AR_final file_2018-19

most of the sheared arcade is relaxed during the extended phase after the eruption. This work has been done in collaboration with N. Vasantharaju, P. Vemareddy, and B. Ravindra. Recurring coronal holes and their rotation rates during the solar cycles 22-24 Coronal holes (CHs) play a significant role in mak- ing the Earth geo-magnetically active during the declining and minimum phases of the solar cycle. In this study, we analysed the evolutionary char- acteristics of the Recurring CHs (RCHs) from the year 1992 to 2016. The extended minimum of Solar Cycle 23 shows unusual characteristics in the num- ber of persistent CHs in the mid- and low-latitude regions of the Sun. Carrington rotation maps of He 10830 ˚A and EUV 195 ˚A observations are used to identify the CHs. The latitude distribution of the RCHs shows that most of them are appeared be- tween ± 20 0 latitudes. In this period, more number of recurring RCHs appeared in and around 100 0 and 200 0 Carrington longitudes. The large sized CHs lived for shorter period and they appeared close to the equator. From the area distribution over the latitude considered, it shows that more number of RCHS with area < 10 21 cm 2 appeared in the southern latitude close to the equator. The rotation rates calculated from the RCHs appeared between ± 60 0 latitude shows rigid body character- istics. The derived rotational profiles of the CHs show that they have anchored to a depth well be- low the tachocline of the interior, and compares well with the helioseismology results. This work has been done in collaboration with K. Prabhu, B. Ravindra, and Manjunath Hegde. Broja Gopal Dutta Evolution of accretion disc geometry of GRS 1915+105 during its χ -state as revealed by TCAF solution The evolution of the low frequency quasi-periodic oscillations (LFQPOs) and associated time lag in transient black hole sources as function of time can be explained by variation of the Compton cloud size in a Two Component Advective Flow (TCAF) so- lution. A similar study of a persistent source, GRS 1915+105, has not been attempted. We fit the evo- lution of QPOs with propagatory oscillating shock (POS) solution for the two sets of χ -state observa- tions, and find that the shock steadily recedes with almost constant velocity v s ∼ t 0 . 1 d and v s ∼ t 0 . 4 d when QPO frequency is declining and the spectral state becomes harder. The shock moves inward with a constant velocity v 0 = 473 . 0 cm s − 1 and v 0 = 400 . 0 cm s − 1 when QPO frequency is rising and the spectral state becomes softer. This prop- agation is similar to what was observed in XTE J1550-564 during 1998 outburst. The time lag mea- sured at the QPO frequency varies in a similar way as the size of the Compton cloud (CENBOL in TCAF). Most interestingly, in both the cases, the lag switches sign (hard lag to soft lag) at a QPO frequency of ∼ 2 . 3 − 2 . 5 Hz irrespective of the energy of photons. We find that at very low frequencies < 1 Hz, the Comptonizing Efficiency (CE) increases with QPO frequency, and at higher frequency, the trend is opposite, and the time lags become mostly positive at all energies when CE is larger than 0 . 85% for both the sources. This work has been done in collaboration with Partha Sarathi Pal, and Sandip K. Chakrabarti. Variability properties of galactic black holes In X-ray binaries, fast variability in X-ray emission on time-scales of milli-seconds to seconds is a com- mon and very complex phenomenon. The variabil- ity of astronomical sources is usually expressed as a time series. The study of variability property im- plies the time series analysis of the light curve ob- tained from various sources. These study includes the study of Power Density Spectrum (PDS) and time/phase lag spectrum. Studies of energy depen- dent temporal properties (i.e.,variability proper- ties) in different variability time scale for the nearly edge on (high inclination) and nearly face on (low inclination) compact binary sources could throw light on the detail structure of the accretion geom- etry. Again, the evolutionary picture of variability parameters, such as QPO frequency, time lag and energy dependent lag for transient and persistent sources can be studied with the simplified frame- work of Two Component Advective Flow (TCAF) solution. However, even with this simplified model and employing Monte-Carlo simulation technique, the simulated spectral behaviour can be tested with the observational results. Thus, it is possible to answer the cause of hard and soft lag and their transitions in higher inclination sources within the framework of a single TCAF solution. Also the evolution of the low frequency quasi-periodic oscil- lations (LFQPOs) and associated time lag in tran- ( 188 )