EP241217a: a likely Type II GRB with an achromatic bump at z = 4.59

TL;DR

EP241217a presents a high-redshift, γ-ray–faint X-ray transient whose X-ray plateau and an achromatic optical/X-ray bump are studied within the standard fireball-shock framework. The authors fit a wind-like circumburst environment with a mildly relativistic jet, finding a best-fit jet with Γ_0 ≈ 46 and a small opening angle, while also highlighting tensions between the model predictions and the observed spectra. They conclude that the wind/coasting scenario cannot fully explain all features and discuss several alternative plateau mechanisms, stressing the need for further theoretical modeling. The work supports the interpretation of EP241217a as a Type II GRB that falls below current γ-ray instrument sensitivities, illustrating how high-z, intrinsically faint events can evade γ-ray detection yet reveal rich afterglow phenomenology with multi-wavelength data.

Abstract

EP241217a is an X-ray transient detected by the Einstein Probe (EP) lasting for about 100 seconds and without accompanying $γ$-ray detection. The optical spectroscopy reveals the redshift of EP241217a is 4.59. By combining the $γ$-ray upper limit provided by GECAM-C, there is a considerable possibility that EP241217a is a typical Type II gamma-ray burst (GRB), but it is fainter than the detection threshold of any available $γ$-ray monitors (i.e., $E_{γ,{\rm iso}}\lesssim10^{53}$ erg). The X-ray light curve exhibits a plateau lasting for $\sim5\times10^4$ seconds. However, the joint analysis with optical data suggests the presence of an achromatic bump peaking at $\sim3\times10^4$ s after the trigger, indicating the actual duration of the X-ray plateau may be significantly shorter than it appears. To interpret the achromatic bump, we adopt the scenario of a mildly relativistic jet coasting in a wind-like medium and encountering a rapid density enhancement of the circumburst medium, which is likely induced by the the interaction of the progenitor's stellar wind and the interstellar medium. However, this model cannot fully explain observed data, and some issues do exist, e.g., the observed spectrum is harder than the model prediction. Consequently, we conclude that the scenario of a mildly relativistic jet coasting in the wind-like medium cannot explain all observed features of EP241217a. In addition, some alternative models commonly invoked to explain X-ray plateaus are discussed, but there are more or less issues when they are applied to EP241217a. Therefore, further theoretical modeling is encouraged to explore the origin of EP241217a.

Figures (10)

• Figure 1: The prompt X-ray light curve of EP241217a. The black line represents the EP-WXT (0.5-4 keV) rate curve of EP241217a, and the gray region represents the 1-$\sigma$ uncertainty. The width of each time bin is 5 s. The vertical dashed line represent the possible start time of EP241217a, i.e., $T_0\sim T_{\rm trig}-114.0$ s.
• Figure 2: The multi-band light curve of EP241217a. The upper panel shows the optical and X-ray light curves with different markers, and 3-$\sigma$ up-limits are marked with downward arrows. The best fitted phenomenal model, i.e., Equation (\ref{['equ:x_model']}), for the X-ray data is plotted with the solid black line. The vertical gray regions show the 2 epochs to extract broadband SEDs shown in Figure \ref{['fig:sed']}. The middle panel shows the residual of the best fitted model for the X-ray data, and the gray region shows the 3-$\sigma$ region. The lower panel shows the X-ray photon index $\Gamma_{\rm X}$, and from $\sim T_0+10^3$ s to $\sim T_0+10^5$ s, the photon index is slightly changed from $\sim1.9$ to $\sim2.1$.
• Figure 3: WXT spectrum of the prompt emission. The black dashed line and the black dotted line represent the best fitted PL model and thermal model for the spectrum, and the gray region shows the 1-$\sigma$ uncertainty of the best fitted PL model. The red solid line shows the 3-$\sigma$ upper limit for a 100-second exposure in the 15-150 keV band, which is almost same as the extrapolation of the best fitted model.
• Figure 4: The Amati relation of EP241217a. Black diamonds represent different EP transients, from left to the right are EP250108a, EP240414a, EP241113a, EP240801a and EP240315a, respectively. The GRBs with redshift $>4.5$ are marked with empty red squares, and the black dashed line represents the up-limit of $E_{\gamma, {\rm iso}}$ of EP241217a calculated with the Equation (\ref{['equ:x_model']}) and Equation (\ref{['equ:eiso']}). The gray dotted-dashed line represents the Amati relation of EP241217a if assuming a typical high-energy index. The orange region shows the 3-$\sigma$ region of the Amati relation for Type II GRBs. There is a considerable possibility that EP241217a could be a Type II GRB at z=4.59.
• Figure 5: Shifted light curves of EP241217a. All optical light curves are shifted to match the X-ray flux density at $\sim T_0+3\times10^4$ s. Black dashed lines represent the PL approximation for each segment of shifted light curves, and the numbers represent the temporal decay indices for each approximation. Temporal decay indices of the panchromatic bump at $\sim T_0+3\times10^4$ s for the rising and the decay phase are $\sim-0.95$ and $\sim2.05$. However, the X-ray light curve decays faster than the optical light curve after the peak, which implies the cooling frequency may locate somewhere between the X-ray and the optical bands. For the X-ray light curve from $\sim T_0+5\times10^3$ s to $\sim T_0+7\times10^4$ s, the temporal decay index is about 2.6.