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.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. 1.

    et al. Trends in 44Ti and 56Ni from core-collapse supernovae. Astrophys. J. Suppl. Ser. 191, 66–95 (2010)

  2. 2.

    , & The expansion center and dynamical age of the galactic supernova remnant Cassiopeia A. Astrophys. J. 122, 297–307 (2001)

  3. 3.

    , , & The three-dimensional structure of the Cassiopeia A supernova remnant. I. The spherical shell. Astrophys. J. 440, 706–721 (1995)

  4. 4.

    et al. The Cassiopeia A supernova was of type IIb. Science 320, 1195–1197 (2008)

  5. 5.

    et al. COMPTEL observations of Ti-44 gamma-ray line emission from Cas A. Astron. Astrophys. 284, L1–L4 (1994)

  6. 6.

    et al. Detection of the 67.9 and 78.4 keV lines associated with the radioactive decay of 44Ti in Cassiopeia A. Astrophys. J. 560, L79–L82 (2001)

  7. 7.

    & Limits to the Cassiopeia A 44Ti line flux and constraints on the ejecta energy and the compact source. Astrophys. J. 582, 257–261 (2003)

  8. 8.

    et al. The signature of 44Ti in Cassiopeia A revealed by IBIS/ISGRI on INTEGRAL. Astrophys. J. 647, L41–L44 (2006)

  9. 9.

    , , , & The high ratio of 44Ti/56Ni in Cassiopeia A and the axisymmetric collapse-driven supernova explosion. Astrophys. J. 492, L45–L48 (1998)

  10. 10.

    et al. Direct confirmation of the asymmetry of the Cas A supernova with light echoes. Astrophys. J. 732, 3 (2011)

  11. 11.

    et al. A million second Chandra view of Cassiopeia A. Astrophys. J. 615, L117–L120 (2004)

  12. 12.

    et al. The expansion asymmetry and age of the Cassiopeia A supernova remnant. Astrophys. J. 645, 283–292 (2006)

  13. 13.

    & The evolution and explosion of massive stars. II. Explosive hydrodynamics and nucleosynthesis. Astrophys. J. Suppl. Ser. 101, 181–235 (1995)

  14. 14.

    et al. Improved measurement of the 44Ti half-life from a 14-year long study. Phys. Rev. C 74, 065803 (2006)

  15. 15.

    et al. The Nuclear Spectroscopic Telescope ARray (NuSTAR) high-energy X-ray mission. Astrophys. J. 770, 103 (2013)

  16. 16.

    , & Hydrodynamical neutron star kicks in three dimensions. Astrophys. J. 725, L106–L110 (2010)

  17. 17.

    et al. Chandra detection of the forward and reverse shocks in Cassiopeia A. Astrophys. J. 552, L39–L43 (2001)

  18. 18.

    & Chandra X-ray survey of ejecta in the Cassiopeia A supernova remnant. Astrophys. J. 746, 130 (2012)

  19. 19.

    & Cassiopeia A and its clumpy presupernova wind. Astrophys. J. 593, L23–L26 (2003)

  20. 20.

    & Where was the iron synthesized in Cassiopeia A? Astrophys. J. 597, 362–373 (2003)

  21. 21.

    et al. The three-dimensional structure of interior ejecta in Cassiopeia A at high spectral resolution. Astrophys. J. 725, 2059–2070 (2010)

  22. 22.

    , & Iron, cobalt, and nickel in SN 1987A. Astrophys. J. 419, 824–836 (1993)

  23. 23.

    et al. Theoretical light curves of Type IIb supernova 1993J. Astrophys. J. 420, 341–347 (1994)

  24. 24.

    , , & Nonlinear growth of Rayleigh-Taylor instabilities and mixing in SN 1987A. Astrophys. J. 358, L57–L61 (1990)

  25. 25.

    , & The evolution and explosion of massive binary stars and Type Ib-Ic-IIb-IIL supernovae. Phys. Rep. 256, 173–191 (1995)

  26. 26.

    , & Gamma-ray lines from asymmetric supernovae. Astrophys. J. 594, 390–403 (2003)

  27. 27.

    Explosion mechanisms of core-collapse supernovae. Annu. Rev. Nucl. Part. Sci. 62, 407–451 (2012)

  28. 28.

    & Core-collapse simulations of rotating stars. Astrophys. J. 541, 1033–1050 (2000)

  29. 29.

    & Late-time convection in the collapse of a 23 star. Astrophys. J. 659, 1438–1448 (2007)

  30. 30.

    , & Stability of standing accretion shocks, with an eye toward core-collapse supernovae. Astrophys. J. 584, 971–980 (2003)

  31. 31.

    et al. The Nuclear Spectroscopic Telescope ARray (NuSTAR) high-energy X-ray mission. Astrophys. J. 770, 103 (2013)

  32. 32.

    , , , & A new method for determining the sensitivity of X-ray imaging observations and the X-ray number counts. Mon. Not. R. Astron. Soc. 388, 1205–1213 (2008)

  33. 33.

    , & The expansion center and dynamical age of the galactic supernova remnant Cassiopeia A. Astrophys. J. 122, 297–307 (2001)

  34. 34.

    , , & The absolute spectrum of CAS A - an accurate flux density scale and a set of secondary calibrators. Astron. Astrophys. 61, 99–106 (1977)

  35. 35.

    et al. The signature of 44Ti in Cassiopeia A revealed by IBIS/ISGRI on INTEGRAL. Astrophys. J. 647, L41–L44 (2006)

  36. 36.

    & The fading of supernova remnant Cassiopeia A from 38 MHz to 16.5 GHz from 1949 to 1999 with new observations at 1405 MHz. Astrophys. J. 537, 904–908 (2000)

  37. 37.

    in Solar and Galactic Composition (ed. ) 411–416 (AIP, 2001)

  38. 38.

    et al. Trends in 44Ti and 56Ni from core-collapse supernovae. Astrophys. J. Suppl. Ser. 191, 66–95 (2010)

  39. 39.

    et al. Constraints on the progenitor of Cassiopeia A. Astrophys. J. 640, 891–900 (2006)

  40. 40.

    , & SNSPH: a parallel three-dimensional smoothed particle radiation hydrodynamics code. Astrophys. J. 643, 292–305 (2006)

  41. 41.

    , , , & First simulations of core- collapse supernovae to supernova remnants with SNSPH. Preprint at (2013)

  42. 42.

    Gamma-ray bursts from stellar mass accretion disks around black holes. Astrophys. J. 405, 273–277 (1993)

  43. 43.

    , , , & The polar regions of Cassiopeia A: the aftermath of a gamma-ray burst? Astrophys. J. 644, 260–273 (2006)

  44. 44.

    & Asymmetric supernovae and gamma-ray bursts. New Astron. Rev. 54, 183–190 (2010)

Download references


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


  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


  1. Search for B. W. Grefenstette in:

  2. Search for F. A. Harrison in:

  3. Search for S. E. Boggs in:

  4. Search for S. P. Reynolds in:

  5. Search for C. L. Fryer in:

  6. Search for K. K. Madsen in:

  7. Search for D. R. Wik in:

  8. Search for A. Zoglauer in:

  9. Search for C. I. Ellinger in:

  10. Search for D. M. Alexander in:

  11. Search for H. An in:

  12. Search for D. Barret in:

  13. Search for F. E. Christensen in:

  14. Search for W. W. Craig in:

  15. Search for K. Forster in:

  16. Search for P. Giommi in:

  17. Search for C. J. Hailey in:

  18. Search for A. Hornstrup in:

  19. Search for V. M. Kaspi in:

  20. Search for T. Kitaguchi in:

  21. Search for J. E. Koglin in:

  22. Search for P. H. Mao in:

  23. Search for H. Miyasaka in:

  24. Search for K. Mori in:

  25. Search for M. Perri in:

  26. Search for M. J. Pivovaroff in:

  27. Search for S. Puccetti in:

  28. Search for V. Rana in:

  29. Search for D. Stern in:

  30. Search for N. J. Westergaard in:

  31. Search for W. W. Zhang in:


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.

Extended data

About this article

Publication history






Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.