We investigate the light-curve properties of a sample of 26 spectroscopically confirmed hydrogen-poor superluminous supernovae (SLSNe-I) in the Palomar Transient Factory survey. These events are ...brighter than SNe Ib/c and SNe Ic-BL, on average, by about 4 and 2 mag, respectively. The peak absolute magnitudes of SLSNe-I in rest-frame g band span −22 Mg −20 mag, and these peaks are not powered by radioactive 56Ni, unless strong asymmetries are at play. The rise timescales are longer for SLSNe than for normal SNe Ib/c, by roughly 10 days, for events with similar decay times. Thus, SLSNe-I can be considered as a separate population based on photometric properties. After peak, SLSNe-I decay with a wide range of slopes, with no obvious gap between rapidly declining and slowly declining events. The latter events show more irregularities (bumps) in the light curves at all times. At late times, the SLSN-I light curves slow down and cluster around the 56Co radioactive decay rate. Powering the late-time light curves with radioactive decay would require between 1 and 10 M of Ni masses. Alternatively, a simple magnetar model can reasonably fit the majority of SLSNe-I light curves, with four exceptions, and can mimic the radioactive decay of 56Co, up to ∼400 days from explosion. The resulting spin values do not correlate with the host-galaxy metallicities. Finally, the analysis of our sample cannot strengthen the case for using SLSNe-I for cosmology.
Supernovae are stellar explosions driven by gravitational or thermonuclear energy that is observed as electromagnetic radiation emitted over weeks or more. In all known supernovae, this radiation ...comes from internal energy deposited in the outflowing ejecta by one or more of the following processes: radioactive decay of freshly synthesized elements (typically (56)Ni), the explosion shock in the envelope of a supergiant star, and interaction between the debris and slowly moving, hydrogen-rich circumstellar material. Here we report observations of a class of luminous supernovae whose properties cannot be explained by any of these processes. The class includes four new supernovae that we have discovered and two previously unexplained events (SN 2005ap and SCP 06F6) that we can now identify as members of the same class. These supernovae are all about ten times brighter than most type Ia supernova, do not show any trace of hydrogen, emit significant ultraviolet flux for extended periods of time and have late-time decay rates that are inconsistent with radioactivity. Our data require that the observed radiation be emitted by hydrogen-free material distributed over a large radius (∼10(15) centimetres) and expanding at high speeds (>10(4) kilometres per second). These long-lived, ultraviolet-luminous events can be observed out to redshifts z > 4.
We study 34 Type Ic supernovae that have broad spectral features (SNe Ic-BL). This is the only SN type found in association with long-duration gamma-ray bursts (GRBs). We obtained our photometric ...data with the Palomar Transient Factory (PTF) and its continuation, the intermediate PTF (iPTF). This is the first large, homogeneous sample of SNe Ic-BL from an untargeted survey. Furthermore, given the high observational cadence of iPTF, most of these SNe Ic-BL were discovered soon after explosion. We present K-corrected Bgriz light curves of these SNe, obtained through photometry on template-subtracted images. We analyzed the shape of the r-band light curves, finding a correlation between the decline parameter Δm15 and the rise parameter Δm−10. We studied the SN colors and, based on g − r, we estimated the host-galaxy extinction for each event. Peak r-band absolute magnitudes have an average of −18.6 ± 0.5 mag. We fit each r-band light curve with that of SN 1998bw (scaled and stretched) to derive the explosion epochs. We computed the bolometric light curves using bolometric corrections, r-band data, and g − r colors. Expansion velocities from Fe II were obtained by fitting spectral templates of SNe Ic. Bolometric light curves and velocities at peak were fitted using the semianalytic Arnett model to estimate ejecta mass Mej, explosion energy EK and 56Ni mass M(56Ni) for each SN. We find average values of Mej = 4 ± 3 M⊙, EK = (7 ± 6)×1051 erg, and M(56Ni)=0.31 ± 0.16 M⊙. The parameter distributions were compared to those presented in the literature and are overall in agreement with them. We also estimated the degree of 56Ni mixing using scaling relations derived from hydrodynamical models and we find that all the SNe are strongly mixed. The derived explosion parameters imply that at least 21% of the progenitors of SNe Ic-BL are compatible with massive (> 28 M⊙), possibly single stars, whereas at least 64% might come from less massive stars in close binary systems.
Precision cosmology with Type Ia supernovae (SNe Ia) makes use of the fact that SN Ia luminosities depend on their light-curve shapes and colours. Using Supernova Legacy Survey (SNLS) and other data, ...we show that there is an additional dependence on the global characteristics of their host galaxies: events of the same light-curve shape and colour are, on average, 0.08 mag (≃4.0σ) brighter in massive host galaxies (presumably metal-rich) and galaxies with low specific star formation rates (sSFR). These trends do not depend on any assumed cosmological model, and are independent of the SN light-curve width: both fast and slow-declining events show the same trends. SNe Ia in galaxies with a low sSFR also have a smaller slope (‘β’) between their luminosities and colours with ∼2.7σ significance, and a smaller scatter on SN Ia Hubble diagrams (at 95 per cent confidence), though the significance of these effects is dependent on the reddest SNe. SN Ia colours are similar between low-mass and high-mass hosts, leading us to interpret their luminosity differences as an intrinsic property of the SNe and not of some external factor such as dust. If the host stellar mass is interpreted as a metallicity indicator using galaxy mass–metallicity relations, the luminosity trends are in qualitative agreement with theoretical predictions. We show that the average stellar mass, and therefore the average metallicity, of our SN Ia host galaxies decreases with redshift. The SN Ia luminosity differences consequently introduce a systematic error in cosmological analyses, comparable to the current statistical uncertainties on parameters such as w, the equation of state of dark energy. We show that the use of two SN Ia absolute magnitudes, one for events in high-mass (metal-rich) galaxies and the other for events in low-mass (metal-poor) galaxies, adequately corrects for the differences. Cosmological fits incorporating these terms give a significant reduction in χ2 (3.8σ–4.5σ); linear corrections based on host parameters do not perform as well. We conclude that all future SN Ia cosmological analyses should use a correction of this (or similar) form to control demographic shifts in the underlying galaxy population.
This paper exploits the gravitational magnification of Type Ia supernovae (SNe Ia) to measure properties of dark matter haloes. Gravitationally magnified and de-magnified SNe Ia should be brighter ...and fainter than average, respectively. The magnification of individual SNe Ia can be computed using observed properties of foreground galaxies and dark matter halo models. We model the dark matter haloes of the galaxies as truncated singular isothermal spheres with velocity dispersion and truncation radius obeying luminosity-dependent scaling laws. A homogeneously selected sample of 175 SNe Ia from the first 3 yr of the Supernova Legacy Survey (SNLS) in the redshift range 0.2 ≲z≲ 1 is used to constrain models of the dark matter haloes associated with foreground galaxies. The best-fitting velocity dispersion scaling law agrees well with galaxy–galaxy lensing measurements. We further find that the normalization of the velocity dispersion of passive and star-forming galaxies are consistent with empirical Faber–Jackson and Tully–Fisher relations, respectively. If we make no assumption on the normalization of these relations, we find that the data prefer gravitational lensing at the 92 per cent confidence level. Using recent models of dust extinction, we deduce that the impact of this effect on our results is very small. We also investigate the brightness scatter of SNe Ia due to gravitational lensing, which has implications for SN Ia cosmology. The gravitational lensing scatter is approximately proportional to the SN Ia redshift. We find the constant of proportionality to be B≃ 0.055+0.039−0.041 mag (B≲ 0.12 mag at the 95 per cent confidence level). If this model is correct, the contribution from lensing to the intrinsic brightness scatter of SNe Ia is small for the SNLS sample. According to the best-fitting dark matter model, gravitational lensing should, however, contribute significantly to the brightness scatter at z≳ 1.6.
Abstract
The late-time spectra of Type Ia supernovae (SNe Ia) are powerful probes of the underlying physics of their explosions. We investigate the late-time optical and near-infrared spectra of ...seven SNe Ia obtained at the VLT with XShooter at >200 d after explosion. At these epochs, the inner Fe-rich ejecta can be studied. We use a line-fitting analysis to determine the relative line fluxes, velocity shifts, and line widths of prominent features contributing to the spectra (Fe ii, Ni ii, and Co iii). By focusing on Fe ii and Ni ii emission lines in the ∼7000–7500 Å region of the spectrum, we find that the ratio of stable Ni ii to mainly radioactively produced Fe ii for most SNe Ia in the sample is consistent with Chandrasekhar-mass delayed-detonation explosion models, as well as sub-Chandrasekhar mass explosions that have metallicity values above solar. The mean measured Ni/Fe abundance of our sample is consistent with the solar value. The more highly ionized Co iii emission lines are found to be more centrally located in the ejecta and have broader lines than the Fe ii and Ni ii features. Our analysis also strengthens previous results that SNe Ia with higher
Si ii velocities at maximum light preferentially display blueshifted Fe ii 7155 Å lines at late times. Our combined results lead us to speculate that the majority of normal SN Ia explosions produce ejecta distributions that deviate significantly from spherical symmetry.
There is a consensus that type Ia supernovae (SNe Ia) arise from the thermonuclear explosion of white dwarf stars that accrete matter from a binary companion. However, direct observation of SN Ia ...progenitors is lacking, and the precise nature of the binary companion remains uncertain. A temporal series of high-resolution optical spectra of the SN Ia PTF 11kx reveals a complex circumstellar environment that provides an unprecedentedly detailed view of the progenitor system. Multiple shells of circumstellar material are detected, and the SN ejecta are seen to interact with circumstellar material starting 59 days after the explosion. These features are best described by a symbiotic nova progenitor, similar to RS Ophiuchi.
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 days. One interpretation of the early light curve is that the excess blue light is 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 H (or helium) emission at late times, possibly along with other species, depending on the companion star and its orbital separation. A search for H emission in our +302 d spectrum yields a nondetection, with a LH < 8.0 × 1035 erg s−1 (given an assumed distance of D = 12.3 Mpc), which we verified by implanting simulated H 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 MH < 1 × 10−4 M . A similar analysis of helium star related lines yields a MHe < 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.
We show that Type Ia supemovae (SNe Ia) are formed within both very young and old stellar populations, with observed rates that depend on the stellar mass and mean star formation rates (SFRs) of ...their host galaxies. Models in which the SN Ia rate depends solely on host galaxy stellar mass are ruled out with >99% confidence. Our analysis is based on 100 spectroscopically confirmed SNe Ia, plus 24 photometrically classified events, all from the Supernova Legacy Survey (SNLS) and distributed over 0.2 < z < 0.75. We estimate stellar masses and SFRs for the SN Ia host galaxies by fitting their broadband spectral energy distributions with the galaxy spectral synthesis code PEGASE.2. We show that the SN Ia rate per unit mass is proportional to the specific SFR of the parent galaxies--more vigorously star-forming galaxies host more SNe Ia per unit stellar mass, broadly equivalent to the trend of increasing SN Ia rate in later type galaxies seen in the local universe. Following earlier suggestions for a simple "two-component" model approximating the SN Ia rate, we find bivariate linear dependencies of the SN Ia rate on both the stellar masses and the mean SFRs of the host systems. We find that the SN Ia rate can be well represented as the sum of 5.3 c 1.1 x 10 super(-14) SNe yr super(-1) M super(-) sub( ) super(1) and 3.9 c 0.7 x 10 super(-4) SNe yr super(-1) (M sub( )yr super(-1)) super(-1) of star formation. We also demonstrate a dependence of distant SN Ia light-curve shapes on star formation in the host galaxy, similar to trends observed locally. Passive galaxies, with no star formation, preferentially host faster declining/dimmer SNe Ia, while brighter events are found in systems with ongoing star formation.
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