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RE: Supernova 2006gy
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Title: Quark nova imprint in the extreme supernova explosion SN 2006gy: the advent of the Quark Star
Authors: Rachid Ouyed, Mathew Kostka, Nico Koning, Denis Leahy, Wolfgang Steffen

The existence of quark stars has until now been purely hypothetical. In this work, we uncover undeniable evidence of these exotic objects with far reaching implications to various branches of physics and astrophysics. The extremely luminous supernova 2006gy (SN 2006gy) has provided photometric and spectroscopic evidence of a new explosion mechanism which signals the birth of a quark star. This supernova is among the most energetic ever observed and spent an unheard of 250 days at magnitude -19 or brighter. This analysis considers the supernova explosion of a massive star followed by the quark nova detonation of a neutron star. Our model naturally explains many aspects of SN 2006gy including the late stage light curve plateau, the broad H alpha line, and the peculiar blue H alpha absorption. In addition, we find that cooling of the re-shocked supernova envelope leads to an explanation for the diminished X-ray production observed by CHANDRA.

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RE: Type IIn Supernova 2006gy
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Title: New Observations of the Very Luminous Supernova 2006gy: Evidence for Echoes
Authors: A. A. Miller (1), N. Smith (1), W. Li (1), J. S. Bloom (1), R. Chornock (1), A. V. Filippenko (1), J. X. Prochaska (2, 3) (1. UC Berkeley, 2. UCO, 3. UC Santa Cruz)

Supernova (SN) 2006gy was a hydrogen-rich core-collapse SN that remains one of the most luminous optical supernovae ever observed. The total energy budget (> 2 x 10^51 erg radiated in the optical alone) poses many challenges for standard SN theory. We present new ground-based near-infrared (NIR) observations of SN 2006gy, as well as a single epoch of Hubble Space Telescope (HST) imaging obtained more than two years after the explosion. Our NIR data taken around peak optical emission show an evolution that is largely consistent with a cooling blackbody, with tentative evidence for a growing NIR excess starting at day ~100. Our late-time Keck adaptive optics (AO) NIR image, taken on day 736, shows little change from previous NIR observations taken around day 400. Furthermore, the optical HST observations show a reduced decline rate after day 400, and the SN is bluer on day 825 than it was at peak. This late-time decline is inconsistent with Co56 decay, and thus is problematic for the various pair-instability SN models used to explain the nature of SN 2006gy. The slow decline of the NIR emission can be explained with a light echo, and we confirm that the late-time NIR excess is the result of a massive (>10 Msun) dusty shell heated by the SN peak luminosity. The late-time optical observations require the existence of a scattered light echo, which may be generated by the same dust that contributes to the NIR echo. Both the NIR and optical echoes originate in the proximity of the progenitor, ~10^18 cm for the NIR echo and <~20 pc for the optical echo, which provides further evidence that the progenitor of SN 2006gy was a very massive star.

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Title: Spectral Evolution of the Extraordinary Type IIn Supernova 2006gy
Authors: Nathan Smith, Ryan Chornock, Jeffrey M. Silverman, Alexei V. Filippenko, Ryan J. Foley
(Version v2)

We present a detailed analysis of the extremely luminous Type IIn supernova SN2006gy using spectra obtained between days 36 and 237 after explosion. We derive the temporal evolution of the effective temperature, radius, expansion speeds, and bolometric luminosity, as well as the progenitor wind density and total swept-up mass overtaken by the shock. SN2006gy can be interpreted in the context of shock interaction with a dense CSM, but with quite extreme values for the CSM mass of 20 Msun and an explosion kinetic energy of at least 5e51 erg. A key difference between SN2006gy and other SNeIIn is that, owing to its large CSM mass, the interaction region remained opaque much longer. At early times, H-alpha widths suggest that the photosphere is ahead of the shock, and photons diffuse out through the opaque CSM. The pivotal transition to optically thin emission begins around day 110, when we start to see a decrease in the blackbody radius and strengthening tracers of the post-shock shell. From the evolution of pre-shock velocities, we deduce that the CSM was ejected by the progenitor in a 1e49 erg precursor event 8yr before explosion. The large CSM mass rules out models involving stars with initial masses around 10Msun. With the full mass budget, even massive M_ZAMS=30-40 Msun progenitor stars are inadequate. At roughly solar metallicity, substantial mass loss probably occurred during the star's life, so SN 2006gy's progenitor is more consistent with LBV eruptions or pulsational pair-instability ejections in stars with initial masses above 100 Msun. This requires significant revision to current paradigms of massive-star evolution.

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Supernova 2006gy
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Title: The Extremely Luminous Supernova 2006gy at Late Phase: Detection of Optical Emission from Supernova
Authors: Koji S. Kawabata, Masaomi Tanaka, Keiichi Maeda, Takashi Hattori, Ken'ichi Nomoto, Nozomu Tominaga, Masayuki Yamanaka

We performed optical spectroscopy and photometry of SN 2006gy at late time, ~400 days after the explosion, with the Subaru/FOCAS in a good seeing condition. We found that the SN faded by ~3 mag from ~200 to ~400 days after the explosion (i.e., by ~5 mag from peak to ~400 days) in R band. The overall light curve is marginally consistent with the 56Ni heating model, although the flattening around 200 days suggests the optical flux declined more steeply between ~200 and ~400 days. The late time spectrum was quite peculiar among all types of SNe. It showed many intermediate width (~2000 km/s FWHM) emission lines, e.g., [Fe II], [Ca II], and Ca II. The absence of the broad [O I] 6300, 6364 line and weakness of [Fe II] and [Ca II] lines compared with Ca II IR triplet would be explained by a moderately high electron density in the line emitting region. This high density assumption seems to be consistent with the large amount of ejecta and low expansion velocity of SN 2006gy. The H-alpha line luminosity was as small as ~1x10^39 erg/s, being comparable with those of normal Type II SNe at similar epochs. Our observation indicates that the strong CSM interaction had almost finished by ~400 days. If the late time optical flux is purely powered by radioactive decay, at least M_Ni ~ 3 M_sun should be produced at the SN explosion. In the late phase spectrum, there were several unusual emission lines at 7400--8800 AA and some of them might be due to Ti or Ni synthesized at the explosion.

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Title: Supernova SN2006gy as a first ever Quark Nova?
Authors: Denis Leahy, Rachid Ouyed (University of Calgary, Calgary, Alberta, Canada)
(Version v4)

The most luminous Supernova SN2006gy (more than a 100 times brighter than a typical supernova) has been a challenge to explain by standard models. For example, pair instability supernovae which are luminous enough seem to have too slow a rise, and core collapse supernovae do not seem to be luminous enough. We present an alternative scenario involving the quark-nova phenomenon (an explosive transition of the newly born neutron star to a quark star) in which a second explosion (delayed) occurs inside the ejecta of a normal supernova. The reheated supernova ejecta can radiate at higher levels for longer periods of time primarily due to reduced adiabatic expansion losses, unlike the standard supernova case. We find an encouraging match between the resulting lightcurve and that observed in the case of SN2006gy suggesting that we might have at hand the first ever signature of a quark-nova. Successful application of our model to SN2005gj and SN2005ap is also presented.

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Title: Late-time observations of SN2006gy: Still Going Strong
Authors: Nathan Smith, Ryan J. Foley, Joshua S. Bloom, Weidong Li, Alexei V. Filippenko, Raphael Gavazzi, Andrea Ghez, Quinn Konopacky, Matthew A. Malkan, Phillip J. Marshall, Tommaso Treu, Jong-Hak Woo

Because of its extremely high luminosity and long duration, SN2006gy radiated more energy in visual light than any other. Two hypotheses to explain its high luminosity at early times -- that it was powered by shock interaction with a large mass of circumstellar material (CSM) as implied by its Type IIn spectrum, or that it was fuelled by radioactive decay from a large mass of 56Ni synthesized in a pair-instability SN -- predicted different late-time behaviour. Here we present observations of SN2006gy obtained more than a year after discovery. We were unable to detect it at visual wavelengths, but clear near-IR detections show that it is still at least as luminous as the peak of a normal Type II supernova. If dust formed, then the late-time luminosity source could be either radioactive decay or additional CSM interaction. However, we find the continued CSM-interaction hypothesis problematic because SN2006gy lacks the strong X-ray and broad H-alpha emission seen in objects like SN 1988Z. A third possible explanation for the late-time IR luminosity is an IR echo, where radiation emitted during peak luminosity heats a pre-existing dust shell at radii near 1 light year. That interpretation, though, requires the star to have ejected another 10 Msun shell about 1500 yr before the SN.

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A Los Alamos National Laboratory astrophysicist and his colleagues have discovered that a superbright supernova observed last year might have exhibited an unusual one-two punch.
The discovery, published this week in the journal Nature, provides a new model for the behaviour of the universes largest and rarest exploding stars.
Alexander Heger of Los Alamos Theoretical Astrophysics Group joined University of California Santa Cruz researcher Stan Woosley and Russian researcher Sergei Blinnikov in developing a new model for supernova behaviour that matches observations of supernova SN 2006gy, which exploded last year in a flash 100 times brighter than typically observed in supernovae.

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Title: A runaway collision in a young star cluster as the origin of the brightest supernova
Authors: Simon Portegies Zwart (UvA) Edward P.J. van den Heuvel (UvA)

Supernova 2006gy in the galaxy NGC 1260 is the most luminous one recorded. Its progenitor might have been a very massive (>100 solar masses) star, but that is incompatible with hydrogen in the spectrum of the supernova, because stars >40 solar masses are believed to have shed their hydrogen envelopes several hundred thousand years before the explosion. Alternatively, the progenitor might have arisen from the merger of two massive stars. Here we show that the collision frequency of massive stars in a dense and young cluster (of the kind to be expected near the centre of a galaxy) is sufficient to provide a reasonable chance that SN 2006gy resulted from such a bombardment. If this is the correct explanation, then we predict that when the supernova fades (in a year or so) a dense cluster of massive stars becomes visible at the site of the explosion.

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A supernova observed last year was so bright--about 100 times as luminous as a typical supernova--that it challenged the theoretical understanding of what causes supernovae. But Stan Woosley, professor of astronomy and astrophysics at the University of California, Santa Cruz, had an idea that he thought could account for it--an extremely massive star that undergoes repeated explosions. When Woosley and two colleagues worked out the detailed calculations for their model, the results matched the observations of the supernova known as SN 2006gy, the brightest ever recorded.

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Title: Shell-shocked diffusion model for the light curve of SN2006gy
Authors: Nathan Smith, Richard McCray

We explore a simple model for the high luminosity of SN 2006gy involving photon diffusion of shock-deposited thermal energy. The distinguishing property of the model is that the large ``stellar'' radius of 160 AU required to prevent adiabatic losses is not the true stellar radius, but rather, the radius of an opaque, unbound circumstellar envelope, created when 10 Msun was ejected in the decade before the supernova in an eruption analogous to that of eta Carinae. The supernova light is produced primarily by diffusion of thermal energy following the passage of the blast wave through this shell. This model differs from traditional models of supernova debris interacting with external CSM in that here the shell is optically thick and the escape of radiation is delayed. We show that any model attempting to account for SN2006gy's huge luminosity with radiation emitted by ongoing CSM interaction fails for the following basic reason: the CSM density required to achieve the observed luminosity makes the same circumstellar envelope opaque, forcing a thermal diffusion solution. In our model, the weaker CSM interaction giving rise to SN2006gy's characteristic Type IIn spectrum and soft X-rays is not linked to the power source of the visual continuum; instead, it arises after the blast wave breaks free of the opaque shell into the surrounding wind. While a simple diffusion model can explain the gross properties of the early light curve of SN2006gy, it predicts that the light curve must plummet rapidly at late-times, unless an additional power source is present.

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