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"Chatterjee, S."
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Handbook of regression analysis
\"Written by two established experts in the field, the purpose of this handbook is to provide a practical, one-stop reference on regression analysis. The focus is on the tools that both practitioners and researchers use in real life. It is intended to be a comprehensive collection of the theory, methods, and applications of the subject matter, but it is deliberately written at an accessible level. The handbook will provide a quick and convenient reference or \"refresher\" on ideas and methods that are useful for the accurate analysis of data and its resulting interpretations. Students can use the book as an introduction to and/or summary of key concepts in regression and related course work (such as linear, nonlinear, and nonparametric regressions). Plentiful references are supplied for the more motivated readers. Theory is presented when necessary, and always supplemented by hands-on examples. Software routines are available via an author-maintained web site\"-- Provided by publisher.
Book
CHIME/FRB Discovery of 25 Repeating Fast Radio Burst Sources
We present the discovery of 25 new repeating fast radio burst (FRB) sources found among CHIME/FRB events detected between 2019 September 30 and 2021 May 1. The sources were found using a new clustering algorithm that looks for multiple events colocated on the sky having similar dispersion measures (DMs). The new repeaters have DMs ranging from ∼220 to ∼1700 pc cm−3, and include sources having exhibited as few as two bursts to as many as twelve. We report a statistically significant difference in both the DM and extragalactic DM (eDM) distributions between repeating and apparently nonrepeating sources, with repeaters having a lower mean DM and eDM, and we discuss the implications. We find no clear bimodality between the repetition rates of repeaters and upper limits on repetition from apparently nonrepeating sources after correcting for sensitivity and exposure effects, although some active repeating sources stand out as anomalous. We measure the repeater fraction over time and find that it tends to an equilibrium of 2.6−2.6+2.9 % over our total time-on-sky thus far. We also report on 14 more sources, which are promising repeating FRB candidates and which merit follow-up observations for confirmation.
Journal Article
Nonaxisymmetric Precession of Magnetars and Fast Radio Bursts
The repeating fast radio bursts (FRBs) 180916.J0158 and 121102 are visible during periodically occurring windows in time. We consider the constraints on internal magnetic fields and geometries if the cyclical behavior observed for FRB 180916.J0158 and FRB 121102 is due to the precession of magnetars. In order to frustrate vortex line pinning we argue that internal magnetic fields must be stronger than about 1016 G, which is large enough to prevent superconductivity in the core and destroy the crustal lattice structure. We conjecture that the magnetic field inside precessing magnetars has three components: (1) a dipole component with characteristic strength ∼ 1014 G; (2) a toroidal component with characteristic strength ∼ 1015–1016 G that only occupies a modest fraction of the stellar volume; and (3) a disordered field with characteristic strength ∼ 1016 G. The disordered field is primarily responsible for permitting precession, which stops once this field component decays away, which we conjecture happens after ∼1000 yr. Conceivably, as the disordered component damps bursting activity diminishes and eventually ceases. We model the quadrupolar magnetic distortion of the star, which is due to its ordered components primarily, as triaxial and very likely prolate. We address the question of whether the spin frequency ought to be detectable for precessing, bursting magnetars by constructing a specific model in which bursts happen randomly in time with random directions distributed in or between cones relative to a single symmetry axis. Within the context of these specific models, we find that there are precession geometries for which detecting the spin frequency is very unlikely.
Journal Article
Imaging stress and magnetism at high pressures using a nanoscale quantum sensor
Pressure alters the physical, chemical, and electronic properties of matter. The diamond anvil cell enables tabletop experiments to investigate a diverse landscape of high-pressure phenomena. Here, we introduce and use a nanoscale sensing platform that integrates nitrogen-vacancy (NV) color centers directly into the culet of diamond anvils. We demonstrate the versatility of this platform by performing diffraction-limited imaging of both stress fields and magnetism as a function of pressure and temperature. We quantify all normal and shear stress components and demonstrate vector magnetic field imaging, enabling measurement of the pressure-driven α ↔ ϵ phase transition in iron and the complex pressure-temperature phase diagram of gadolinium. A complementary NV-sensing modality using noise spectroscopy enables the characterization of phase transitions even in the absence of static magnetic signatures.
Journal Article
Disparities in exposure and accessibility to health: A Spatial Assessment of Covid 19 Pandemic Situation of South Bengal
Inequality and disparities of health is prevalent in a developing nation like India. The emergence of the Novel Corona virus has further aggravated this existing problem. The major issue that has been observed in dealing with the pandemic situation is the lack of facilities for testing and inadequate health infrastructure to support the huge population of this nation. This infrastructural deficiency has evidently become severe as we move away from the city areas. In this study, we have tried to assess this rural-urban disparity of health infrastructure in the settings of the COVID 19 pandemic situation. The study has been conducted to measure rural-urban disparities of eight districts i.e. Howrah, Hooghly, Kolkata, North 24 Parganas and South 24 Parganas concerning risk factors and access to health care infrastructure. To achieve the desired objectives, we have used a multidisciplinary technique incorporating statistical, GIS techniques, composite index with several aspects e.g., geographical coverage and sufficiency of COVID care and testing facilities (CCF, CTF), economic status of people, medical costs, and susceptibility to covid risk factors i.e., number of infected populations, and population density. A significant correlation between access to CCF and CTF and the proportion of urban inhabitants has been found, i.e., urban populations mostly enjoy better access to treatment and testing with dense CTFs and CCFs over space. The density and serviceability of care facilities over rural regions decrease with less urbanized areas. Urban areas are found to be at higher risk in terms of the number of active cases, population density. The study helps us to geo visualize the current COVID scenario of South Bengal in terms of regional disparities. This will help us identify the nearest testing and care facilities to any location within the study area and will be useful in mitigating infrastructural lacuna.
Journal Article
Near room temperature sensing of nitric oxide using SnO2/Ni-decorated natural cellulosic graphene nanohybrid film
In recent years, metal oxide nanoparticles and their composites with graphene have received significant research attention in toxic gas sensor applications. Herein, we demonstrate a novel approach to develop a sensor by combining SnO
2
nanoparticles and Ni-decorated natural cellulosic graphene (Ni-NCG) derived from lotus petals to form SnO
2
/Ni-NCG nanohybrid. The morphology, microstructure and elemental composition of the nanohybrids were investigated by a number of techniques which confirmed presence of nanometer sized SnO
2
particles having large surface area on sheets of few layered Ni-decorated NCG. Upto 15% response was observed when exposed to 40 ppm of NO with high reproducibility at temperature as low as 60 °C which is remarkable when compared to previously reported SnO
2
based NO sensors operating at high temperatures (~ 200 °C or more). Further, the nanohybrid showed excellent selectivity to NO when tested against other gases. A mechanism have been proposed for the improved sensitivity at low temperature based on the improved surface area of SnO
2
nanoparticles leading to larger adsorption of gas molecules combined with an improved conduction of charges provided by the Ni-decorated NCG network. The results show enormous potential for the SnO
2
/Ni-NCG nanohybrid film as near room temperature NO sensor.
Journal Article
Effect of Keyhole Gas Tungsten Arc Welding and Post-welding Heat Treatment on Microstructure and Hardness of Inconel 740H
Keyhole mode gas tungsten arc welding (GTAW) is used to obtain crack-free full-penetration welds in Ni-base superalloy Inconel 740H. Associated spatial variation in microstructure and microhardness are correlated. Local softening in the heat-affected zone (HAZ) is observed after welding. This cannot be explained by changes in grain size, twin fraction or heterogeneous strain distribution, but instead can be attributed to the dissolution of ultra-fine grain boundary carbides. Fusion zone microstructure consists of large, columnar
γ
dendrites and Ti- and Nb-rich carbides at the interdendritic region. The relatively large grain size in the FZ results in a reduction in hardness compared to the base metal. Elemental partitioning during solidification is estimated from composition measurements in the FZ and compared against existing reports for conventional GTAW. The effect of different post-weld heat treatments (PWHTs) on the precipitation of
γ
′
, disappearance of the dendritic microstructure, composition homogenization and spatial variation of microhardness is also investigated. Specifically, grain growth in the HAZ is observed after two-step PWHTs, and not for the single-step direct aging, which could have important implications for the creep response of the material.
Journal Article
Asymmetric mass ratios for bright double neutron-star mergers
The discovery of a radioactively powered kilonova associated with the binary neutron-star merger GW170817 remains the only confirmed electromagnetic counterpart to a gravitational-wave event
1
,
2
. Observations of the late-time electromagnetic emission, however, do not agree with the expectations from standard neutron-star merger models. Although the large measured ejecta mass
3
,
4
could be explained by a progenitor system that is asymmetric in terms of the stellar component masses (that is, with a mass ratio
q
of 0.7 to 0.8)
5
, the known Galactic population of merging double neutron-star systems (that is, those that will coalesce within billions of years or less) has until now consisted only of nearly equal-mass (
q
> 0.9) binaries
6
. The pulsar PSR J1913+1102 is a double system in a five-hour, low-eccentricity (0.09) orbit, with an orbital separation of 1.8 solar radii
7
, and the two neutron stars are predicted to coalesce in
470
−
11
+
12
million years owing to gravitational-wave emission. Here we report that the masses of the pulsar and the companion neutron star, as measured by a dedicated pulsar timing campaign, are 1.62 ± 0.03 and 1.27 ± 0.03 solar masses, respectively. With a measured mass ratio of
q
= 0.78 ± 0.03, this is the most asymmetric merging system reported so far. On the basis of this detection, our population synthesis analysis implies that such asymmetric binaries represent between 2 and 30 per cent (90 per cent confidence) of the total population of merging binaries. The coalescence of a member of this population offers a possible explanation for the anomalous properties of GW170817, including the observed kilonova emission from that event.
Pulsar timing measurements show a mass ratio of about 0.8 for the double neutron-star system PSR J1913+1102, and population synthesis models indicate that such asymmetric systems represent 2–30% of merging binaries.
Journal Article
A repeating fast radio burst associated with a persistent radio source
The dispersive sweep of fast radio bursts (FRBs) has been used to probe the ionized baryon content of the intergalactic medium
1
, which is assumed to dominate the total extragalactic dispersion. Although the host-galaxy contributions to the dispersion measure appear to be small for most FRBs
2
, in at least one case there is evidence for an extreme magneto-ionic local environment
3
,
4
and a compact persistent radio source
5
. Here we report the detection and localization of the repeating FRB 20190520B, which is co-located with a compact, persistent radio source and associated with a dwarf host galaxy of high specific-star-formation rate at a redshift of 0.241 ± 0.001. The estimated host-galaxy dispersion measure of approximately
903
−
111
+
72
parsecs per cubic centimetre, which is nearly an order of magnitude higher than the average of FRB host galaxies
2
,
6
, far exceeds the dispersion-measure contribution of the intergalactic medium. Caution is thus warranted in inferring redshifts for FRBs without accurate host-galaxy identifications.
A repeating fast radio burst co-located with a persistent radio source and associated with a dwarf host galaxy of a high star-formation rate has been detected.
Journal Article