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

TL;DR

The paper reports the identification of a candidate caustic-crossing massive binary in a lensed galaxy at $z=0.94$, inferred from four epochs of JWST/NIRCam photometry of the Warhol arc. Time-dependent microlensing, combined with the binary evolution modeled by POSYDON, supports a red supergiant plus B-type companion with near-equal masses and a wide, eccentric orbit, producing distinct magnification patterns across filters. Quantitative modeling ties the observed color changes and light-curve evolution to orbital motion across the microlens caustic, yielding a source-plane size of tens of AU and a most likely configuration around $M1_{init} \approx 23.6^{+5.3}_{-4.3}$ M⊙ and $q_{init} \approx 0.96^{+0.02}_{-0.03}$. The work demonstrates the feasibility of constraining massive binaries at cosmic noon via microlensing in lensing clusters, with implications for binary demographics and stellar evolution, and anticipates many more such detections with JWST.

Abstract

Massive stars at cosmological distances can be individually detected during transient microlensing events, when gravitational lensing magnifications may exceed ~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 MACSJ0416.1-2403. 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 AU. The most likely binary configurations consistent with the observational constraints on the temperature and luminosity of each star are stars with initial masses M1=23.6+5.3-4.3 Msol and an initial mass ratio between the two stars close to unity. A kinematic model that reproduces the observed light curve in all filters gives a relatively small transverse velocity of 50 km/s. This requires the dominant velocity component of several hundreds of km/s 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.

Figures (10)

• Figure 1: Top: composite-color images of the Warhol arc the four epochs of JWST NIRCam imaging in which source W2 is detected. Bottom: Magnified difference images between the epochs in the F090W filter. (The Visit 1 image is the single-epoch image, not a difference image.) W2 is indicated with a white arrow. North is up and east to the left in all images, and scale bars are provided.
• Figure 2: Observed NIRCam light curve of the transient source in the F115W, F200W, and F356W filters (black points). The red diamonds and blue squares show the POSYDON evolutionary model flux densities for one of the most likely binary configurations. The open purple diamonds show the sum of the two model binary components.
• Figure 3: Observed SED of the lensed transient source (black circles). The red and blue lines show the simulated spectra, normalized by their best-fitting magnification factors, for a likely binary configuration from the weighted POSYDON sample. The open black diamonds indicate the synthetic NIRCam photometry of the sum of the two model spectra.
• Figure 4: Change in color of the lensed transient over the 126-day light curve for two pairs of filters: F277W$-$F090W and F356W$-$F115W.
• Figure 5: Left: Initial masses and periods of all 100,000 binary systems in the simulated population. Middle: Initial masses and periods for the 1001 systems that can reproduce the observed transient SED with $\mu<10,000$. Right: Same as middle, but zoomed in on the selected population. The color of each point indicates the configuration's initial mass ratio $q$.