We study optical light curve(LC) relations of type Ia supernovae(SNe Ia) for their use in cosmology using high-quality photometry published by the Carnegie-Supernovae-Project(CSP-I). We revisit the classical luminosity-decline-rate (∆m 15 ) relation and the Lira-relation, as well as investigate the time evolution of the (B −V ) color and B(B −V ), which serves as the basis of the color-stretch relation and Color-MAGnitude-Intercept-Calibrations(CMAGIC). Our analysis is based on explosion and radiation transport simulations for spherically-symmetric delayed-detonation models(DDT) producing normalbright and subluminous SNe Ia. Empirical LC-relations can be understood as having the same physical underpinnings: i.e. the opacities, ionization balances in the photosphere, and radioactive energy deposition changing with time from below to above the photosphere. Some 3−4 weeks past maximum, the photosphere recedes to 56 Ni-rich layers of similar density structure, leading to a similar color evolution. An important secondary parameter is the central density ρ c of the WD because at higher densities more electron capture elements are produced at the expense of 56Ni production. This results in a ∆m 15 spread of 0.1 mag for normal-bright and 0.7 mag in sub-luminous SNe Ia and ≈ 0.2 mag in the Lira-relation. We show why color-magnitude diagrams emphasize the transition between physical regimes, and enables the construction of templates which depend mostly on ∆m 15 with little dispersion in both the CSP-I sample and our DDT-models. This allows to separate intrinsic SN Ia variations from the interstellar reddening characterized by E(B −V ) and R B . Invoking different scenarios causes a wide spread in empirical relations which may suggest one dominant scenario.

Accreting supermassive black holes (SMBHs) can exhibit variable emission across the electromagnetic spectrum and over a broad range of time-scales. The variability of active galactic nuclei (AGN) in the ultra-violet (UV) and optical is usually at the few tens of percent level over time-scales of hours to weeks 1 . Recently, rare, more dramatic changes to the emission from accreting SMBHs have been observed, including tidal disruption events (TDEs) 2-5 , "changing look" AGN 6-9 , and other extreme variability objects 10,11 . The physics behind the "re-ignition", enhancement, and "shutdown" of accretion onto SMBHs is not entirely understood. Here we present a rapid increase in ultraviolet-optical emission in the centre of a nearby galaxy marking the onset of sudden increased accretion onto a SMBH. The optical spectrum of this flare, dubbed AT 2017bgt, exhibits a mix of emission features. Some are typical of luminous, unobscured AGN, but others are likely driven by Bowen fluorescence -robustly linked here, for the first time, with highvelocity gas in the vicinity of the accreting SMBH. The spectral features and increased UV flux show little evolution over a period of at least 14 months. This disfavours the tidal disruption of a star as their origin, and instead suggests a longer-term event of intensified accretion. Together with two other recently reported events with similar properties, we define a new class of SMBH-related flares. This has important implications for the classification of different types of enhanced accretion onto SMBHs.AT 2017bgt was discovered by the All Sky Automated Survey for Supernovae (ASAS-SN 12 ) as ASASSN-17cv on 2017 February 21 in the early-type galaxy 2MASX J16110570+0234002, at z=0.064 (ref. 13; see Methods §). The long-term ASAS-SN optical data show that the total emission from the galaxy brightened by a factor of ∼50% over a period of about two months, with half of the rise occurring within three weeks (see Supplementary Fig. 1). Follow-up Swift observations show that the UV emission increased by a factor of ∼75 compared to GALEX data from 2004, reaching a luminosity of νLν (NUV) 8.9×10 44 erg s −1 , and that the X-ray emission increased by a factor of ∼2−3 compared to ROSAT data from 1990 August (see 'Detection and photometric monitoring' and 'Archival multi-wavelength data' in Methods for details on all new and archival data). The archival X-ray luminosity, of L(2−10 keV) 7×10 42 erg s −1 , and the archival UV to X-ray luminosity ratio are consistent with what is commonly observed in AGN (that is, a UV-to-X-ray spectral slope of αox≈ − 1.2; see Methods §). Archival detections in the radio (from 1998; obtained by the Very Large Array) and in the mid-infrared (from 2010; obtained with the Wide-field Infrared Survey Explorer) can be accounted for by star formation in the host galaxy. Thus, AT 2017bgt experienced a dramatic increase in its UV emission, accompanied by a smaller increase in optical and X-ray emission, sometime between 2004 and 2017.The X-ray spectral energy distribution...

We present late-time (200 − 400 day) near-infrared spectral evolution for the Type Ia supernova SN 2005df. The spectra show numerous strong emission features of [Co II], [Co III] and [Fe II] throughout the 0.8 − 1.8 µm region. As the spectrum ages, the cobalt features fade as would be expected from the decay of 56 Co to 56 Fe. We show that the strong and isolated [Fe II] emission line at 1.644 µm provides a unique tool to analyze near-infrared spectra of Type Ia supernovae. Normalization of spectra to this line allows separation of features produced by stable versus unstable isotopes of iron group elements. We develop a new method of determining the initial central density, ρ c , and the magnetic field, B, of the white dwarf using the width of the 1.644 µm line. The line width is sensitive because of electron capture in the early stages of burning, which increases as a function of density. The sensitivity of the line width to B increase with time and the effects of the magnetic field shift towards later times with decreasing ρ c . The initial central density for SN 2005df is measured as ρ c = 0.9(±0.2) (in 10 9 g cm −3 ), which corresponds to a white dwarf close to the Chandrasekhar mass (M Ch ) with M WD = 1.313(±0.034) M ⊙ and systematic error less than 0.04 M ⊙ . Within M Ch explosions, however, the central density found for SN 2005df is very low for a H-accretor, possibly suggesting a helium star companion or a tidally-disrupted white dwarf companion. As an alternative, we suggest mixing of the central region. We find some support for high initial magnetic fields of strength 10 6 G for SN 2005df, however, 0 G cannot be ruled out because of noise in the spectra combined with low ρ c .

We present nebular phase optical and near-infrared spectroscopy of the Type Ia supernova (SN) 2017cbv. The early light curves of SN 2017cbv showed a prominent blue bump in the U , B and g bands lasting for ∼5 d. One interpretation of the early light curve was that the excess blue light was due to shocking of the SN ejecta against a nondegenerate companion star -a signature of the single degenerate scenario. If this is the correct interpretation, the interaction between the SN ejecta and the companion star could result in significant Hα (or helium) emission at late times, possibly along with other species, depending on the companion star and its orbital separation. A search for Hα emission in our +302 d spectrum yields a nondetection, with a L Hα <8.0×10 35 erg/s (given an assumed distance of D=12.3 Mpc), which we have verified by implanting simulated Hα emission into our data. We make a quantitative comparison to models of swept-up material stripped from a nondegenerate companion star, and limit the mass of hydrogen that might remain undetected to M H < 1 × 10 −4 M ⊙ . A similar analysis of helium star related lines yields a M He < 5 × 10 −4 M ⊙ . Taken at face value, these results argue against a nondegenerate H or He-rich companion in Roche lobe overflow as the progenitor of SN 2017cbv. Alternatively, there could be weaknesses in the envelope-stripping and radiative transfer models necessary to interpret the strong H and He flux limits.