A MODEST review

We present an account of the state of the art in the fields explored by the research community invested in 'Modeling and Observing DEnse STellar systems'. For this purpose, we take as a basis the activities of the MODEST-17 conference, which was held at Charles University, Prague, in September 2017. Reviewed topics include recent advances in fundamental stellar dynamics, numerical methods for the solution of the gravitational N-body problem, formation and evolution of young and old star clusters and galactic nuclei, their elusive stellar populations, planetary systems, and exotic compact objects, with timely attention to black holes of different classes of mass and their role as sources of gravitational waves. Such a breadth of topics reflects the growing role played by collisional stellar dynamics in numerous areas of modern astrophysics. Indeed, in the next decade, many revolutionary instruments will enable the derivation of positions and velocities of individual stars in the Milky Way and its satellites and will detect signals from a range of astrophysical sources in different portions of the electromagnetic and gravitational spectrum, with an unprecedented sensitivity. On the one hand, this wealth of data will allow us to address a number of long-standing open questions in star cluster studies; on the other hand, many unexpected properties of these systems will come to light, stimulating further progress of our understanding of their formation and evolution.Comment: 42 pages; accepted for publication in 'Computational Astrophysics and Cosmology'. We are much grateful to the organisers of the MODEST-17 conference (Charles University, Prague, September 2017). We acknowledge the input provided by all MODEST-17 participants, and, more generally, by the members of the MODEST communit

Serum hepatitis B virus RNA detectability, composition and clinical significance in patients with ab initio hepatitis B e antigen negative chronic hepatitis B

Background: Serum hepatitis B virus (HBV) RNA is a surrogate biomarker for intrahepatic covalently closed circular DNA (cccDNA) transcriptional activity and persistence. In this retrospective study, we investigated its presence, levels and composition in ab initio Hepatitis B e antigen (HBeAg) negative chronically infected patients and examined possible associations with disease activity and the outcome of nucleos(t)ide analogue (NA) discontinuation. Methods: We developed a sensitive real time polymerase chain reaction (RT-PCR) for the specific detection of HBV pregenomic RNA (pgRNA) and precore (preC) mRNA and analyzed 220 serum specimens, 160 under NA treatment, from 116 Greek patients initially negative for HBeAg. Results: HBV pgRNA was detected in 31% and preC mRNA in 15% of samples, at lower levels representing a small fraction (3.4%) of total core promoter produced transcripts. In the absence of NAs, pgRNA was detected in 57% of samples with median value of 5.19 (2.61–8.35) log10 cp/mL, at lower levels than HBV DNA and correlated significantly with ALT (r = 0.764) and serum HBV DNA (r = 0.906). A wide range of HBV DNA/pgRNA ratio was observed with significant inter- and intra-patient variation. During NA treatment, pgRNA displayed low detectability (22%) and variable levels, median 3.97 (2.30– 8.13) log10 cp/mL, as well as, a significant inverse correlation with the duration of treatment (r = − 0.346, p < 0.01). In 74 events of NA discontinuation, end-of-treatment pgRNA-positive compared to pgRNA-negative cases, experienced more frequently virological (p = 0.016) and clinical (p = 0.011) relapse. Conclusions: In genotype D ab initio HBeAg negative patients, serum HBV RNA is primarily composed of pgRNA plus a minor fraction of preC mRNA transcripts. Serum pgRNA is associated with disease activity, suggesting lysis of infected hepatocytes as a possible source of serum HBV RNA in untreated patients and in the early phase of NA treatment. During long term NA treatment, detectable serum pgRNA predicts viral rebound and clinical relapse following treatment discontinuation and may thus serve as a marker for the decision of cessation of therapy. © 2022, The Author(s)

Stress-induced birefringence in the lenses of Wide-Area Linear Optical Polarimeter-South

Two unique wide-field and high-accuracy polarimeters named WALOP (Wide-Area Linear Optical Polarimeter)- North and WALOP-South are currently under development at the Inter-University Center for Astronomy and Astrophysics (IUCAA), India, to create a large area optical polarization map of the sky for the upcoming PASIPHAE sky survey. These instruments are designed to achieve a linear polarimetric measurement accuracy of 0.1% across a field of view (FoV) of 30×30 arcminutes. The WALOP-South instrument will be installed first on a 1 m telescope at the Sutherland Observatory, where the temperatures during the night can vary between 10 to -5°C. These temperature variations and the instrument's pointing to various non-zenithal positions in the sky can introduce stress birefringence in the lenses, leading to time-varying instrumental polarization. This work estimates stress-induced birefringence due to thermal, and gravity stresses on WALOP-South lenses. Using the optomechanical model of the WALOP-South, we carried out Finite Element Analysis (FEA) simulations in SolidWorks software to estimate the stresses for various scenarios of temperature, telescope pointing airmass, and lens mount material (aluminum and titanium). Further, we use the stress tensor analysis to estimate the principal stresses and their directions and consequent birefringence and retardance introduced in the lenses. The stressinduced birefringence will change the optical path length for orthogonal polarization states of the beam passing through the lenses and introduce phase retardation. Overall, with the lens mount design of the instrument, we find that the retardation and consequent instrumental polarization will be within the instrumental accuracy requirements. Additionally, the stress birefringence is found to be higher for aluminum compared to titanium mounts. We further incorporated this retardance in the instrument Mueller matrix estimation to understand its effects on the polarization measurements. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.Immediate accessThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

WALOP-South: a four-camera one-shot imaging polarimeter for PASIPHAE survey. Paper II – polarimetric modeling and calibration

The Wide-Area Linear Optical Polarimeter (WALOP)-South instrument is an upcoming wide-field and high accuracy optical polarimeter to be used as a survey instrument for carrying out the Polar-Areas Stellar Imaging in Polarization High-Accuracy Experiment program. Designed to operate as a one-shot four-channel and four-camera imaging polarimeter, it will have a field of view of 35 × 35 arcminutes and will measure the Stokes parameters I, q, and u in a single exposure in the Sloan Digital Sky Survey-r broadband filter. The design goal for the instrument is to achieve an overall polarimetric measurement accuracy of 0.1% over the entire field of view. We present here the complete polarimetric modeling of the instrument, characterizing the amount and sources of instrumental polarization. To accurately retrieve the real Stokes parameters of a source from the measured values, we have developed a calibration method for the instrument. Using this calibration method and simulated data, we demonstrate how to correct for instrumental polarization and obtain 0.1% accuracy in degree of polarization, p. In addition, we tested and validated the calibration method by implementing it on a table-top WALOP like test-bed polarimeter in the laboratory. © 2022 SPIE.Immediate accessThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

RoboPol: AGN polarimetric monitoring data

We present uniformly reprocessed and re-calibrated data from the RoboPol programme of optopolarimetric monitoring of active galactic nuclei (AGNs), covering observations between 2013, when the instrument was commissioned, and 2017. In total, the data set presented in this paper includes 5068 observations of 222 AGN with Dec. > -25a -. We describe the current version of the RoboPol pipeline that was used to process and calibrate the entire data set, and we make the data publicly available for use by the astronomical community. Average quantities summarizing optopolarimetric behaviour (average degree of polarization, polarization variability index) are also provided for each source we have observed and for the time interval we have followed it. © 2020 The Author(s)