AstroNote 2023-199

We report the discovery of a candidate highly magnified star (AT 2023mlz) in an arc in GLASS (PI: Treu) NIRISS observations of the Abell 2744 galaxy-cluster field taken on 2023 July 7. The coordinates of the transient are RA = 0:14:22.4798 and Dec = -30:24:38.699 (J2000). The source is located between two mirror images in an arc labeled 34.1 by Mahler et al. (2018) with a spectroscopic redshift of z = 3.785. The arc was also reported in Richard et al. 2021, and with updated position and redshift in Bergamini et al. 2023a (z = 3.784). The arc is object 3864 in the Paris et al. (2023) catalog with a star formation rate of ~11 M_sun/yr and a stellar mass of ~3.6E7 M_sun (before corrected for lensing magnification). A GLAFIC model (Kawamata et al. 2016) predicts a magnification due to the cluster of ~100 at the transient’s position. The most recent Bergamini et al. (2023b) model yields a magnification greater than 100. A fainter source at the transient’s position is visible in the GLASS pre-imaging taken on 2022 June 28. A persistent image detected across observations taken over many years has previously been identified with multiple examples of lensed stars, and variability can arise from microlensing or intrinsic variability of the lensed star or stellar system.

Photometry JWST NIRISS 2022-Jun-28: 

F115W = 29.49 +- 0.69

F150W = 29.37 +- 0.65

Photometry JWST NIRISS 2023-Jul-07: 

F115W = 28.19 +- 0.11

F150W = 28.76 +- 0.20

References

Bergamini et al. 2023a, A&A, 670, 60

Bergamini et al. 2023b, eprint arXiv:2303.10210

Kawamata et al 2016, ApJ, 819, 114

Mahler et al. 2018, MNRAS, 473:1, 663-692

Paris et al. 2023, eprint arXiv:2301.02179

Richard et al. 2021, A&A, 646, A83