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Title: Extremely late photometry of SN 2011fe
Author: W.E. Kerzendorf, C. McCully, S. Taubenberger, A. Jerkstrand, I. Seitenzahl, A. J. Ruiter, J. Spyromilio, K. S. Long, C. Fransson

Type Ia supernovae are widely accepted to be the outcomes of thermonuclear explosions in white dwarf stars. However, many details of these explosions remain uncertain (e.g. the mass, ignition mechanism, and flame speed). Theory predicts that at very late times (beyond 1000 d) it might be possible to distinguish between explosion models. Few very nearby supernovae can be observed that long after the explosion. The Type Ia supernova SN 2011fe located in M101 and along a line of sight with negligible extinction, provides us with the once-in-a-lifetime chance to obtain measurements that may distinguish between theoretical models. In this work, we present the analysis of photometric data of SN 2011fe taken between 900 and 1600 days after explosion with Gemini and HST. At these extremely late epochs theory suggests that the light curve shape might be used to measure isotopic abundances which is a useful model discriminant. However, we show in this work that there are several currently not well constrained physical processes introducing large systematic uncertainties to the isotopic abundance measurement. We conclude that without further detailed knowledge of the physical processes at this late stage one cannot reliably exclude any models on the basis of this dataset.

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Title: Multi-epoch Spectropolarimetry of SN 2011fe
Author: Peter A. Milne, G. Grant Williams, Amber Porter, Paul S. Smith, Nathan Smith, Mark D. Leising, Buell T. Jannuzi, E.M. Green

We present multiple spectropolarimetric observations of the nearby Type Ia supernova (SN) 2011fe in M101, obtained before, during, and after the time of maximum apparent visual brightness. The excellent time coverage of our spectropolarimetry has allowed better monitoring of the evolution of polarisation features than is typical, which has allowed us new insight into the nature of normal SNe Ia. SN 2011fe exhibits time-dependent polarisation in both the continuum and strong absorption lines. At early epochs, red wavelengths exhibit a degree of continuum polarisation of up to 0.4%, likely indicative of a mild asymmetry in the electron-scattering photosphere. This behaviour is more common in sub-luminous SNe Ia than in normal events, such as SN2011fe. The degree of polarisation across a collection of absorption lines varies dramatically from epoch to epoch. During the earliest epoch a lambda 4600-5000 \AA\ complex of absorption lines shows enhanced polarisation at a different position angle than the continuum. We explore the origin of these features, presenting a few possible interpretations, without arriving at a single favoured ion. During two epochs near maximum, the dominant polarisation feature is associated with the Siii lambda 6355 \AA\ absorption line. This is common for SNeIa, but for SN2011fe the polarisation of this feature increases after maximum light, whereas for other SNeIa, that polarisation feature was strongest before maximum light.

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Unusual Supernova is Doubly Unusual for Being Perfectly Normal

August, 2011, saw the dazzling appearance of the closest and brightest Type Ia supernova since Type Ia's were established as "standard candles" for measuring the expansion of the universe. The brilliant visitor, labelled SN 2011fe, was caught by the Palomar Transient Factory less than 12 hours after it exploded in the Pinwheel Galaxy in the Big Dipper.
Easy to see through binoculars, 2011fe was soon dubbed the Backyard Supernova. Major astronomical studies from the ground and from space followed close on its heels, recording its luminosity and colours as it rapidly brightened and then slowly faded away.

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Title: Hubble Space Telescope spectra of the type Ia supernova SN 2011fe: A low-energy delayed detonation of a white dwarf with Z<Z_solar
Authors: Paolo Mazzali, Mark Sullivan, Stephan Hachinger, Richard Ellis, Peter E. Nugent, D. Andrew Howell, Avishay Gal-Yam, Kate Maguire, Jeff Cooke, Rollin Thomas

Hubble Space Telescope spectroscopic observations of the nearby type Ia supernova (SN Ia) SN 2011fe, taken on 10 epochs from -13.5 to +41 days relative to B-band maximum light, and spanning the far-ultraviolet (UV) to the near-infrared (IR) are presented. This spectroscopic coverage makes SN 2011fe the best-studied local SN Ia to date. SN 2011fe is a typical moderately-luminous SN Ia with no evidence for dust extinction. Its near-UV spectral properties are representative of a larger sample of local events studied in Maguire et al. (2012). As a result, conclusions inferred from our detailed investigations are likely representative of those for other normal SNe Ia. The near-UV to optical spectra of SN 2011fe are modelled with a Monte Carlo radiative transfer code using the technique of 'abundance tomography', providing tight constraints on the density structure and abundance stratification of the event. SN 2011fe was a relatively weak explosion, with moderate Fe-group yields. Although its density structure is close to the 'standard' SN Ia pure deflagration explosion model W7, an improved model was developed which demonstrates that the ejecta of SN 2011fe have a more pronounced high-velocity tail, typical of a detonation wave affecting the outer layers. This improved model has a lower energy than typical delayed-detonation models. The derived Fe abundance in the outermost layer is consistent with the metallicity of ~0.5 solar at the SN explosion site in M101. Importantly, the spectroscopic rise time of ~19 days is significantly longer than that measured from the early optical light curve, implying a 'dark phase' of ~1-1.5 days. Such an extension in the rise time has significant implications when deducing the properties of the white dwarf and binary system from the early photometric behaviour.

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Title: Observations of SN2011fe with INTEGRAL
Authors: J. Isern, P. Jean, E. Bravo, R. Diehl, J. Knödlseder, A. Domingo, A. Hirschmann, P. Hoeflich, F. Lebrun, M. Renaud, S. Soldi, N. Elias--Rosa, M. Hernanz, B. Kulebi, X. Zhang, C. Badenes, I. Domínguezk, D. Garcia-Senz, C. Jordi, G. Lichti, G. Vedrenneb, P. Von Ballmoos

SN2011fe was detected by the Palomar Transient Factory on August 24th 2011 in M101 few hours after the explosion. From the early spectra it was immediately realized that it was a Type Ia supernova thus making this event the brightest one discovered in the last twenty years. In this paper the observations performed with the instruments on board of INTEGRAL (SPI, IBIS/ISGRI, JEM-X and OMC) before and after the maximum of the optical light as well as the interpretation in terms of the existing models of gamma--ray emission from such kind of supernovae are reported. All INTEGRAL high-energy have only been able to provide upper limits to the expected emission due to the decay of ^{56}Ni. These bounds allow to reject explosions involving a massive white dwarf in the sub--Chandrasekhar scenario. On the other hand, the optical light curve obtained with the OMC camera suggests that the event was produced by a delayed detonation of a CO white dwarf that produced ~ 0.5 solar masses of ^{56}Ni. In this particular case, INTEGRAL would have only been able to detect the early gamma--ray emission if the supernova had occurred at a distance of 2 -3 Mpc, although the brightest event could be visible up to distances larger by a factor two.

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Title: Observation of SN2011fe with INTEGRAL. I. Pre--maximum phase
Authors: J. Isern, P. Jean, E. Bravo, R. Diehl, J. Knödlseder, A. Domingo, A. Hirschmann, P. Hoeflich, F. Lebrun, M. Renaud, S. Soldi, N. Elias-Rosa, M. Hernanz, B. Kulebi, X. Zhang, C. Badenes, I. Domínguez, D. Garcia-Senz, C. Jordi, G. Lichti, G. Vedrenne, P. Von Ballmoos

SN2011fe was detected by the Palomar Transient Factory on August 24th 2011 in M101 a few hours after the explosion. From the early optical spectra it was immediately realised that it was a Type Ia supernova thus making this event the brightest one discovered in the last twenty years. The distance of the event offered the rare opportunity to perform a detailed observation with the instruments on board of INTEGRAL to detect the gamma-ray emission expected from the decay chains of ^{56}Ni. The observations were performed in two runs, one before and around the optical maximum, aimed to detect the early emission from the decay of ^{56}Ni and another after this maximum aimed to detect the emission of ^{56}Co. The observations performed with the instruments on board of INTEGRAL (SPI, IBIS/ISGRI, JEMX and OMC) have been analysed and compared with the existing models of gamma-ray emission from such kind of supernovae. In this paper, the analysis of the gamma-ray emission has been restricted to the first epoch. Both, SPI and IBIS/ISGRI, only provide upper-limits to the expected emission due to the decay of ^{56}Ni. These upper-limits on the gamma-ray flux are of 7.1 x 10^{-5} ph/s/cm² for the 158 keV line and of 2.3 x 10^{-4} ph/s/cm² for the 812 keV line. These bounds allow to reject at the 2\sigma level explosions involving a massive white dwarf, ~ 1 solar masses in the sub--Chandrasekhar scenario and specifically all models that would have substantial amounts of radioactive ^{56}Ni in the outer layers of the exploding star responsible of the SN2011fe event. The optical light curve obtained with the OMC camera also suggests that SN2011fe was the outcome of the explosion, possibly a delayed detonation although other models are possible, of a CO white dwarf that synthesised ~0.55 solar masses of ^{56}Ni. For this specific model.

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Title: The Distance to M101 Hosting Type Ia SN 2011fe Based on the Tip of the Red Giant Branch
Authors: Myung Gyoon Lee, In Sung Jang

We present a new determination of the distance to M101, host of the type Ia SN 2011fe, based on the tip of the red giant branch method (TRGB). Our determination is based on Hubble Space Telescope archival F555W and F814W images of nine fields within the galaxy. Colour-magnitude diagrams of arm-free regions in all fields show a prominent red giant branch (RGB). We measure the I-band magnitudes of the TRGB, obtaining a mean value of I_TRGB=25.28±0.01 (where the error is a standard error), using an edge-detection method. We derive a weighted mean value of distance modulus (m-M)_0=29.30±0.01 (random)±0.12 (systematic), corresponding to a linear distance of 7.24±0.03±0.40 Mpc. While previous estimates for M101 show a large range (TRGB distances of (m-M)_0=29.05 to 29.42 and Cepheid distances of (m-M)_0=29.04 to 29.71), our measurements of the TRGB distances for nine fields show a small dispersion of only 0.02. We combine our distance estimate and photometry in the literature to derive absolute peak magnitudes in optical and near-infrared bands of SN 2011fe. Absolute maximum magnitudes of SN 2011fe are \sim0.2 mag brighter in the optical band and much more in the NIR than the current calibrations of SNe Ia in the literature. From the optical maximum magnitudes of SN 2011fe we obtain a value of the Hubble constant, H_0=65.0±0.5(random)±5.7(systematic) \kmsMpc, slightly smaller than other recent determinations of H_0.

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Title: No Stripped Hydrogen in the Nebular Spectra of Nearby Type Ia Supernova 2011fe
Authors: Benjamin J. Shappee, K. Z. Stanek, R. W. Pogge, P. M. Garnavich

A generic prediction of the single-degenerate model for Type Ia supernovae (SNe Ia) is that a significant amount of material will be stripped from the donor star (~0.5 solar masses for a giant donor and ~0.15 solar masses for a main sequence donor) by the supernova ejecta. This material, excited by gamma-rays from radioactive decay, would then produce relatively narrow (<1000 km s^-1) emission features observable once the supernova enters the nebular phase. Such emission has never been detected, which already provides strong constraints on Type Ia progenitor models. In this Letter we report the deepest limit yet on the presence of H alpha emission originating from the stripped hydrogen in the nebular spectrum of a Type Ia supernova obtained using a high signal-to-noise spectrum of the nearby normal SN Ia 2011fe 274 days after B-band maximum light with the Large Binocular Telescope's Multi-Object Double Spectrograph. We put a conservative upper limit on the H alpha flux of 3.14x10^-17 erg/s/cm^2, which corresponds to a luminosity of 1.57x10^35 erg/s. Assuming the models of Mattila et al. (2005) and the methods of Leonard (2007), this translates into an upper limit of <0.001 solar masses of stripped material, which is an order of magnitude stronger than previous limits by Leonard (2007). SN 2011fe was a typical Type Ia supernova, special only in its proximity, and we argue that lack of hydrogen emission in its nebular spectrum adds yet another strong constraint on the single degenerate class of models for SNe Ia.

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Title: Analysis of the Early-Time Optical Spectra of SN 2011fe in M101
Authors: J. T. Parrent, D. A. Howell, B. Friesen, R. C. Thomas, R. A. Fesen, D. Milisavljevic, F. B. Bianco, B. Dilday, P. Nugent, E. Baron, I. Arcavi, S. Ben-Ami, D. Bersier, L. Bildsten, J. Bloom, Y. Cao, S. B. Cenko, A. V. Filippenko, A. Gal-Yam, M. M. Kasliwal, N. Konidaris, S. R. Kulkarni, N. M. Law, D. Levitan, K. Maguire, P. A. Mazzali, E. O. Ofek, Y. Pan, D. Polishook, D. Poznanski, R. M. Quimby, J. M. Silverman, A. Sternberg, M. Sullivan, E. S. Walker, C. Buton, R. Pereira

The nearby Type Ia supernova SN 2011fe in M101 (cz=241 km s^-1) provides a unique opportunity to study the early evolution of a "normal" Type Ia supernova, its compositional structure, and its elusive progenitor system. We present 18 high signal-to-noise spectra of SN 2011fe during its first month beginning 1.2 days post-explosion and with an average cadence of 1.8 days. This gives a clear picture of how various line-forming species are distributed within the outer layers of the ejecta, including that of unburned material (C+O). We follow the evolution of C II absorption features until they diminish near maximum light, showing overlapping regions of burned and unburned material between ejection velocities of 10,000 and 16,000 km s^-1. This supports the notion that incomplete burning, in addition to progenitor scenarios, is a relevant source of spectroscopic diversity among SNe Ia. The observed evolution of the highly Doppler-shifted O I 7774 absorption features detected within five days post-explosion indicate the presence of O I with expansion velocities from 11,500 to 21,000 km s^-1. The fact that some O I is present above C II suggests that SN 2011fe may have had an appreciable amount of unburned oxygen within the outer layers of the ejecta.

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Title: Constraining Type Ia supernova models: SN 2011fe as a test case
Authors: F. K. Roepke, M. Kromer, I. R. Seitenzahl, R. Pakmor, S. A. Sim, S. Taubenberger, F. Ciaraldi-Schoolmann, W. Hillebrandt, G. Aldering, P. Antilogus, C. Baltay, S. Benitez-Herrera, S. Bongard, C. Buton, A. Canto, F. Cellier-Holzem, M. Childress, N. Chotard, Y. Copin, H. K. Fakhouri, M. Fink, D. Fouchez, E. Gangler, J. Guy, S. Hachinger, E. Y. Hsiao, C. Juncheng, M. Kerschhaggl, M. Kowalski, P. Nugent, K. Paech, R. Pain, E. Pecontal, R. Pereira, S. Perlmutter, D. Rabinowitz, M. Rigault, K. Runge, C. Saunders, G. Smadja, N. Suzuki, C. Tao, R. C. Thomas, A. Tilquin, C. Wu

The nearby supernova SN 2011fe can be observed in unprecedented detail. Therefore, it is an important test case for Type Ia supernova (SN Ia) models, which may bring us closer to understanding the physical nature of these objects. Here, we explore how available and expected future observations of SN 2011fe can be used to constrain SN Ia explosion scenarios. We base our discussion on three-dimensional simulations of a delayed detonation in a Chandrasekhar-mass white dwarf and of a violent merger of two white dwarfs-realizations of explosion models appropriate for two of the most widely-discussed progenitor channels that may give rise to SNe Ia. Although both models have their shortcomings in reproducing details of the early and near-maximum spectra of SN 2011fe obtained by the Nearby Supernova Factory (SNfactory), the overall match with the observations is reasonable. The level of agreement is slightly better for the merger, in particular around maximum, but a clear preference for one model over the other is still not justified. Observations at late epochs, however, hold promise for discriminating the explosion scenarios in a straightforward way, as a nucleosynthesis effect leads to differences in the 55Co production. SN 2011fe is close enough to be followed sufficiently long to study this effect.

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