AR-2019-2020

produces lognormal distribution in flux. However, for two BL Lacs, viz., Mrk 501 and Mrk 421, AD test shows that their flux distributions are neither Gaussian nor lognormal, and their index distributions are non-Gaussian. The histogram fitting of Mrk 501 and Mrk 421 suggests that their flux distributions are more likely to be a bimodal, and their index distributions are double Gaussian. Since, Gaussian distribution of index produces a lognormal distribution in flux, double Gaussian distribution of index in Mrk 501 and Mrk 421 indicates that their flux distributions are probably double lognormal. Observation of double lognormal flux distribution with double Gaussian distribution in index reaffirms two flux states hypothesis. Further, the difference observed in the flux distribution of FSRQ (3C 273) and BL Lacs (Mrk 501 and Mrk 421) at X-rays suggests that the low-energy emitting electrons have a single lognormal flux distribution, while the high-energy ones have a double lognormal flux distribution. The long-term X-ray flux distibution of Cygnus X-1 using RXTE-ASM and MAXI observations Cygnus X-1 is one of the brightest galactic black hole binary systems in the sky. Generally, the X- ray emission of Cygnus X-1 falls into one of the two distinct states, viz. “low-Hard” and “high-Soft”. Kabita Deka, Zahir Shah, Ranjeev Misra and Gazi Ameen Ahmed carried a detailed X-ray flux distribution study of Cygnus X-1 using the RXTE-ASM and MAXI long-term B and C band light curves. They characterized the X-ray flux distribution of Cygnus X-1 by constructing the normalized histograms of B-band and C-band light curves such that each bin in the histogram has equal number of flux points. Since the long-term X-ray emission of Cygnus X-1 is predominantly in Hard-state and Soft-state, they fitted the flux histograms of B and C bands simultaneously with the double probability density function (PDF). The reduced- χ 2 obtained from different double PDF shows that the lognormal+Gaussian PDF provides a better fit to the RXTE B and C band histograms, while double-lognormal PDF results in better fit to the MAXI B and C-band histograms. Also, histogram fitting shows that the Hard-state flux distribution in both RXTE and MAXI observations is lognormal, while the Soft-state has the Gaussian distribution in case of RXTE observation and lognormal distribution for the MAXI observation. Moreover, they also checked the tri-model fit of the histograms, as Cygnus X-1 sometimes is also found in the intermediate state. However, the fitting did not improve upon using three component PDF, suggesting that the intermediatory spectral class may not correspond to a distinct component of the flux distribution. Moreover, the histogram fitting together with spectral state definition of Grinberg V., et al., 2013 , suggests that the intermediate state is not a separate state, rather it is a extreme extension of Soft-sate. Stars, Interstellar Medium and Planetary Studies Unveiling Vela variability of interstellar lines in the direction of the Vela supernova remnant High-resolution optical spectra were obtained by Ranjan Gupta , during the period 2017-2019 using the Southern African Large Telescope (SALT) for 15 stars in the direction (lines of sight) of the Vela supernova remnant. These observations are on the interstellar CaII H and K and NaI D lines, and are discussed in details in our recent publication (Rao, et al., MNRAS, 493 497, 2020). In particular, the line profiles are compared with profiles at a comparable spectral resolution obtained in the period 1993 - 1996 by Cha and Sembach (2000). Ten of the sight-lines show changes to one or more of the components in those respective line of sights. Changes include small (1-2 kms − 1 ) in radial velocity and/or increases/decreases in the equivalent widths over the two decades between the periods of observation. Changes are more obvious in the Ca K lines than in the NaD lines and are attributed to gas disturbed by interactions between the supernova ejecta and the surrounding interstellar medium. A representative time-scale may be 20-50 years. Small-scale variations in line profiles across the face of the remnant suggest, as previously remarked, that a linear scale for interactions is a small fraction of the 40 pc size of the present remnant. Figs. 19 and 20 are samples