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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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.

Author information

Affiliations

  1. Cahill Center for Astrophysics, 1216 East California Boulevard, California Institute of Technology, Pasadena, California 91125, USA

    • B. W. Grefenstette
    • , F. A. Harrison
    • , K. K. Madsen
    • , K. Forster
    • , P. H. Mao
    • , H. Miyasaka
    •  & V. Rana
  2. Space Sciences Laboratory, University of California, Berkeley, California 94720, USA

    • S. E. Boggs
    • , A. Zoglauer
    •  & W. W. Craig
  3. Physics Department, North Carolina State University, Raleigh, North Carolina 27695, USA

    • S. P. Reynolds
  4. CCS-2, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

    • C. L. Fryer
  5. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

    • D. R. Wik
    •  & W. W. Zhang
  6. Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA

    • C. I. Ellinger
  7. Department of Physics, Durham University, Durham DH1 3LE, UK

    • D. M. Alexander
  8. Department of Physics, McGill University, Rutherford Physics Building, Montreal, Quebec H3A 2T8, Canada

    • H. An
    •  & V. M. Kaspi
  9. Université de Toulouse, UPS-OMP, IRAP, 9 Avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France

    • D. Barret
  10. CNRS, Institut de Recherche en Astrophysique et Planétologie, 9 Avenue colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France

    • D. Barret
  11. DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby, Denmark

    • F. E. Christensen
    • , A. Hornstrup
    •  & N. J. Westergaard
  12. Lawrence Livermore National Laboratory, Livermore, California 94550, USA

    • W. W. Craig
    •  & M. J. Pivovaroff
  13. Agenzia Spaziale Italiana (ASI) Science Data Center, Via del Politecnico snc, I-00133 Roma, Italy

    • P. Giommi
    • , M. Perri
    •  & S. Puccetti
  14. Columbia Astrophysics Laboratory, Columbia University, New York, New York 10027, USA

    • C. J. Hailey
    •  & K. Mori
  15. RIKEN, Nishina Center, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan

    • T. Kitaguchi
  16. Kavli Institute for Particle Astrophysics and Cosmology, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    • J. E. Koglin
  17. INAF – Osservatorio Astronomico di Roma, via di Frascati 33, I-00040 Monteporzio, Italy

    • M. Perri
    •  & S. Puccetti
  18. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA

    • D. Stern

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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.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

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

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https://doi.org/10.1038/nature12997

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