In this work, we study the synthetic explosions of a massive star. We take a 100 $M_{\odot}$ zero-age main-sequence (ZAMS) star and evolve it until the onset of core-collapse using $\mathtt{MESA}$. Then, the resulting star model is exploded using the publicly available stellar explosion code, $\mathtt{STELLA}$. The outputs of $\mathtt{STELLA}$ calculations provide the bolometric light curve and photospheric velocity evolution along with other physical properties of the underlying supernova. In this paper, the effects of having a large Hydrogen-envelope on the supernova light curve have been explored.We also explore the effects of the presence of different amounts of nickel mass and the variation of the explosion energy of the supernovae from such heavy progenitors, on thebolometric light curves and photospheric velocities.

Optical follow-up observations of optical afterglows of gamma-ray bursts are crucial to probe the geometry of outflows, emission mechanisms, energetics and burst environments. We performed the follow-up observations of GRB 210205A and ZTF21aaeyldq (AT2021any) using the 3.6m Devasthal opticaltelescope (DOT) around one day after the burst to deeper limits due to the longitudinal advantage of the place. This paper presents our analysis of the two objects using data from other collaborative facilities, i.e., 2.2m Calar Alto Astronomical Observatory (CAHA) and other archival data. Our analysis suggests that GRB 210205A is a potential dark burst once compared with the X-ray afterglow data. Also, comparing results with other known and well-studied dark GRBs samples indicate that the reason for the optical darkness of GRB210205A could either be intrinsic faintness or a high redshift event. Based on our analysis, we also found that ZTF21aaeyldq is the third known orphan afterglow with a measured redshift except for ZTF20aajnksq (AT2020blt) and ZTF19abvizsw (AT2019pim). The multiwavelength afterglow modeling of ZTF21aaeyldq using the afterglowpy package demands a forward shock model for an ISM-like ambient medium with a rather wider jet opening angle. We determine circumburst density of $n_0 =0.87$ cm$^{-3}$, kinetic energy $E_k=3.80 \times 10^{52}$ erg and the afterglow modeling also indicates that ZTF21aaeyldq is observed on-axis ($\theta_{\rm obs}$ < $\theta_{\rm core}$) and a gamma-ray counterpart was missed by GRBs satellites. Our results emphasize that the 3.6m DOT has a unique capability for deep follow-up observations of similar and other new transients for deeper observations as a part of time-domain astronomy in the future.

In this paper, we present multi-band photometric observations and analysis of the host galaxies for a sample of five interesting gamma-ray bursts (GRBs) observed using the 3.6mDevasthal optical telescope (DOT) and the back-end instruments. The host galaxy observations of GRBs provide unique opportunities to estimatethe stellar mass, ages, star-formation rates and other vital properties of the burst environments and hence, progenitors. We performed a detailed spectral energy distribution (SED) modeling of the five host galaxies using an advanced tool called $\mathtt{Prospector}$, a stellar population synthesis model. Furthermore, we comparedthe results with a larger sample of well-studied host galaxies of GRBs, supernovae and normal star-forming galaxies. Our SED modeling suggests that GRB 130603B, GRB 140102A, GRB 190829A and GRB 200826A have massive host galaxies with high star-formation rates (SFRs). On the other hand, a supernovae-connectedGRB 030329 has a rare low-mass galaxy with a low star-formation rate.We also find that GRB 190829A has the highest (in our sample) amount of visual dust extinction and gas in its local environment of the host, suggesting that the observed very high-energy emission from this burst might have a unique local environment. Broadly,the five GRBs in our sample satisfy the typical correlations between host galaxies parameters and these physical parameters are more common to normal star-forming galaxies at the high-redshift Universe. Our results also demonstrate the capabilities of 3.6m DOT and the back-end instruments for the deeper photometric studies ofthe host galaxies of energetic transients, such as GRBs, supernovae and other transients in the long run.

In this work, we employ two publicly available analysis tools to study four hydrogen (H)–stripped core–collapse supernovae (CCSNe), namely, SN 2009jf, iPTF13bvn, SN 2015ap and SN 2016bau. We use the modular open-source fitter for transients (${\texttt{MOSFiT}}$) to model the multi-band light curves. ${\texttt{MOSFiT}}$ analyses show ejecta masses ($\log M_{ej}$) of $0.80^{+0.18}_{−0.13}$ $M_{\odot}$, $0.15^{+0.13}_{−0.09}$ $M_{\odot}$, $0.19^{+0.03}_{−0.03}$ $M_{\odot}$ and $0.19^{−0.01}_{+0.02}$ $M_{\odot}$ for SN 2009jf, iPTF13vn, SN 2015ap and SN 2016au, respectively. Later, modules for experiments in stellar astrophysics(MESA), is used to construct models of stars from pre-main sequence up to core collapse, which serve as the possible progenitors of these H-stripped CCSNe. Based on literature, we model a 12 $M_{\odot}$ ZAMS star as the possible progenitor for iPTF13vn, SN 2015ap and SN 2016bau, while a 20 $M_{\odot}$ ZAMS star is modeled as the possible progenitor for SN 2009jf. Glimpses of stellar engineering and physical properties of models at various stages of their lifetime have been presented to demonstrate the usefulness of these analysis threads to understand the observed properties of several classes of transients in detail.