AR-2019-2020

cross-correlation to be about 2.5 σ . We also used the cross-correlation method to search for short duration signals at all other physical values of the time lag, within this 4,096 second time interval, but do not find evidence for any statistically significant events in the off-source region. This work Has been done in Collaboration with Rahul Maroju, Sristi Ram Dyuthi, and Anumandla Sukrutha. Shanti Priya Devarapalli Comprehensive study of a neglected contact binary TYC 5532-1333-1 A comprehensive photometric and spectroscopic analysis of the variable TYC5532-1333-1 (TYC) along with an investigation of its orbital period variation are presented for the first time. The B and V band photometric study indicates that TYC is an intermediate contact binary with de- gree of contact and mass ratio of 32% and ∼ 0.24, respectively. The derived equivalent widths from the spectroscopic study of H α and Na-I lines re- veal phase dependent variation and mutual corre- lation. Using the available times of minimum light, an investigation of orbital period variation shows a long-term decrease at a rate of 3 . 98 × 10 − 6 days/yr. Expected causes for such decline in the orbital pe- riod could be angular momentum loss and a quasi- sinusoidal variation due to light-time effect, proba- bly caused by a third-body companion. The mini- mum mass of the third body ( M 3 ) was derived to be 0.65 M . Our study is an attempt to evaluate and understand the evolutionary state of above- mentioned neglected contact binary. This analy- sis has been done in collaboration with Rukmini Jagirdar, Rajendra M. Prasad, Vinoy S. Thomas, Shehab A. Ahmed, R. Gralapally, et al. Broja Gopal Dutta Evidence of outflow-induced soft lags of galactic black holes The nature of lag variation of galactic black holes remains enigmatic mostly because of non-linear and non-local physical mechanisms, which contribute to the lag of the photons coming from the region close to the central black holes. One of the widely ac- cepted major sources of the hard lag is the inverse Comptonization mechanism. However, the exact reason, for soft lags has yet to be identified. In this work, we report a possible correlation between ra- dio intensities of several outbursting galactic black hole candidates and amounts of soft lag. The cor- relation suggests that the presence of major out- flows or jets changes the disk morphology along the line of sight of the observer which produces soft lags. This work has been done in collaboration with Dusmanta Patra, Arka Chatterjee, Sandip K. Chakrabarti, and Prantik Nandi. Discovery of jet-induced soft lags of XTE J1550- 564 during its 1998 outburst X-ray time lags are complicated in nature. The exact reasons for complex lag spectra are as yet unknown. However, the hard lags, in general, are believed to be originated due to inverse Comp- tonization process. Recent studies on “Disk-Jet Connections” revealed that the jets are also con- tributing in the X-ray spectral and timing prop- erties in a magnitude, which was more than what was predicted earlier. In this work , we first show an exact anti-correlation between X-ray time lag and radio flux for XTE J1550-546 during its 1998 outburst. We propose that the soft lags might be generated due to the change in the accretion disk structure along the line of sight during higher jet activity. This work has been done in collaboration with Arka Chaterjee, Dusmanta Patra, Sandip K. Chakrabarti, and Prantik Nandi. Jibitesh Dutta Linear growth index of matter perturbations in Rastall gravity Rastall gravity theory shows notable features con- sistent with physical observations in comparison to the standard Einstein theory. Recently, there has been a debate about the equivalence of Rastall gravity and general relativity. Motivated by this open issue, we attempt to shed some light on this debate by analyzing the evolution of the Rastall based cosmological model at the background as well as perturbation level. Employing the dynamical system techniques, we found that at late times, the dynamics of the model resembles the ΛCDM model at the background level irrespective of the choice of Rastall’s parameter. However, at the per- turbation level, we found that the evolution of the growth index heavily depends on the Rastall’s pa- rameter and displays a significant deviation from