JWST’s PEARLS: A Candidate Massive Binary Star System in a Lensed Galaxy at Redshift 0.94

Abstract Massive stars at cosmological distances can be individually detected during transient microlensing events, when gravitational lensing magnifications may exceed μ ≈ 1000. Nine such sources were identified in JWST NIRCam imaging of a single galaxy at redshift z = 0.94 known as the “Warhol arc,” which is mirror imaged by the galaxy cluster MACS J0416.1−2403. Here we present the discovery of two coincident and well-characterized microlensing events at the same location followed by a third event observed in a single filter approximately 18 months later. The events can be explained by microlensing of a binary star system consisting of a red supergiant ( T ≈ 4000 K) and a B-type ( T ≳ 13,000 K) companion star. The timescale of the coincident microlensing events constrains the estimated projected source-plane size to tens of astronomical units. The most likely binary configurations consistent with the observational constraints on the temperature and luminosity of each star are stars with initial masses <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>M</mml:mi> <mml:msub> <mml:mn>1</mml:mn> <mml:mi>init</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>23</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mn>6</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>5.3</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> M ⊙ and an initial mass ratio between the two stars close to unity. A kinematic model that reproduces the observed light curves in all filters gives a relatively small transverse velocity of ∼50 km s −1 . This requires the dominant velocity component of several hundreds of kilometers per second to be roughly parallel to the microcaustic. An alternative possibility would be that the three microlensing events correspond to unrelated stars crossing distinct microcaustics, but this would imply a highly elevated rate of events at their common position, even though no underlying knot is present at the location.