Quark nova imprint in the extreme supernova explosion SN 2006gy

Mon. Not. R. Astron. Soc. 423, 1652–1662 (2012) doi:10.1111/j.1365-2966.2012.20986.x Quark nova imprint in the extreme supernova explosion SN 2006gy R. Ouyed,1 M. Kostka,1 N. Koning,1 D. A. Leahy1 and W. Steffen2 1 Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, AB T2N 1N4, Canada 2 Instituto de Astronomı́a, Universidad Nacional Autonoma de México, 22860 Ensenada, BC, Mexico Accepted 2012 March 23. Received 2012 January 10; in original form 2011 September 14 ABSTRACT Key words: dense matter – radiative transfer – stars: evolution – supernovae: individual: 2006gy. 1 I N T RO D U C T I O N The supernova (SN) 2006gy discovered by Robert Quimby in 2006 August has challenged our understanding of stellar evolution (Quimby et al. 2007). SN 2006gy was 100 times more luminous than a typical SN and at the time the most energetic ever recorded. For almost a year it continued to radiate at a pace in which an ordinary SN could only sustain for at most a few days. Explaining this tremendous energy budget pushes the limits of current SN theory. SN 2006gy reveals many singular features in both its light curve (left-hand panel of Fig. 1) and hydrogen spectrum (right-hand panel of Fig. 1). The light curve exhibits a luminous peak, broad shape and energetic plateau, while the hydrogen spectrum displays a curious evolution and a peculiar structure. Models have been proposed to describe individual characteristics of this event but all have been left wanting. 1.1 Supernova explosion models The significance of SN 2006gy was first discussed independently by Smith et al. (2007) and Ofek et al. (2007). The model proposed by Ofek et al. (2007) involved a Type Ia SN exploding during the common-envelope phase of a binary system. The collision between the SN ejecta (SNE) and dense circumstellar material (CSM) re E-mail: leases the amount of energy required to explain the observations of SN 2006gy. However, this model demands the CSM to be extraordinarily massive (Ofek et al. 2007) suggesting a mass-loss rate several orders of magnitude greater than expected (Yungelson et al. 2008). In addition, the spectrum of SN 2006gy indicates the presence of elements not seen in a Type Ia SN (Smith et al. 2010). Smith et al. (2010) championed a CSM model which considered a wind from an exotic luminous blue variable (LBV) to account for the massive CSM. Although this model can account for the first five months of the observed light curve, the diffusion process requires a rapid drop off in luminosity (Agnoletto et al. 2009) rather than the observed plateau. Attempts to reconcile the CSM model with the late-stage plateau invoke the decay of radioactive 56 Ni and 56 Co to generate the needed luminosity. This necessitates 10–27 times the maximum amount of 56 Ni that can be created by a SN (Umeda & Nomoto 2008). This description for the late-stage light curve of SN 2006gy has been rebuked by recent near-infrared observations which show that the decline in luminosity is inconsistent with 56 Co decay (Miller et al. 2010). A further challenge to the CSM model lies in the fact that a high mass-loss rate is deduced using an equation that requires the stellar wind to be constant in time; however, the conclusion made by Smith et al. (2010) is that the stellar wind must vary with time, contradicting their initial assumptions (Dwarkadas 2011). For this model to achieve the amount of radiated energy observed in SN 2006gy, the initial kinetic energy of the SNE must be at least 5 × 1051 erg. This implies that SN 2006gy was one of  C 2012 The Authors C 2012 RAS Monthly Notices of the Royal Astronomical Society  The extremely luminous supernova 2006gy (SN 2006gy) is among the most energetic ever observed. The peak brightness was 100 times that of a typical supernova and it spent an unheard of 250 d at magnitude −19 or brighter. Efforts to describe SN 2006gy have pushed the boundaries of current supernova theory. In this work we aspire to simultaneously reproduce the photometric and spectroscopic observations of SN 2006gy using a quark nova (QN) model. This analysis considers the supernova explosion of a massive star followed days later by the QN detonation of a neutron star. We lay out a detailed model of the interaction between the supernova envelope and the QN ejecta paying special attention to a mixing region which forms at the inner edge of the supernova envelope. This model is then fitted to photometric and spectroscopic observations of SN 2006gy. This QN model naturally describes several features of SN 2006gy including the late-stage light-curve plateau, the broad Hα line and the peculiar blue Hα absorption. We find that a progenitor mass between 20 and 40 M provides ample energy to power SN 2006gy in the context of a QN. Quark nova imprint in SN 2006gy 1653 the most energetic SNe and requires a massive (M CSM > 20 M ) CSM cloud (Smith et al. 2010). As noted by Smith et al. (2010), the transformation of kinetic energy into radiation should translate into substantial narrowing of the Hα line as the fast-moving ejecta slows, which is contrary to that observed. Finally any model that involves the collision of SNE with a dense CSM should be a strong emitter of X-ray radiation since the shock temperature would be very high (Blinnikov 2008). Observations from the Chandra X-ray Observatory of SN 2006gy have seen very limited X-ray emission (Smith et al. 2010). The CSM model explains the lack of X-rays through self-absorption by the cold, outer layers of the CSM (Blinnikov 2008). A pulsational pair-instability (pPISN) model for SN 2006gy was considered by Woosley, Blinnikov & Heger (2007). In this model an unusually massive star (>100 M ) becomes prone to the γ = 4/3 instability, triggering a SN explosion. The pair-instability process must occur twice, leading to a collision between ejected shells which releases the energy needed for SN 2006gy. In order to reach the peak luminosity, it was necessary for Woosley et al. (2007) to artificially increase the kinetic energy of the second ejection. Like the CSM model, the pPISN model fails to achieve on several levels. First it is unable to properly account for the light-curve plateau, as it falls off too rapidly (Woosley et al. 2007). Woosley et al. (2007) explain that the extra energy required to fit the plateau must by generated by radioactive decay of 56 Co, a conclusion contradicted by latestage near-infrared observations (Miller et al. 2010). Secondly, the multicomponent structure of the Hα line is difficult to reconcile with the pPISN model which demands all the hydrogen to be contained in an outer shell (Woosley et al. 2007; Blinnikov 2008). Thirdly, a progenitor star of 110 M is required for the pPISN model to achieve the power output of SN 2006gy. A star this massive is expected to lose its hydrogen long before it goes SN (Yungelson  C 2012 The Authors, MNRAS 423, 1652–1662 C 2012 RAS Monthly Notice (...truncated)