SN 2006gy Research Articles

The CO White Dwarf + Intermediate-mass/Massive Star Binary Evolution: Possible Merger Origins for Peculiar Type Ia and II Supernovae

Binary stellar evolution has been studied as an important pathway to initiate various transient events like supernovae (SNe). Although the common envelope (CE) phase in a binary, outcomes of the CE phase, and conditions for SN explosion during the CE phase are uncertain, it has been suggested that SN explosions can be triggered during the CE phase. In this work, we explore the formation and evolution routes of carbon/oxygen (CO) white dwarf (WD) binaries in order to investigate mergers of CO WDs and cores of nondegenerate stars during the CE phase as possible origins for SNe under the core merger detonation (CMD) scenario by considering several physical binary models. The evolution of CO WD + intermediate-mass normal (nondegenerate and hydrogen-rich) star binaries lead to mergers during the CE phases and may still trigger Type Ia SNe (SNe Ia) interacting with circumstellar material in different models. Mergers between CO WDs and the cores of He (nondegenerate and helium-rich) stars within the common envelope (CE) phase are rare compared to other CE merger events. These two channels may produce peculiar SNe Ia such as overluminous/super-Chandrasekhar-mass SNe Ia as a certain fraction of them have combined core masses ≥2 M ⊙. In the channel of CO WD + massive star (≥8 M ⊙) CE events, we find that rates of mergers between CO WDs and He cores of massive stars are (0.85–12.18) , which may initiate Type II superluminous SNe like SN 2006gy, and delay times from this scenario are in the range of 34–120 Myr. Our results based on the CMD model are comparable with observational results of peculiar SNe.

The objects in observations of Perseus cluster region

AbstractThe results of 20‐year observations of the Perseus cluster centering on the NGC 1275 including IC 310 radio galaxy and extragalactic supernova SN 2006gy at energies 800 GeV to 45 TeV by the SHALON telescope are presented. Also, the emission from the galactic source of non‐thermal radio and X‐ray emission GK Per (Nova 1901) of classical nova type was found as it accompanied to the observations. For NGC 1275, it was found that the TeV γ‐ray emission at energies >800 GeV has an extended structure with a distinct core centered at the NGC 1275 nucleus and well correlates with the photon emission regions viewed in X‐rays by Chandra and anti‐correlates with radio‐structures. Also, the variations of TeV γ‐ray flux both at year‐ and day scales were found. The obtained data indicate that the part of TeV γ‐ray emission is generated by relativistic jets in the nucleus of NGC 1275. Whereas the presence of an extended structure around NGC 1275 and the slow rise of the γ‐ray flux is the evidence of the interaction of cosmic rays and magnetic fields generated in the jets at the galactic center with the gas of the Perseus cluster.

TeV gamma-rays from the region of Perseus Cluster

The results of 20-year observations of the Perseus cluster centering on the NGC 1275 including IC 310 radio galaxy and extragalactic supernova SN 2006gy at energies 800 GeV - 45 TeV by the SHALON telescope are presented. It was found, that the TeV γ-ray emission at energies > 800 GeV has an extended structure with a distinct core centered at the NGC 1275 nucleus and well correlates with the photon emission regions viewed in X-rays by Chandra and anti-correlates with radio-structures. Also, the variations of TeV γ-ray flux both at year- and day- scales were found. The obtained data indicate that the part of TeV γ-ray emission is generated by relativistic jets in the nucleus of NGC 1275. Whereas, the presence of an extended structure around NGC 1275 and the slow rise of the γ-ray flux is the evidence of the interaction of cosmic rays and magnetic fields generated in the jets at the galactic center with the gas of the Perseus cluster.

A type Ia supernova at the heart of superluminous transient SN 2006gy.

Superluminous supernovae radiate up to 100 times more energy than normal supernovae. The origin of this energy and the nature of the stellar progenitors of these transients are poorly understood. We identify neutral iron lines in the spectrum of one such supernova, SN 2006gy, and show that they require a large mass of iron (≳0.3 solar masses) expanding at 1500 kilometers per second. By modeling a standard type Ia supernova hitting a shell of circumstellar material, we produce a light curve and late-time iron-dominated spectrum that match the observations of SN 2006gy. In such a scenario, common envelope evolution of a progenitor binary system can synchronize envelope ejection and supernova explosion and may explain these bright transients.

What powers the 3000-day light curve of SN 2006gy?

SN 2006gy was the most luminous SN ever observed at the time of its discovery and the first of the newly defined class of superluminous supernovae (SLSNe). The extraordinary energetics of SN 2006gy and all SLSNe (>10^51 erg) require either atypically large explosion energies (e.g., pair-instability explosion) or the efficient conversion of kinetic into radiative energy (e.g., shock interaction). The mass-loss characteristics can therefore offer important clues regarding the progenitor system. For the case of SN 2006gy, both a scattered and thermal light echo from circumstellar material (CSM) have been reported at later epochs (day ~800), ruling out the likelihood of a pair-instability event and leading to constraints on the characteristics of the CSM. Owing to the proximity of the SN to the bright host-galaxy nucleus, continued monitoring of the light echo has not been trivial, requiring the high resolution offered by the Hubble Space Telescope (HST) or ground-based adaptive optics (AO). Here we report detections of SN 2006gy using HST and Keck AO at ~3000 days post-explosion and consider the emission mechanism for the very late-time light curve. While the optical light curve and optical spectral energy distribution are consistent with a continued scattered-light echo, a thermal echo is insufficient to power the K'-band emission by day 3000. Instead, we present evidence for late-time infrared emission from dust that is radiatively heated by CSM interaction within an extremely dense dust shell, and we consider the implications on the CSM characteristics and progenitor system.

Calculating Pre-SN Mass-Loss Histories from Post-Explosion Observations and the Special Case of SN 2006gy

AbstractA growing number of SNe now show evidence for significant interaction between the forward shock and a pre-existing circumstellar medium (CSM) formed by the pre-SN mass-loss. The SN community likes to argue that it can indirectly constrain the progenitor systems for these SNe by measuring the wind speeds, densities, compositions, and asymmetries of the CSM. The measurements, however, rely on a number of assumptions and can result in degeneracies that straddle the divide between RSGs and more massive stars. Furthermore, the implications of the measured properties may sometimes be simplified to fit a SN expert's relatively basic understanding of stellar wind parameters. Given the opportunity to have experts in massive stars, cool stars, and supernovae together in the same room, I thought it would be a good opportunity to review the different methods employed by the SN community to measure mass-loss rates. I will then provide an update from a recent Spitzer survey on the measured CSM properties from different subclasses of SNe with known shock interaction (including SLSNe, SNe II, IIn, Ibc, and even Ia-CSM). Finally, I will present new HST data on the SLSN Type IIn SN 2006gy and discuss the implications on the progenitor system and pre-SN mass-loss.

Study of supernovae important for cosmology

The dense shell method for the determination of distances to type IIn supernovae has been briefly reviewed. Applying our method to SN 2006gy, SN 2009ip, and SN 2010jl supernovae, we have obtained distances in excellent agreement with the previously known distances to the parent galaxies. The dense shell method is based on the radiation hydrodynamic model of a supernova. The method of the blackbody model, as well as the correctness of its application for simple estimates of distances from observation data, has been justified.

Synthetic light curves of shocked dense circumstellar shells

We numerically investigate light curves (LCs) of shocked circumstellar shells which are suggested to reproduce the observed LC of superluminous SN 2006gy analytically. In the previous analytical model, the effects of the recombination and the bolometric correction on LCs are not taken into account. To see the effects, we perform numerical radiation hydrodynamic calculations of shocked shells by using STELLA, which can numerically treat multigroup radiation transfer with realistic opacities. We show that the effects of the recombination and the bolometric correction are significant and the analytical model should be compare to the bolometric LC instead of a single band LC. We find that shocked circumstellar shells have a rapid LC decline initially because of the adiabatic expansion rather than the luminosity increase and the shocked shells fail to explain the LC properties of SN 2006gy. However, our synthetic LCs are qualitatively similar to those of superluminous SN 2003ma and SN 1988Z and they may be related to shocked circumstellar shells.

Supernova interaction with dense mass loss

AbstractSupernovae of Type IIn (narrow line) appear to be explosions that had strong mass loss before the event, so that the optical luminosity is powered by the circumstellar interaction. If the mass loss region has an optical depth >c/vs, where vs is the shock velocity, the shock breakout occurs in the mass loss region and a significant fraction of the explosion energy can be radiated. The emission from the superluminous SN 2006gy and the normal luminosity SN 2011ht can plausibly be attributed to shock breakout in a wind, with SN 2011ht being a low energy event. Superluminous supernovae of Type I may derive their luminosity from interaction with a mass loss region of limited extent. However, the distinctive temperature increase to maximum luminosity has not been clearly observed in Type I events. Suggested mechanisms for the strong mass loss include pulsational pair instability, gravity-waves generated by instabilities in late burning phases, and binary effects.

Light-curve modelling of superluminous supernova 2006gy: collision between supernova ejecta and a dense circumstellar medium

We show model light curves of superluminous supernova 2006gy on the assumption that the supernova is powered by the collision of supernova ejecta and its dense circumstellar medium. The initial conditions are constructed based on the shock breakout condition, assuming that the circumstellar medium is dense enough to cause the shock breakout within it. We perform a set of numerical light curve calculations by using a one-dimensional multigroup radiation hydrodynamics code STELLA. We succeeded in reproducing the overall features of the early light curve of SN 2006gy with the circumstellar medium whose mass is about 15 Msun (the average mass-loss rate ~ 0.1 Msun/yr). Thus, the progenitor of SN 2006gy is likely a very massive star. The density profile of the circumstellar medium is not well constrained by the light curve modeling only, but our modeling disfavors the circumstellar medium formed by steady mass loss. The ejecta mass is estimated to be comparable to or less than 15 Msun and the explosion energy is expected to be more than 4e51 erg. No 56Ni is required to explain the early light curve. We find that the multidimensional effect, e.g., the Rayleigh-Taylor instability, which is expected to take place in the cool dense shell between the supernova ejecta and the dense circumstellar medium, is important in understanding supernovae powered by the shock interaction. We also show the evolution of the optical and near-infrared model light curves of high-redshift superluminous supernovae. They can be potentially used to identify SN 2006gy-like superluminous supernovae in the future optical and near-infrared transient surveys.

Direct determination of the hubble parameter using type IIn supernovae

A novel approach, a Dense Shell Method, is proposed for measuring distances for cosmology. It is based on original Baade idea to relate absolute difference of photospheric radii with photospheric velocity. We demonstrate that this idea works: the new method does not rely on the Cosmic Distance Ladder and gives satisfactory results for the most luminous Type IIn Supernovae. This allows one to make them good primary distance indicators for cosmology. Fixing correction factors for illustration, we obtain with this method the median distance of ≈ 68 −15 +19 (68%CL) Mpc to SN 2006gy and median Hubble parameter 79 −17 +23 (68%CL) km/(s Mpc).

SUPERLUMINOUS LIGHT CURVES FROM SUPERNOVAE EXPLODING IN A DENSE WIND

Observations from the last decade have indicated the existence of a general class of superluminous supernovae (SLSNe), in which the peak luminosity exceeds 10^{44} erg/s. Here we focus on a subclass of these events, where the light curve is also tens of days wide, so the total radiated energy is order 10^{51} erg. If the origin of these SLSNe is a core-collapse-driven explosion of a massive star, then the mechanism which converts the explosion energy into radiation must be very efficient (much more than in typical core collapse SNe, where this efficiency is of order one percent). We examine the scenario where the radiated luminosity is due to efficient conversion of kinetic energy of the ejected stellar envelope into radiation by interaction with an optically thick, pre-existing circumstellar material (CSM), presumably the product of a steady wind from the progenitor. We base the analysis on a simple, numerically solved, hydrodynamic diffusion model, which allows us to identify the qualitative behavior of the observable light curves, and to relate them to the parameters of the wind. We specifically show that a wide and superluminous supernova requires the mass of the relevant wind material to be comparable to that of ejected material from the exploding progenitor. We find the wind parameters which explain the peak luminosity and width of the bolometric light curves of three particular SLSNe, namely, SN 2005ap, SN 2006gy, and SN 2010gx, and show that they are best fitted with a wind that extends to a radius of order 10^{15} cm. These results serve as an additional indication that at least some SLSNe are powered by interaction of the ejected material with a steady wind of similar mass.

Quark nova imprint in the extreme supernova explosion SN 2006gy

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 days 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 model. This analysis considers the supernova explosion of a massive star followed days later by the quark nova detonation of a neutron star. We lay out a detailed model of the interaction between the supernova envelope and the quark nova ejecta paying special attention to a mixing region which forms at the inner edge of the supernova envelope. This model is then fit 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 H{\alpha} line and the peculiar blue H{\alpha} absorption. We find that a progenitor mass between 20Msun and 40Msun provides ample energy to power SN 2006gy in the context of a QN.

X-RAYS FROM SUPERNOVA SHOCKS IN DENSE MASS LOSS

Type IIn and related supernovae show evidence for an interaction with a dense circumstellar medium that produces most of the supernova luminosity. X-ray emission from shock heated gas is crucial for the energetics of the interaction and can provide diagnostics on the shock interaction. Provided that the shock is at an optical depth tau_w\la c/v_s in the wind, where c is the speed of light and v_s is the shock velocity, a viscous shock is expected that heats the gas to a high temperature. For tau_w\ga 1, the shock wave is in the cooling regime; inverse Compton cooling dominates bremsstrahlung at higher densities and shock velocities. Although tau_w\ga 1, the optical depth through the emission zone is \la 1 so that inverse Compton effects do not give rise to significant X-ray emission. The electrons may not reach energy equipartition with the protons at higher shock velocities. As X-rays move out through the cool wind, the higher energy photons are lost to Compton degradation. If bremsstrahlung dominates the cooling and Compton losses are small, the energetic radiation can completely photoionize the preshock gas. However, inverse Compton cooling in the hot region and Compton degradation in the wind reduce the ionizing flux, so that complete photoionization is not obtained and photoabsorption by the wind further reduces the escaping X-ray flux. We conjecture that the combination of these effects led to the low observed X-ray flux from the optically luminous SN 2006gy.

SN 2008am: A SUPER-LUMINOUS TYPE IIn SUPERNOVA

We present observations and interpretation of the Type IIn supernova SN 2008am discovered by the ROTSE Supernova Verification Project (RSVP). SN 2008am peaked at approximately -22.3 mag at a redshift of z=0.2338, giving it a peak luminosity of 3 x 10^{44}erg/s and making it one of the most luminous supernovae ever observed. The total radiated energy is ~ 2 x 10^{51} erg. Photometric observations in the ultraviolet, optical and infrared bands (J,H,Ks) constrain the SED evolution. We obtained six optical spectra of the supernova, five on the early decline from maximum light and a sixth nearly a year later plus a very late-time spectrum (~2 yr) of the host galaxy. The spectra of SN 2008am show strong Balmer-line and He I lambda 5876A emission with intermediate widths (~25A) in the first ~40 days after optical maximum. We examine a variety of models for the line wings and conclude that multiple scattering is most likely, implying that our spectra contain no specific information on the bulk flow velocity. We examine a variety of models for the ROTSE light curve subject to the rise time and the nature of the spectra, including radioactive decay, shocks in optically-thick and optically-thin circumstellar media (CSM) and a magnetar. The most successful model is one for which the CSM is optically-thick and in which diffusion of forward shock-deposited luminosity gives rise to the observed light curve. Diffusion of the shock-deposited energy from the forward shock is found to be important to account for the rising part of the light curve. Although there are differences in detail, SN 2008am appears to be closely related to other super-luminous Type IIn supernovae, SN 2006gy, SN 2006tf and perhaps SN 2008iy, that may represent the deaths of very massive LBV-type progenitors and for which the luminosity is powered by the interaction of the ejecta with a dense circumstellar medium.

SHOCK BREAKOUT IN DENSE MASS LOSS: LUMINOUS SUPERNOVAE

We examine the case where a circumstellar medium around a supernova is sufficiently opaque that a radiation dominated shock propagates in the circumstellar region. The initial propagation of the shock front into the circumstellar region can be approximated by a self-similar solution that determines the radiative energy in a shocked shell; the eventual escape of this energy gives the maximum luminosity of the supernova. If the circumstellar density is described by \rho=Dr^{-2} out to a radius R_w, where D is a constant, the properties of the shock breakout radiation depend on R_w and R_d\equiv\kappa Dv_{sh}/c, where \kappa is the opacity and v_{sh} is the shock velocity. If R_w>R_d, the rise to maximum light begins at ~ R_d/v_{sh}; the duration of the rise is also ~ R_d/v_{sh}; the outer parts of the opaque medium are extended and at low velocity at the time of peak luminosity; and a dense shell forms whose continued interaction with the dense mass loss gives a characteristic flatter portion of the declining light curve. If R_w<R_d, the rise to maximum light begins at R_w/v_{sh}; the duration of the rise is R_w^2/v_{sh}R_d; the outer parts of the opaque medium are not extended and are accelerated to high velocity by radiation pressure at the time of maximum luminosity; and a dense shell forms but does not affect the light curve near maximum. We argue that SN 2006gy is an example of the first kind of event, while SN 2010gx and related supernovae are examples of the second.

SPECTRAL EVOLUTION OF THE EXTRAORDINARY TYPE IIn SUPERNOVA 2006gy

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. (abridged)

The extragalactic Supernova Remnant SN2006gy and flaring activity of Mkn 421 and Mkn 501

The observation results of the explosion of the extragalactic Supernova remnant SN2006 gy (z=0.019) are presented. NGC 1275 has been regularly observed by SHALON since 1996. NGC 1275 is being intensively studied and the γ-ray flux is found to be ( 0.78 ± 0.13 ) × 10 − 12 c m − 2 s − 1 . A flux increase was detected from the region NGC 1275 in autumn 2006. The detailed analysis of γ-shower direction resulted in the detection of a metagalactic object. This object was identified with the supernova SN 2006gy that is about 10′ away from NGC 1275. The integral γ-ray flux for SN 2006gy is found to be ( 3.51 ± 0.20 ) × 10 − 12 c m − 2 s − 1 at energies of > 0.8 TeV . The integral average γ-ray fluxes of Mkn 421 and Mkn 501 were estimated as ( 0.63 ± 0.14 ) c m − 2 s − 1 and ( 0.86 ± 0.13 ) c m − 2 s − 1 respectively. An increase of the flux over the average value was detected in the 1997 and 2004 observations of Mkn 421 by SHALON and estimated to be ( 1.01 ± 0.25 ) c m − 2 s − 1 and ( 0.96 ± 0.2 ) c m − 2 s − 1 , respectively. A significant increase of Mkn 501 flux was detected in 1997 with VHE ground telescopes worldwide.

Nucleosynthesis of the Elements in Faint Supernovae and Hypernovae

AbstractWe review the properties of supernovae (SNe) as a function of the progenitor's massM. (1)Mup- 10M⊙stars are super-AGB stars and resultant electron capture SNe may be Faint supernovae like Type IIn SN 2008S. (2) 10 - 12M⊙stars undergo Fe-core collapse to form neutron stars (NSs) and Faint supernovae. (3) 12M⊙-MBNstars undergo Fe-core collapse to form NSs and normal core-collapse supernovae. (4)MBN- 90M⊙stars undergo Fe-core collapse to form Black Holes. Resultant supernovae are bifurcate into Hypernovae and Faint supernovae. The observed properties of SN 2008ha can be explained with this type of Faint supernovae. (5) 90 - 140M⊙stars produce Luminous SNe, like SNe 2007bi and 2006gy. (6) 140 - 300M⊙stars become pair-instability supernovae which could be Luminous supernovae (SNe 2007bi and 2006gy). (7) Very massive stars withM≳ 300M⊙undergo core-collapse to form intermediate mass black holes. Some SNe could be more Luminous supernovae (like SN 2006gy).

EXTREMELY LUMINOUS SUPERNOVA 2006gy AT LATE PHASE: DETECTION OF OPTICAL EMISSION FROM SUPERNOVA

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. (abridged)