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SUPERNOVA EMISSION FROM GAMMA RAYS TO THE INFRARED Anders Jerkstrand, Stockholm University 1 Supernovae A supernova is the explosion of a star, which occurs about once per century per galaxy. When they occur, they often outshine the entire galaxy for a few weeks. SN1994D in NGC 4526 2 Supernovae These stellar explosions can be of two types Gravitational collapse of a massive star (M > 8 Msun). Neutron star or black hole forms. Labelled Type II, Ib or Ic. Thermonuclear explosion of a white dwarf approaching the Chandrasekhar mass. No remnant. Labelled Type 1A. 3 Supernovae In both cases , about 1051 ergs is liberated in the form of radiation (~1%) and kinetic energy (~99%). This gives expansion velocities of ~104 km/s, giving broad spectral lines. 4 Supernovae This energy (1051 ergs) agrees well with the thermonuclear energy of burning ~ 1.4 Msun of carbon/oxygen to iron. (~0.001*Mc²) But the binding energy of a neutron star is >~1053 ergs. (3/5GM2/R with M=1.4 Msun and R=10 km). Where does that energy go? 5 Supernovae In 1987, Kamiokande II, IMB and Baksan gave us the definite answer by registering 24 neutrinos from SN1987A, three hours before the explosion was observed in electromagnetic bands. Some 99% of the 1053 ergs released in the neutron star formation emerges as neutrinos. 6 Kamiokande II Outbreak As the shock wave breaks through the surface, there is an initial and brief (~minutes) burst of hard radiation at UV and Xrays. The only such outburst observed (!), lasting 6 minutes, occured in 2008 as Swift was pointed in the right direction at the right time (SN2008D). Xrays Optical a few days later 7 Lightcurve After the breakout, the supernova cools quickly and behaves as an expanding fireball at ~500010,000 K. The luminosity increases as the surface area of the photosphere grows, then peaks and falls off as the gas becomes optically thin. 8 Lightcurve After a few weeks or months, the lightcurve is seen to settle down into a strict exponential decay L ~ exp( t/) with ~111 days. Since this matches the decay time of the radioactive 56Co, Colgate & McKee suggested in 1969 that this isotope powers supernovae in this phase. 9 Nucleosynthesis Modern hydrodynamical explosion models confirm that substantial amounts of radioactive elements are created in the explosion, the most important being 56 Ni (which quickly decays to 56Co), 57Ni and 44Ti. Explosion model of A 25 Msun star from Woosley & Weaver 1995. 10 THE ENERGY INPUT BY THE THREE DOMINANT RADIOACTIVE ELEMENTS Power [ergs/second] Total power Using the masses determined From SN1987A: M(Co56) = 0.07 Msun M(Co57) = 3.3*103 Msun M(Ti44) = 1*104 Msun Co56 Co57 Ti44 Co56phase Co57phase Ti44phase Time [years] 11 Radioactive decays The decays usually occur by electron capture, (electron) or + (positron) channels to an excited state in the (A,Z+1), (A,Z1) daughter nucleus. This state deexcites by the emission of gamma rays, with energies often exceeding the first step. Ex for 56Co: 3% of the energy emitted in positrons. 97% emitted as gamma rays (mainly at 847 and 1238 keV). 12 Gamma rays The gamma rays Compton scatter off free and bound electrons. From the KleinNishina cross section it can be seen that the ejecta becomes optically thin to the gamma rays after a few years. Detection of the gamma rays Hard Xrays, being Compton downscattered gamma rays from 56Co decay, were detected from 1987A by the Ginga satellite and by the Kvant module on Mir already after ~6 months. 14 Detection of the gamma rays Soon, also direct detection of the 56Co gamma rays (at 847 and 1,238 keV) from SN1987A were made by the Solar Maximum Mission (SMM). 15 Detection of the gamma rays The detection of Xrays and gamma rays occured much too early for the 56 Co to reside deep in the core, protected by the envelope. The conclusion was that mixing has occured where 56 Coclumps have been propelled far out into the envelope. Carbon Oxygen Nickel 3D explosion of a 16 ZAMS progenitor. Three components displayed. 16 Detection of the gamma rays For the 44Ti gamma rays, the emission from 1987A has been too weak to be detected. However, in 1994 COMPTEL detected the 1157 keV line from 320yearold supernova remnant Cas A. Later, the 68 and 78 keV lines were detected with BeppoSAX. . 17 Supernova model spectra In Stockholm, part of the work is aimed at computing supernova model spectra in late phases (t >~ 1 year), and comparing those with observations. To do that, a large set of coupled, nonlinear equations have to be solved. 18 Energy degradation The first step is to compute how the gamma rays (and the fast electrons and positrons) produce a set of fast ”secondary” electrons that in turn cause heating, excitation and ionization in the supernova gas. Fast electron hits free electron → HEATING Fast electron hits internal transition in atom → EXCITATION + Fast electron hits collisional ionization 19 channel in atom → IONIZATION Thermal and statistical equlibrium Next, the temperature, the ionization balance and the nonLTE level populations are solved for in each zone by setting up the equations for thermal and statistical equilibrium. These equations also depend on the internal radiation field, which is solved for by doing radiative transfer with a MonteCarlo technique. For a 1D explosion model, typical run times to convergence are ~ 1 day on a 100 CPU parallell computer. 20 Radiative transfer The supernova gas is in a Hubble flow just like the galaxies in the universe. A photon is continously redshifting with respect to the comoving frame. 5000 Å 5100 Å EXPLOSION CENTER Gas parcels 5150 Å 21 Radiative transfer 104 km/s The fact that each photon has to pass many lines (all that it redshifts past before reaching the edge of the nebula) makes line absorptions very important in supernovae. Ex : a photon emitted at 3000 Å redshifts to 3100 before reaching the edge of the nebula at ~ 104 km/s. The line opacity is the sum of all lines between 3000 and 3100 Å. 3000 Å 3100 Å 22 Radiative transfer Especially in the UV, the irongroup elements have thousands of lines that together form a formidable opacity. Photons at UV wavelengths are therefore easily absorbed and fluoresce to longer wavelengths. 23 Radiative transfer This line opacity commonly makes supernovae very weak in the UV band, while at the same time adding fluorescence to the optical/NIR bands. 24 The 44 Ti-phase After ~5 years, supernovae enter the 44Ti dominated phase. The ejecta is by now largely optically thin to the gamma rays, but the positrons are efficiently trapped. Ti has a long lifetime (~86 years) and the output therefore changes slowly in this phase. 44 25 The 44 Ti-phase The nebula is now cold (~100 K), mostly neutral, and at densities of ~ 105 particles cm3 (t/10 years)3. The cooling is done by midIR lines such as [Fe II] 26 um. The dust is probably still optically thick and emits a blackbody spectrum of similar temperature. All in all, about 5070% of the deposited energy probably emerges in the midIR (350 um) in this phase. 26 The 44 Ti-phase However, excitations and ionizations still produce emission in the UV/optical/NIR. We have shown by modeling of the 1987A spectrum at 8 years (Jerkstrand et al, in prep.) and 20 years (Kjaer et al 2010, in press), that ~1*104 Msun of 44Ti is powering the deepest core regions in the supernova, producing emission in newly synthesized iron, silicon and oxygen. 27 The 44 Ti-phase Kjaer et al, 2010, in press Observed spectrum Model ● Silicon Calcium Iron Helium 28 Dust in supernovae Conditions favorable for dust formation include high densities (>~ 107 cm3) and low temperatures (<~ 1500 K). In supernovae, the temperature is initially too high, and eventually the density becomes too low. But there is an intermediate epoch (~12 years after explosion) when conditions are appropriate. 29 Dust in supernovae In 1987A, a systematic shift of the line peaks towards the blue coincided with a rapid luminosity decrease starting at day ~530. The interpretation was that dust was forming, preferentially blocking out the far (receding) side of the nebula. Line profile Frequency 30 Soon, a 300 K blackbody continuum was rising in the mid IR, as detected by the Kuiper Airborn Observatory. 31 Dust in supernovae Dust formation has also been observed in other (gravitationalcollapse) supernovae (for example 1999em, 2004et, Cas A), usually with Spitzer. Cas A in optical and at 24 um. The dust temperature is 80 K. It appears that the amount of dust formed in supernovae is quite small (~104 102 Msun), and supernovae are currently not believed to be a major 32 production site for the dust in the universe. Dust in supernovae However, it is possible that most of the dust is very cold in supernova remnants, and escapes detection by Spitzer. Submm detection by SCUBA of Cas A and Kepler have been controversially interpreted as the presence of large (~1 Msun) dust masses. Herschel imaging of Cas A has recently verified a cold (35 K) dust component with M ~ 0.075 Msun. But modeling is tricky! 33 Barlow et al, 2010 : Cas A farIR with Herschel. Many components, also from the ISM! 34 Summary Supernovae are stellar explosions that produce emission in all electromagnetic bands for many decades or centuries. Except for the first few weeks, they are powered by radioactive decays from 56Co, 57Co and 44Ti. The computation of model spectra requires computing the degradation of gamma rays and positrons, solving the equations of thermal and statistical equilibrium, and performing Hubbleflow 35 radiative transfer. Summary At late time, the supernova emits mainly gamma rays from the 44Ti decay and midIR emission from gas and dust at ~100 K, but also some UV/optical/NIR emission from fastelectron excitations and ionizations. THANK YOU 36