The First Photometric Evidence of a Transient/Variable Source at z>5 with JWST

Christa DeCoursey, Eiichi Egami, Fengwu Sun, Arshia Akhtarkavan, Rachana Bhatawdekar, Andrew J. Bunker, David A. Coulter, Michael Engesser, Ori D. Fox, Sebastian Gomez, Kohei Inayoshi, Benjamin D. Johnson, Mitchell Karmen, Conor Larison, Xiaojing Lin, Jianwei Lyu, Seppo Mattila, Takashi J. Moriya, Justin D. R. Pierel, Dávid Puskás, Armin Rest, George H. Rieke, Brant Robertson, Sepehr Salamat, Louis-Gregory Strolger, Sandro Tacchella, Christian Vassallo, Christina C. Williams, Yossef Zenati, Junyu Zhang

First listed 2025-04-23 | Last updated 2025-07-31

Abstract

The James Webb Space Telescope (JWST) discovered 79 transients out to $z$$\sim$4.8 through the JADES Transient Survey (JTS), but the JTS did not find any $z$$>$5 transients. Here, we present the first photometric evidence of a $z$$>$5 transient/variable source with JWST. The source, AT 2023adya, resides in a $z_{\mathrm{spec}}$$=$5.274 galaxy in GOODS-N, which dimmed from $m_{\rm F356W}$$=$26.05$\pm$0.02 mag to 26.24$\pm$0.02 mag in the rest-frame optical over approximately two rest-frame months, producing a clear residual signal in the difference image ($m_{\rm F356W}$$=$28.01$\pm$0.17 mag; SN$_\mathrm{var}$$=$6.09) at the galaxy center. Shorter-wavelength bands (F090W/F115W) show no rest-frame ultraviolet brightness change. Based on its rest-frame V-band absolute magnitude of M$_\mathrm{V}$$=$$-$18.48 mag, AT 2023adya could be any core-collapse supernova (SN) subtype or an SN Ia. However, due to low SN Ia rates at high redshift, the SN Ia scenario is unlikely. Alternatively, AT 2023adya may be a variable active galactic nucleus (AGN). However, the JWST NIRCam/Grism spectrum shows no broad H$α$ emission line (FWHM$=$130$\pm$26 km s$^{-1}$), disfavoring the variable AGN scenario. It is also unlikely that AT 2023adya is a tidal disruption event (TDE) because the TDE models matching the observed brightness changes have low event rates. Although it is not possible to determine AT 2023adya's nature based on the two-epoch single-band photometry alone, this discovery indicates that JWST can push the frontier of transient/variable science past $z$$=$5 and towards the epoch of reionization.

Short digest

Reports AT 2023adya, a transient/variable source in a z_spec=5.274 GOODS-N galaxy, detected via a 0.19 mag fade in F356W over ~2 rest-frame months with a significant difference-image residual (SN_var≈6). No contemporaneous variability is seen in the rest-UV (F090W/F115W), and the rest-frame V-band absolute magnitude is MV≈−18.48. NIRCam/Grism data show no broad Hα (FWHM=130±26 km s−1), disfavoring an AGN origin; Ia is possible but unlikely given low high‑z rates, with CCSN remaining plausible while TDEs are disfavored by low event rates. The result establishes practical JWST sensitivity to transient/variable phenomena beyond z>5, nudging discovery space toward the reionization era.

Key figures to inspect

• Figure 1: Compare Epoch1 vs. Epoch2 NIRCam cutouts and the RGB composites to verify the F356W‑only residual centered on the host nucleus and the lack of F090W/F115W changes; also note the labeled companion that could affect blending in Epoch1.
• Figure 2: Use the overplotted MV≈−18.5 marker against B‑band peak distributions to gauge which CCSN subclasses or Ia could match the luminosity, keeping in mind it is a lower limit if Epoch1 was pre‑peak/post‑peak.
• Figure 3: Inspect the NIRCam/WFSS extractions of Hα (F444W) and [O III]5008 (F356W) to confirm the absence of broad components (Hα FWHM ~130 km s−1) and assess the caveat that [O III] lies at the blue edge with imperfect background subtraction.