Nonlinear coherent transport of waves in disordered media

We present a diagrammatic theory for coherent backscattering from disordered dilute media in the nonlinear regime. The approach is non-perturbative in the strength of the nonlinearity. We show that the coherent backscattering enhancement factor is strongly affected by the nonlinearity, and corroborate these results by numerical simulations. Our theory can be applied to several physical scenarios like scattering of light in nonlinear Kerr media, or propagation of matter waves in disordered potentials.Comment: 4 pages, 3 figure

Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar

The relativistic plasma flows onto neutron stars that are accreting material from stellar companions can be used to probe strong-field gravity as well as the physical conditions in the supranuclear-density interiors of neutron stars. Plasma inhomogeneities orbiting a few kilometres above the stars are observable as X-ray brightness fluctuations on the millisecond dynamical timescale of the flows. Two frequencies in the kilohertz range dominate these fluctuations: the twin kilohertz quasi-periodic oscillations (kHz QPOs). Competing models for the origins of these oscillations (based on orbital motions) all predict that they should be related to the stellar spin frequency, but tests have been difficult because the spins were not unambiguously known. Here we report the detection of kHz QPOs from a pulsar whose spin frequency is known. Our measurements establish a clear link between kHz QPOs and stellar spin, but one not predicted by any current model. A new approach to understanding kHz QPOs is now required. We suggest that a resonance between the spin and general relativistic orbital and epicyclic frequencies could provide the observed relation between QPOs and spin.Comment: Published in the 2003 July 3 issue of Natur

Kilohertz quasi-periodic oscillations in low-mass X-ray binaries

In early 1996 a series of discoveries begun with NASA's Rossi X-ray Timing Explorer of a new, up to then unknown astrophysical phenomenon. It turned out that accreting low magnetic-field neutron stars show quasi-periodic oscillations in their X-ray flux at rates of up to more than a kilohertz. These kHz QPO, now reported from eleven different systems, are among the fastest phenomena in the sky and can provide us with new information about the fundamental properties of neutron stars and help testing general relativity in the strong-field regime. If, for example, their frequencies can be identified with the Keplerian frequencies of matter in orbit around a 1.4 Solar-mass neutron star, then the radius of the star would have to be less than 15 km, which directly constrains the equation of state of bulk nuclear-density matter, and for an only slightly tighter orbit or slightly more massive neutron star the orbital radius would equal the Schwarzschild-geometry general-relativistic marginally stable orbit (12.5 km for a 1.4 Solar mass object). So far all models that have been put forward for explaining the new phenomenon have encountered problems. In this paper I review the relatively simple and highly suggestive phenomenology as it has emerged from the data up to now, and discuss some of the proposed models.Comment: To be published in the Proceedings of the Wise Observatory 25th Anniversary Symposium "Astronomical Time Series", Tel Aviv, January 199

Accreting Neutron Stars in Low-Mass X-Ray Binary Systems

Using the Rossi X-ray Timing Explorer (RossiXTE), astronomers have discovered that disk-accreting neutron stars with weak magnetic fields produce three distinct types of high-frequency X-ray oscillations. These oscillations are powered by release of the binding energy of matter falling into the strong gravitational field of the star or by the sudden nuclear burning of matter that has accumulated in the outermost layers of the star. The frequencies of the oscillations reflect the orbital frequencies of gas deep in the gravitational field of the star and/or the spin frequency of the star. These oscillations can therefore be used to explore fundamental physics, such as strong-field gravity and the properties of matter under extreme conditions, and important astrophysical questions, such as the formation and evolution of millisecond pulsars. Observations using RossiXTE have shown that some two dozen neutron stars in low-mass X-ray binary systems have the spin rates and magnetic fields required to become millisecond radio-emitting pulsars when accretion ceases, but that few have spin rates above about 600 Hz. The properties of these stars show that the paucity of spin rates greater than 600 Hz is due in part to the magnetic braking component of the accretion torque and to the limited amount of angular momentum that can be accreted in such systems. Further study will show whether braking by gravitational radiation is also a factor. Analysis of the kilohertz oscillations has provided the first evidence for the existence of the innermost stable circular orbit around dense relativistic stars that is predicted by strong-field general relativity. It has also greatly narrowed the possible descriptions of ultradense matter.Comment: 22 pages, 7 figures, updated list of sources and references, to appear in "Short-period Binary Stars: Observation, Analyses, and Results", eds. E.F. Milone, D.A. Leahy, and D. Hobill (Dordrecht: Springer, http://www.springerlink.com

Crisis Visits and Psychiatric Hospitalizations Among Patients Attending a Community Clinic in Rural Southern California

Ethnic minorities from disadvantaged socioeconomic backgrounds report increased utilization of mental health emergency services; however findings have been inconsistent across ethnic/racial groups. In this study we describe patients who present to a rural crisis unit in Southern California, examine rates of psychiatric hospitalizations across ethnic/racial groups, and investigate factors that are associated with increased psychiatric hospitalizations in this sample. This is a retrospective study of 451 racially and ethnically diverse patients attending a crisis unit in Imperial County, California. Chart review and data abstraction methods were used to characterize the sample and identify factors associated with psychiatric crises and subsequent hospitalizations. The sample was predominantly Latino/Hispanic (58.5%). Based on chart review, common psychosocial stressors which prompted a crisis center visit were: (a) financial problems; (b) homelessness; (c) partner or family conflict; (d) physical and health problems; (e) problems at school/work; (f) medication compliance; (g) aggressive behavior; (h) delusional behavior; (i) addiction and (j) anxiety/depression. Bivariate analyses revealed that Hispanics had a disproportionately lower rate of psychiatric hospitalizations while African Americans had a higher rate. Multivariate analyses which included demographic, clinical and psychosocial stressor variables revealed that being African American, having a psychotic disorder, and presenting as gravely disabled were associated with a higher likelihood of hospitalization while partner/family conflict was associated with a lesser likelihood in this rural community. These data elucidate the need for longitudinal studies to understand the interactions between psychosocial stressors, ethnicity and social support as determinants of psychiatric hospitalizations

Millisecond Oscillations in X-Ray Binaries

The first millisecond X-ray variability phenomena from accreting compact objects have recently been discovered with the Rossi X-ray Timing Explorer. Three new phenomena are observed from low-mass X-ray binaries containing low-magnetic-field neutron stars: millisecond pulsations, burst oscillations and kiloHertz quasi-periodic oscillations. Models for these new phenomena involve the neutron star spin, and orbital motion closely around the neutron star and rely explicitly on our understanding of strong gravity and dense matter. I review the observations of these new neutron-star phenomena and possibly related ones in black-hole candidates, and describe the attempts to use them to perform measurements of fundamental physical interest in these systems.Comment: 40 pages, 17 figures, 4 tables - submitted to the Annual Review of Astronomy and Astrophysics; to appear September 200

Generalized Flows around Neutron Stars

In this chapter, we present a brief and non-exhaustive review of the developments of theoretical models for accretion flows around neutron stars. A somewhat chronological summary of crucial observations and modelling of timing and spectral properties are given in sections 2 and 3. In section 4, we argue why and how the Two-Component Advective Flow (TCAF) solution can be applied to the cases of neutron stars when suitable modifications are made for the NSs. We showcase some of our findings from Monte Carlo and Smoothed Particle Hydrodynamic simulations which further strengthens the points raised in section 4. In summary, we remark on the possibility of future works using TCAF for both weakly magnetic and magnetic Neutron Stars.Comment: 15 pages, 7 figures. arXiv admin note: text overlap with arXiv:1901.0084

Accreting Millisecond X-Ray Pulsars

Accreting Millisecond X-Ray Pulsars (AMXPs) are astrophysical laboratories without parallel in the study of extreme physics. In this chapter we review the past fifteen years of discoveries in the field. We summarize the observations of the fifteen known AMXPs, with a particular emphasis on the multi-wavelength observations that have been carried out since the discovery of the first AMXP in 1998. We review accretion torque theory, the pulse formation process, and how AMXP observations have changed our view on the interaction of plasma and magnetic fields in strong gravity. We also explain how the AMXPs have deepened our understanding of the thermonuclear burst process, in particular the phenomenon of burst oscillations. We conclude with a discussion of the open problems that remain to be addressed in the future.Comment: Review to appear in "Timing neutron stars: pulsations, oscillations and explosions", T. Belloni, M. Mendez, C.M. Zhang Eds., ASSL, Springer; [revision with literature updated, several typos removed, 1 new AMXP added

A persistent neutrophil-associated immune signature characterizes post-COVID-19 pulmonary sequelae.

Interstitial lung disease and associated fibrosis occur in a proportion of individuals who have recovered from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection through unknown mechanisms. We studied individuals with severe coronavirus disease 2019 (COVID-19) after recovery from acute illness. Individuals with evidence of interstitial lung changes at 3 to 6 months after recovery had an up-regulated neutrophil-associated immune signature including increased chemokines, proteases, and markers of neutrophil extracellular traps that were detectable in the blood. Similar pathways were enriched in the upper airway with a concomitant increase in antiviral type I interferon signaling. Interaction analysis of the peripheral phosphoproteome identified enriched kinases critical for neutrophil inflammatory pathways. Evaluation of these individuals at 12 months after recovery indicated that a subset of the individuals had not yet achieved full normalization of radiological and functional changes. These data provide insight into mechanisms driving development of pulmonary sequelae during and after COVID-19 and provide a rational basis for development of targeted approaches to prevent long-term complications