An energetic dirty fireball detected in soft X-rays

The collapse of massive stars drives explosions that power relativistic fireballs. If only a small amount of matter is entrained, such clean fireballs can expand with Lorentz factors $Γ> 100$, accounting for gamma-ray bursts (GRBs). It has been hypothesized that energetic explosions with more baryon contamination, dubbed ``dirty fireballs’’, may exist in nature, but they have not been observed. Here we report the observation of an extragalactic fast X-ray transient, EP241113a, detected by Einstein Probe. Compared to GRBs, it has a similar isotropic energy of $1.4\times 10^{51}$ erg, but significantly lower spectral peak energy. Theoretical modeling of its early X-ray afterglow suggests a relativistic jet with a low Lorentz factor of $Γ\sim 20$ aligned close to the line-of-sight, signifying the prototype of a dirty fireball.

💡 Research Summary

The authors report the discovery and comprehensive multi‑wavelength analysis of EP241113a, an extragalactic fast X‑ray transient (EFXT) detected by the Einstein Probe (EP) on 2024‑11‑13. The event lasted about 204 s in the 0.5–4 keV band, showing multiple pulses but no contemporaneous gamma‑ray emission in Fermi/GBM, indicating an unusually soft spectrum. Time‑averaged unabsorbed X‑ray flux is 1.1 × 10⁻⁹ erg cm⁻² s⁻¹, with a photon index Γ_X ≈ 2.7 and intrinsic absorption N_int ≈ 3.2 × 10²² cm⁻². Spectral fitting with three models (Band, smoothly broken power‑law, cutoff power‑law) favors the cutoff power‑law, yielding a peak energy E_p = 1.3 keV (68 % confidence) and an upper limit E_p ≤ 2.4 keV (95 % confidence), far below the typical hundreds‑keV peaks of long GRBs.

Follow‑up observations with EP‑FXT began 2 min after the trigger, revealing a steep decay (temporal index α₁ ≈ 3.05), a flat plateau (α₂ ≈ ‑0.16), and a normal decay (α₃ ≈ 0.95) over ~30 days. The steep decay matches the high‑latitude emission prediction (α ≈ 2 + β) for an on‑axis view of a uniform jet core, ruling out an off‑axis geometry. The plateau is unusually flat, requiring either an energy‑injection index q ≈ ‑0.3 (unphysical for standard spin‑down) or, more plausibly, a freely expanding jet in a wind‑like circumburst medium (density ρ ∝ R⁻²).

Optical imaging with the Large Binocular Telescope (LBT) at 1.52 days post‑burst detected a faint r′ ≈ 23.35 mag source within the X‑ray error circle. Subsequent observations with SVOM/VT, Keck/LRIS, and GTC/HiPERCAM confirmed a decaying optical afterglow and identified a host galaxy at redshift z = 1.53 via a weak