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The observed diversity in Type Ia supernovae (SNe Ia) – the thermonuclear explosions of carbon–oxygen white dwarf stars used as cosmological standard candles – is currently met with a variety of explosion models and progenitor scenarios. To help improve our understanding of whether and how often different models contribute to the occurrence of SNe Ia and their assorted properties, we present a comprehensive analysis of seven nearby SNe Ia. We obtained one to two epochs of optical spectra with Gemini Observatory during the nebular phase (>200 d past peak) for each of these events, all of which had time series of photometry and spectroscopy at early times (the first ∼8 weeks after explosion). We use the combination of early- and late-time observations to assess the predictions of various models for the explosion (e.g. double-detonation, off-centre detonation, stellar collisions), progenitor star (e.g. ejecta mass, metallicity), and binary companion (e.g. another white dwarf or a non-degenerate star). Overall, we find general consistency in our observations with spherically symmetric models for SN Ia explosions, and with scenarios in which the binary companion is another degenerate star. We also present an in-depth analysis of SN 2017fzw, a member of the subgroup of SNe Ia which appear to be transitional between the subluminous '91bg-like' events and normal SNe Ia, and for which nebular-phase spectra are rare. [ABSTRACT FROM AUTHOR]

Multipulsed GRB 190530A, detected by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope onboard Fermi , is the sixth most fluent GBM burst detected so far. This paper presents the timing, spectral, and polarimetric analysis of the prompt emission observed using AstroSat and Fermi to provide insight into the prompt emission radiation mechanisms. The time-integrated spectrum shows conclusive proof of two breaks due to peak energy and a second lower energy break. Time-integrated (55.43 ± 21.30 per cent) as well as time-resolved polarization measurements, made by the Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat , show a hint of high degree of polarization. The presence of a hint of high degree of polarization and the values of low energy spectral index (αpt) do not run over the synchrotron limit for the first two pulses, supporting the synchrotron origin in an ordered magnetic field. However, during the third pulse, αpt exceeds the synchrotron line of death in few bins, and a thermal signature along with the synchrotron component in the time-resolved spectra is observed. Furthermore, we also report the earliest optical observations constraining afterglow polarization using the MASTER (P < 1.3  per cent) and the redshift measurement (z  = 0.9386) obtained with the 10.4 m GTC (Gran Telescopio Canarias) telescopes. The broad-band afterglow can be described with a forward shock model for an ISM (interstellar medium)-like medium with a wide jet opening angle. We determine a circumburst density of n 0 ∼ 7.41, kinetic energy E K ∼ 7.24 × 1054 erg, and radiated gamma-ray energy E γ,iso ∼ 6.05 × 1054 erg. [ABSTRACT FROM AUTHOR]

Optical, near-infrared (NIR) photometric and spectroscopic studies, along with the optical imaging polarimetric results for SN 2012au, are presented in this article to constrain the nature of the progenitor and other properties. Well-calibrated multiband optical photometric data (from –0.2 to +413 d since B -band maximum) were used to compute the bolometric light curve and to perform semi-analytical light-curve modelling using the minim code. A spin-down millisecond magnetar-powered model explains the observed photometric evolution of SN 2012au reasonably. Early-time imaging polarimetric follow-up observations (–2 to +31 d) and comparison with other similar cases indicate signatures of asphericity in the ejecta. Good spectral coverage of SN 2012au (from –5 to +391 d) allows us to trace the evolution of layers of SN ejecta in detail. SN 2012au exhibits higher line velocities in comparison with other SNe Ib. Late nebular phase spectra of SN 2012au indicate a Wolf–Rayet star as the possible progenitor for SN 2012au, with oxygen, He-core, and main-sequence masses of ∼1.62 ± 0.15 M⊙, ∼4–8 M⊙, and ∼17–25 M⊙, respectively. There is a clear absence of a first overtone of carbon monoxide (CO) features up to +319 d in the K -band region of the NIR spectra. Overall analysis suggests that SN 2012au is one of the most luminous slow-decaying Type Ib SNe, having comparatively higher ejecta mass (∼ 4.7–8.3 M⊙) and kinetic energy (∼ [4.8–5.4] × 1051 erg). Detailed modelling using mesa and the results obtained through stella and snec explosions also strongly support spin-down of a magnetar with mass of around 20 M⊙ and metallicity Z  = 0.04 as a possible powering source of SN 2012au. [ABSTRACT FROM AUTHOR]

We present direct spectroscopic modelling of 11 high signal-to-noise ratio observed spectra of the Type Ia supernova (SN) 2010kg, taken between-10 and+5 d with respect to B-maximum. The synthetic spectra, calculated with the SYN++ code, span the range between 4100 and 8500 Å. Our results are in good agreement with previous findings for other Type Ia SNe. Most of the spectral features are formed at or close to the photosphere, but some ions, like Fe II and MgII, also form features at ~2000-5000 km s-1 above the photosphere. The well-known high-velocity features of the Ca II IR-triplet as well as Si II λ6355 are also detected. The single absorption feature at ~4400 Å, which usually has been identified as due to Si III, is poorly fit with Si III in SN 2010kg. We find that the fit can be improved by assuming that this feature is due to either C III or OII, located in the outermost part of the ejecta, ~4000-5000 km s-1 above the photosphere. Since the presence of CIII is unlikely, because of the lack of the necessary excitation/ionization conditions in the outer ejecta, we identify this feature as due to O II. The simultaneous presence of O I and O II is in good agreement with the optical depth calculations and the temperature distribution in the ejecta of SN 2010kg. This could be the first identification of singly ionized oxygen in a Type Ia SN atmosphere. [ABSTRACT FROM AUTHOR]

Superluminous supernovae (SLSNe) are very bright explosions that were only discovered recently and that show a preference for occurring in faint dwarf galaxies. Understanding why stellar evolution yields different types of stellar explosions in these environments is fundamental in order to both uncover the elusive progenitors of SLSNe and to study star formation in dwarf galaxies. In this paper, we present the first results of our project to study SUperluminous Supernova Host galaxIES, focusing on the sample for which we have obtained spectroscopy. We show that SLSNe-I and SLSNe-R (hydrogen-poor) often (~50 per cent in our sample) occur in a class of galaxies that is known as Extreme Emission Line Galaxies (EELGs). The probability of this happening by chance is negligible and we therefore conclude that the extreme environmental conditions and the SLSN phenomenon are related. In contrast, SLSNe-II (hydrogen-rich) occur in more massive, more metal-rich galaxies with softer radiation fields. Therefore, if SLSNe-II constitute a uniform class, their progenitor systems are likely different from those of H-poor SLSNe. Gamma-ray bursts (GRBs) are, on average, not found in as extreme environments as H-poor SLSNe. We propose that H-poor SLSNe result from the very first stars exploding in a starburst, even earlier than GRBs. This might indicate a bottom-light initialmass function in these systems. SLSNe present a novelmethod of selecting candidate EELGs independent of their luminosity. [ABSTRACT FROM AUTHOR]

We present the low-resolution spectroscopic and UBVRI broad-band photometric investigations of the Type IIb supernova (SN) 2011fu, discovered in UGC 01626. The photometric follow-up of this event was initiated a few days after the explosion and covers a period of about 175 d. The early-phase light curve shows a rise, followed by steep decay in all bands, and shares properties very similar to that seen for SN 1993J, with a possible detection of the adiabatic cooling phase. Modelling of the quasi-bolometric light curve suggests that the progenitor had an extended (∼1 × 1013 cm), low-mass (∼0.1 M⊙) H-rich envelope on top of a dense, compact (∼2 × 1011 cm), more massive (∼1.1 M⊙) He-rich core. The nickel mass synthesized during the explosion was found to be ∼0.21 M⊙, slightly larger than that seen for other Type IIb SNe. The spectral modelling performed with synow suggests that the early-phase line velocities for H and Fe ii features were ∼16 000 and ∼14 000 km s−1, respectively. Then, the velocities declined up to day +40 and became nearly constant at later epochs. [ABSTRACT FROM AUTHOR]