Based on observations collected at the European Southern Observatory under ESO program(s) 113.26ET.002 (PI: Jonker), 113.26ET.008 (PI: Jonker), 110.24CF.022 1110.A-4348(V) (PI: Tanvir), 111.259Q.001 (PI: Jonker), and 113.26ET.004 (PI: Jonker); the GTC, under the International Time Programme of the CCI (International Scientific Committee of the Observatorios de Canarias of the IAC), operated on the island of La Palma by the Roque de los Muchachos under programm ID ITP23 (PIs: Jonker and Torres); and on observations made in part with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and 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. Based in part on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia e Inovações (MCTI/LNA) do Brasil, the US National Science Foundation's NOIRLab, the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU); and based in part on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere, Chile as part of the Public ESO Spectroscopic Survey for Transient Objects Survey (PESSTO) ESO programs 188.D-3003, 191.D-0935, and 197.D-1075. The scientific results reported in this Letter are based in part on observations made by the Chandra X-ray Observatory under program number 29420 (PI: Jonker), contained in the Chandra Data Collection (CDC) 331 doi:10.25574/cdc.331. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. J.N.D.v.D., P.G.J., J.Q.V., and A.P.C.v.H. are supported by the European Union (ERC, StarStruck, 101095973). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. J.Q.V. additionally acknowledges support by the IAU-Gruber foundation. D.M.S. and M.A.P.T. acknowledge support by the Spanish Ministry of Science via the Plan de Generacion de conocimiento PID2021-124879NB-I00. F.E.B. acknowledges support from ANID-Chile BASAL CATA FB210003, FONDECYT Regular 1241005, and Millennium Science Initiative, AIM23-0001 and ICN12_009. J.P.A.'s work was funded by ANID, Millennium Science Initiative, ICN12_009. L.G. acknowledges financial support from AGAUR, CSIC, MCIN, and AEI 10.13039/501100011033 under projects PID2023-151307NB-I00, PIE 20215AT016, CEX2020-001058-M, and 2021-SGR-01270. M.E.R. acknowledges support from the research program Athena with project number 184.034.002, which is financed by the Dutch Research Council (NWO). P.O.B. acknowledges support from the UK Science and Technology Facilities Council through grant ST/W000857/1. T.E.M.B. acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación (MCIN), the Agencia Estatal de Investigación (AEI) 10.13039/501100011033, and the European Union Next Generation EU/PRTR funds under the 2021 Juan de la Cierva program FJC2021-047124-I and the PID2023-151307NB-I00 SNNEXT project, from Centro Superior de Investigaciones Cientìficas (CSIC) under the PIE project 20215AT016, and the program Unidad de Excelencia Marìa de Maeztu CEX2020-001058-M. N.S. acknowledges support from the Knut and Alice Wallenberg foundation through the "Gravity Meets Light" project. S.S. is partially supported by LBNL Subcontract 7707915.

Detections of fast X-ray transients (FXTs) have accrued over the last few decades. However, their origin has remained mysterious. Rapid progress is now being made thanks to timely discoveries and localizations with the Einstein Probe mission. Early results indicate that FXTs may frequently, but not always, be associated with gamma-ray bursts (GRBs). Here, we report on the multiwavelength counterpart of FXT EP240414a, which has no reported gamma-ray counterpart. The transient is located 25.7 kpc in projection from a massive galaxy at z = 0.401. We perform comprehensive photometric and spectroscopic follow-up. The optical light curve shows at least three distinct emission episodes with timescales of ~1, 4, and 15 days and peak absolute magnitudes of MR ∼ −20, –21, and –19.5, respectively. The optical spectrum at early times is extremely blue, inconsistent with afterglow emission. It may arise from the interaction of both jet and supernova (SN) shock waves with the stellar envelope and a dense circumstellar medium, as has been suggested for some luminous fast blue optical transients (LFBOTs). At late times, the spectrum evolves to a broad-lined Type Ic SN, similar to those seen in collapsar long GRBs. This implies that the progenitor of EP240414a is a massive star creating a jet-forming SN inside a dense envelope, resulting in an X-ray outburst with a luminosity of ~1048 erg s−1 and the complex observed optical/IR light curves. If correct, this argues for a causal link between the progenitors of long GRBs, FXTs, and LFBOTs.

With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (CEX2020-001058-M)

Joyce N. D. van Dalen et al.

Peer reviewed