35th Annual Report (2022-2023) - ENGLISH

12). Apart from this, the AstroSat Science Support Cell (ASSC) was also set up as a joint venture of the Indian Space Research Organisation (ISRO) and IUCAA with the primary purpose of facilitating the use of AstroSat, both for making observing proposals and for utilizing archival data. In this regard, the ASSC website provides different required tools, documentation and updates related to the instruments onboard AstroSat. ASSC also runs a help desk to address user queries, provides utility tools, and disseminates analysis software through a consolidated web portal. It also maintains the AstroSat Proposal Processing System (APPS) deployed at ISSDC, a software platform central to the workflow management of AstroSat operations. In addition, ASSC organizes meetings, workshops and webinars from time to time to train the users in utilizing the AstroSat archival data and writing proposals. AstroSat has already completed 7 years of successful operation and is aiming to go far beyond. The successful operation of AstroSat has established India as a significant player in the field of multi- wavelength astronomy and continues to contribute to groundbreaking discoveries in astrophysics. Till date, the number of refereed publications using AstroSat observations hassurpassed350. Inaddition,more than 22 PhD theses (both nationally and internationally) have been produced based on scientific conclusions obtained from AstroSat observations. Some of the important research undertaken by IUCAA members using AstroSat data are highlightedbelow. The internal dust extinction of a galaxy can be inferred from the measurement of its UV continuum slope ( β ). There have been several studies that computed β of galaxies starting from the nearby universe up to redshift z~10. The observed β −z relation signifies a gradual dust enrichment in AstroSat View of Galaxies Z=0.40-0.75 in the GOODS-North- Field at galaxies with cosmic time. Due to the lack of wide-field near-UV imaging and higher angular resolution than what was possible by GALEX satellite, the β measurement of galaxies from z = 1 to 0 has not been explored well; except the very local galaxies. Hence, a study has been carried out by IUCAA Astronomers (Chayan Mondal and Kanak Saha) along with their international collaborators using the high-resolutionNUV images of the GOODS-North field observed with AstroSat/UVIT to fill this ~8 Gyr age gap in the global β − z relation. Combining with HST F275W, F336W, and KPNO U bands, the AstroSat/UVIT data has helped to estimate β of 465 galaxies between redshift 0.40 and 0.75. The mean (median) and 1 σ scatter in the observed β are found to be − 1.33±0.07 (−1.32) and 0.60 within the considered redshift range. The β measurements found from this study add new data points to the least-explored redshift regime, further reinforcing the gradual reddening of the galaxy UV continuumwith cosmic time. The sensitivity of UVIT instrument allowed to sample β of faint galaxies down to M1500=−15.6 mag (i.e., 0.028L* at z=0.5). No specific trend between β and M1500 for the entire luminosity range −21< M1500<−15 mag was observed, although the majority of the most luminous galaxies (M1500<−19 mag) are found to have relatively redder slopes. This study highlights the unique capability of AstroSat/UVIT near-UV imaging to characterize the rest-frame far-UV properties of galaxies at redshift z ~ 0.5, and is published in the Astrophysical Journal (ApJ). A group of IUCAA Astronomers (Shrabani Kumar, G. C. Dewangan and Prakash Tripathi) along with their national and international collaborators have carried out a study on the accretion disk emission from a sample of eight Seyfert 1 − 1.5 active galactic nuclei (AGN) using far ultraviolet (1300 − 1800 Å) slit-less grating spectra acquired with AstroSat/UVIT. The emission from accretion disk is contaminated and Insights to the intrinsic accretion disk emission from Seyfert type 1 AGNusingAstroSat/UVIT 21 RESEARCH HIGHLIGHTS AstroSat: A Multi-wavelength explorationof theUniverse India's first multi-wavelength satellite, AstroSat, marks a significant milestone in the country's space exploration efforts. It was launched by the Indian Space Research Organisation (ISRO) on September 28, 2015. AstroSat is specially designed to study high energy phenomena, detect transient sources, measure neutron star's magnetic fields and conduct deep field surveys in the ultraviolet frequencies. It has five astronomy payloads onboard for simultaneous multi-band observations: Ultraviolet Imaging Telescopes (UVIT), Large Area X-ray Proportional Counters (LAXPC), Soft X-ray Telescope (SXT), Cadmium-Zinc-Telluride coded-mask Imager (CZTI) and Scanning Sky Monitor (SSM). These instruments are being handled by different payload operation centers (POCs), situated at various institutes at different parts of the country. The role of the Inter-University Centre for Astronomy and Astrophysics (IUCAA) for AstroSat has been a crucial one. IUCAA hosts the POC for the CZTI instrument. CZTI is designed to detect high-energy X-rays and gamma rays from celestial sources. One of the most significant accomplishments of CZTI has been the detection of over 550 Gamma Ray Bursts (GRBs), which are explosive events accompanying the birth of Black Holes. The CZTI team, led by Prof. Dipankar Bhattacharya and Prof. Varun Bhalerao, has continuously improved the search and detection methods, involving young scientists, including undergraduate students and Ph.D. scholars, who actively contribute to the analysis of the data. An important feature of CZTI is its capability to measure the polarization of high energy X- rays, which is instrumental in unraveling the dynamics just outside newly formed black holes. This unique ability distinguishes CZTI from flagship GRB missions like NASA's Neil Gehrels Swift Telescope and the US-Europe Fermi Space Telescope. Till date the CZTI team has reported the detection of strongly polarized emission from several GRBs and has deduced the source geometry and magnetic orientation based on these results (Chattopadhyay et al 2022 ApJ 936,