Quiescent times in gamma-ray bursts: I. An observed correlation between the durations of subsequent emission episodes

Although more than 2000 astronomical gamma-ray bursts (GRBs) have been detected, the precise progenitor responsible for these events is unknown. The temporal phenomenology observed in GRBs can significantly constrain the different models. Here we analyse the time histories of a sample of bright, long GRBs, searching for the ones exhibiting relatively long (more than 5 per cent of the total burst duration) quiescent times, defined as the intervals between adjacent episodes of emission during which the gamma-rays count rate drops to the background level. We find a quantitative relation between the duration of an emission episode and the quiescent time elapsed since the previous episode. We suggest here that the mechanism responsible for the extraction and the dissipation of energy has to take place in a meta-stable configuration, such that the longer the accumulation period, the higher is the stored energy available for the next emission episode.Comment: 5 pages, 3 figures, with final revision

Bondi-Hoyle-Lyttleton Accretion onto Star Clusters

An isolated star moving supersonically through a uniform gas accretes material from its gravitationally-induced wake. The rate of accretion is set by the accretion radius of the star and is well-described by classical Bondi-Hoyle-Lyttleton theory. Stars, however, are not born in isolation. They form in clusters where they accrete material that is influenced by all the stars in the cluster. We perform three-dimensional hydrodynamic simulations of clusters of individual accretors embedded in a uniform-density wind in order to study how the accretion rates experienced by individual cluster members are altered by the properties of the ambient gas and the cluster itself. We study accretion as a function of number of cluster members, mean separation between them, and size of their individual accretion radii. We determine the effect of these key parameters on the aggregate and individual accretion rates, which we compare to analytic predictions. We show that when the accretion radii of the individual objects in the cluster substantially overlap, the surrounding gas is effectively accreted into the collective potential of the cluster prior to being accreted onto the individual stars. We find that individual cluster members can accrete drastically more than they would in isolation, in particular when the flow is able to cool efficiently. This effect could potentially modify the luminosity of accreting compact objects in star clusters and could lead to the rejuvenation of young star clusters as well as globular clusters with low-inclination and low-eccentricity.Comment: 15 pages, 12 figures. Accepted to Ap

Accretion Disk Assembly During Common Envelope Evolution: Implications for Feedback and LIGO Binary Black Hole Formation

During a common envelope episode in a binary system, the engulfed companion spirals to tighter orbital separations under the influence of drag from the surrounding envelope material. As this object sweeps through material with a steep radial gradient of density, net angular momentum is introduced into the flow, potentially leading to the formation of an accretion disk. The presence of a disk would have dramatic consequences for the outcome of the interaction because accretion might be accompanied by strong, polar outflows with enough energy to unbind the entire envelope. Without a detailed understanding of the necessary conditions for disk formation during common envelope, therefore, it is difficult to accurately predict the population of merging compact binaries. This paper examines the conditions for disk formation around objects embedded within common envelopes using the `wind tunnel' formalism developed by MacLeod et al. (2017). We find that the formation of disks is highly dependent on the compressibility of the envelope material. Disks form only in the most compressible of stellar envelope gas, found in envelopes' outer layers in zones of partial ionization. These zones are largest in low-mass stellar envelopes, but comprise small portions of the envelope mass and radius in all cases. We conclude that disk formation and associated accretion feedback in common envelope is rare, and if it occurs, transitory. The implication for LIGO black hole binary assembly is that by avoiding strong accretion feedback, common envelope interactions should still result in the substantial orbital tightening needed to produce merging binaries.Comment: 12 pages, 10 figures, submitted to Ap

Challenges Veterans Encounter Receiving or Seeking Mental Health Services

The goal of this research was to identify the challenges U.S. Veterans faced when accessing mental health services within the Veterans Affairs (V.A.) Healthcare System. The V.A. serves approximately six million U.S. military veterans annually and is considered a leader in providing high-quality care in the USA. Despite the V.A.\u27s efforts to improve its patient care system, many veterans do not seek treatment or follow through with mental health services. The study found that participants had difficulty getting appointments with V.A. mental health services. When they did, the appointments were usually convenient and took place as scheduled. While some participants were satisfied with V.A. services, others preferred outside providers because they were easier to access. Participants who did not seek mental health services felt they did not need them but would be more likely to seek them if the process were simplified. The main challenges preventing participants from seeking services were appointments being scheduled too far in advance, societal stigma, and potential obstacles from leadership. Many participants believed that seeking help would make them appear weak and that their peers would have less confidence. Talking about mental health was difficult for many, and they feared it could negatively impact their careers

Use of m-Health Technology for Preventive Interventions to Tackle Cardiometabolic Conditions and Other Non-Communicable Diseases in Latin America- Challenges and Opportunities

In Latin America, cardiovascular disease (CVD) mortality rates will increase by an estimated 145% from 1990 to 2020. Several challenges related to social strains, inadequate public health infrastructure, and underfinanced healthcare systems make cardiometabolic conditions and non-communicable diseases (NCDs) difficult to prevent and control. On the other hand, the region has high mobile phone coverage, making mobile health (mHealth) particularly attractive to complement and improve strategies toward prevention and control of these conditions in low- and middle-income countries. In this article, we describe the experiences of three Centers of Excellence for prevention and control of NCDs sponsored by the National Heart, Lung, and Blood Institute with mHealth interventions to address cardiometabolic conditions and other NCDs in Argentina, Guatemala, and Peru. The nine studies described involved the design and implementation of complex interventions targeting providers, patients and the public. The rationale, design of the interventions, and evaluation of processes and outcomes of each of these studies are described, together with barriers and enabling factors associated with their implementation.Fil: Beratarrechea, Andrea Gabriela. Instituto de Efectividad Clínica y Sanitaria; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Diez Canseco, Francisco. Universidad Peruana Cayetano Heredia; PerúFil: Irazola, Vilma. Instituto de Efectividad Clínica y Sanitaria; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Miranda, Jaime. Universidad Peruana Cayetano Heredia; PerúFil: Ramirez Zea, Manuel. Institute of Nutrition of Central America and Panama; GuatemalaFil: Rubinstein, Adolfo Luis. Instituto de Efectividad Clínica y Sanitaria; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Quiescent times in gamma-ray bursts: II. Dormant periods in the central engine?

Within the framework of the internal-external shocks model for gamma-ray bursts, we study the various mechanisms that can give rise to quiescent times in the observed gamma-ray light-curves. In particular, we look for the signatures that can provide us with evidence as to whether or not the central engine goes dormant for a period of time comparable to the duration of the gaps. We show that the properties of the prompt gamma-ray and X-ray emission can in principle determine whether the quiescent episodes are due to a modulated relativistic wind or a switching off of the central engine. We suggest that detailed observations of the prompt afterglow emission from the reverse shock will strongly constrain the possible mechanisms for the production of quiescent times in gamma-ray bursts.Comment: 12 pages, 8 figures, with final revisions, MNRAS in pres

Helicopter Rotor Blade Monitoring using Autonomous Wireless Sensor Network

The advancement on Wireless Sensor Networks for vibration monitoring presents important possibilities for helicopter rotor health and usage monitoring. While main rotor blades account for the main source of lift for helicopters, rotor induced vibration establishes an important source for understanding the rotor performance and blade condition. A discussion on the dual character of blades as rotating structures results in two different interrogation strategies for external and internal dynamic loading on the blade. The first strategy aims for in-flight rotor performance monitoring, while the second pursues health assessment. An overview of different measurements performed on an actual helicopter blade is presented. The measurements include a complete modal analysis using a full wired instrumented blade and a comparison between wireless sensor nodes and wired instrumentation. Additionally, a numerical multibody dynamics model for damage simulation is presented. The experimental and numerical work contribute to the identification of several implications on the migration of condition and health monitoring techniques to a wireless setting