Chandra Observation of PSR B1823-13 and its Pulsar Wind Nebula

We report on an observation of the Vela-like pulsar B1823-13 and its synchrotron nebula with Chandra.The pulsar's spectrum fits a power-law model with a photon index Gamma_PSR=2.4 for the plausible hydrogen column density n_H=10^{22} cm^{-2}, corresponding to the luminosity L_PSR=8*10^{31} ergs s^{-1} in the 0.5-8 keV band, at a distance of 4 kpc. The pulsar radiation likely includes magnetospheric and thermal components, but they cannot be reliably separated because of the small number of counts detected and strong interstellar absorption. The pulsar is surrounded by a compact, 25''x 10'', pulsar wind nebula (PWN) elongated in the east-west direction, which includes a brighter inner component, 7''x 3'', elongated in the northeast-southwest direction. The slope of the compact PWN spectrum is Gamma_comp=1.3, and the 0.5-8 keV luminosity is L_comp~3*10^{32} ergs s^{-1}. The compact PWN is surrounded by asymmetric diffuse emission (extended PWN) seen up to at least 2.4' south of the pulsar, with a softer spectrum (Gamma_ext=1.9), and the 0.5-8 keV luminosity L_ext~10^{33}-10^{34} ergs s^{-1}. We also measured the pulsar's proper motion using archival VLA data: \mu_\alpha=23.0+/-2.5 mas yr^{-1}, \mu_\delta=-3.9+/-3.3 mas yr^{-1}, which corresponds to the transverse velocity v_perp=440 km s^{-1}. The direction of the proper motion is approximately parallel to the elongation of the compact PWN, but it is nearly perpendicular to that of the extended PWN and to the direction towards the center of the bright VHE gamma-ray source HESS J1825-137, which is likely powered by PSR B1823-13.Comment: 13 pages, 8 figures and 3 tables; submitted to Ap

Damping behaviour of thin-walled composite columns under impact in-plane loading

In this paper the analysis of the damping behaviour of thin-walled composite columns with open stiffened cross-sections subjected to in-plane pulse loading is described. The pulse loading of a rectangular shape is concerned. The discussed problem of the dynamic interactive buckling is solved by the analytical-numerical method (ANM) using the Koiter’s perturbations method. A critical value of the dynamic load factors is determined according to the Budiansky-Hutchinson’s criterion for different value of the viscous damping ratio. The detailed calculations confirm that small damping does not affect the dynamic response of the thin-walled composite columns under the impact in-plane loading

X-ray emission from PSR J1809-1917 and its pulsar wind nebula, possibly associated with the TeV gamma-ray source HESS J1809-193

We detected X-ray emission from the 50-kyr-old pulsar J1809-1917 and resolved its pulsar wind nebula (PWN) with Chandra. The pulsar spectrum fits PL+BB model with the photon index of 1.2 and the BB temperature of 2 MK for n_{H}=0.7\times 10^{22} cm^{-2}. The luminosities are(4\pm 1)\times 10^{31} ergs s^{-1} for the PL component (in the 0.5-8 keV band) and ~1\times 10^{32} ergs s^{-1} for the BB component (bolometric) at a plausible distance of 3.5 kpc. The bright inner PWN component of a 3''\times12'' size is elongated in the north-south direction, with the pulsar close to its south end. This component is immersed in a larger (20''\times40''), similarly elongated outer PWN component of lower surface brightness. The elongated shape of the compact PWN can be explained by the ram pressure confinement of the pulsar wind due to the supersonic motion of the pulsar. The PWN spectrum fits a PL model with photon index of 1.4\pm0.1 and 0.5-8 keV luminosity of 4\times10^{32} ergs s^{-1}. The compact PWN appears to be inside a large-scale (~4'\times4') emission more extended to the south of the pulsar, i.e. in the direction of the alleged pulsar motion. To explain the extended X-ray emission ahead of the moving pulsar, one has to invoke strong intrinsic anisotropy of the pulsar wind or assume that this emission comes from a relic PWN swept by the asymmetrical reverse SNR shock. The pulsar and its PWN are located within the extent of the unidentified TeV source HESS J1809-193 whose brightest part is offset by ~8' to the south of the pulsar, i.e. in the same direction as the large-scale X-ray emission. Although the association between J1809-1917 and HESS J1809-193 is plausible, an alternative source of relativistic electrons powering HESS J1809-193 might be the serendipitously discovered X-ray source CXOU J180940.7-192544.Comment: 13 pages, 10 figures and 3 tables, submitted to ApJ. Version with the high-resolution figures is available at http://www.astro.psu.edu/users/green/J1809/ms_astroph.pd

X-ray emission from PSR B1800-21, its wind nebula, and similar systems

We detected X-ray emission from PSR B1800-21 and its synchrotron nebula with the Chandra X-ray Observatory. The pulsar's observed flux is (1.4+/-0.2) 10^{-14} ergs cm^{-2} s^{-1} in the 1-6 keV band. The spectrum can be described by a two-component PL+BB model, suggesting a mixture of thermal and magnetospheric emission. For a plausible hydrogen column density n_{H}=1.4 10^{22} cm^{-2}, the PL component has a slope Gamma=1.4+/-0.6 and a luminosity L_{psr}^{nonth}=4 10^{31}(d/4 kpc)^2 ergs s^{-1}. The properties of the thermal component (kT=0.1-0.3 keV, L^{bol}=10^{31}-10^{33} ergs s^{-1}) are very poorly constrained because of the strong interstellar absorption. The compact, 7''\times4'', inner pulsar-wind nebula (PWN), elongated perpendicular to the pulsar's proper motion, is immersed in a fainter asymmetric emission. The observed flux of the PWN is (5.5+/-0.6) 10^{-14} ergs cm^{-2} s^{-1} in the 1-8 keV band. The PWN spectrum fits by a PL model with Gamma=1.6+/-0.3, L=1.6 10^{32} (d/4 kpc})^2 ergs s^{-1}. The shape of the inner PWN suggests that the pulsar moves subsonically and X-ray emission emerges from a torus associated with the termination shock in the equatorial pulsar wind. The inferred PWN-pulsar properties (e.g., the PWN X-ray efficiency, L_{pwn}/\dot{E}~10^{-4}; the luminosity ratio, L_{pwn}/L_{psr}^{nonth}=4; the pulsar wind pressure at the termination shock, p_s=10^{-9} ergs cm^{-3}) are very similar to those of other subsonically moving Vela-like objects detected with Chandra (L_{pwn}/\dot{E}=10^{-4.5}-10^{-3.5}, L_{pwn}/L_{psr}^{nonth}~5, p_s=10^{-10}-10^{-8} ergs cm^{-1}).Comment: 11 pages, 10 figures, 2 tables; submitted to ApJ. Version with the high-resolution figures is available at http://www.astro.psu.edu/users/green/B1800/B1800_ApJ.pd

Compressibility and structural stability of ultra-incompressible bimetallic interstitial carbides and nitrides

We have investigated by means of high-pressure x-ray diffraction the structural stability of Pd2Mo3N, Ni2Mo3C0.52N0.48, Co3Mo3C0.62N0.38, and Fe3Mo3C. We have found that they remain stable in their ambient-pressure cubic phase at least up to 48 GPa. All of them have a bulk modulus larger than 330 GPa, being the least compressible material Fe3Mo3C, B0 = 374(3) GPa. In addition, apparently a reduction of compressibility is detected as the carbon content increased. The equation of state for each material is determined. A comparison with other refractory materials indicates that interstitial nitrides and carbides behave as ultra-incompressible materials.Comment: 14 pages, 3 figures, 1 tabl

Structure and Stability of Si(114)-(2x1)

We describe a recently discovered stable planar surface of silicon, Si(114). This high-index surface, oriented 19.5 degrees away from (001) toward (111), undergoes a 2x1 reconstruction. We propose a complete model for the reconstructed surface based on scanning tunneling microscopy images and first-principles total-energy calculations. The structure and stability of Si(114)-(2x1) arises from a balance between surface dangling bond reduction and surface stress relief, and provides a key to understanding the morphology of a family of surfaces oriented between (001) and (114).Comment: REVTeX, 4 pages + 3 figures. A preprint with high-resolution figures is available at http://cst-www.nrl.navy.mil/papers/si114.ps . To be published in Phys. Rev. Let

Confronting Neutron Star Cooling Theories with New Observations

With the successful launch of Chandra and XMM/Newton X-ray space missions combined with the lower-energy band observations, we are in the position where careful comparison of neutron star cooling theories with observations will make it possible to distinguish among various competing theories. For instance, the latest theoretical and observational developments already exclude both nucleon and kaon direct URCA cooling. In this way we can now have realistic hope for determining various important properties, such as the composition, degree of superfluidity, the equation of state and steller radius. These developments should help us obtain better insight into the properties of dense matter.Comment: 11 pages, 1 figur

DECREASE Final Technical Report: Development of a Commercial Ready Enzyme Application System for Ethanol

Conversion of biomass to sugars plays a central in reducing our dependence on petroleum, as it allows production of a wide range of biobased fuels and chemicals, through fermentation of those sugars. The DECREASE project delivers an effective enzyme cocktail for this conversion, enabling reduced costs for producing advanced biofuels such as cellulosic ethanol. Benefits to the public contributed by growth of the advanced biofuels industry include job creation, economic growth, and energy security. The DECREASE primary project objective was to develop a two-fold improved enzyme cocktail, relative to an advanced cocktail (CZP00005) that had been developed previously (from 2000- 2007). While the final milestone was delivery of all enzyme components as an experimental mixture, a secondary objective was to deploy an improved cocktail within 3 years following the close of the project. In February 2012, Novozymes launched Cellic CTec3, a multi-enzyme cocktail derived in part from components developed under DECREASE. The externally validated performance of CTec3 and an additional component under project benchmarking conditions indicated a 1.8-fold dose reduction in enzyme dose required for 90% conversion (based on all available glucose and xylose sources) of NREL dilute acid pretreated PCS, relative to the starting advanced enzyme cocktail. While the ability to achieve 90% conversion is impressive, targeting such high levels of biomass digestion is likely not the most cost effective strategy. Novozymes techno economic modeling showed that for NREL's dilute acid pretreated corn stover (PCS), 80% target conversion enables a lower total production cost for cellulosic ethanol than for 90% conversion, and this was also found to be the case when cost assumptions were based on the NREL 2002 Design Report. A 1.8X dose-reduction was observed for 80% conversion in the small scale (50 g) DECREASE benchmark assay for CTec3 and an additional component. An upscaled experiment (in 0.5 kg kettle reactors) was performed to compare the starting enzyme mixture CZP00005 with CTec3 alone; these results indicated a 1.9X dose- reduction for 80% conversion. The CTec3 composition does not include the best available enzyme components from the DECREASE effort. While these components are not yet available in a commercial product, experimental mixtures were assayed in a smaller scale assay using DECREASE PCS, at high solids loadings (21.5% TS). The results indicated that the newer mixtures required 2.9X-less enzyme for 90% conversion, and 3.2X-less enzyme for 80% conversion, relative to the starting enzyme cocktail. In conclusion, CTec3 delivers a 1.8-1.9X dose reduction on NREL PCS at high solids loadings, and the next generation enzyme from Novozymes will continue to show dramatically improved biochemical performance. CTec3 allows reduced costs today, and the experimental cocktails point to continued biotechnological improvements that will further drive down costs for biorefineries of tomorrow