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Register a new userAn Unusually Fast-Evolving Supernova
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Analyses of supernovae (SNe) have revealed two main types of progenitors: exploding white dwarfs and collapsing massive stars. We present SN2002bj, which stands out as different from any SN reported to date. Its light curve rises and declines very rapidly, yet reaches a peak intrinsic brightness greater than -18 mag. A spectrum obtained 7 days after discovery shows the presence of helium and intermediate-mass elements, yet no clear hydrogen or iron-peak elements. The spectrum only barely resembles that of a Type Ia supernova, with added carbon and helium. Its properties suggest that SN2002bj may be representative of a class of progenitors that previously has been only hypothesized: a helium detonation on a white dwarf, ejecting a small envelope of material. New surveys should find many such objects, despite their scarcity.
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We present optical and near-infrared photometry and spectroscopy of SN 2009ib, a Type II-P supernova in NGC 1559. This object has moderate brightness, similar to those of the intermediate-luminosity SNe 2008in and 2009N. Its plateau phase is unusually long, lasting for about 130 days after explosion. The spectra are similar to those of the subluminous SN 2002gd, with moderate expansion velocities. We estimate the $^{56}$Ni mass produced as $0.046 pm 0.015,{rm M}_{sun}$. We determine the distance to SN 2009ib using both the expanding photosphere method (EPM) and the standard candle method. We also apply EPM to SN 1986L, a type II-P SN that exploded in the same galaxy. Combining the results of different methods, we conclude the distance to NGC 1559 as $D=19.8 pm 3.0$ Mpc. We examine archival, pre-explosion images of the field taken with the Hubble Space Telescope, and find a faint source at the position of the SN, which has a yellow colour ($(V-I)_0 = 0.85$ mag). Assuming it is a single star, we estimate its initial mass as $M_{rm ZAMS}=20,{rm M}_{sun}$. We also examine the possibility, that instead of the yellow source the progenitor of SN 2009ib is a red supergiant star too faint to be detected. In this case we estimate the upper limit for the initial zero-age main sequence mass of the progenitor to be $sim 14-17,{rm M}_{sun}$. In addition, we infer the physical properties of the progenitor at the explosion via hydrodynamical modelling of the observables, and estimate the total energy as $sim 0.55 times 10^{51}$~erg, the pre-explosion radius as $sim 400,{rm R}_{sun}$, and the ejected envelope mass as $sim 15,{rm M}_{sun}$, which implies that the mass of the progenitor before explosion was $sim 16.5-17,{rm M}_{sun}$.
We present the detailed optical evolution of a type Ib SN 2015dj in NGC 7371, using data spanning up to $sim$ 170 days after discovery. SN 2015dj shares similarity in light curve shape with SN 2007gr and peaks at M$_{V}$ = $-17.37pm$0.02 mag. Analytical modelling of the quasi bolometric light curve yields 0.06$pm$0.01 M$_{odot}$ of $^{56}$Ni, ejecta mass $M_{rm ej} = 1.4^{+1.3}_{-0.5}$ msol, and kinetic energy $E_{rm k} = 0.7^{+0.6}_{-0.3} times 10^{51}$ erg. The spectral features show a fast evolution and resemble those of spherically symmetric ejecta. The analysis of nebular phase spectral lines indicate a progenitor mass between 13-20 M$_{odot}$ suggesting a binary scenario.
I have discovered a prominent light echo around the low-luminosity Type II-Plateau Supernova (SN) 2008bk in NGC 7793, seen in archival images obtained with the Wide Field Channel of the Advanced Camera for Surveys on-board the Hubble Space Telescope (HST). The echo is a partial ring, brighter to the north and east than to the south and west. The analysis of the echo I present suggests that it is due to the SN light pulse scattered by a sheet, or sheets, of dust located ~15 pc from the SN. The composition of the dust is assumed to be of standard Galactic diffuse interstellar grains. The visual extinction of the dust responsible for the echo is A_V ~ 0.05 mag, in addition to the extinction due to the Galactic foreground toward the host galaxy. That the SN experienced much less overall extinction implies that it is seen through a less dense portion of the interstellar medium in its environment. The late-time HST photometry of SN 2008bk also clearly demonstrates that the progenitor star has vanished.
We present observations of the unusually luminous Type II supernova (SN) 2016gsd. With a peak absolute magnitude of V = $-$19.95 $pm$ 0.08, this object is one of the brightest Type II SNe, and lies in the gap of magnitudes between the majority of Type II SNe and the superluminous SNe. Its light curve shows little evidence of the expected drop from the optically thick phase to the radioactively powered tail. The velocities derived from the absorption in H$alpha$ are also unusually high with the blue edge tracing the fastest moving gas initially at 20000 km s$^{-1}$, and then declining approximately linearly to 15000 km s$^{-1}$ over $sim$100 d. The dwarf host galaxy of the SN indicates a low-metallicity progenitor which may also contribute to the weakness of the metal lines in its spectra. We examine SN 2016gsd with reference to similarly luminous, linear Type II SNe such as SNe 1979C and 1998S, and discuss the interpretation of its observational characteristics. We compare the observations with a model produced by the JEKYLL code and find that a massive star with a depleted and inflated hydrogen envelope struggles to reproduce the high luminosity and extreme linearity of SN 2016gsd. Instead, we suggest that the influence of interaction between the SN ejecta and circumstellar material can explain the majority of the observed properties of the SN. The high velocities and strong H$alpha$ absorption present throughout the evolution of the SN may imply a circumstellar medium configured in an asymmetric geometry.
We investigate the observational properties of a hydrogen-deficient superluminous supernova (SLSN) SN 2020ank (at z = 0.2485), with the help of early phase observations carried out between $-$21 and +52 d since $g$-band maximum. Photometrically, SN 2020ank is one of the brightest SLSN ($M_{g,peak}$ $sim$ $-$21.84 $pm$ 0.10 mag), having fast pre-peak rising and post-peak decaying rates. The bolometric light curve of SN 2020ank exhibits a higher peak luminosity ($L_{max}$) of $sim$(3.9 $pm$ 0.7) $times$ 10$^{44}$ erg s$^{-1}$ and appears to be symmetric around the peak with $L^{rise}_{max}$/e $approx$ $L^{fall}_{max}$/e $approx$ 15 d. The semi-analytical light-curve modelling using the MINIM code suggests a spin down millisecond magnetar with $P_i$ $sim$2.2 $pm$ 0.5 ms and $B$ $sim$(2.9 $pm$ 0.1) $times$ $10^{14}$ G as a possible powering source for SN 2020ank. The possible magnetar origin and excess ultraviolet flux at early epochs indicate a central-engine based powering source for SN 2020ank. Near-peak spectra of SN 2020ank are enriched with the W-shaped O II features but with the weaker signatures of C II and Fe III. Using the estimated rise time of $sim$27.9 d and the photospheric velocity of $sim$12050 km s$^{-1}$, we constrain the ejecta mass to $sim$7.2 $M_{odot}$ and the kinetic energy of $sim$6.3 $times$ 10$^{51}$ erg. The near-peak spectrum of SN 2020ank exhibits a close spectral resemblance with that of fast-evolving SN 2010gx. The absorption features of SN 2020ank are blueshifted compared to Gaia16apd, suggesting a higher expansion velocity. The spectral similarity with SN 2010gx and comparatively faster spectral evolution than PTF12dam (a slow-evolving SLSN) indicate the fast-evolving behavior of SN 2020ank.
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