Tuesday, September 09, 2014

Looking into the heart of a supernova explosion

Fig. 1: Top: The decay chain 56Ni -> 56Co -> 56Fe releases large amounts of energy as gamma-ray photons and positrons. Bottom: Predicted spectrum of emerging gamma-rays. Initially, most of the energy from the nickel decay chain is re-processed in the expanding, ejected material of the supernova, giving rise to powerful optical emission. After some time, the ejected material becomes transparent enough that the majority of gamma-ray photons can escape to form characteristic spectral features (shown here 75 days after the explosion).

Fig. 2: Spectrum of type II supernova SN1987A in Large Magellanic Cloud, observed 27 years ago using X-Ray devices aboard of MIR space station (Sunyaev et al., 1987). A detailed analysis of the observed spectrum proved that the scientists were dealing with gamma-rays of radioactive cobalt decay down-scattered in the optically thick envelope to the 20-200 keV spectral band due to multiple Compton scatterings and recoil effect.

Fig. 3: Spectrum of type Ia SN2014J obtained by INTEGRAL, 50 to 100 days after the explosion (Churazov et al., 2014). Red and blue points show data from the two instruments SPI and ISGRI/IBIS respectively. The black curve shows a fiducial model of the supernova spectrum for day 75 after the explosion. The top row shows images obtained in three high-energy spectral bands by INTEGRAL. A gamma-ray source is clearly visible in all images at the (optical) position of SN2014J. 

Fig. 4: Gamma-ray lines from 56Co decay in the expanding ejecta, broadened by the Doppler effect. 

First detection of Cobalt gamma-ray lines from a type Ia supernova (SN2014J) with INTEGRAL

The exceptional brightness and regular light-curves in the optical band have made supernova explosions of Type Ia (SNIa) a valuable "standard candle" in modern Cosmology. However, SNIa’s have never been detected directly in gamma-rays, limiting the analysis to re-processed emission and the outer layers of the material ejected in the stellar explosion. This year, however, a new supernova exploded in the nearby spiral galaxy M82. SN2014J, as the supernova was called, was close enough to ensure the first ever detection of gamma-ray lines by scientists at the Max Planck Institute for Astrophysics (MPA) - providing an unambiguous proof of the theoretical concept of SNIa's

A Type Ia supernova (SNIa) is believed to be the thermonuclear explosion of a white dwarf star - the stellar remnant of a normal star, such as our Sun, after the star has exhausted its hydrogen fuel. Such a white dwarf consists mainly of carbon and oxygen - the ashes of hydrogen and helium burning - and during the supernova explosion large amounts of a radioactive nickel isotope (56Ni) are produced. The subsequent decay chain from nickel to cobalt and eventually iron (see Fig. 1) releases huge amounts of energy in the form of highly energetic gamma-ray photons. These are re-processed in the expanding material ejected by the explosion, giving rise to a powerful optical emission, which has become an invaluable tool in cosmological studies as a distance indicator. 

Despite a long history of observations and simulations, the detailed physics of a SNIa explosion and the evolutionary path, which the compact object has to follow towards the explosion, remain a matter of debate. The majority of models predict that during the first 10-20 days after the explosion the ejected material is opaque for gamma-ray lines due to Compton scattering. As the ejected material becomes progressively more transparent, a large fraction of gamma-rays can finally escape. 

However, gamma-ray emission from SNIa's has never been detected, primarily because the objects were too far away. More than 27 years ago, hard X-rays and direct gamma-rays were detected from the supernova SN1987A in the Large Magellanic Cloud; this was the nearest core collapse supernova (a Type II) in recent history. Even though Type Ia events are intrinsically brighter, they are more rare and have remained elusive in gamma-rays until now. 

"In August 1987 we were very lucky", remembers MPA Director Rashid Sunyaev, "when we - together with the group of Prof. Joachim Trümper at the Max Planck Institute for Extraterrestrial Physics - detected extremely unusual hard X-Ray radiation (Fig. 2) coming from the Type II supernova SN1987A. Back then, we were able to use X-Ray devices aboard the MIR space station for these observations. And we are lucky again this year: three million seconds of INTEGRAL spacecraft observations permitted us to detect a Type Ia supernova with huge luminosity in two narrow gamma-ray lines." 

On 15 January 2014, a SNIa exploded in the spiral galaxy M82 and was discovered by S.J. Fossey and a team of students from University College London a few days later. At a distance of just over 10 million light-years, this is the nearest SNIa in at least four decades. The proximity of this supernova, dubbed SN2014J, triggered many follow-up observations, including those by ESA's gamma-ray observatory INTEGRAL. The data, taken by INTEGRAL between 50 and 100 days after the explosion, clearly show the two brightest of the expected cobalt gamma-ray lines at 847 and 1238 keV (see Fig. 3). Additionally, the flux at lower energies (200-400 keV) agrees with theoretical predictions as well. 

"The line fluxes suggest that a large amount of radioactive nickel was synthesized during the explosion, more than half the mass of our Sun," explains Eugene Churazov, the lead author of this study. Both observed gamma-ray lines are strongly broadened due to Doppler effect. This suggests that the cloud of radioactive materials expands with velocities of about 10000 km/s. Initially, the material is so dense that the gamma-rays produced by the radioactive decay of nickel to cobalt (with a typical time scale of 9 days) loose most of their energy due to Compton scattering and recoil effect. The subsequent decay of cobalt to iron takes much longer, about 111 days. During this time the ejected material becomes increasingly transparent, ultimately allowing the gammy-rays to escape and making SNIa a long-lasting source of gamma-rays. 

Further comparisons with several popular theoretical models, based on detailed calculations of the nucleosynthesis processes during the explosion, reveal good agreement of the SN2014J data with "canonical" models of SNIa explosions, where a white dwarf reaches the critical Chandrasekhar-mass and detonates. Strongly sub-Chandrasekhar-mass models or pure detonation models can already be ruled out by these observations. 

The overall, good agreement with the "canonical" models shows that in gamma-rays SN2014J looks like a proto-typical SNIa, even if strong and complicated extinction in the optical band makes the analysis challenging. "The INTEGRAL data provide unambiguous proof that SN2014J and therefore Type Ia supernovae are a thermonuclear explosion," concludes Eugene Churazov. "The data are consistent with the explosion of a white dwarf just massive enough to be unstable to gravitational collapse, but do not exclude merger scenarios that fuse comparable amounts of nickel."

E.Churazov, R.Sunyaev, J.Isern, J.Knödlseder, P.Jean, F.Lebrun, N.Chugai, S.Grebenev, E.Bravo, S.Sazonov, M.Renaud
Dr. Eugene Churazov
Max-Planck-Institut für Astrophysik, Garching
Telefon: +49 98 30000-2219

Original publications:

E.Churazov, R.Sunyaev, J.Isern, J.Knödlseder, P.Jean, F.Lebrun, N.Chugai, S.Grebenev, E.Bravo, S.Sazonov, M.Renaud 56CO gamma-ray emission lines from the type Ia supernova SN 2014J, Nature, Aug 28th, 2014

R. Sunyaev, A. Kaniovsky, V. Efremov, M. Gilfanov, E. Churazov, S. Grebenev, A.  Kuznetsov, A. Melioranskiy, N. Yamburenko, S. Yunin, D. Stepanov, I. Chulkov, N.  Pappe, M. Boyarskiy, E. Gavrilova, V. Loznikov, A. Prudkoglyad, V. Rodin, C.  Reppin, W. Pietsch, J. Engelhauser, J. Trümper, W. Voges, E. Kendziorra, M.  Bezler, R. Staubert, A. C. Brinkman, J. Heise, W. A. Mels, R. Jager, G. K.  Skinner, O. Al-Emam, T. G. Patterson & A. P. Willmore.Discovery of hard X-ray emission from supernova 1987A, Nature 330, 227 - 229 (19 November 1987)