Abstract
Massive stars end their lives as core-collapse supernovae, among which some extremes are broad-lined type Ic supernovae from Wolf–Rayet stars associated with long-duration gamma-ray bursts (LGRBs) with powerful relativistic jets. Their less-extreme brethren make unsuccessful jets that are choked inside the stars, appearing as X-ray flashes or low-luminosity GRBs. However, there exists a population of extragalactic fast X-ray transients with timescales ranging from seconds to thousands of seconds, whose origins remain obscure. Here we report the discovery of the bright X-ray transient EP240414a detected by the Einstein Probe, which is associated with the type Ic supernova SN 2024gsa at a redshift of 0.401. The X-ray emission evolution is characterized by a very soft energy spectrum peaking at <1.3 keV, which makes it different from known LGRBs, X-ray flashes or low-luminosity GRBs. Follow-up observations at optical and radio bands revealed the existence of a weak relativistic jet that interacts with an extended shell surrounding the progenitor star. Located on the outskirts of a massive galaxy, this event reveals a population of explosions of Wolf–Rayet stars characterized by a less powerful engine that drives a successful but weak jet, possibly owing to a progenitor star with a smaller core angular momentum than in traditional LGRB progenitors.
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Data availability
The light curves and spectra of EP-WXT and EP-FXT and the spectroscopic data are available at https://github.com/huisungh/EP240414a.git. The light curves of Swift-BAT GRBs are public and can be found at https://www.swift.ac.uk/burst_analyser.
Code availability
Upon reasonable request, the code (mostly in Python) used to produce the results and figures will be provided.
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Acknowledgements
This work is based on data obtained with the Einstein Probe, a space mission supported by the Strategic Priority Program on Space Science of the Chinese Academy of Sciences, in collaboration with ESA, MPE and CNES (grant XDA15310000); the Strategic Priority Program on Space Science of the Chinese Academy of Sciences (grant number E02212A02S) and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant number XDB0550200). We acknowledge the support by the National Natural Science Foundation of China (NSFC grants 12288102, 12373040, 12021003, 12103065, 12333004, 12203071, 12033003, 12233002 and 12303047). This work is also supported by the National Key R&D Program of China (grant 2022YFF0711500). W.-X.L., S.-J.X., H.Z. and W.-J.G. acknowledge the support from the Strategic Priority Research Program of the Chinese Academy of Sciences (grant numbers XDB0550100 and XDB0550000), National Key R&D Program of China (grant numbers 2023YFA1607804, 2022YFA1602902 and 2023YFA1608100) and National Natural Science Foundation of China (NSFC; grant numbers 12120101003, 12373010 and 12233008) X.-F. Wang’s group at Tsinghua University is supported by NSFC (grants 12288102 and 12033003), and the Tencent Xplorer Prize. A.V.F.’s group at UC Berkeley is grateful for financial assistance from the Christopher R. Redlich Fund, G. and C. Bengier, C. and S. Winslow, A. Eustace (W.-K.Z. is a Bengier–Winslow–Eustace Specialist in Astronomy), W. Draper, T. and M. Draper, B. and K. Wood, S. Robertson (T.G.B. is a Draper–Wood–Robertson Specialist in Astronomy), and many other donors. S.A. has received support from the Carlsberg Foundation (CF18-0183, principal investigator I. Tamborra). This work is supported by the ANID FONDECYT project number 3220029. Z.G. is funded by ANID, Millennium Science Initiative, AIM23-001. Partly based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. A.J.C.T. acknowledges support from the Spanish Ministry projects PID2020-118491GB-I00 and PID2023-151905OB-I00 and Junta de Andalucía grant P20_010168 and from the Severo Ochoa grant CEX2021-001131-S funded by MCIN/AEI/10.13039/501100011033. We acknowledge the support of the staff of the 10.4 m Gran Telescopio Canarias (GTC) and Keck I 10 m telescope. This work makes use of the Las Cumbres Observatory global network of robotic telescopes. The LCO group is supported by NSF grants AST-1911225 and AST-1911151. S.B. and N.E.-R. acknowledge support from the PRIN-INAF 2022, ‘Shedding light on the nature of gap transients: from the observations to the models’. We gratefully acknowledge the China National Astronomical Data Center (NADC), the Astronomical Data Center of the Chinese Academy of Sciences, and the Chinese Virtual Observatory (China-VO) for providing data resources and technical support. The work of D.S.S., A.V.R. and D.D.F. was supported by the basic funding programme of the Ioffe Institute number FFUG-2024-0002. The radio data processing was conducted using China SKA Regional Center compute system. The Australia Telescope Compact Array is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by CSIRO. This research has made use of the Common Astronomy Software Applications (CASA). T. An acknowledges the support of the Xinjiang Tianchi Talent Program. T. An and Y.-Q.L. are supported by the National SKA Program of China grant numbers 2022SKA0130103 and FAST special funding (NSFC 12041301).
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Contributions
W.Y. has been leading the Einstein Probe project as principal investigator since the mission proposal stage. H.G., H.S., B.Z., W.-X.L., X.-F. Wang and Y.L. initiated the study. H.G., X.-F. Wang, B.Z., X.-F. Wu, H.S., W.-X.L. and L.-D. Liu coordinated the scientific investigations of the event and led the discussions. H.S., Y.L., T.-Y.L. processed and analysed the WXT data. Q.-Y.W. and H.S. processed and analysed the FXT data. Q.-Y.W. processed and analysed the XRT data. W.-X.L., X.-F. Wang and D.X. led the optical and near-infrared data taking and data analysis. A.V.F., W.-K.Z. Y.Y., T.G.B. and N.E.-R. obtained and reduced the optical/NIR spectroscopy and photometry. T. An and Y.-Q.L. helped with the radio data taking and analysis. S.B. helped with the reduction of GTC photometry and spectroscopy. D.M., S.A.E. and D.A.H. contributed to the optical data taking with AZT-22 telescope and LCO 1 m telescopes, respectively. L.-D. Liu, H.G., B.Z. and S.A. led the theoretical investigation of the event. W.-X.L, H.S., B.-B.Z., X.-F. Wang and D.X. contributed to the theoretical investigation of the event. C.-Y.W., B.-B.Z. and B.Z. contributed to comparing this event with GRBs. H.S. contributed to the event rate density. J.D. performed the GRB search in Swift/BAT data and the upper limit. D.S.S., A.V.R. and D.D.F. performed GRB search in the Konus-Wind data and the upper limit. T.-Y.L., X.P., Y.-F.L., J.Y. and C.-Y.D. are the transient advocates on 14 April 2024 and contributed to the discovery and preliminary analysis of the event. Z.-X.L., C.Z., S.-N.Z., X.-J.S., S.-L.S., X.-F.Z., Y.-H.Z., Z.-M.C. F.-S.C. and W.Y. contributed to the development of the WXT instrument. C.Z., Z.-X.L., H.-Q.C., D.-H.Z. and Y.L. contributed to the calibration of WXT data. Y.L., H.-Q.C., C.-C.J., W.-D.Z., D.-Y.L., J.-W.H., H.-Y.L., H.S., H.-W.P. and M.-J.L. contributed to the development of WXT data analysis software. Y.C., S.-M.J., W.-W.C., C.-K.L., D.-W.H., J.W., W.L., Y.-J.Y., Y.-S.W., H.-S.Z., J.G., J.Z., X.-F.Z., J.-J.X., J.M., L.-D. Luo, H.W., X.-T.Y., T.-X.C., J.H., Z.-J.Z., Z.-L.Z., M.-S.L., Y.-X.Z., D.-J.H., L.-M.S., F.-J.L., C.-Z.L., Q.-J.T. and H.-L.C. contributed to the development of the FXT instrument. S.-M.J., H.-S.Z., C.-K.L., J.Z. and J.G. contributed to the development of FXT data analysis software. W.-X.L., H.S., L.-D. Liu, H.G., X.-F. Wang, B.Z. and S.A. drafted the paper with help from all authors. A.V.F. assisted with editing the paper.
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Extended data
Extended Data Fig. 1 The WXT spectrum in the time interval of T90.
a, The WXT observed spectrum and the predicted best-fit absorbed power-law model. Data are presented as the count rate spectrum with 1σ uncertainties. b, Best-fit values of photon index and intrinsic absorption Nint and the 1σ, 2σ confidence contours. c, The spectral energy distribution. Data are presented as the energy flux density with 1σ uncertainties. The upper limits displayed in brown and green represent those of Konus Wind and Swift/BAT, respectively.
Extended Data Fig. 2 Fitting of the theoretical afterglow model to the X-ray, optical, and radio data.
Left panel: The early X-ray (1 keV) and optical (r and i band) afterglow light curves and the modeling with the classical GRB afterglow model77,78. Data are presented as the measured flux density with 1σ uncertainties. Right panel: The synchrotron spectrum in the radio band at T0 + 19 days. The νa and νm are the selfabsorption frequency and the synchrotron frequency related to the accelerated electrons at the low-energy end, respectively.
Extended Data Fig. 3 Radio luminosity of EP240414a/SN 2024g sa.
The 9 GHz radio luminosity of EP240414a/SN 2024gsa is compared to low-frequency (1–10 GHz) light curves of different classes of energetic explosions: tidal disruption events100,101, SNe102,103, relativistic Ic-BL SNe104,105, long-duration GRBs106,107,108, and fast blue optical transients109,110,111.
Extended Data Fig. 4 Redshift-corrected optical and NIR spectra of SN 2024gsa.
The optical spectrum was obtained on April 19.9 using the GTC/OSIRIS+ instrument (light blue) and the NIR spectrum was obtained on April 19.3 using the Keck/NIRES instrument (light red). Rebinned versions of both spectra, generated using a bin width of 20 Å, are also overplotted in blue and red, respectively. Two photometric data points taken at similar phases in the i and z bands, converted to flux density, are also plotted along with their transmission curves. The uncertainties associated with the two data points are shown at the 1σ confidence level. The effective wavelengths have been redshift-corrected, while the flux density remains uncorrected for redshift.
Extended Data Fig. 5 Projected offsets of SNe Ic-BL and GRBSNe.
The cumulative distributions of the projected offsets from the host-galaxy centres for a sample of SNe Ic-BL (blue) and GRBSNe (yellow) are shown as solid lines35, with SN 2024gsa marked by a red star. An I-band image of the host galaxy, obtained with the Keck-I telescope, is overlaid as an inset; Object 1 corresponds to J1246, the faint Object 2 to SN 2024gsa, and Object 3 to a foreground point source.
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Sun, H., Li, WX., Liu, LD. et al. A fast X-ray transient from a weak relativistic jet associated with a type Ic-BL supernova. Nat Astron 9, 1073–1085 (2025). https://doi.org/10.1038/s41550-025-02571-1
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DOI: https://doi.org/10.1038/s41550-025-02571-1
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