Two peculiar fast transients in a strongly lensed host galaxy


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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Fig. 1: Detection of HFF14Spo-NW and HFF14Spo-SE in HST imaging from the HFF.
Fig. 2: Predictions for the reappearance episodes of HFF14Spo-NW and HFF14Spo-SE caused by gravitational lensing time delays.
Fig. 3: Locations of the lensing critical curves relative to the positions of the two HFF14Spo sources.
Fig. 4: Light curves for the two transient events.
Fig. 5: Peak luminosity versus decline time for HFF14Spo and assorted categories of explosive transients.


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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 ( 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.

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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.

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Rodney, S.A., Balestra, I., Bradac, M. et al. Two peculiar fast transients in a strongly lensed host galaxy. Nat Astron 2, 324–333 (2018).

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