resonators composed of a heavy core surrounded by a rubber shell dispersed in an epoxy matrix. At the resonant frequency, the resonators vibrate 180° out-of-phase with the matrix and a band gap in transmission is observed making these materials excellent sound absorbers. The resonant frequency of the resonators scales with the core mass; therefore, it can be tailored by increasing the core diameter or the density of the core material. A significant challenge in the study and adoption of these materials is the lack of techniques to easily fabricate resonators with a wide range of sizes, and properties. Here, we present a robust yet simple technique to fabricate resonators with diameters ranging from 50 µm to 5 mm from core-shell drops generated in microfluidic and millifluidic devices. We started by fabricating resonators with core diameters ranging from 50 µm to 1 mm at rates ranging from 2000 to 200 drops/second respectively, from double emulsion drops composed of a concentrated ceramic suspension in the core (inner drop) surrounded by a UV-crosslinkable rubber shell (outer drop) using microcapillary microfluidic devices. The double emulsion drops were collected and exposed to UV to crosslink the shell material forming resonators with resonant frequencies ranging from 100 kHz to 25 kHz depending on their size. Lower resonant frequencies down to 6 kHz were obtained by fabricating resonators with core diameters ranging from 1.2 mm to 2 mm from core-shell drops extruded in air from a coaxial nozzle at rates up to 6 drops/minute. The effects of core density were studied by utilizing suspensions composed of ceramic particles of increasing density including silica, alumina, and lead zirconate titanate (PZT). Resonators were harvested, dried and mixed with epoxy to fabricate acoustic metamaterials. The transmission properties of the acoustic metamaterials made with resonators with different core diameters, core materials, and ordering within the matrix, were measured using a shaker/accelerometer setup in the frequency range from 1 kHz to 12 kHz. For example, acoustic metamaterials composed of randomly dispersed 1.8 mm alumina-core resonators at a 30 vol% concentration showed a well defined band-gap at 8.5 kHz. A finite element model was also developed to capture the acoustic transmission physics of these materials. This technique offers a robust path for the fabrication of acoustic resonators and locally resonant acoustic metamaterials

The Low-Spin Black Hole in LMC X-3

Building upon a new dynamical model for the X-ray binary LMC X-3, we measure the spin of its black hole primary via the continuum-fitting method. We consider over one thousand thermal-state RXTE X-ray spectra of LMC X-3. Using a large subset of these spectra, we constrain the spin parameter of the black hole to be spin = 0.21(+0.18,-0.22), 90% confidence. Our estimate of the uncertainty in spin takes into account a wide range of systematic errors. We discuss evidence for a correlation between a black hole's spin and the complexity of its X-ray spectrum.Comment: Submitted to ApJL, 5 pages emulateapj, 2 figures and 1 tabl

Modeling the Optical-X-ray Accretion Lag in LMC X-3: Insights Into Black-Hole Accretion Physics

The X-ray persistence and characteristically soft spectrum of the black hole X-ray binary LMC X-3 make this source a touchstone for penetrating studies of accretion physics. We analyze a rich, 10-year collection of optical/infrared (OIR) time-series data in conjunction with all available contemporaneous X-ray data collected by the ASM and PCA detectors aboard the Rossi X-ray Timing Explorer. A cross-correlation analysis reveals an X-ray lag of ~2 weeks. Motivated by this result, we develop a model that reproduces the complex OIR light curves of LMC X-3. The model is comprised of three components of emission: stellar light; accretion luminosity from the outer disk inferred from the time-lagged X-ray emission; and light from the X-ray-heated star and outer disk. Using the model, we filter a strong noise component out of the ellipsoidal light curves and derive an improved orbital period for the system. Concerning accretion physics, we find that the local viscous timescale in the disk increases with the local mass accretion rate; this in turn implies that the viscosity parameter alpha decreases with increasing luminosity. Finally, we find that X-ray heating is a strong function of X-ray luminosity below ~50% of the Eddington limit, while above this limit X-ray heating is heavily suppressed. We ascribe this behavior to the strong dependence of the flaring in the disk upon X-ray luminosity, concluding that for luminosities above ~50% of Eddington, the star lies fully in the shadow of the disk.Comment: Accepted in ApJ (12 pages long in emulateapj format

Measuring the Spins of Stellar Black Holes: A Progress Report

We use the Novikov-Thorne thin disk model to fit the thermal continuum X-ray spectra of black hole X-ray binaries, and thereby extract the dimensionless spin parameter a* = a/M of the black hole as a parameter of the fit. We summarize the results obtained to date for six systems and describe work in progress on additional systems. We also describe recent methodological advances, our current efforts to make our analysis software fully available to others, and our theoretical efforts to validate the Novikov-Thorne model.Comment: 6 pages, conference proceedings, X-ray Astronomy 2009: Present Status, Multi-Wavelength Approach and Future Perspectives, AIP, eds. A. Comastri et al.; list of authors revise