Khagol Bulletin # 132 (Apr 2024) - ENG

| 10 | KHAG L | No. 132 - APRIL 2024 in WLQs may be related to a rather evolutionary scenario, where their Broad Emission line Region (BLR, responsible for strong emission line in AGNs) is perhaps not well developed as can happen in the early stage of AGN formation. To ascertain this possibility, another PhD student of the same team, Ritish Kumar, at Central University of Dharamshala, carried out the Spectral Energy Distribution (SED) of 61 RQWLQs and compared them with that of SED of control samples of normal QSOs using SDSS and WISE data. In their detailed SED decomposition, one main difference they found is that the luminosity from the "dusty torus component" in their fitting, is about 42 percent smaller in WLQs compared to the value found in their control sample of normal Quasars. As dusty torus and BLR covering factors are expected to be of a similar order in AGN, so their analysis has clearly shown that the BLR in the WLQs is underdeveloped and could be a dominant cause of theweakness of their emission line. These series of experiments, mainly using photometric data from 1-2m class telescopes, among various possible models has given support to the evolution scenario as the cause of weak emission lines in WLQs, and have helped to resolve this puzzle beyond doubt. Like this team-driven specific programme, more ambitious large survey driven programs, such as the ongoing Zwicky Transient Facility (ZTF) over the last five years, are also found extremely important to address the various cutting edge mysteries in AGNs research. These surveys allow statistical study of even large representative samples of AGN class. For instance, another member of the same team, with lead by Vibhore Negi for his PhD work, uses a large sample of about 455 Bl- Lac and 442 Flat Spectrum Radio Quasar (FSRQ) to carry out their color variability using ZTF 'g' and ' r' band light curve. Until now such studies are carried out on individual sources (rather with biased source selections) and with surveys such as ZTF using 1-2 m telescopes, it has become possible to carry out such detailed investigation for the first time with such a large sample for various AGNclasses . In conclusion, this article underscores the impor tance o f 1-2m c l ass opt i ca l telescopes in advancing AGN research by highlighting an example of a programme recently carried out using 1.3m DFOT. Owing to their relatively smaller running cost such a network of small telescopes in general can enhance the astronomy research and training activities especially at the University sector (having limited access to large aperture telescopes), in conjunction with the various survey Figure 1: Illustration of the best-fitting SED comparison of different components of emission in one of theWLQs fromour sample along with its control sample of normal QSOs matched in redshift and optical luminosity [Kumar, Chand&Joshi 2023, MNRAS2023, 519, 3656]. Figure 2: The plot shows emission redshift (zemi), computed for each member of the WLQ with respect to its control sample of normal QSOs. It can be noticed that (i) except for a few outliers, all the WLQs show smaller torus luminosity in comparison to their redshift and r-band magnitude-matched sample of normal QSOs, and (ii) there is no significant trend in the Δ Ltor/Ltor as a function of emission redshift [Kumar, Chand & Joshi, MNRAS, 2023 519, 3656]