EP241107a is a fast X-ray transient (FXT) discovered by the Einstein Probe satellite whose optical and radio afterglow, host-galaxy properties, and afterglow modelling indicate a gamma-ray burst (GRB) viewed close to the jet axis, yet no prompt gamma-ray emission was detected and the 1.3 GHz radio upper limits conflict with the best-fit afterglow model. The source paper (Eappachen et al. 2025, MNRAS) leaves the origin of the radio tension open and interprets the absent prompt emission, via the low beaming-corrected energetics, as evidence that EP241107a is an intrinsically faint GRB, but no afterglow model including low-frequency absorption was fitted and that interpretation is not settled; the question whether such events form a common intrinsically faint GRB channel or a heterogeneous subset of the FXT population is raised by the subsequent literature. This problem asks for (i) a physically consistent multiwavelength afterglow model for EP241107a that includes low-frequency absorption (synchrotron self-absorption and/or free-free absorption) and reconciles the radio non-detections, (ii) a physical account of the absent prompt gamma-ray emission compatible with the on-axis geometry, (iii) refined host-galaxy constraints from the existing four broadband photometric points (or a statement of which host properties—stellar mass, star-formation rate, dust extinction, metallicity—those data can constrain), and (iv) population-level inference from uniformly modelled Einstein Probe FXT samples constraining the distributions of kinetic energy, jet opening angle, viewing angle, and microphysical shock parameters, bearing on whether gamma-ray-quiet FXTs are intrinsically faint or low-Lorentz-factor GRBs or a heterogeneous mix. Follow-up literature through August 2026 actively studies the FXT population but leaves both the event-specific and population questions open.

Background

Fast X-ray transients (FXTs) are brief, luminous bursts of soft X-rays (roughly 0.3-10 keV) lasting minutes to hours, whose physical origins remain uncertain; proposed progenitors include supernova shock breakouts, white-dwarf tidal disruption events, and gamma-ray bursts (GRBs) seen off-axis or at low intrinsic luminosity. The Einstein Probe (EP) satellite, a wide-field soft X-ray survey mission launched in January 2024, now detects FXTs promptly and enables rapid multiwavelength follow-up, transforming a field previously dependent on archival X-ray discoveries. EP241107a, detected on 2024 November 7, is one of the first EP FXTs with prompt optical and radio follow-up: a radio counterpart was found with the VLA at 10 GHz and 6 GHz, while the Giant Metrewave Radio Telescope (uGMRT) yielded non-detections at 1.3 GHz and 648 MHz, the optical afterglow and host galaxy gave a spectroscopic redshift z=0.457z=0.457, and comparison with other transients placed it in the parameter space of GRB afterglows. Gamma-ray burst afterglows arise when a relativistic jet decelerates in circum-burst gas, producing synchrotron emission across the electromagnetic spectrum. Standard afterglow modelling fits broadband light curves with parameters including the isotropic-equivalent kinetic energy EK,isoE_{K,\mathrm{iso}} (the energy the jet would carry if emitted isotropically), the jet half-opening angle θc\theta_{c}, the observer's viewing angle θobs\theta_{\mathrm{obs}} relative to the jet axis, the circumburst density, and microphysical shock parameters: the fractions ϵe\epsilon_{e} and ϵB\epsilon_{B} of shock energy in electrons and magnetic fields and the power-law index pp of the electron energy distribution. Eappachen et al. (2025) modelled EP241107a with the public afterglow code afterglowpy (Ryan et al. 2020) using Markov-chain Monte Carlo fitting, obtaining EK,iso∼1051E_{K,\mathrm{iso}}\sim10^{51} erg, θc≈15∘\theta_{c}\approx15^{\circ}, and θobs≈9∘\theta_{\mathrm{obs}}\approx9^{\circ}, with a jet break (a steepening of the light curve when the spreading jet edge becomes visible) near 10 days in the 6 GHz band. Two anomalies persist. First, the 1.3 GHz upper limits disagree with the best-fit model light curve (a shallower 648 MHz upper limit from the same uGMRT campaign was also obtained); the authors note that afterglowpy does not include synchrotron self-absorption (SSA, absorption of the synchrotron radiation by the emitting electron population itself, which suppresses emission below a self-absorption frequency) and that low-frequency non-detections may plausibly be due to absorption processes, but no model with this physics was fitted. Second, although the inferred geometry is on-axis (θobs<θc\theta_{\mathrm{obs}}<\theta_{c}), no prompt gamma-ray emission was reported by any monitor at the time of writing, though a dedicated multi-instrument search of first-year EP/WXT transients including EP241107a reported combined gamma-ray upper limits (Zhang et al. 2025); the authors interpret this, via the low beaming-corrected kinetic energy (≈4.8×1049\approx4.8\times10^{49} erg), as evidence that EP241107a is an intrinsically low-energy GRB, and also note energetics overlapping relativistic tidal disruption events. The host-galaxy properties rest on only four broadband photometric points, so stellar mass, star-formation rate, and dust constraints remain coarse. At the population level, subsequent work has begun to address whether EP FXTs are GRB-related: Guo et al. (2025) derived a nonparametric luminosity function and a local formation rate (≈154\approx154 Gpc−3^{-3} yr−1^{-1}) exceeding the long-GRB rate, concluding that a component of FXTs is associated with long GRBs; Li et al. (2026) reconstructed EP-FXT formation histories with the Lynden-Bell c−c^{-} method (a nonparametric statistical technique for estimating distributions from truncated data) and found evidence for a common progenitor channel with long GRBs, though EP241107a itself was excluded from their sample; Chen, Wang & Zhang (2026) showed that off-axis viewing of standard GRB jet structures cannot account for the low spectral peak energies of some energetic EP FXTs, arguing they may occupy previously unexplored jet parameter space such as low Lorentz factors; Choza et al. (2026) found from radio follow-up of 20 EP FXTs that the population is heterogeneous, containing both relativistic and non-relativistic explosions; and Dai et al. (2026) identified EP241113a as an on-axis, low-Lorentz-factor (Γ∼20\Gamma\sim20) 'dirty fireball' (an energetic, baryon-loaded relativistic explosion too contaminated to produce ordinary prompt gamma-rays) with no detected gamma-ray emission. None of these works treats EP241107a itself, and together they leave the classification question open and in tension.

Problem Statement

Determine the physical origin of EP241107a and of gamma-ray-quiet Einstein Probe fast X-ray transients like it, jointly addressing the following linked questions: items (1)-(3) are left open by Eappachen et al. (2025), while item (4) is the population-level classification question raised by subsequent Einstein Probe FXT studies. A complete answer must address all four items jointly; omitting any one of them leaves the origin question unresolved. (1) Construct an afterglow model for EP241107a that includes low-frequency absorption physics, synchrotron self-absorption and/or free-free absorption (absorption of radio emission by ionized circum-burst gas), and determine whether it reconciles the low-frequency radio upper limits (1.3 GHz and 648 MHz) with the 6 GHz and 10 GHz detections and the X-ray and optical afterglow within a single GRB-jet interpretation, including how the required absorption parameters relate to the circumburst environment implied by the other bands. (2) Provide a physical account of the absence of any detected prompt gamma-ray emission that is consistent with the inferred on-axis viewing geometry (θobs≈9∘<θc≈15∘\theta_{\mathrm{obs}}\approx9^{\circ}<\theta_{c}\approx15^{\circ}) and the beaming-corrected kinetic energy of about 4.8×10494.8\times10^{49} erg, evaluating whether EP241107a is an intrinsically faint gamma-ray burst, a low-Lorentz-factor (dirty-fireball-type) jet, or better explained by a non-GRB mechanism, taking into account the existing multi-instrument gamma-ray upper limits for this event (Zhang et al. 2025) and stating what prompt gamma-ray limits the favoured interpretation implies. (3) Refine the host-galaxy constraints beyond the four broadband photometric points currently available, or show what subset of host properties (stellar mass, star formation rate, dust extinction, metallicity) can be constrained from existing data, and assess whether the host is consistent with the long-GRB host population. (4) At the population level, addressing the classification question raised by subsequent EP-FXT studies (Guo et al. 2025; Li et al. 2026; Chen, Wang & Zhang 2026; Choza et al. 2026), use uniformly modelled EP-FXT samples to constrain the distributions of isotropic-equivalent kinetic energy EK,isoE_{K,\mathrm{iso}}, jet half-opening angle θc\theta_{c}, viewing angle θobs\theta_{\mathrm{obs}}, and microphysical shock parameters, and decide whether events like EP241107a represent a common intrinsically faint or low-Lorentz-factor GRB channel or a heterogeneous subset of the FXT population.

References

  1. D. Eappachen, A. Balasubramanian, Vishwajeet Swain, G. C. Anupama, D. K. Sahu, V. Bhalerao, T. Ahumada, I. Andreoni, Sudhanshu Barway, J. Carney, J. Freeburn, M. M. Kasliwal, Tanishk Mohan, A. C. Rodriguez, G. Waratkar (2025), 'Characterizing EP241107a: multiwavelength observations of an Einstein Probe-detected fast X-ray transient', Monthly Notices of the Royal Astronomical Society 545, DOI: 10.1093/mnras/staf2062, https://doi.org/10.1093/mnras/staf2062 (preprint arXiv:2511.02562, https://arxiv.org/abs/2511.02562).
  2. Geoffrey Ryan, Hendrik van Eerten, Luigi Piro, Eleonora Troja (2020), 'Gamma-Ray Burst Afterglows in the Multimessenger Era: Numerical Models and Closure Relations', The Astrophysical Journal 896, 166, DOI: 10.3847/1538-4357/ab93cf, https://doi.org/10.3847/1538-4357/ab93cf (preprint arXiv:1909.11691, https://arxiv.org/abs/1909.11691).
  3. Sen-Lin Pang, Biao Zhang, Zi-Gao Dai (2026), 'Fast X-Ray Transients from Magnetar-driven Relativistic Electron–Positron Pair Winds', The Astrophysical Journal 1005, 214, DOI: 10.3847/1538-4357/ae7a62, https://doi.org/10.3847/1538-4357/ae7a62
  4. Connery Chen, Yihan Wang, Bing Zhang (2026), 'Einstein Probe Fast X-Ray Transients Extend the Physical Parameter Space of Relativistic Jets', The Astrophysical Journal Letters 1006, L44, DOI: 10.3847/2041-8213/ae89a4, https://doi.org/10.3847/2041-8213/ae89a4 (preprint arXiv:2607.14317, https://arxiv.org/abs/2607.14317)
  5. Carmen Choza, Joe S. Bright, Francesco Carotenuto, Alex Pollak, Rob Fender, Andrew Siemion (2026), 'Radio follow-up of Einstein Probe fast X-ray transients', Monthly Notices of the Royal Astronomical Society 551, DOI: 10.1093/mnras/stag1370, https://doi.org/10.1093/mnras/stag1370 (preprint arXiv:2607.27439, https://arxiv.org/abs/2607.27439)
  6. Yizhou Guo, Houdun Zeng, Junjie Wei, Hao Zhou, Zhiping Jin, Xuefeng Wu, Daming Wei (2025), 'The Formation Rate and Luminosity Function of Fast X-Ray Transients from Einstein Probe', The Astrophysical Journal Letters 995, L53, DOI: 10.3847/2041-8213/ae2745, https://doi.org/10.3847/2041-8213/ae2745 (preprint arXiv:2510.13533, https://arxiv.org/abs/2510.13533)
  7. Qin-Mei Li, Qi-Bin Sun, Sheng-Bang Qian, Fu-Xing Li (2026), 'New Evidence for Extragalactic Einstein Probe Transients Associated with Long Gamma-Ray Bursts', The Astrophysical Journal Letters 997, L15, DOI: 10.3847/2041-8213/ae2012, https://doi.org/10.3847/2041-8213/ae2012 (preprint arXiv:2510.10267, https://arxiv.org/abs/2510.10267)
  8. C.-Y. Dai et al. (2026), 'An energetic dirty fireball detected in soft X-rays', arXiv:2603.26213, https://arxiv.org/abs/2603.26213 (67-author collaboration list; manuscript under consideration at Science)
  9. Yan-Qiu Zhang, Wang-Chen Xue, Jin-Peng Zhang, Ce Cai, Shao-Lin Xiong, Cheng-Kui Li, et al. (2025), 'Multi-Instrument Search for Gamma-Ray Counterpart of X-ray Transients detected by EP/WXT', The Astrophysical Journal Letters (accepted), arXiv:2506.05920, https://arxiv.org/abs/2506.05920