AR-2019-2020

exact solutions to the Navier-Stokes equation as derived by Singh and Sridhar (2017), and deter- mined the average response tensor governing the evolution of mean magnetic field. The growth rate and cycle period of the growing dynamo wave revealed new scaling relations with the shear rate. These have implications for magnetic activity cycles of stars in recent observations. Their study essentially generalizes the standard alpha-omega dynamo as also the alpha-effect is affected by shear and the modelled random flow has a finite memory. The shear dynamo problem with a magnetic background In recent years, the possibility of large-scale dynamo action through the shear-current effect in flows where more conventional dynamo effects, such as the alpha-effect arising through stratification and rotation, cannot operate, has gained a lot of interest. Latest work of Nishant Singh and his collaborators involved a study of shear-current effect in the presence of magnetized burgulence. The main purpose of this work was to check if recently claimed magnetic shear current effect could lead to dynamo growth. By measuring the turbulent transport coefficients, importantly the diffusivity tensor, using the non-linear test-field method, they found that the magnetic shear current effect is not operative in most cases which show a dynamo instability. Further analysis revealed that the main driver of the dynamo in those cases is the so-called incoherent-alpha shear mechanism in which alpha-coefficients are allowed to fluctuate about a zero mean. High Energy Astrophysics Polarisation measurements Polarisation measurements of the prompt emission of 11 bright Gamma Ray Bursts detected during the first year of AstroSat operations have been presented by Dipankar Bhattacharya , Vidushi Sharma and collaborators. There were six detections and five upper limits, indicating a wide range in the degree of polarisation of the emission integrated over the burst. It appears likely that temporal averaging reduces the measurable polarisation in a significant number of GRBs. Time resolved analysis of one of the brightest events, GRB160821A, carried out by Sharma , Bhattacharya , Shabnam Iyyani and collaborators, revealed near-orthogonal transition of polarisation angle twice through the burst, accompanied by a change in spectrum. The observed behaviour indicates the presence of strongly ordered magnetic field in the emitting material. AstroSat observation of an accreting black hole Detailed AstroSat observations of an accreting black hole binary MAXI J1535-571 led to the dis- covery of a very strong correlation between the spectral slope and the frequency of quasi-periodic oscillation in the intensity of the X-ray radiation. This poses significant constraints on the ori- gin of the observed X-ray emission, and suggests that a Comptonising region controls both the emergent spectrum and the intensity variations. This observation has been done by Yash Bhar- gava , Dipankar Bhattacharya , Ranjeev Misra and collaborators. AstroSat observations of another well-known black hole X-ray binary GRS 1915+105 by Bhattacharya and collaborators have showed, for the first time, a variation in the frequency of the ∼ 70 Hz quasi-periodic oscillation of its X-ray luminosity being correlated with the spectral state of the source Sustained timing observations of eight radio pulsars with the Ooty Radio Telescope and the Giant Metrewave Radio Telescope by A. Basu, Dipankar Bhattacharya and collaborators revealed 11 incidents of sudden change in the spin frequency of these pulsars, called glitches. Three of these glitch events were new discoveries, the remaining being reported also from other observatories