Characterising EP241107a: Multiwavelength Observations of an Einstein Probe-detected Fast X-ray Transient

TL;DR

This paper presents a comprehensive, multiwavelength follow-up of the Einstein Probe FXT EP241107a, combining optical photometry, spectroscopy, and radio imaging to identify a host at $z \approx 0.457$ and to characterize the transient's afterglow. Through Afterglowpy modelling, the authors infer a mildly relativistic GRB jet with $E_{K,iso} \sim 10^{51}$ erg, $\theta_{c} \approx 15^{\circ}$, and $\theta_{obs} \approx 9^{\circ}$, placing EP241107a near the on-axis GRB population but with a relatively faint gamma-ray output. Host-galaxy analysis using Bagpipes yields a modest stellar mass and star-formation rate, with a ~4.8 kpc offset consistent with long GRB hosts. The results suggest EP241107a is an intrinsically faint GRB, and the study situates EP241107a within the broader FXT landscape, highlighting EP’s capability to uncover diverse, high-energy transients and constrain jet properties through prompt, multiwavelength follow-up.

Abstract

Fast X-ray Transients (FXTs) represent a new class of highly luminous transients in soft X-rays ($\sim$0.3-10 keV) associated with violent astrophysical processes. They manifest as short, singular flashes of X-ray photons with durations lasting from minutes to hours. Their origin remains unclear, and they have been associated with various progenitor mechanisms. The newly launched X-ray survey, Einstein-Probe (EP), is revolutionising this field by enabling the discovery and immediate follow-up of FXTs. Here we present the multiwavelength observations of EP-discovered FXT EP241107a and the discovery of its radio counterpart. Comparison of the optical and radio observations of EP241107a and its host properties with other extragalactic transients suggests a gamma-ray burst (GRB) origin. Through our afterglow modelling, we infer the GRB jet properties for EP241107a, yielding a jet of the isotropic-equivalent kinetic energy $E_{\mathrm{K,iso}} \sim10^{51}$ erg, with a half opening angle $θ_{c}$ $\approx$15$^{\circ}$, viewed at an angle of $θ_{\rm obs}$~$\approx$9$^{\circ}$. We also evaluate EP241107a in the landscape of both EP-discovered FXTs as well as the FXTs discovered from Chandra, XMM-Newton, and Swift-XRT.

Figures (10)

• Figure 1: The location of the counterparts to EP241107a in optical and radio bands is shown. The left panel shows the GIT $r{^\prime}$-filter image of the field of EP241107a, 0.165 days after the EP-trigger. The EP-FoXT uncertainty region of 10$^{\prime\prime}$ is marked with a dashed red circle, while the optical counterpart of EP241107a is marked with black lines. The VLA C-band image of the field of EP241107a is shown in the right panel. The VLA beam size for this observation is given in the bottom left of the image. Note that the scales of the optical and radio images are different.
• Figure 2: Optical photometry of the counterpart to EP241107a. We present the data obtained from our follow-up observations along with the data obtained from the GCNs. Different telescopes are denoted with different markers, while filters are indicated by various colours. The upper limits obtained from our observations are represented by open markers. We consider the T$_0$ to be the time of the EP-WXT trigger. Chinese Ground Follow-up Telescope at Changchun Observatory started the observation of the field of EP241107a about six minutes after the trigger and detected the counterpart in $i$-filter at a magnitude of 17.12$\pm$0.10. The secondary x and y axes assume a redshift of $z$ = 0.457$\pm$0.003.
• Figure 3: The flux calibrated Keck LRIS spectrum of the counterpart of EP241107a taken 0.767 days after the EP-trigger is shown. For display purposes, the boxcar smoothed spectrum (Box1DKernel with a width of 5 pixels) is shown in the black solid line, whereas the light grey shows the spectrum. The location of important emission lines in the spectrum is marked with dashed lines. The emission lines are the host contributions, and we derive a redshift of 0.457$\pm$0.003 from the emission lines.
• Figure 4: The best-fitting SED model of the host of EP241107a using BAGPIPES package is shown. The blue markers give the photometric data, and their 1$\sigma$ uncertainties, whereas the 16th–84th percentile range for the posterior probability for the spectrum and photometry are shown in light and dark orange colours, respectively. The bottom panel shows the posterior probability distributions for the five fitted parameters, SFR, age, galaxy stellar mass, metallicity and dust extinction. In each subplot, the 16th, 50th, and 84th percentile posterior values are represented by vertical dashed black lines marked from left to right.
• Figure 5: The optical light curve of EP241107a ($r$-filter; shown in skyblue) in comparison with the light curves of SN2008D (2008Natur.453..469S), AT2017gfo (2017ApJ...848L..17C), Type II SNe, Type Ia SNe, Type Ic SNe (2000ApJ...534..660I; 2006AJ....131..527J; 2006ApJ...645L..21M; 2017ApJS..233....6H), GRB afterglows (2010ApJ...720.1513K; 2011ApJ...734...96K), the optical counterpart of EP240315a (2024arXiv240416350L; 2024ApJ...969L..14G), EP240414a (2024arXiv240919056V; 2024arXiv240919070S), AT2018cow (the "Cow"; 2019MNRAS.484.1031P), ZTF18abvkwla (the "Koala"; 2020ApJ...895...49H) and the jetted TDE AT2022cmc (2022Natur.612..430A). The magnitudes are corrected for the galactic extinction. The light curve of EP241107a is consistent with the GRB afterglow light curves.