A characterization of the central shell-focusing singularity in spherical gravitational collapse

We give a characterization of the central shell-focusing curvature singularity that can form in the spherical gravitational collapse of a bounded matter distribution obeying the dominant energy condition. This characterization is based on the limiting behaviour of the mass function in the neighbourhood of the singularity. Depending on the rate of growth of the mass as a function of the area radius R, the singularity may be either covered or naked. The singularity is naked if this growth rate is slower than R, covered if it is faster than R, and either naked or covered if the growth rate is same as R.Comment: 12 pages, Latex, significantly revised version, including change of title. Revised version to appear in Classical and Quantum Gravit

An X-ray view of quasars

I present an overview of observational studies of quasars of all types, with particular emphasis on X-ray observational studies. The presentation is based on the most popularly accepted unified picture of quasars - collectively referred to as AGN (active galactic nuclei) in this review. Characteristics of X-ray spectra and X-ray variability obtained from various X-ray satellites over the last 5 decades have been presented and discussed. The contribution of AGN in understanding the cosmic X-ray background is discussed very briefly. Attempt has been made to provide up-to-date information; however, this is a vast subject and this presentation is not intended to be comprehensive.Comment: Accepted for publication in the Bulletin of the Astronomical Society of India (22 pages, 6 figures

How to serve our ethnic minority communities better

On Jan 9, 2017, Theresa May, the British Prime Minister, gave a speech about “the burning injustice of mental illness”, mentioning “injustices in the way black people with mental ill health in particular are treated”, and promising that politicians would “take action to put things right.” In response to three decades of UK research on ethnic differences in mental health, such emotionally charged rhetoric has been commonplace, but has rarely produced meaningful change. Mental health care in ethnic minorities is complex, and needs dispassionate and objective scrutiny of evidence and its limitations, with careful disentanglement of the interactions between ethnicity, culture, community histories, legacies of racism, and the labyrinthine service structures that people with mental illness and their families must navigate to get appropriate help. In The Lancet Psychiatry, Phoebe Barnett and colleagues present findings from a systematic review and meta-analysis of ethnicity and legal detention of people with mental illness, an impressive attempt at providing just such scrutiny. Although the findings are not strikingly different from what is known, this comprehensive paper is a timely reminder of how far we are from fully understanding the problem—let alone solving it—and why the stated political intention to put things right might be easy to promise but hard to deliver