AR-2019-2020

K. Surendra Nadh Somala Effect of induced seismicity on advanced gravita- tional wave interferometers Advanced LIGO and the next generation of ground-based detectors aim to capture many more binary coalescences through improving sensitivity and duty cycle. Earthquakes have always been a limiting factor at low frequency where neither the pendulum suspension nor the active controls pro- vide sufficient isolation to the test mass mirrors. Several control strategies have been proposed to re- duce the impact of teleseismic events by switching to a robust configuration with less aggressive feed- back. The continental United States has witnessed a huge increase in the number of induced earth- quake events primarily associated with hydraulic fracking-related waste water re-injection. Effects from these differ from teleseismic earthquakes pri- marily because of their depth which is in turn linked to their triggering mechanism. In this work, we discuss the impact caused due to these low mag- nitude regional earthquakes and explore ways to minimize the impact of induced seismicity on the detector. This work has been done in collaboration with K. Nikhil Mukund, Brian OReilly, and Sanjit Mitra. Karthik Sriram Constraining the coronal heights and readjustment velocities based on the detection of a few hundred seconds delays in the Z source GX 17+2 Neutron star Z-type sources provide a unique plat- form in order to understand the structure of ac- cretion disk-corona geometry emitting close to the Eddington luminosity. Using RXTE and Nuclear Spectroscopic Telescope Array mission (NuSTAR) satellite data, we performed cross correlation func- tion (CCF) studies in GX 17+2 in order to con- strain the size of the corona responsible for hard X- rays. From the RXTE data, we found that during horizontal and normal branches, the CCFs showed anti-correlated hard (16-30 keV) and soft (2-5 keV) X-ray delays of the order of a few tens to hun- dred seconds with a mean correlation coefficient of 0 . 42 ± 0 . 11. A few observations show correlated lags and on one occasion, coincident with radio emis- sion. We also report an anti-correlated hard X- ray delay of 113 ± 51 s using the NuSTAR data of GX 17+2. Based on RXTE data, we find that soft and hard X-ray fluxes are varying, indicating the changes in the disk-corona structure during delays. We bridle the size of the corona using relativistic precession, transition layer models, and boundary layer models. Assuming the delays to be a readjust- ment time scale of the disk-corona structure, the height of the corona was estimated to be 17-100 km. Assuming that the inner region of the truncated disk is occupied by the corona, we constrain the coronal readjustment velocities ( v corona = βv disk , where v disk is the radial velocity component of the disk) of the order of β = 0 . 06 − 0 . 12 . This study indicates that the observed delays are primarily de- pendent on the varying coronal readjustment veloc- ities. This has been studied in collaboration with Siddhart Malu, and Changhwan S. Choi. L. Sriramkumar Viable scalar spectral tilt and tensor-to-scalar ratio in near-matter bounces In a recent work, we constructed a model consist- ing of two fields—a canonical scalar field and a non-canonical ghost field—that sourced a symmet- ric matter bounce scenario. The model involved only one parameter, viz. the scale associated with the bounce. For a suitable value of the parame- ter, the model led to strictly scale-invariant power spectra with a COBE normalized scalar amplitude and a rather small tensor-to-scalar ratio. In this work, we extend the model to achieve near-matter bounces, which contain a second parameter apart from the bounce scale. As the new model does not seem to permit analytical evaluation of the scalar modes near the bounce, with the aid of techniques that we used in our earlier work, we compute the scalar and the tensor power spectra numerically. For appropriate values of the additional parame- ter, we find that the model produces red spectra with a scalar spectral tilt and a small tensor-to- scalar ratio, which are consistent with the recent observations of the anisotropies in the cosmic mi- crowave background by Planck. This work is done in collaboration with Rathul Nath Raveendran. Can non-minimal coupling restore the consistency condition in bouncing universes? An important property of the three-point functions generated in the early universe is the so-called con- sistency condition. According to the condition, in the squeezed limit wherein the wave number of one