A massive galaxy cluster can serve as a magnifying glass for distant stellar populations, as strong gravitational lensing magnifies background galaxies and exposes details that are otherwise undetectable. In time-domain astronomy, imaging programmes with a short cadence are able to detect rapidly evolving transients, previously unseen by surveys designed for slowly evolving supernovae. Here, we describe two unusual transient events discovered in a Hubble Space Telescope programme that combined these techniques with high-cadence imaging on a field with a strong-lensing galaxy cluster. These transients were faster and fainter than any supernovae, but substantially more luminous than a classical nova. We find that they can be explained as separate eruptions of a luminous blue variable star or a recurrent nova, or as an unrelated pair of stellar microlensing events. To distinguish between these hypotheses will require clarification of the cluster lens models, along with more high-cadence imaging of the field that could detect related transient episodes. This discovery suggests that the intersection of strong lensing with high-cadence transient surveys may be a fruitful path for future astrophysical transient studies.

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The authors thank M. Livio and L. Chomiuk for discussion of this paper, as well as S. Murray and the late N. Gehrels for assistance with the Chandra and Swift data, respectively. Some of the data in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). Support for MAST for non-HST data is provided by the NASA (National Aeronautics and Space Administration) Office of Space Science via grant NNX09AF08G, and by other grants and contracts. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Financial support for this work was provided to S.A.R., O.G. and L.-G.S. by NASA through grant HST-GO-13386 from the Space Telescope Science Institute (STScI), which is operated by Associated Universities for Research in Astronomy, Inc. (AURA), under NASA contract NAS 5-26555. G.B.C. and P.R. acknowledge financial support from PRIN-INAF 2014 J.M.D. acknowledges support of projects AYA2015-64508-P (MINECO/FEDER, UE) and AYA2012-39475-C02-01 and the consolider project CSD2010-00064 funded by the Ministerio de Economia y Competitividad. A.V.F. and P.L.K. are grateful for financial assistance from the Christopher R. Redlich Fund, the TABASGO Foundation and NASA/STScI grants GO-14208, GO-14528, GO-14872 and GO-14922; A.V.F. is also grateful to the Miller Institute for Basic Research in Science (U.C. Berkeley). The work of A.V.F. was conducted in part at the Aspen Center for Physics, which is supported by NSF grant PHY-1607611. R.J.F. and the UCSC group are supported in part by NSF grant AST-1518052 and from fellowships to R.J.F. from the Alfred P. Sloan Foundation and the David and Lucile Packard Foundation. C.G. acknowledges support by the VILLUM FONDEN Young Investigator Programme through grant 10123. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). M.J. was supported by the Science and Technology Facilities Council (grant ST/L00075X/1) and used the DiRAC Data Centric system at Durham University, operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). M.J. was funded by BIS National E-infrastructure capital grant ST/K00042X/1, STFC capital grant ST/H008519/1, STFC DiRAC Operations grant ST/K003267/1 and Durham University. DiRAC is part of the National E-Infrastructure. R.K. was supported by Grant-in-Aid for JSPS Research Fellow (16J01302). A.M. acknowledges the financial support of the Brazilian funding agency FAPESP (Postdoc fellowship, process 2014/11806-9). M.O. acknowledges support in part by World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan, and JSPS KAKENHI grants 26800093 and 15H05892. J.R. acknowledges support from the ERC starting grant 336736-CALENDS. G.C. and S.H.S. thank the Max Planck Society for support through the Max Planck Research Group of S.H.S. The GLASS team and T.T. were funded by NASA through NASA/STScI grant GO-13459. L.L.R.W. thanks the Minnesota Supercomputing Institute at the University of Minnesota for providing resources and support.

Author information


  1. Department of Physics and Astronomy, University of South Carolina, Columbia, SC, USA

    • S. A. Rodney
  2. University Observatory Munich, Munich, Germany

    • I. Balestra
  3. Department of Physics, University of California, Davis, CA, USA

    • M. Bradac
    •  & A. Hoag
  4. Space Telescope Science Institute, Baltimore, MD, USA

    • G. Brammer
    • , A. G. Riess
    •  & L.-G. Strolger
  5. Fisika Teorikoa, Zientzia eta Teknologia Fakultatea, Euskal Herriko Unibertsitatea UPV/EHU, Leioa, Spain

    • T. Broadhurst
  6. IKERBASQUE, Basque Foundation for Science, Alameda Urquijo, Bilbao, Spain

    • T. Broadhurst
  7. Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Ferrara, Italy

    • G. B. Caminha
    •  & P. Rosati
  8. Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV, Groningen, The Netherlands

    • G. B. Caminha
  9. Max-Planck-Institut für Astrophysik, Garching, Germany

    • G. Chirivì
    •  & S. H. Suyu
  10. IFCA, Instituto de Física de Cantabria (UC-CSIC), Santander, Spain

    • J. M. Diego
  11. Department of Astronomy, University of California, Berkeley, CA, USA

    • A. V. Filippenko
    •  & P. L. Kelly
  12. Miller Institute for Basic Research in Science, University of California, Berkeley, CA, USA

    • A. V. Filippenko
  13. Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA

    • R. J. Foley
  14. Center for Cosmology and Particle Physics, New York University, New York, NY, USA

    • O. Graur
  15. Department of Astrophysics, American Museum of Natural History, New York, NY, USA

    • O. Graur
  16. Harvard/Smithsonian Center for Astrophysics, Cambridge, MA, USA

    • O. Graur
  17. Dipartimento di Fisica, Università degli Studi di Milano, Milan, Italy

    • C. Grillo
  18. Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

    • C. Grillo
    • , J. Hjorth
    •  & J. Selsing
  19. Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA, USA

    • S. Hemmati
  20. Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham, UK

    • M. Jauzac
  21. Institute for Computational Cosmology, Durham University, Durham, UK

    • M. Jauzac
  22. Astrophysics and Cosmology Research Unit, School of Mathematical Sciences, University of KwaZulu-Natal, Durban, South Africa

    • M. Jauzac
  23. Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ, USA

    • S. W. Jha
  24. Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo, Japan

    • R. Kawamata
  25. School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA

    • P. L. Kelly
    •  & L. L. R. Williams
  26. Las Cumbres Observatory Global Telescope Network, Goleta, CA, USA

    • C. McCully
  27. Department of Physics, University of California, Santa Barbara, CA, USA

    • C. McCully
    •  & K. B. Schmidt
  28. Department of Physics and Astronomy, University of California, Riverside, CA, USA

    • B. Mobasher
  29. Universidade de São Paulo, Cidade Universitária, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, São Paulo, Brazil

    • A. Molino
  30. Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain

    • A. Molino
  31. Research Center for the Early Universe, University of Tokyo, Tokyo, Japan

    • M. Oguri
  32. Department of Physics, University of Tokyo, Tokyo, Japan

    • M. Oguri
  33. Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), University of Tokyo, Kashiwa, Japan

    • M. Oguri
  34. Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, F-69230, Saint-Genis-Laval, France

    • J. Richard
  35. Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD, USA

    • A. G. Riess
  36. Leibniz-Institut für Astrophysik Potsdam (AIP), Potsdam, Germany

    • K. B. Schmidt
  37. Department of Astronomy, University of Michigan, Ann Arbor, MI, USA

    • K. Sharon
  38. Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan

    • S. H. Suyu
  39. Physik-Department, Technische Universität München, Garching, Germany

    • S. H. Suyu
  40. Department of Physics and Astronomy, University of California, Los Angeles, CA, USA

    • T. Treu
  41. Department of Astronomy, University of Arizona, Tucson, AZ, USA

    • B. J. Weiner
  42. Ben-Gurion University of the Negev, Beer-Sheva, Israel

    • A. Zitrin


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S.A.R. designed the observations, processed the HST data, organized the analysis and wrote the manuscript. M.B., T.B., G.B.C., G.C., J.M.D., A.H., M.J., R.K., M.O., J.R., K.S., S.H.S., L.L.R.W. and A.Z. contributed to the lensing analysis with construction and/or interpretation of a cluster lens model. I.B., G.B., G.B.C., C.G., S.H., B.M., A.M., P.R., K.B.S., J.S. and B.J.W. collected, processed and/or analysed data on the host galaxy and other galaxies in the cluster field. R.J.F., S.W.J., P.L.K., C.M., O.G., J.H., A.G.R. and L.-G.S. contributed to the evaluation of models of astrophysical transients. A.V.F. and T.T. assisted with the observational programme design and editing of the manuscript.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to S. A. Rodney.

Supplementary information

  1. Supplementary Information

    Supplementary Figures 1–10, Supplementary Table 1–4, Supplementary References 1–19, Supplementary Text.

  2. Supplementary Information

    Supplementary Dataset

  3. Supplementary Information

    Supplementary Dataset

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