Early ⁵⁶ Ni decay gamma rays from SN2014J suggest an unusual explosion

Abstract

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 56 Ni. The 56 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 56 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 56 Ni. Some mechanism must break the spherical symmetry of the supernova, and at the same time create a major amount of 56 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 56 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...

The e-ASTROGAM mission

“…INTEGRAL could detect the long awaited gamma-ray signatures of the thermonuclear runaway, through the early emission from the decay of 56 Ni (mean lifetime τ 8.8 days) about 20 days after the explosion, and the main gamma-ray lines at 847, 1238, and 511 keV from the decay of 56 Co (τ 111 days). These data suggested either a surface explosion or some unusual morphology of the runaway [44,45,53,54,76], either case in stark contrast to the conventional Chandrasekhar model. Clearly, the glimpse offered by SN2014J observations underlines the importance of gamma-ray line diagnostics in these systems and emphasize that more and better observations hold the key to a deeper understanding of how the thermonuclear explosion of a white dwarf star unfolds.…”

Revisiting INTEGRAL/SPI observations of44Ti from Cassiopeia A

“…We then fix the shapes of the spectral templates during one orbit of the selected data set and scale the amplitude of each component for the entire Ge camera by pointing to properly trace intensity variations of each background component. Long-term investigations showed that the relative intensities of a particular spectral background feature present in one detector with respect to the mean intensity of the same feature in all detectors (detector ratios) are constant in time and thus are maximally independent of possible celestial signals (Diehl et al 2014(Diehl et al , 2015 1 . Therefore, the spectral shape and detector ratio constancy that is expected for each individual background component is imprinted onto the shortterm (pointing-to-pointing) variations, while the mean amplitude (over all detectors) of a particular spectral feature is still allowed to vary to normalize the predicted background count rate.…”

Supernova 2014J at M82 – II. Direct analysis of a middle-class Type Ia supernova

“…The general results from INTEGRAL, which detected the 847 and 1238 keV γ -ray lines of radioactive 56 Co (Churazov et al 2014(Churazov et al , 2015Diehl et al 2014Diehl et al , 2015, showed that the general nickel distribution was stratified with a total 56 Ni mass of ∼0.5 M . Due to the low signal to noise, there is some ambiguity about just how to interpret the results, that is, whether the γ -rays favour symmetric nickel distributions (Churazov et al 2014(Churazov et al , 2015 or whether the lines are significantly Doppler-broadened asymmetric ones (Diehl et al 2014(Diehl et al , 2015. We are not terribly sensitive to small asymmetries, and therefore we cannot distinguish between these different interpretations.…”

“…modern SN Ia, SN 2014J has been extremely well observed: in γ -rays (Churazov et al 2014;Diehl et al 2014), X-rays (Margutti et al 2014), by Hubble Space Telescope Kelly et al 2014), optical (Amanullah et al 2014;Ashall et al 2014;Goobar et al 2014;Nielsen et al 2014) at high resolution (Welty et al 2014;Graham et al 2015;Jack, Baron & Hauschildt 2015;Ritchey et al 2015), near-infrared (near-IR; Friesen et al 2014;Marion et al 2015), mid-IR (Johansson et al 2014;Telesco et al 2015), polarimetry (Kawabata et al 2014), with rapid photometry (Siverd et al 2015), and in radio (Pérez-Torres et al 2014).…”