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An energetic dirty fireball detected in soft X-rays

C. -Y. Dai, J. Quirola-Vásquez, Y. -H. Wang, H. -L. Li, J. Yang, X. -L. Chen, A. -L. Wang, H. Sun, X. -Y. Wang, B. Zhang, P. G. Jonker, Y. Liu, W. Yuan, D. Xu, Z. -G. Dai, M. E. Ravasio, L. Piro, P. O'Brien, D. Stern, H. -M. Zhang, Y. -P. Yang, T. An, Y. -L. Qiu, L. -P. Xin, W. -X. Li, R. -Y. Liu, X. -F. Wu, C. -Y. Wang, D. -M. Wei, Y. -F. Huang, F. E. Bauer, W. -H. Lei, B. -B. Zhang, N. -C. Sun, H. Gao, V. S. Dhillon, J. An, C. -H. Bai, A. Martin-Carrillo, H. -Q. Cheng, J. A. Chacon Chavez, Y. Chen, G. -W. Du, J. N. D. van Dalen, A. Esamdin, Y. -Z. Fan, X. Gao, F. Harrison, J. -W. Hu, M. -Q. Huang, S. -M. Jia, A. J. Levan, C. -K. Li, D. -Y. Li, E. -W. Liang, S. Littlefair, X. -W. Liu, Z. -Y. Liu, Z. -X. Ling, D. B. Malesani, H. -W. Pan, A. Rodriguez, A. Rossi, D. Mata Sánchez, J. Sánchez-Sierras, X. -J. Sun, M. A. P. Torres, A. P. C. van Hoof, X. -F. Wang, Q. -Y. Wu, X. -P. Xu, Y. -F. Xu, Y. -W. Yu, C. Zhang, M. -H. Zhang, S. -N. Zhang, Y. Zhang, Y. -H. Zhang, Z. -P. Zhu

Abstract

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.

An energetic dirty fireball detected in soft X-rays

Abstract

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 , 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 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 aligned close to the line-of-sight, signifying the prototype of a dirty fireball.

Paper Structure

This paper contains 12 sections, 13 equations, 14 figures, 10 tables.

Table of Contents

  1. Introduction
  2. Observations of EP241113a
  3. Deviation from the Amati relation
  4. Jet properties inferred from the steep decay and plateau
  5. Conclusions and implications
  6. Materials and Methods
  7. Observations and Data Reduction.
  8. Spectral Analysis.
  9. The Amati relation.
  10. Analysis of the X-ray Light Curve.
  11. Theoretical Modelling.
  12. Event rate.

Figures (14)

  • Figure 1: Characteristics of the prompt X-ray emission of EP241113a observed by EP-WXT. (a) Upper panel: The light curve of the net count rate of EP241113a in the $0.5$--$4\, \rm keV$ band, with a time bin size of $3\, \rm s$. Middle panel: Cumulative X-ray photon count distribution as a function of time. Lower panel: The photon index in the absorbed power-law model for three time bins. The two vertical dashed lines mark the epochs $T_{05} = T_0 + 3 \, \rm s$ and $T_{95} = T_0 + 207 \, \rm s$, corresponding to the times when the cumulative fluence reaches $5\%$ and $95\%$ of the total fluence, respectively. The horizontal dashed line marks these count thresholds. (b) The fitting results of the EP-WXT spectrum in the $0.5$–$4 \, \rm keV$ band during $T_{90}$, assuming an absorbed power-law modelsupplementary. The dashed line in the upper panel represents the best-fit model. (c) The best-fit values of the photon index $\Gamma_X$ and the intrinsic absorption ($N_{\rm{int}}$) for the WXT emission assuming the absorbed power-law model (indicated by a cross), along with the confidence contours corresponding to the $1\sigma$ and $2\sigma$ levels. (d) The intrinsic SED of EP-WXT during $T_{90}$ (black data point). The dashed line represents the same absorbed power-law model with the best-fit parameters from panel (b), but corrected for absorption. The blue upper limit represents the differential flux limit in the energy range $[8,\,900]~\rm{keV}$ derived from the Fermi/GBM integrated flux limit (shown in the Table \ref{['tab_GBM_up']}), assuming a single power-law spectrum with a photon index of $-2$.
  • Figure 2: X-ray light curve of EP241113a and the photon indices of the spectra for various time intervals. (a) The upper panel shows the X-ray luminosity and flux observed by EP-WXT (blue) and EP-FXT (black) in the $0.5$–$10 \, \rm keV$ band. For EP-WXT, the flux in the $0.5$--$10\,\rm{keV}$ band is derived by extrapolating the observed count rate in the $0.5$--$4\,\rm{keV}$ band, using a count rate-to-flux conversion factor derived from the best-fit spectral parameters of the absorbed power-law model (see Table \ref{['tab_spectrum_fitting']}). The data during the steep decay and plateau phases were binned into logarithmically spaced time intervals with a bin size of $\Delta \log{t\,(\rm s)} = 0.1$. The lower panel shows the photon index fitted with an absorbed power-law model during different time intervals, as listed in the Table \ref{['tab_spectrum_fitting']}. All errors are shown at the $1\sigma$ confidence level. (b) MCMC fitting results for the temporal decay indices of the X-ray flux density during the steep decay, plateau, and normal decay phasessupplementary. The red line denotes the best-fitting model, obtained using the parameter set that yields the minimum reduced $\chi^2$ among all completed runs. The blue shaded region illustrates 70 representative MCMC realizations drawn from the posterior distribution within the $1\sigma$ credible interval. The uncertainties of the temporal indices, $\alpha_1$, $\alpha_2$, and $\alpha_3$, are quoted at the $1\sigma$ confidence level.
  • Figure 3: Images and host-galaxy spectrum of EP241113a. The top left panel shows $r'$-band image of the field of EP241113a using the LBT telescope taken 1.52 days after the triggersupplementary, where the X-ray position uncertainty of the transient measured by the EP-FXT telescope and its optical counterpart are marked by red-dashed and magenta circles, respectively. The top right panel shows a zoom-in at the position of the transient taken by GTC-HiPERCAM ($i$-band) 54.2 days after the X-ray trigger, with the host galaxy of the transient marked by cyan ticks. The bottom panel shows a Keck-LRIS spectrum taken of the optical transient at 16.78 days after the transient detection. The strong emission line at 9429Å is interpreted as $[\rm O_{\rm II}]\lambda3727$ at $z=1.53$.
  • Figure 4: The rest-frame peak energy versus isotropic energy for EFXTs detected by EP and GRBs (the Amati relation). Type I and Type II GRBs correspond to those originating from compact object mergers and massive star core collapses, respectively. All data points are plotted with $1\sigma$ uncertainties. $E_{\gamma, \rm iso}$ is calculated over the $1-10^4\,\rm keV$ energy range in the rest frame. The data for GRB 100816D, EP240414a, and EP240801a are adopted from Refs.Sun2025Jiang2025, while the remaining GRB samples are taken from Ref.Liu2025. The solid lines represent the best-fit Amati relations for Type I and Type II GRBs, and the dashed lines indicate the corresponding $3\sigma$ intrinsic scatter regions (see Ref.supplementaryLiu2025). As illustrated, EP241113a is located outside the $3\sigma$ scatter region of the Amati relation.
  • Figure 5: Modeling of the multi-band afterglow of EP241113a. (a) Modeling the multi-band light curve as synchrotron emission from a forward shock expanding in a wind environment, assuming a power-law structured jet with $k_{\rm c}=2$. The best-fit parameters, derived from the MCMC samples with the minimum reduced $\chi^2$, are as follows: $E_{\rm k} = 1.0 \times 10^{53} \, \rm erg$, $\Gamma_c = 23$, $\theta_{\rm c} = 0.48$, $p = 2.4$, $\epsilon_e = 1.3\times 10^{-2}$, $\epsilon_B = 3.5\times 10^{-2}$, $A_* = 1.5 \times 10^{-2}$, and $\xi = 4.8\times 10^{-3}$ (for parameter definitions, see Ref.supplementary). The marginalized distributions of the parameters are shown in Fig. \ref{['fig_af_contour']} and the Table \ref{['tab_afterglow_mcmc_results']}. Note that, for clarity, the comparison between the upper limits of the other optical data and radio data with the model is displayed in Fig. \ref{['fig_af_up']}. (b) Modeling the spectral data of EP241113a during $1.0$–$8.1 \, \rm ks$ with the forward shock synchrotron emission, using the same parameters as in panel (a). The spectral data are derived using the best-fit absorbed power-law model during the corresponding time interval.
  • ...and 9 more figures