Flashlights: an off-caustic lensed star at redshift z = 1.26 in Abell 370

Ashish Kumar Meena, Wenlei Chen, Adi Zitrin, Patrick L. Kelly, Miriam Golubchik, Rui Zhou, Amruth Alfred, Tom Broadhurst, Jose M. Diego, Alexei V. Filippenko, Sung Kei Li, Masamune Oguri, Nathan Smith, Liliya L.R. Williams

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6 Scopus citations


We report the discovery of a transient seen in a strongly lensed arc at redshift zs = 1.2567 in Hubble Space Telescope imaging of the Abell 370 galaxy cluster. The transient is detected at 29.51 ± 0.14 AB mag in a WFC3/UVIS F200LP difference image made using observations from two different epochs, obtained in the framework of the Flashlights programme, and is also visible in the F350LP band (mF350LP ≈ 30.53 ± 0.76 AB mag). The transient is observed on the negative-parity side of the critical curve at a distance of ∼0.6 arcsec from it, greater than previous examples of lensed stars. The large distance from the critical curve yields a significantly smaller macromagnification, but our simulations show that bright, O/B-type supergiants can reach sufficiently high magnifications to be seen at the observed position and magnitude. In addition, the observed transient image is a trailing image with an observer-frame time delay of ∼+0.8 d from its expected counterpart, so that any transient lasting for longer than that should have also been seen on the minima side and is thus excluded. This, together with the blue colour we measure for the transient (mF200LP - mF350LP ≈ [-0.3, -1.6] AB), rules out most other transient candidates such as (kilo)novae, for example, and makes a lensed star the prime candidate. Assuming that the transient is indeed a lensed star as suggested, many more such events should be detected in the near future in cluster surveys with the Hubble Space Telescope and JWST.

Original languageEnglish (US)
Pages (from-to)5224-5231
Number of pages8
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Jun 1 2023

Bibliographical note

Publisher Copyright:
© 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.


  • galaxies: clusters: individual: Abell 370
  • gravitational lensing: micro
  • gravitational lensing: strong


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