35th Annual Report (2022-2023) - ENGLISH

22 suffers extinction on its way to the telescope. They corrected for the Galactic and intrinsic extinction, contamination from the host galaxies, narrow and broad-line regions, Fe II emission, and Balmer continuum to derive the intrinsic continua. The astronomers have also used the HST COS/FOS spectra to account for the emission/absorption lines in the low- resolutionUVIT spectra, and found generally α redder power-law (f ∝ ν ) slopes ( α ~ −1.1 - ν 0.3) in the far UV band than predicted by the standard accretion disk model in the optical/UV band. Different accretion disk models such as multi-temperature disk blackbody (DISKBB) and relativistic disk (ZKERRBB, OPTXAGNF) have been fitted to the observed intrinsic continuum emission. They measured the inner disk temperatures ( ~ 3.6 - 5.8 eV) using the DISKBB model for seven AGN. These temperatures are lower than the peak temperatures predicted for standard disks around maximally spinning supermassive black holes accreting at Eddington rates. The inner disks in two AGN, NGC 7469 and Mrk 352, appear to be truncated at ~ 35 - 125rg and 50 - 135rg, respectively. While these results show that the intrinsic FUV emission from the AGN are consistent with the standard disks, it is possible that UV continua may be affected by the presence of soft X-ray excess emission, X-ray reprocessing, and thermal Comptonisation in the hot corona. Hence, the astronomers have suggested that joint spectral modeling of simultaneously acquired UV/X-ray data will be required to further investigate the nature of accretion disks in AGN. This part of the work is accepted for publication in the Astrophysical Journal (ApJ). The dynamics of transient Black Hole X-ray binaries and their luminosity outbursts during their evolutionary process have intrigued researchers. These systems transition through hard and soft spectral states, each displaying varying variability p a t t e r n s a n d t h e eme r g e n c e o r disappearance of localized peaks in the Investigating the dynamics of Quas i -per i od i c Osc i l l at i ons (QPOs)usingAstroSatdata power density spectrum. These localized peaks, known as Quasi-periodic Oscillations (QPOs), are believed to originate in the inner accretion regions and provide valuable insights into the extreme gravity near black holes. However, the exact origin of QPOs remains a subject of ongoing research. In recent studies, national researchers including Dr. Akash Garg and Prof. Ranjeev Misra from IUCAA, along with Dr. Somasri Sen from Jamia Millia Islamia University, have developed a generic technique to model the energy-dependent properties of QPOs by analyzing variations in the radiative components of the time-averaged photon spectrum. They applied this technique to study low-frequency QPOs detected through an extensive collection of AstroSat observations of the black hole system MAXI J1535-571. During their analysis, the researchers fitted the spectra emitted from a truncated disc with an inner hot corona. They investigated the dependence of QPO frequency on other spectral parameters and modeled the energy-dependent root mean square (rms) and time lag of the QPOs. Their objectivewas to identify the physical spectral parameters responsible for the observedQPObehavior. Interestingly, the researchers observed that the time lag between hard and soft photons is negative for QPO frequencies above 2.2 Hz, while it becomes positive for lower values. This behavior, previously seen only in the black hole system GRS 1915+105, represents a significant finding. By modeling the fractional rms and time lag, the researchers discovered a correlated variation among the accretion rate, inner disc radii, and coronal heating rate. They observed that the coronal heating rate exhibits a time lag compared to the other two parameters for QPO frequencies below 2.2 Hz. However, for higher frequencies, the sign of the time lag changes, implying that the coronal heating variation precedes the accretion rate variation. This finding suggests different dynamic origins for QPOs based on the observed time lags. Nevertheless, the researchers emphasize that the underlying scenario may be more intricate andwarrant detailedmodeling. The research findings have been published in the MNRAS journal, contributing to the ongoing understanding of QPO dynamics and their implications in the realm of astrophysics. Galaxies are the basic building blocks of the Universe- they come in all sizes. Our galaxy, the Milky Way, is one of the giant galaxies with billions of stars, but little current star formation. Giant galaxies such as ours are surrounded by tens of dwarf galaxies- irregular in shape, often forming stars. As we look backwards in time we see that galaxies were smaller and more irregular (since light takes time to travel, a galaxy seen 3 billion light-years away from a Universe that is 3 billion years younger). How these dwarf and giant galaxies assemble their stars and evolve into modern-day galaxies, like our own Galaxy, is still one of themajor puzzles. A recent study by a team of scientists using AstroSat shows how the star-forming clumps in the outskirts of a dwarf galaxy migrate towards the central region and contribute to its growth in mass and luminosity. This process that is now witnessed in several dwarf galaxies is a very important link in understanding the bigger picture of galaxy growth and evolution. The key challenge has been to firmly establish the detection of these faint, extremely blue, star-forming clumps which are very far away to see although they have a million solar masses of material within them. At slightly larger distances, the UVIT would not resolve these galaxies and we do not have an example of an extended disk seen inUV in any present-day dwarf galaxies. The redshift (cosmological distance) of these 12 dwarfs has been just optimal to probe these blue clumpy structures in their outskirts. The discovery was made by an international team of astronomers from India, the USA and France. The study was conceived by Professor Kanak Saha at the Inter-University Centre f o r A s t r onomy and A s t r oph y s i c s (IUCAA), Pune, and was published as Witnessing the 'live' formation of dwarf galaxies with AstroSat's ultraviolet-eye