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Asymmetries in core-collapse supernovae from maps of radioactive 44Ti in Cassiopeia A

Abstract

Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive 44Ti, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium1, directly probes the explosion asymmetries. Cassiopeia A is a young2, nearby3, core-collapse4 remnant from which 44Ti emission has previously been detected5,6,7,8 but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed 44Ti emission to estimated 56Ni emission9, from optical light echoes10, and from jet-like features seen in the X-ray11 and optical12 ejecta. Here we report spatial maps and spectral properties of the 44Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the 44Ti and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae.

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Figure 1: The broadband hard-X-ray spectrum of Cas A.
Figure 2: A comparison of the spatial distribution of the 44Ti with the known jet structure in Cas A.
Figure 3: A comparison of the spatial distribution of 44Ti with known Fe K-shell emission in Cas A.

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Acknowledgements

This work was supported by NASA under grant no. NNG08FD60C, and made use of data from the Nuclear Spectroscopic Telescope Array (NuSTAR) mission, a project led by Caltech, managed by the Jet Propulsion Laboratory and funded by NASA. We thank the NuSTAR operations, software and calibration teams for support with execution and analysis of these observations.

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Authors and Affiliations

Authors

Contributions

B.W.G.: reduction and modelling of the NuSTAR Cas A observations, interpretation, manuscript preparation. F.A.H.: NuSTAR principal investigator, observation planning, interpretation of results and manuscript preparation. S.E.B.: interpretation, manuscript review. S.P.R.: interpretation, manuscript preparation and review. C.L.F.: interpretation of results, manuscript review. K.K.M.: observation planning, data analysis, manuscript review. D.R.W.: background modelling, data analysis, manuscript review. A.Z.: background modelling, manuscript review. C.I.E.: supernova simulations, manuscript review. H.A.: image deconvolution, manuscript review. T.K.: detector modelling, data analysis, manuscript review. H.M., V.R., P.H.M.: detector production, response modelling, manuscript review. M.J.P.: optics calibration, manuscript review. S.P., M.P.: analysis software, calibration, manuscript review. K.F.: observation planning. F.E.C.: optics production and calibration, manuscript review. W.W.C.: optics and instrument production and response, observation planning, manuscript review. C.J.H.: optics production and response, interpretation, manuscript review. J.E.K.: optics production and response, manuscript review. N.J.W.: manuscript review, calibration. W.W.Z.: optics production and response, manuscript review. D.M.A., D.B., P.G., A.H., V.M.K., D.S.: science planning, manuscript review.

Corresponding authors

Correspondence to B. W. Grefenstette or F. A. Harrison.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 The background-subtracted image of Cas A in the 65–70-keV band containing the 68-keV 44Ti line showing the significance of the 44Ti knots.

The data have been smoothed with a 20′′-radius top-hat function (dashed circle) and are shown with 3σ and 4σ significance contours (green). In addition to the features shown in Fig. 1, here we also show locations of the forward (R ≈ 150′′) and reverse (R ≈ 100′′) shocks17 (white dashed circles), for context. The 44Ti clearly resolves into several significantly identified clumps that are non-uniformly distributed around the centre of expansion.

Extended Data Figure 2 The radial profile of the 44Ti emission.

We collect each photon in annular bins of increasing radius in the plane of the sky without any spatial smoothing. a, Radial profile of the 44Ti data in the 65–70-keV band (black) and the radial profile expected from the background images (red), scaled by the area of each annulus and shown in units of counts per square arcsec. b, Background-subtracted radial profile. c, Percentage of enclosed flux in annuli of increasing radii as observed on the plane of the sky. All error bars are 1σ.

Extended Data Figure 3 Simulated 44Ti intensity contours for a symmetric explosion and a bipolar explosion.

The vertical line shows a 4′ scale (note the different spatial scale between the symmetric (left) and bipolar (right) explosions). The non-uniformities in the observed 44Ti spatial distribution rule out the purely symmetric explosion, even with extensive mixing. Similarly, the presence of 44Ti away from the jet axis argues against the rapidly rotating progenitor that produced the bipolar explosion. We therefore argue that the explosion that produced Cas A is somewhere between these two extremes and that this is the first clear example of a low-mode convection explosion.

Extended Data Figure 4 The background spectral model fit for one of the Cas A epochs.

Shown are the data from the background regions (black points with 1σ error bars included but not visible), the instrumental background (green), the CXB components (blue, dashed is the focused CXB component), the phenomenological ‘source’ model (magenta) and the total background model (red). Inset, background spectrum near the 44Ti emission lines showing the features that we model. The broad lines at 65 and 75 keV are probably neutron-capture emission features, and the narrow line near 67 keV is an internal activation line in the CdZnTe detectors. See Methods for more details.

Extended Data Figure 5 The significant signals observed in the spectrum near 68 and 78 keV.

Top, the black points (1σ error bars) are the data shown after the background model spectrum has been subtracted from the source data. The red continuum is the best-fit power-law continuum over the 20–80-keV band pass. Bottom, the contribution to the C-stat statistics for each spectral bin. The large signals near 68 and 78 keV (the 44Ti emission lines) suggest that an additional spectral component is required. See Methods for details.

Extended Data Table 1 List of observations used in this analysis
Extended Data Table 2 Best-fit continuum parameters
Extended Data Table 3 Results from spectral analysis

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Grefenstette, B., Harrison, F., Boggs, S. et al. Asymmetries in core-collapse supernovae from maps of radioactive 44Ti in Cassiopeia A. Nature 506, 339–342 (2014). https://doi.org/10.1038/nature12997

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