The Einstein Probe Transient EP240414a: Linking Fast X-Ray Transients, Gamma-Ray Bursts, and Luminous Fast Blue Optical Transients

Abstract 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 M R ∼ −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 ~10 48 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.

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Publication Details

Journal
The Astrophysical Journal Letters
Published
2025-03-27
DOI
https://doi.org/10.3847/2041-8213/adbc7e
Citations
33
Primary Topic
Gamma-ray bursts and supernovae
Type
article
Field-Weighted Citation Impact
38.11

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article

The Einstein Probe Transient EP240414a: Linking Fast X-Ray Transients, Gamma-Ray Bursts, and Luminous Fast Blue Optical Transients

J. P. Anderson, S. P. Littlefair, M. Fraser, J. Terwel et al.
33 citations
The Astrophysical Journal Letters
Gamma-ray bursts and supernovae
38.11
article

The Einstein Probe Transient EP240414a: Linking Fast X-Ray Transients, Gamma-Ray Bursts, and Luminous Fast Blue Optical Transients

J. P. Anderson, S. P. Littlefair, M. Fraser, J. Terwel, L. Galbany, P. Jakobsson, M. Gromadzki, T. E. Müller-Bravo, G. Pugliese, P. G. Jonker, A. de Ugarte Postigo, F. E. Bauer, A. J. Levan, A. Martín-Carrillo, J. Sollerman, Tayyaba Zafar, D. Malesani, N. R. Tanvir, A. J. van der Horst, D. Mata Sánchez, J. Quirola-Vásquez, A A Chrimes, Luca Izzo, M. A. P. Torres, Steve Schulze, Fabio Ragosta, M. E. Ravasio, Nikhil Sarin, Joyce N. D. van Dalen, Agnes P. C. van Hoof, Avishay Gal-Yam, Massimiliano de Pasquale, P. O’Brien
article en
33 citations

Abstract

Abstract 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 M R ∼ −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 ~10 48 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.

The Astrophysical Journal LettersVol. 982(2)
Laboratoire d’Astrophysique de Marseille (FR), Château Gombert (FR), Observatoire de la Côte d’Azur (FR)
National Science Foundation, Aarhus Universitet, Michigan State University, Gruber Foundation, University of Leicester, European Commission, Nederlandse Organisatie voor Wetenschappelijk Onderzoek, Ministério da Ciência, Tecnologia e Inovação, European Southern Observatory, Ministerio de Ciencia e Innovación, Universitetet i Oslo, Háskóli Íslands, Turun Yliopisto, Stockholms Universitet, Agencia Nacional de Investigación y Desarrollo, Centers for Disease Control and Prevention, College of Engineering, Michigan State University, University of North Carolina at Chapel Hill, Science and Technology Facilities Council, Agència de Gestió d'Ajuts Universitaris i de Recerca, Agencia Estatal de Investigación
Openalex Percentile: Top 0%
Gamma-ray bursts and supernovae
38.11
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