* Astronomy

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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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: 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: 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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