Searching for Highly Magnified Stars at Cosmological Distances: Discovery of a Redshift 0.94 Blue Supergiant in Archival Images of the Galaxy Cluster MACS J0416.1-2403

Wenlei Chen, Patrick L. Kelly, Jose M. Diego, Masamune Oguri, Liliya L.R. Williams, Adi Zitrin, Tommaso L. Treu, Nathan Smith, Thomas J. Broadhurst, Nick Kaiser, Ryan J. Foley, Alexei V. Filippenko, Laura Salo, Jens Hjorth, Jonatan Selsing

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17 Scopus citations


Individual highly magnified stars have been recently discovered at lookback times of more than half the age of the universe, in lensed galaxies that straddle the critical curves of massive galaxy clusters. Having established their detectability, it is now important to carry out systematic searches for them in order to establish their frequency, and in turn learn about the statistical properties of high-redshift stars and of the granularity of matter in the foreground deflector. Here we report the discovery of a highly magnified star at redshift z = 0.94 in a strongly lensed arc behind a Hubble Frontier Field (HFF) galaxy cluster, MACS J0416.1-2403, discovered as part of a systematic archival search. The bright transient (dubbed "Warhol") was discovered in Hubble Space Telescope data taken on 2014 September 15 and 16. The point source faded over a period of two weeks, and observations taken on 2014 September 1 show that the duration of the microlensing event was at most four weeks in total. The magnified stellar image that exhibited the microlensing peak may also exhibit slow changes over a period of years at a level consistent with that expected for microlensing by the stars responsible for the intracluster light of the cluster. Optical and infrared observations taken near peak brightness can be fit by a stellar spectrum with moderate host-galaxy extinction. A blue supergiant matches the measured spectral energy distribution near peak, implying a temporary magnification of at least several thousand. The short timescale of the event and the estimated effective temperature indicate that the lensed source is an extremely magnified star. Finally, we detect the expected counterimage of the background lensed star at an offset by ∼0.″1 in a deep coaddition of HFF imaging.

Original languageEnglish (US)
Article number8
JournalAstrophysical Journal
Issue number1
StatePublished - Aug 10 2019

Bibliographical note

Funding Information:
2019-08-10 2019-08-06 15:45:19 cgi/release: Article released cgi/article: new .article bin/incoming: New from .zip NSF AST-1518052 JSPS KAKENHI JP15H05892 JSPS KAKENHI JP18K03693 NASA/HST GO-14922 NASA/HST GO-14872 Ministerio de Economia y Competitividad AYA2015-64508-P VILLUM FONDEN 16599 yes

Publisher Copyright:
© 2019. The American Astronomical Society. All rights reserved..


  • galaxies: clusters: general
  • galaxies: clusters: individual: MACS J0416.1-2403
  • gravitational lensing: strong
  • stars: massive


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