Ni decay γ-rays from SN 2014 J suggest an unusual explosion

Type-Ia supernovae result from binary systems that include a carbon-oxygen white dwarf, and these thermonuclear explosions typically produce 0.5 M ! of radioactive Ni. The Ni is commonly believed to be buried deeply in the expanding supernova cloud. Surprisingly, in SN2014J we detected the lines at 158 and 812 keV from Ni decay (τ~8.8 days) earlier than the expected several-week time scale, only ~20 days after the explosion, and with flux levels corresponding to roughly 10% of the total expected amount of Ni. Some mechanism must break the spherical symmetry of the supernova, and at the same time create a major amount of Ni at the outskirts. A plausible explanation is that a belt of helium from the companion star is accreted by the white dwarf, where this material explodes and then triggers the supernova event. SN2014J was discovered on January 22, 2014 (1), in the nearby starburst galaxy M82, and was classified as a supernova of type Ia (SN Ia) (2). This is the closest SN Ia since the advent of gamma-ray astronomy. It reached its optical brightness maximum on Feb 3, 20 days after the explosion on January 14.75 UT (3). At a distance of 3.5 Mpc (4), a most-detailed comparison of models to observations across a wide range of wavelengths appears feasible, including gammaray observations from the Ni decay chain. Calibrated lightcurves of SNe Ia have become standard tools to determine cosmic distances and the expansion history of the universe (5), but we still do not understand the physics that drives their explosion (6,7). Their extrapolation as distance indicators at high redshifts, where their population has not been empirically studied, can only be trusted if a physical model is established (5). Unlike core-collapse supernovae, which obtain their explosion energy from their gravitational energy, SNe Ia are powered by the release of nuclear binding energy through fusion reactions. It is generally believed that carbon fusion reactions ignited in the degenerate matter inside a white dwarf star lead to a runaway. This sudden release of a large amount of nuclear energy is enough to overcome the binding energy of such a compact star, and thus causes a supernova explosion of type Ia. A consensus had been for years that the instability of a white dwarf at the Chandrasekhar-mass limit in a binary system with a main sequence or (red-) giant companion star was the most plausible model to achieve the apparent homogeneity (6). However, observations have revealed an unexpected diversity in type-Ia supernovae in recent years (8), and increasing model sophistication along with the re-evaluations of more exotic explosion scenarios have offered plausible alternatives. The consensus now leans towards a broader range of binary systems and more methods of igniting a white dwarf, independent of its mass. Destabilizing events such as accretion flow instabilities, He detonations, mergers or collisions with a degenerate companion star are being considered (9-12). # This manuscript has been accepted for publication in Science. This version has not undergone final editing. Please refer to the complete version of record at http://www.sciencemag. org/. The manuscript may not be reproduced or used in any manner that does not fall within the fair use provisions of the Copyright Act without the prior, written permission of AAAS.