The polarized image of a synchrotron-emitting ring of gas orbiting a black hole

Synchrotron radiation from hot gas near a black hole results in a polarized image. The image polarization is determined by effects including the orientation of the magnetic field in the emitting region, relativistic motion of the gas, strong gravitational lensing by the black hole, and parallel transport in the curved spacetime. We explore these effects using a simple model of an axisymmetric, equatorial accretion disk around a Schwarzschild black hole. By using an approximate expression for the null geodesics derived by Beloborodov and conservation of the Walker–Penrose constant, we provide analytic estimates for the image polarization. We test this model using currently favored general relativistic magnetohydrodynamic simulations of M87*, using ring parameters given by the simulations. For a subset of these with modest Faraday effects, we show that the ring model broadly reproduces the polarimetric image morphology. Our model also predicts the polarization evolution for compact flaring regions, such as those observed from Sgr A* with GRAVITY. With suitably chosen parameters, our simple model can reproduce the EVPA pattern and relative polarized intensity in Event Horizon Telescope images of M87*. Under the physically motivated assumption that the magnetic field trails the fluid velocity, this comparison is consistent with the clockwise rotation inferred from total intensity images.http://iopscience.iop.org/0004-637Xam2023Physic

“After viral load testing, I get my results so I get to know which path my life is taking me”: qualitative insights on routine centralized and point-of-care viral load testing in western Kenya from the Opt4Kids and Opt4Mamas studies

Abstract Background Viral suppression (VS) is a marker of effective HIV therapy, and viral load (VL) testing is critical for treatment monitoring, especially in high-risk groups such as children and pregnant/postpartum women. Although routine VL testing, via centralized laboratory networks, was implemented in Kenya starting in 2014, optimization and sustainable scale up of VL testing are still needed. Methods We conducted a mixed methods study to evaluate the impact of higher frequency, point-of-care (POC) VL testing in optimizing VS among children and pregnant/postpartum women on antiretroviral treatment (ART) in five HIV treatment facilities in western Kenya in the Opt4Kids and Opt4Mamas studies. We conducted 68 key informant interviews (KIIs) from December 2019 to December 2020 with children and pregnant women living with HIV, child caregivers, providers, laboratory/facility leadership, and county- or national-level policymakers. Our KII guide covered the following domains: (1) barriers and facilitators to ART use and VS, (2) literacy and experiences with VL in routine care and via study, and (3) opinions on how to scale up VL testing for optimal programmatic use. We used inductive coding and thematic analysis to identify dominant themes with convergent and divergent subthemes. Results Three main themes regarding VL testing emerged from our analysis. (1) Key informants uniformly contrasted POC VL testing’s faster results turnaround, higher accessibility, and likely cost-effectiveness against centralized VL testing. (2) Key informants also identified areas of improvement for POC VL testing in Kenya, such as quality control, human resource and infrastructure capacity, supply chain management, and integration of VL testing systems. (3) To enable successful scale-up of VL testing, key informants proposed expanding the POC VL testing scheme, electronic medical records systems, conducting quality checks locally, capacity building and developing strong partnerships between key stakeholders. Conclusion The more accessible, decentralized model of POC VL testing was deemed capable of overcoming critical challenges associated with centralized VL testing and was considered highly desirable for optimizing VS for children and pregnant/postpartum women living with HIV. While POC VL testing has the potential to improve VS rates among these populations, additional research is needed to develop strategies for ensuring the sustainability of POC VL testing programs. Trial registration NCT03820323, 29/01/201

A universal power-law prescription for variability from synthetic images of black hole accretion flows

We present a framework for characterizing the spatiotemporal power spectrum of the variability expected from the horizon-scale emission structure around supermassive black holes, and we apply this framework to a library of general relativistic magnetohydrodynamic (GRMHD) simulations and associated general relativistic ray-traced images relevant for Event Horizon Telescope (EHT) observations of Sgr A*. We find that the variability power spectrum is generically a red-noise process in both the temporal and spatial dimensions, with the peak in power occurring on the longest timescales and largest spatial scales. When both the time-averaged source structure and the spatially integrated light-curve variability are removed, the residual power spectrum exhibits a universal broken power-law behavior. On small spatial frequencies, the residual power spectrum rises as the square of the spatial frequency and is proportional to the variance in the centroid of emission. Beyond some peak in variability power, the residual power spectrum falls as that of the time-averaged source structure, which is similar across simulations; this behavior can be naturally explained if the variability arises from a multiplicative random field that has a steeper high-frequency power-law index than that of the time-averaged source structure. We briefly explore the ability of power spectral variability studies to constrain physical parameters relevant for the GRMHD simulations, which can be scaled to provide predictions for black holes in a range of systems in the optically thin regime. We present specific expectations for the behavior of the M87* and Sgr A* accretion flows as observed by the EHT.South African Research Chairs Initiative, National Research Foundation and Department of Science and Innovation (DSI) of South Africa.http://iopscience.iop.org/0004-637XPhysic