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Item SN 2008gz − most likely a normal type IIP event(Mon. Not. R. Astron. Soc., 2011-02-08) Roy, Rupak; Kumar, Brijesh; Moskvitin, Alexander S; et.alWe present BV RI photometric and low-resolution spectroscopic investigation of a type II core-collapse supernova (SN) 2008gz, which occurred in a star forming arm and within a half-light radius (solar metallicity region) of a nearby spiral galaxy NGC 3672. The SN event was detected late, and a detailed investigation of its light curves and spectra spanning 200 days suggest that it is an event of type IIP similar to archetypal SNe 2004et and 1999em. However, in contrast to other events of its class, the SN 2008gz exhibits rarely observed V magnitude drop of 1.5 over the period of a month during plateau to nebular phase. Using 0.21 mag of AV as a lower limit and a distance of 25.5 Mpc, we estimate synthesized 56Ni mass of 0.05 ± 0.01M⊙ and a mid-plateau MV of −16.6 ± 0.2 mag. The photospheric velocity is observed to be higher than that was observed for SN 2004et at similar epochs, indicating explosion energy was comparable to or higher than SN 2004et. Similar trend was also seen for the expansion velocity of H-envelopes. By comparing its properties with other well studied events as well as by using a recent simulation of pre-SN models of Dessart, Livne & Waldman (2010), we infer an explosion energy range of 2−3×1051 erg and this coupled with the observed width of the forbidden [O i] 6300-6364°A line at 275 days after the explosion gives an upper limit for the main-sequence (non-rotating, solar metallicity) progenitor mass of 17M⊙. Our narrow-band Hα observation, taken nearly 560 days after the explosion and the presence of an emission kink at zero velocity in the Doppler corrected spectra of SN indicate that the event took place in a low luminosity star forming Hii region.Item SN 2008in − BRIDGING THE GAP BETWEEN NORMAL AND FAINT SUPERNOVAE OF TYPE IIP(2011-06-10) Roy, Rupak; Kumar, Brijesh Kumar; Benetti, Stefano; et.alWe present optical photometric and low-resolution spectroscopic observations of the Type II plateau supernova (SN) 2008in, which occurred in the outskirts of the nearly face-on spiral galaxy M 61. Photometric data in the X-rays, ultraviolet and near-infrared bands have been used to characterize this event. The SN field was imaged with the ROTSE-IIIb optical telescope about seven days before the explosion. This allowed us to constrain the epoch of the shock breakout to JD = 2454825.6. The duration of the plateau phase, as derived from the photometric monitoring, was 98 days. The spectra of SN 2008in show a striking resemblance to those of the archetypal low-luminosity IIP SNe 1997D and 1999br. A comparison of ejecta kinematics of SN 2008in with the hydrodynamical simulations of Type IIP SNe by Dessart et al. (2010) indicates that it is a less energetic event ( 5 × 1050 erg). However, the light curve indicates that the production of radioactive 56Ni is significantly higher than that in the low-luminosity SNe. Adopting an interstellar absorption along the SN direction of AV 0.3 mag and a distance of 13.2 Mpc, we estimated a synthesized 56Ni mass of 0.015M⊙. Employing semi-analytical formulae (Litvinova & Nadezhin 1985), we derived a pre-SN radius of 126R⊙ , an explosion energy of 5.4 × 1050 erg and a total ejected mass of 16.7M⊙ . The latter indicates that the zero age main-sequence mass of the progenitor did not exceed 20M⊙ . Considering the above properties of SN 2008in and its occurrence in a region of sub-solar metallicity ([O/H] 8.44 dex), it is unlikely that fall-back of the ejecta onto a newly formed black hole occurred in SN 2008in. We therefore favor a low-energy explosion scenario of a relatively compact, moderate-mass progenitor star that generates a neutron star.Item THE DISCOVERY AND NATURE OF THE OPTICAL TRANSIENT CSS100217:102913+404220(The Astrophysical Journal, 2011-07-10) Drake, A.J; Djorgovski, S.G; Mahabal, A; et.alWe report on the discovery and observations of the extremely luminous optical transient CSS100217:102913+404220 (CSS100217 hereafter). Spectroscopic observations showed that this transient was coincident with a galaxy at redshift z = 0.147 and reached an apparent magnitude of V ∼ 16.3. After correcting for foreground Galactic extinction we determine the absolute magnitude to be MV = −22.7 approximately 45 days after maximum light. Over a period of 287 rest-frame days, this event had an integrated bolometric luminosity of 1.3 × 1052 erg based on time-averaged bolometric corrections of ∼15 from V- and R-band observations. Analysis of the pre-outburst Sloan Digital Sky Survey (SDSS) spectrum of the source shows features consistent with a narrow-line Seyfert 1 galaxy. High-resolution Hubble Space Telescope and Keck follow-up observations show that the event occurred within 150 pc of the nucleus of the galaxy, suggesting a possible link to the active nuclear region. However, the rapid outburst along with photometric and spectroscopic evolution are much more consistent with a luminous supernova. Line diagnostics suggest that the host galaxy is undergoing significant star formation. We use extensive follow-up of the event along with archival Catalina Sky Survey NEO search and SDSS data to investigate the three most likely sources of such an event: (1) an extremely luminous supernova, (2) the tidal disruption of a star by the massive nuclear black hole, and (3) variability of the central active galactic nucleus (AGN). We find that CSS100217 was likely an extremely luminous Type IIn supernova and occurred within the range of the narrow-line region of an AGN. We discuss how similar events may have been missed in past supernova surveys because of confusion with AGN activity.