SN2014J gamma rays from the⁵⁶Ni decay chain

Abstract

Context. The detection and measurement of gamma-ray lines from the decay chain of 56 Ni provides unique information about the explosion in supernovae. SN2014J at 3.3 Mpc is a sufficiently-nearby supernova of type Ia so that such measurements have been feasible with the gamma-ray spectrometer SPI on ESA's INTEGRAL gamma-ray observatory. Aims. The 56 Ni freshly produced in the supernova is understood to power the optical light curve, because it emits gamma rays upon its radioactive decay first to 56 Co and then to 56 Fe. Gamma-ray lines from 56 Co decay are expected to become directly visible through the white dwarf material several weeks after the explosion, as they progressively penetrate the overlying material of the supernova envelope, which is diluted as it expands. The lines are expected to be Doppler-shifted or broadened from the kinematics of the 56 Ni ejecta. We aim to exploit high-resolution gamma-ray spectroscopy with the SPI spectrometer on INTEGRAL toward constraining the 56 Ni distribution and kinematics in this supernova. Methods. We use the observations with the SPI spectrometer on INTEGRAL, together with an improved instrumental background method. Results. We detect the two main lines from 56 Co decay at 847 and 1238 keV, which are significantly Doppler-broadened, and at intensities (3.65 ± 1.21) × 10 −4 and (2.27 ± 0.69) × 10 −4 ph cm −2 s −1 , respectively, at their brightness maximum. We measure their rise toward a maximum after about 60-100 days and a decline thereafter. The intensity ratio of the two lines is found to be consistent with expectations from 56 Co decay (0.62 ± 0.28 at brightness maximum, the expected ratio is 0.68). We find that the broad lines seen in the late, gamma-ray transparent phase are not representative of the early gamma-ray emission, and notice instead that the emission spectrum is complex and irregular until the supernova is fully transparent to gamma rays, with progressive uncovering of the bulk of 56 Ni. We infer that the explosion morphology is not spherically symmetric, both in the distribution of 56 Ni and in the unburnt material which occults the 56 Co emission. After we compare light curves from different plausible models, the resulting 56 Ni mass is determined to be 0.49 ± 0.09 M .

A reddening-free method to estimate the56Ni mass of Type Ia supernovae

“…Churazov et al (2014) derive 56 Ni = 0.62 ± 0.13 M . Diehl et al (2015) find a slightly lower mass of M56 Ni = 0.56 ± 0.10 M .…”

“…Since t 2 is independent of reddening, we can use the derived correlation to determine the peak luminosity and estimate the 56 Ni mass for heavily reddened SNe Ia. We tested this relation on SN 2014J, which has a direct γ-ray detection from the 56 Ni → 56 Co decay chain (Churazov et al 2014;Diehl et al 2015). Using the best fit relation for the reddening-free sample, we obtain M56 Ni = 0.64 ± 0.15 M for a t 2 = 31.99 ± 1.15 days and a rise time of 19 days.…”

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.…”

“…4 in Ref. [54], red data points) are compared to a family of candidate models [137]. A simulation of the response to a time evolution such as in the W7 model [99] shows that the sensitivity improvement by e-ASTROGAM (blue points) will lead to breakthrough science.…”

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.…”