arXiv subjects

M. M. Phillips

Publications and source records attributed to M. M. Phillips.

At least 73 records · Page 4Linked to original sources

arXiv2019

Comparison of the optical light curves of hydrogen-rich and hydrogen-poor type II supernovae

Type II supernovae (SNe II) show strong hydrogen features in their spectra throughout their whole evolution while type IIb supernovae (SNe IIb) spectra evolve from dominant hydrogen lines at early times to increasingly strong helium features later on. However, it is currently unclear whether the progenitors of these supernova (SN) types form a continuum in pre-SN hydrogen mass or whether they are physically distinct. SN light-curve morphology directly relates to progenitor and explosion properties such as the amount of hydrogen in the envelope, the pre-SN radius, the explosion energy and the synthesized mass of radioactive material. In this work we study the morphology of the optical-wavelength light curves of hydrogen-rich SNe II and hydrogen-poor SNe IIb to test whether an observational continuum exists between the two. Using a sample of 95 SNe (73 SNe II and 22 SNe IIb), we define a range of key observational parameters and present a comparative analysis between both types. We find a lack of events that bridge the observed properties of SNe II and SNe IIb. Light curve parameters such as rise times and post-maximum decline rates and curvatures clearly separate both SN types and we therefore conclude that there is no continuum, with the two SN types forming two observationally distinct families. In the V-band a rise time of 17 days (SNe II lower, SNe IIb higher), and a magnitude difference between 30 and 40 days post explosion of 0.4 mag (SNe II lower, SNe IIb higher) serve as approximate thresholds to differentiate both types.

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arXiv2019

ASASSN-15pz: Revealing Significant Photometric Diversity among 2009dc-like, Peculiar SNe Ia

We report comprehensive multi-wavelength observations of a peculiar Type Ia-like supernova ("SN Ia-pec") ASASSN-15pz. ASASSN-15pz is a spectroscopic "twin" of SN 2009dc, a so-called "Super-Chandrasekhar-mass" SN, throughout its evolution, but it has a peak luminosity M_B,peak = -19.69 +/- 0.12 mag that is \approx 0.6 mag dimmer and comparable to the SN 1991T sub-class of SNe Ia at the luminous end of the normal width-luminosity relation. The synthesized Ni56 mass of M_Ni56 = 1.13 +/- 0.14 M_sun is also substantially less than that found for several 2009dc-like SNe. Previous well-studied 2009dc-like SNe have generally suffered from large and uncertain amounts of host-galaxy extinction, which is negligible for ASASSN-15pz. Based on the color of ASASSN-15pz, we estimate a host extinction for SN 2009dc of E(B-V)_host=0.12 mag and confirm its high luminosity (M_B, peak[2009dc] \approx -20.3 mag). The 2009dc-like SN population, which represents ~1% of SNe Ia, exhibits a range of peak luminosities, and do not fit onto the tight width-luminosity relation. Their optical light curves also show significant diversity of late-time (>~ 50 days) decline rates. The nebular-phase spectra provide powerful diagnostics to identify the 2009dc-like events as a distinct class of SNe Ia. We suggest referring to these sources using the phenomenology-based "2009dc-like SN Ia-pec" instead of "Super-Chandrasekhar SN Ia," which is based on an uncertain theoretical interpretation.

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arXiv2019

A Physical Basis for the H-band Blue-edge Velocity and Light-Curve Shape Correlation in Context of Type Ia Supernova Explosion Physics

Our recent work demonstrates a correlation between the high-velocity blue edge, $v_{edge}$, of the iron-peak Fe/Co/Ni $H$-band emission feature and the optical light curve shape of normal, transitional and sub-luminous type Ia Supernovae (SNe Ia). We explain this correlation in terms of SN Ia physics. $v_{edge}$ corresponds to the sharp transition between the complete and incomplete silicon burning regions in the ejecta. It measures the point in velocity space where the outer $^{56}$Ni mass fraction, $X_{\rm{Ni}}$, falls to the order of 0.03-0.10. For a given $^{56}$Ni mass, $M(^{56}Ni)$, $v_{edge}$ is sensitive to the specific kinetic energy $E_{\rm kin}$($M(^{56}Ni)/M_{WD}$) of the corresponding region. Combining $v_{edge}$ with light curve parameters (i.e., s$_{BV}$, $Δm_{15,s}$ in $B$ and $V$) allows us to distinguish between explosion scenarios. The correlation between $v_{edge}$ and light-curve shape is consistent with explosion models near the Chandrasekhar limit. However, the available sub-$M_{Ch}$ WD explosion model based on SN 1999by exhibits velocities which are too large to explain the observations. Finally, the sub-luminous SN 2015bo exhibits signatures of a dynamical merger of two WDs demonstrating diversity among explosion scenarios at the faint end of the SNe Ia population.

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arXiv2019

Carnegie Supernova Project-II: Using Near-Infrared Spectroscopy to determine the location of the outer $^{56}$Ni in Type Ia Supernovae

We present the $H$-band wavelength region of thirty post-maximum light near-infrared (NIR) spectra of fourteen transitional and sub-luminous type Ia supernovae (SNe Ia), extending from $+$5d to +20d relative to the epoch of $B$-band maximum. We introduce a new observable, the blue-edge velocity, $v_{edge}$, of the prominent Fe/Co/Ni-peak $H$-band emission feature which is quantitatively measured. The $v_{edge}$ parameter is found to slowly decrease over sub-type ranging from around $-$13,000km/s for transitional SNe~Ia, down to $-$5,000km/s for the sub-luminous SNe Ia. Furthermore, inspection of the +10$\pm$3d spectra indicates that $v_{edge}$ is correlated with the color-stretch parameter, s$_{BV}$, and hence with peak luminosity. These results follow the previous findings that brighter SNe Ia tend to have $^{56}$Ni located at higher velocities as compared to sub-luminous objects. As $v_{edge}$ is a model-independent parameter, we propose it can be used in combination with traditional observational diagnostics to provide a new avenue to robustly distinguish between leading SNe Ia explosion models.

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arXiv2019

H-$α$ emission in the nebular spectrum of the Type Ia supernova ASASSN-18tb

As part of the 100IAS survey, a program aimed to obtain nebular-phase spectra for a volume-limited and homogeneous sample of Type Ia supernovae (SNe Ia), we observed ASASSN-18tb (SN 2018fhw) at 139 days past maximum light. ASASSN-18tb was a fast-declining, sub-luminous event that fits well within the observed photometric and spectroscopic distributions of the SN Ia population. We detect a prominent H$α$ emission line of $L_{{\rm H}α}=2.2\pm0.2\times10^{38}$ ergs s$^{-1}$ with FWHM $\approx1100$ km s$^{-1}$ in the nebular-phase spectrum of this SN Ia. High luminosity H$α$ emission ($L_{{\rm H}α}\gtrsim 10^{40}$ ergs~s$^{-1}$) has previously been discovered in a rare class of SNe Ia-like objects showing CSM interactions (SNe Ia-CSM). They predominantly belong to over-luminous ($M_{\rm max}<-19$ mag in optical) 1991T-like SNe Ia and are exclusively found in star-forming galaxies. By contrast, ASASSN-18tb is a sub-luminous SN Ia ($M_{B, {\rm max}}\sim -17.7$ mag) found in an early-type galaxy dominated by old stellar populations. We discuss possible origins for the observed hydrogen. Out of 75 SNe Ia for which we have so far obtained nebular spectra in 100IAS, no other SN shows a $\sim 1000 \,{\rm km s^{-1}}$ H$α$ emission line with comparable line luminosity as ASASSN-18tb, emphasizing the rarity of such emission in the nebular phase. Based on preliminary results from our survey, the rate for ASASSN-18tb-like nebular H$α$ emission could be as high as $\sim 10\%$ level among sub-luminous SNe Ia.

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arXiv2018

Type II Supernovae as Distance Indicators at Near-IR Wavelengths

Motivated by the advantages of observing at near-IR wavelengths, we investigate Type II supernovae (SNe II) as distance indicators at those wavelengths through the Photospheric Magnitude Method (PMM). For the analysis, we use $BVIJH$ photometry and optical spectroscopy of 24 SNe II during the photospheric phase. To correct photometry for extinction and redshift effects, we compute total-to-selective broadband extinction ratios and $K$-corrections up to $z=0.032$. To estimate host galaxy colour excesses, we use the colour-colour curve method with the $V\!-\!I$ versus $B\!-\!V$ as colour combination. We calibrate the PMM using four SNe II in galaxies having Tip of the Red Giant Branch distances. Among our 24 SNe II, nine are at $cz>2000$ km s$^{-1}$, which we use to construct Hubble diagrams (HDs). To further explore the PMM distance precision, we include into HDs the four SNe used for calibration and other two in galaxies with Cepheid and SN Ia distances. With a set of 15 SNe II we obtain a HD rms of 0.13 mag for the $J$-band, which compares to the rms of 0.15-0.26 mag for optical bands. This reflects the benefits of measuring PMM distances with near-IR instead of optical photometry. With the evidence we have, we can set the PMM distance precision with $J$-band below 10 per cent with a confidence level of 99 per cent.

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arXiv2018

Carnegie Supernova Project-II: The Near-infrared Spectroscopy Program

Shifting the focus of Type Ia supernova (SN Ia) cosmology to the near-infrared (NIR) is a promising way to significantly reduce the systematic errors, as the strategy minimizes our reliance on the empirical width-luminosity relation and uncertain dust laws. Observations in the NIR are also crucial for our understanding of the origins and evolution of these events, further improving their cosmological utility. Any future experiments in the rest-frame NIR will require knowledge of the SN Ia NIR spectroscopic diversity, which is currently based on a small sample of observed spectra. Along with the accompanying paper, Phillips et al. (2018), we introduce the Carnegie Supernova Project-II (CSP-II), to follow up nearby SNe Ia in both the optical and the NIR. In particular, this paper focuses on the CSP-II NIR spectroscopy program, describing the survey strategy, instrumental setups, data reduction, sample characteristics, and future analyses on the data set. In collaboration with the Harvard-Smithsonian Center for Astrophysics (CfA) Supernova Group, we obtained 661 NIR spectra of 157 SNe Ia. Within this sample, 451 NIR spectra of 90 SNe Ia have corresponding CSP-II follow-up light curves. Such a sample will allow detailed studies of the NIR spectroscopic properties of SNe Ia, providing a different perspective on the properties of the unburned material, radioactive and stable nickel produced, progenitor magnetic fields, and searches for possible signatures of companion stars.

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arXiv2018

Carnegie Supernova Project-II: Extending the Near-Infrared Hubble Diagram for Type Ia Supernovae to $z\sim0.1$

The Carnegie Supernova Project-II (CSP-II) was an NSF-funded, four-year program to obtain optical and near-infrared observations of a "Cosmology" sample of $\sim100$ Type Ia supernovae located in the smooth Hubble flow ($0.03 \lesssim z \lesssim 0.10$). Light curves were also obtained of a "Physics" sample composed of 90 nearby Type Ia supernovae at $z \leq 0.04$ selected for near-infrared spectroscopic time-series observations. The primary emphasis of the CSP-II is to use the combination of optical and near-infrared photometry to achieve a distance precision of better than 5%. In this paper, details of the supernova sample, the observational strategy, and the characteristics of the photometric data are provided. In a companion paper, the near-infrared spectroscopy component of the project is presented.

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arXiv2018

Investigating the Unusual Spectroscopic Time-Evolution in SN 2012fr

The type Ia supernova (SN) 2012fr displayed an unusual combination of its Si II λλ5972, 6355 features. This includes the ratio of their pseudo equivalent widths, placing it at the border of the Shallow Silicon (SS) and Core Normal (CN) spectral subtype in the Branch diagram, while the Si II λ6355 expansion velocities places it as a High-Velocity (HV) object in the Wang et al. spectral type that most interestingly evolves slowly, placing it in the Low Velocity Gradient (LVG) typing of Benetti et al. Only 5% of SNe Ia are HV and located in the SS+CN portion of the Branch diagram and less than 10% of SNe Ia are both HV and LVG. These features point towards SN 2012fr being quite unusual, similar in many ways to the peculiar SN 2000cx. We modeled the spectral evolution of SN 2012fr to see if we could gain some insight into its evolutionary behavior. We use the parameterized radiative transfer code SYNOW to probe the abundance stratification of SN 2012fr at pre-maximum, maximum, and post-maximum light epochs. We also use a grid of W7 models in the radiative transfer code PHOENIX to probe the effect of different density structures on the formation of the Si II λ6355 absorption feature at post-maximum epochs. We find that the unusual features observed in SN 2012fr are likely due to a shell-like density enhancement in the outer ejecta. We comment on possible reasons for atypical Ca II absorption features, and suggest that they are related to the Si II features.

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arXiv2018

A nearby superluminous supernova with a long pre-maximum 'plateau' and strong CII features

Super-luminous supernovae (SLSNe) are rare events defined as being significantly more luminous than normal terminal stellar explosions. The source of the extra powering needed to achieve such luminosities is still unclear. Discoveries in the local Universe (i.e. $z<0.1$) are scarce, but afford dense multi-wavelength observations. Additional low-redshift objects are therefore extremely valuable. We present early-time observations of the type I SLSN ASASSN-18km/SN~2018bsz. These data are used to characterise the event and compare to literature SLSNe and spectral models. Host galaxy properties are also analysed. Optical and near-IR photometry and spectroscopy were analysed. Early-time ATLAS photometry was used to constrain the rising light curve. We identified a number of spectral features in optical-wavelength spectra and tracked their time evolution. Finally, we used archival host galaxy photometry together with HII region spectra to constrain the host environment. ASASSN-18km/SN~2018bsz is found to be a type I SLSN in a galaxy at a redshift of 0.0267 (111 Mpc), making it the lowest-redshift event discovered to date. Strong CII lines are identified in the spectra. Spectral models produced by exploding a Wolf-Rayet progenitor and injecting a magnetar power source are shown to be qualitatively similar to ASASSN-18km/SN~2018bsz, contrary to most SLSNe-I that display weak/non-existent CII lines. ASASSN-18km/SN~2018bsz displays a long, slowly rising, red 'plateau' of $>$26 days, before a steeper, faster rise to maximum. The host has an absolute magnitude of --19.8 mag ($r$), a mass of M$_{*}$ = 1.5$^{+0.08}_{-0.33}$ $\times$10$^{9}$ M$_{\odot}$ , and a star formation rate of = 0.50$^{+2.22}_{-0.19}$ M$_{\odot}$ yr$^{-1}$. A nearby HII region has an oxygen abundance (O3N2) of 8.31$\pm$0.01 dex.

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arXiv2018

The Carnegie Supernova Project: Absolute Calibration and the Hubble Constant

We present an analysis of the final data release of the Carnegie Supernova Project I, focusing on the absolute calibration of the luminosity-decline-rate relation for Type Ia supernovae (SNeIa) using new intrinsic color relations with respect to the color-stretch parameter, $s_{BV}$, enabling improved dust extinction corrections. We investigate to what degree the so-called fast-declining SNeIa can be used to determine accurate extragalactic distances. We estimate the intrinsic scatter in the luminosity-decline-rate relation, and find it ranges from $\pm 0.13$ mag to $\pm 0.18$ mag with no obvious dependence on wavelength. Using the Cepheid variable star data from the SH0ES project (Riess et al., 2016), the SNIa distance scale is calibrated and the Hubble constant is estimated using our optical and near-infrared sample, and these results are compared to those determined exclusively from a near-infrared sub-sample. The systematic effect of the supernova's host galaxy mass is investigated as a function of wavelength and is found to decrease toward redder wavelengths, suggesting this effect may be due to dust properties of the host. Using estimates of the dust extinction derived from optical and NIR wavelengths, and applying these to H band, we derive a Hubble constant $H_0 = 73.2 \pm 2.3$ km/s/Mpc, whereas using a simple $B-V$ color-correction applied to B band yields $H_0 = 72.7 \pm 2.1$ km/s/Mpc. Photometry of two calibrating SNeIa from the CSP-II sample, SN2012ht and SN2015F, is presented and used to improve the calibration of the SNIa distance ladder.

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arXiv2018

The first 48: Discovery and progenitor constraints on the Type Ia supernova 2013gy

We present an early-phase $g$-band light curve and visual-wavelength spectra of the normal Type Ia supernova (SN) 2013gy. The light curve is constructed by determining the appropriate S-corrections to transform KAIT natural-system $B$- and $V$-band photometry and Carnegie Supernova Project natural-system $g$-band photometry to the Pan-STARRS1 $g$-band natural photometric system. A Markov Chain Monte Carlo calculation provides a best-fit single power-law function to the first ten epochs of photometry described by an exponent of $2.16^{+0.06}_{-0.06}$ and a time of first light of MJD 56629.4$^{+0.1}_{-0.1}$, which is $1.93^{+0.12}_{-0.13}$ days (i.e., $<48$~hr) before the discovery date (2013 December 4.84 UT) and $-19.10^{+0.12}_{-0.13}$ days before the time of $B$-band maximum (MJD 56648.5$\pm0.1$). The estimate of the time of first light is consistent with the explosion time inferred from the evolution of the Si II $λ$6355 Doppler velocity. Furthermore, discovery photometry and previous nondetection limits enable us to constrain the companion radius down to $R_c \leq 4\,R_{\odot}$. In addition to our early-time constraints, we use a deep +235 day nebular-phase spectrum from Magellan/IMACS to place a stripped H-mass limit of $< 0.018\,M_{\odot}$. Combined, these limits effectively rule out H-rich nondegenerate companions.

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arXiv2018

Red vs Blue: Early observations of thermonuclear supernovae reveal two distinct populations?

We examine the early phase intrinsic $(B-V)_{0}$ color evolution of a dozen Type~Ia supernovae discovered within three days of the inferred time of first light ($t_{first}$) and have $(B-V)_0$ color information beginning within 5 days of $t_{first}$. The sample indicates there are two distinct early populations. The first is a population exhibiting blue colors that slowlybevolve, and the second population exhibits red colors and evolves more rapidly. We find that the early-blue events are all 1991T/1999aa-like with more luminous slower declining light curves than those exhibiting early-red colors. Placing the first sample on the Branch diagram (i.e., ratio of \ion{Si}{2} $λλ$5972, 6355 pseudo-Equivalent widths) indicates all blue objects are of the Branch Shallow Silicon (SS) spectral type, while all early-red events except for the 2000cx-like SN~2012fr are of the Branch Core-Normal (CN) or CooL (CL) type. A number of potential processes contributing to the early emission are explored, and we find that, in general, the viewing-angle dependance inherent in the companion collision model is inconsistent with all SS objects with early-time observations being blue and exhibiting an excess. We caution that great care must be taken when interpreting early-phase light curves as there may be a variety of physical processes that are possibly at play and significant theoretical work remains to be done.

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arXiv2018

A Significantly off-center Ni56 Distribution for the Low-Luminosity Type Ia Supernova SN 2016brx from the 100IAS survey

We present nebular-phase spectra of the Type Ia supernova (SN Ia) 2016brx, a member of the 1991bg-like subclass that lies at the faint end of the SN Ia luminosity function. Nebular spectra are available for only three other 1991bg-like SNe, and their Co line centers are all within <~ 500 km/s of each other. In contrast, the nebular Co line center of SN 2016brx is blue-shifted by >1500 km/s compared to them and by ~1200 km/s compared to the rest frame. This is a significant shift relative to the narrow nebular line velocity dispersion of <~ 2000 km/s of these SNe. The large range of nebular line shifts implies that the Ni56 in the ejecta of SN 1991bg-like events is off-center by ~1000 km/s rather than universally centrally confined as previously suggested. With the addition of SN 2016brx, the Co nebular line shapes of 1991bg-like objects appear to connect with the brighter SNe Ia that show double-peak profiles, hinting at a continuous distribution of line profiles among SNe Ia. One class of models to produce both off-center and bi-modal Ni56 distributions is collisions of white dwarfs with unequal and equal masses.

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arXiv2018

Near-infrared spectral evolution of the Type Ia supernova 2014J in the nebular phase: implications for the progenitor system

As the closest Type Ia supernova in decades, SN 2014J provides a unique opportunity for detailed investigation into observational signatures of the progenitor system and explosion mechanism in addition to burning product distribution. We present a late-time near-infrared spectral series from Gemini-N at $307-466$ days after the explosion. Following the $H$-band evolution probes the distribution of radioactive iron group elements, the extent of mixing, and presence of magnetic fields in the expanding ejecta. Comparing the isolated $1.6440$ $μ$m [Fe II] emission line with synthetic models shows consistency with a Chandrasekhar-mass white dwarf of $ρ_c=0.7\times10^9$ g cm${}^{-3}$ undergoing a delayed detonation. The ratio of the flux in the neighboring $1.54$ $μ$m emission feature to the flux in the $1.6440$ $μ$m feature shows evidence of some limited mixing of stable and radioactive iron group elements in the central regions. Additionally, the evolution of the $1.6440$ $μ$m line shows an intriguing asymmetry. When measuring line-width of this feature, the data show an increase in line width not seen in the evolution of the synthetic spectra, corresponding to $\approx1{,}000$ km s${}^{-1}$, which could be caused by a localized transition to detonation producing asymmetric ionization in the ejecta. Using the difference in width between the different epochs, an asymmetric component in the central regions, corresponding to approximately the inner $2\times10^{-4}$ of white dwarf mass suggests an off-center ignition of the initial explosion and hence of the kinematic center from the chemical center. Several additional models investigated, including a He detonation and a merger, have difficulty reproducing the features seen these spectra.

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arXiv2018

The lowest metallicity type II supernova from the highest mass red-supergiant progenitor

Red supergiants have been confirmed as the progenitor stars of the majority of hydrogen-rich type II supernovae. However, while such stars are observed with masses >25M$_\odot$, detections of >18M$_\odot$ progenitors remain elusive. Red supergiants are also expected to form at all metallicities, but discoveries of explosions from low-metallicity progenitors are scarce. Here, we report observations of the type II supernova, SN 2015bs, for which we infer a progenitor metallicity of $\leq$0.1Z$_\odot$ from comparison to photospheric-phase spectral models, and a Zero Age Main-Sequence mass of 17-25M$_\odot$ through comparison to nebular-phase spectral models. SN 2015bs displays a normal 'plateau' light-curve morphology, and typical spectral properties, implying a red supergiant progenitor. This is the first example of such a high mass progenitor for a 'normal' type II supernova, suggesting a link between high mass red supergiant explosions and low-metallicity progenitors.

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arXiv2018

SN 2012fr: Ultraviolet, Optical, and Near-Infrared Light Curves of a Type Ia Supernova Observed Within a Day of Explosion

We present detailed ultraviolet, optical and near-infrared light curves of the Type Ia supernova (SN) 2012fr, which exploded in the Fornax cluster member NGC 1365. These precise high-cadence light curves provide a dense coverage of the flux evolution from $-$12 to $+$140 days with respect to the epoch of $B$-band maximum (\tmax). Supplementary imaging at the earliest epochs reveals an initial slow, nearly linear rise in luminosity with a duration of $\sim$2.5 days, followed by a faster rising phase that is well reproduced by an explosion model with a moderate amount of $^{56}$Ni mixing in the ejecta. From an analysis of the light curves, we conclude: $(i)$ explosion occurred $< 22$ hours before the first detection of the supernova, $(ii)$ the rise time to peak bolometric ($λ> 1800 $Å) luminosity was $16.5 \pm 0.6$ days, $(iii)$ the supernova suffered little or no host-galaxy dust reddening, $(iv)$ the peak luminosity in both the optical and near-infrared was consistent with the bright end of normal Type Ia diversity, and $(v)$ $0.60 \pm 0.15 M_{\odot}$ of $^{56}$Ni was synthesized in the explosion. Despite its normal luminosity, SN 2012fr displayed unusually prevalent high-velocity \ion{Ca}{2} and \ion{Si}{2} absorption features, and a nearly constant photospheric velocity of the \ion{Si}{2} $λ$6355 line at $\sim$12,000 \kms\ beginning $\sim$5 days before \tmax. Other peculiarities in the early phase photometry and the spectral evolution are highlighted. SN 2012fr also adds to a growing number of Type Ia supernovae hosted by galaxies with direct Cepheid distance measurements.

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arXiv2018

The nebular spectra of the transitional Type Ia Supernovae 2007on and 2011iv: broad, multiple components indicate aspherical explosion cores

The nebular-epoch spectrum of the rapidly declining, "transitional" type Ia supernova (SN) 2007on showed double emission peaks, which have been interpreted as indicating that the SN was the result of the direct collision of two white dwarfs. The spectrum can be reproduced using two distinct emission components, one red-shifted and one blue shifted. These components are similar in mass but have slightly different degrees of ionization. They recede from one another at a line-of-sight speed larger than the sum of the combined expansion velocities of their emitting cores, thereby acting as two independent nebulae. While this configuration appears to be consistent with the scenario of two white dwarfs colliding, it may also indicate an off-centre delayed detonation explosion of a near Chandrasekhar-mass white dwarf. In either case, broad emission line widths and a rapidly evolving light curve can be expected for the bolometric luminosity of the SN. This is the case for both SNe 2007on and 2011iv, also a transitional SN Ia which exploded in the same elliptical galaxy, NGC 1404. Although SN 2011iv does not show double-peaked emission line profiles, the width of its emission lines is such that a two-component model yields somewhat better results than a single-component model. Most of the mass ejected is in one component, however, which suggests that SN 2011iv was the result of the off-centre ignition of a Chandrasekhar-mass white dwarf.

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