JWST and HST observations of the host galaxy and supernova, SN 2024aihh in EP240801a at z=1.67

We present James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) observations of the counterpart of EP240801a, at z=1.67, the first fast X-ray transient (FXT) identified as an X-ray flash (XRF) by the Einstein Probe (EP) and Fermi-GBM. Our observations reveal strong photometric and spectroscopic evidence for an associated broad-lined Type Ic supernova (SN) SN 2024aihh, the most distant spectroscopically identified gamma ray-burst (GRB)-SN to date. The SN exhibits similar luminosity and light curve evolution to the proto-type GRB-SN 1998bw with an absolute magnitude of the SN at $\sim$23 days rest-frame of $M_{F140W} \approx$-19 mag. The SN is located in a host galaxy with complex morphology at a large ($\sim$6 kpc) offset in a region of relatively low surface brightness. The region around the SN has a modest star formation rate and is dominated by an intermediate mass-weighted age (1.4$\pm$0.3 Gyr) population, despite the apparent presence of a young, massive broad-lined Type Ic SN progenitor. These observations demonstrate that observations with HST and JWST can greatly extend the redshift range over which the GRB/FXT-SN connection can be studied, including in relatively low luminosity, X-ray rich events. They demonstrate little apparent evolution in the SN properties from local examples despite EP240801a originating from an epoch 10 Gyr ago.

💡 Research Summary

The paper reports on a multi‑wavelength follow‑up campaign of EP 240801a, the first fast X‑ray transient (FXT) identified as an X‑ray flash (XRF) by the Einstein Probe (EP) and Fermi‑GBM. The event occurred at a redshift of z = 1.67, placing it at a look‑back time of roughly 10 Gyr. Using the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST), the authors obtained deep imaging in the near‑infrared (F140W) and optical (F606W) bands, as well as a low‑resolution JWST/NIRSpec prism spectrum covering 0.6–5.3 µm.

HST observations were taken at three epochs (≈ 25, 62, and 293 days after trigger). Difference imaging revealed a transient that faded only slightly in the F140W band (≈ 0.05 mag) while fading by >2 mag in the F606W band, consistent with the emergence of a supernova (SN) component that dominates the near‑infrared flux. The JWST/NIRSpec spectrum, obtained at 87 days (32.6 days rest‑frame), shows a spatially extended trace containing both host galaxy emission lines (e.g., H α, H β,