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Abstract With a small sample of fast X-ray transients (FXTs) with multiwavelength counterparts discovered to date, their progenitors and connections toγ-ray bursts (GRBs) and supernovae (SNe) remain ambiguous. Here, we present photometric and spectroscopic observations of SN 2025kg, the SN counterpart to the FXT EP 250108a. Atz= 0.17641, this is the closest known SN discovered following an Einstein Probe (EP) FXT. We show that SN 2025kg’s optical spectra reveal the hallmark features of a broad-lined Type Ic SN. Its light-curve evolution and expansion velocities are comparable to those of GRB-SNe, including SN 1998bw, and two past FXT-SNe. We present JWST/NIRSpec spectroscopy taken around SN 2025kg’s maximum light, and find weak absorption due to HeI1.0830μm and 2.0581μm and a broad, unidentified emission feature at ∼4–4.5μm. Further, we observe broadened Hαin optical data at 42.5 days that is not detected at other epochs, indicating interaction with H-rich material. From its light curve, we derive a56Ni mass of 0.2–0.6M⊙. Together with our companion Letter, our broadband data are consistent with a trapped or low-energy (≲1051erg) jet-driven explosion from a collapsar with a zero-age main-sequence mass of 15–30M⊙. Finally, we show that the sample of EP FXT-SNe supports past estimates that low-luminosity jets seen through FXTs are more common than successful (GRB) jets, and that similar FXT-like signatures are likely present in at least a few percent of the brightest Type Ic-BL SNe.
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This content will become publicly available on February 1, 2026
Constraints on Relativistic Jets from the Fast X-Ray Transient 210423 Using Prompt Radio Follow-up Observations
Abstract Fast X-ray transients (FXTs) are a new observational class of phenomena with no clear physical origin. This is at least partially a consequence of limited multiwavelength follow-up of this class of transients in real time. Here we present deep optical (g- andi-band) photometry with Keck, and prompt radio observations with the Very Large Array of FXT 210423 obtained atδt≈ 14–36 days since the X-ray trigger. We use these multiband observations, combined with publicly available data sets, to constrain the presence and physical properties of on-axis and off-axis relativistic jets such as those that can be launched by neutron star mergers and tidal disruption events, which are among the proposed theoretical scenarios of FXTs. Considering a wide range of possible redshiftsz≤ 3.5, circumstellar medium densityn= 10−6–10−1cm−3, and isotropic-equivalent jet kinetic energyEk,iso= 1048–1055erg, we find that we can rule out wide jets with opening angleθj= 15° viewed within 10° off-axis. For more collimated jets (θj= 3°) we can only rule out on-axis (θobs= 0°) orientations. This study highlights the constraining power of prompt multiwavelength observations of FXTs discovered in real time by current (e.g., Einstein Probe) and future facilities.
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- Award ID(s):
- 2224255
- PAR ID:
- 10613941
- Publisher / Repository:
- ApJ
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 980
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 92
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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