AR-2019-2020

To investigate the observational implications, we consider dissipative shocks and estimate the maxi- mum accessible energy from the post-shock corona (PSC) for nine stellar mass black hole candidates. We compare this with the observed radio jet ki- netic power reported in the literature, whenever available. We find close agreement between the es- timated values from our model with those reported in the literature This work has been done in col- laboration with Anjali Rao. Rathin Sarma and Amit Pathak NuSTAR observation of Ark 564 reveals the varia- tion of coronal temperature with flux The hard X-ray spectral index of some AGNs has been observed to steepen with the source flux. This has been interpreted in a thermal Comptonization scenario, where an increase in the soft flux de- creases the temperature of the corona, leading to steepening of the photon index. However, the vari- ation of the coronal temperature with flux has been difficult to measure due to the presence of complex reflection component in the hard X-rays and the lack of high-quality data at that energy band. Re- cently, a 200 ks NuSTAR observation of Ark 564 in 3–50 keV band, revealed the presence of one of the coolest coronae with temperature kT e ∼ 15 keV in the time-averaged spectrum. Here, we re-analyse the data and examined the spectra in four flux lev- els. Our analysis shows that the coronal tempera- ture decreased from ∼ 17 to ∼ 14 keV as the flux increased. The high energy photon index Γ ∼ 2 . 3 varied by less than 0 . 1, implying that the optical depth of the corona increased by about 10% as the flux increased. This first reporting of coronal tem- perature variation with flux shows that further long observation by NuSTAR of this and other sources would shed light on the geometry and dynamics of the inner regions of the accretion flow. This work is done in collaboration with Samuzal Barua, Vadakkumthani Jithesh, Ranjeev Misra, and Gu- lab C. Dewangan. Asoke Kumar Sen Spectroscopic survey of H-alpha emission line stars associated with bright-rimmed clouds The results of a spectroscopic survey of H α emis- sion line stars associated with fourteen bright rimmed clouds are presented. Slit-less optical spec- troscopy was carried out with the IUCAA Girawali Obervatory 2 m telescope, and IUCAA Faint Ob- ject Spectrograph and Camera (IFOSC). H α emis- sion line was detected from 173 objects. Among them, 85 objects have a strong H α emission line with its equivalent width larger than 10 A. Those are classical T Tauri stars. 52 objects have a weak H α emission line with its equivalent width less than 10 A and do not show intrinsic near-infrared ex- cess. Those are weak-line T Tauri stars. On the other hand, 36 objects have a weak H α emission line ( < 10 A ), although they show intrinsic near- infrared excess. Such objects are not common in low-mass star forming regions. Those are misfits of the general concept on formation process of a low-mass star, in which it evolves from a classi- cal T Tauri star to a weak-line T Tauri star. Those might be weak-line T Tauri stars with a flared disk, in which gas is heated by ultraviolet radiation from a nearby early-type star. Alternatively, we pro- pose pre-transitional disk objects as their evolu- tional stage. This work has been done in collabo- ration with Kensuke Hosoya, Yoichi Itoh, Yumiko Oasa, and Ranjan Gupta. Photometry and colour index of Comet 67P/Churyumov-Gerasimenko on 2015 December 12 Comet 67P / Churyumov-Gerasimenko (hereafter, 67P) was observed on 12 December 2015, from 2 m Himalayan Chandra Telescope in India in pho- tometry to study its dust properties, using Bessell R and I filters. We study the photometric images to highlight coma structures and jets. The radial decrease in intensity in the different coma struc- tures are compared to the azimuthally integrated intensities. The observations of the slopes show a quasi-steady-state coma to an optocentric distance of about 20,000 km. The change in the slopes in the structures indicates changing properties of the dust particles and/or change in their local size dis- tributions. Comparison of the radial decrease in the two wavelengths suggests a change in the local colour-index. Based on the absolute photometry carried out in the present work, we calculate the reddening of the comet dust, which helps to char- acterize variations in the size and the materials of the particles. The colour-index is calculated for different apertures and regions in the coma (0.40 ± 0.07 mag) for a 22,000 diameter aperture. A