Contributions to the mixed-alkali effect in molecular dynamics simulations of alkali silicate glasses

The mixed-alkali effect on the cation dynamics in silicate glasses is analyzed via molecular dynamics simulations. Observations suggest a description of the dynamics in terms of stable sites mostly specific to one ionic species. As main contributions to the mixed--alkali slowdown longer residence times and an increased probability of correlated backjumps are identified. The slowdown is related to the limited accessibility of foreign sites. The mismatch experienced in a foreign site is stronger and more retarding for the larger ions, the smaller ions can be temporarily accommodated. Also correlations between unlike as well as like cations are demonstrated that support cooperative behavior.Comment: 10 pages, 12 figures, 1 table, revtex4, submitted to Phys. Rev.

Cross-spectral modelling of the black hole X-ray binary XTEJ1550-564: challenges to the propagating fluctuations paradigm

Timing properties of black hole X-ray binaries in outburst can be modeled with mass accretion rate fluctuations propagating towards the black hole. Such models predict time lags between energy bands due to propagation delays. First application of a propagating fluctuations model to black hole power spectra showed good agreement with the data. Indeed, hard lags observed from these systems appear to be in agreement with this generic prediction. Our PROPFLUC code allows to simultaneously predict power spectra, time lags, and coherence of the variability as a function of energy. This was successfully applied to Swift data on the black hole MAXIJ1659-152, fitting jointly the power spectra in two energy bands and the cross-spectrum between these two bands. In the current work, we attempt to to model two high signal to noise Rossi X-ray Timing Explorer (RXTE) observations of the black hole XTE J1550-564. We find that neither observation can be adequately explained by the model even when considering, additionally to previous PROPFLUC versions, different propagation speeds of the fluctuations. After extensive exploration of model extensions, we tentatively conclude that the quantitative and qualitative discrepancy between model predictions and data is generic to the propagating fluctuations paradigm. This result encourages further investigation of the fundamental hypotheses of the propagating fluctuations model. We discuss some of these hypotheses with an eye to future works.Comment: 14 pages, 12 figures, accepted for publication in MNRA

Evolution of the hot flow of MAXI J1543-564

We present a spectral and timing analysis of the black hole candidate MAXI J1543-564 during its 2011 outburst. As shown in previous work, the source follows the standard evolution of a black hole outburst. During the rising phase of the outburst we detect an abrupt change in timing behavior associated with the occurrence of a type-B quasi-periodic oscillation (QPO). This QPO and the simultaneously detected radio emission mark the transition between hard and soft intermediate state. We fit power spectra from the rising phase of the outburst using the recently proposed model propfluc. This assumes a truncated disc / hot inner flow geometry, with mass accretion rate fluctuations propagating through a precessing inner flow. We link the propfluc physical parameters to the phenomenological multi-Lorentzian fit parameters. The physical parameter dominating the QPO frequency is the truncation radius, while broad band noise characteristics are also influenced by the radial surface density and emissivity profiles of the flow. In the outburst rise we found that the truncation radius decreases from $r_o \sim 24$ to $10 R_g$, and the surface density increases faster than the mass accretion rate, as previously reported for XTE J1550-564. Two soft intermediate state observations could not be fitted with propfluc, and we suggest that they are coincident with the ejection of material from the inner regions of the flow in a jet or accretion of these regions into the BH horizon, explaining the drop in QPO frequency and suppression of broad band variability preferentially at high energy bands coincident with a radio flare.Comment: 13 pages, 11 figures, 2 table

Hidden in Plain Sight: Homeless Students In America's Public Schools

Student homelessness is on the rise, with more than 1.3 million homeless students identified during the 2013-14 school year. This is a 7 percent increase from the previous year and more than double the number of homeless students in 2006-07. As high as these numbers seem, they are almost certainly undercounts.Despite increasing numbers, these students - as well as the school liaisons and state coordinators who support them - report that student homelessness remains an invisible and extremely disruptive problem.Students experiencing homelessness struggle to stay in school, to perform well, and to form meaningful connections with peers and adults. Ultimately, they are much more likely to fall off track and eventually drop out of school more often than their non-homeless peers.This study:provides an overview of existing research on homeless students,sheds light on the challenges homeless students face and the supports they say they need to succeed,reports on the challenges adults - local liaisons and state coordinators - face in trying to help homeless students, andrecommends changes in policy and practice at the school, community, state and national level to help homeless students get on a path to adult success.This is a critical and timely topic. The recent reauthorization of the Every Student Succeeds Act (ESSA) provides many new and stronger provisions for homeless students (effective Oct. 1, 2016); requires states, district and schools for the first time to report graduation rates for homeless students (effective beginning with the 2016-17 school year); and affirms the urgency and importance of dealing with homelessness so that all children can succeed

Definition of smolder experiments for Spacelab

The feasibility of conducting experiments in space on smoldering combustion was studied to conceptually design specific smoldering experiments to be conducted in the Shuttle/Spacelab System. Design information for identified experiment critical components is provided. The analytical and experimental basis for conducting research on smoldering phenomena in space was established. Physical descriptions of the various competing processes pertaining to smoldering combustion were identified. The need for space research was defined based on limitations of existing knowledge and limitations of ground-based reduced-gravity experimental facilities

Voyager Mars planetary quarantine Basic math model report

Basic math model study of planetary quarantine effects on Voyager Mars missio

Low-frequency QPO from the 11 Hz accreting pulsar in Terzan 5: not frame dragging

We report on 6 RXTE observations taken during the 2010 outburst of the 11 Hz accreting pulsar IGR J17480-2446 located in the globular cluster Terzan 5. During these observations we find power spectra which resemble those seen in Z-type high-luminosity neutron star low-mass X-ray binaries, with a quasi-periodic oscillation (QPO) in the 35-50 Hz range simultaneous with a kHz QPO and broad band noise. Using well known frequency-frequency correlations, we identify the 35-50 Hz QPOs as the horizontal branch oscillations (HBO), which were previously suggested to be due to Lense-Thirring precession. As IGR J17480-2446 spins more than an order of magnitude more slowly than any of the other neutron stars where these QPOs were found, this QPO can not be explained by frame dragging. By extension, this casts doubt on the Lense-Thirring precession model for other low-frequency QPOs in neutron-star and perhaps even black-hole systems.Comment: 6 pages, 5 figures, Accepted for publication in ApJ

Keeping One\u27s Friends Close: The Maintenance of Cooperation in Supposedly Fragmenting Alliances

From the perspective of an outside observer, interactions between sovereign states within the international arena seem to mirror the often paradoxical interpersonal complexities of their populace and those who govern them. Why did the Russian Empire and Poland-Lithuania choose to maintain a cooperative relationship in the early 1700s despite a shared desire to take control of the Baltic for themselves (Masse 1980)? How can one decipher the simultaneous desire of Turkey to stay in good graces with its NATO partners while also foraging ahead in its siding with the authoritarians of the world (Bekdil 2018)? All of these questions play into understanding at a theoretical level why nations in alliances still choose to remain in their frameworks despite their own individual goals, to which some scholars attribute to the base desires of nations to either spread their influence or seek security. Others invoke the power of the frameworks themselves to prevent collapse, and there are also those who highlight states’ subjective attitudes’ roles in this maintenance. This study of alliances that analyzes cases ranging from contemporary to historical alliance structures presents a further addition to this debacle, utilizing a constructed model and close scrutiny to effectively determine that the stability of cooperation between alliances rests in the extent of their structural alignment, holding vast implications for contemporary arrangements and crises that have captured the public’s attention in the process