Unveiling the nature of the <i>Einstein Probe</i> transient EP241021a

ABSTRACT We present a multiwavelength analysis of the fast X-ray transient EP241021a, discovered by the Wide-field X-ray Telescope aboard the Einstein Probe satellite on 2024 October 21. The event was not detected in gamma-rays. Follow-up observations from $\sim$1.5 to 100 d post-trigger were obtained across X-ray, ultraviolet, optical, near-infrared, and radio bands with ground- and space-based facilities. The redshift is constrained to $z = 0.7485$ from prominent optical spectral features. The optical light curve shows complex evolution: an initial $\sim t^{-0.7}$ decay, followed by a rapid re-brightening peaking at day 7.7 with $\sim t^{-1.7}$ decay, and a third phase peaking near day 19 with $\sim t^{-1.3}$ decay. The spectral energy distribution (SED) and its temporal evolution are consistent with a mix of non-thermal and thermal components. Early optical-to-X-ray spectral indices agree with optically thin synchrotron emission, while steepening of the optical SED after $\sim$20 d indicates either a shift in emission mechanism or the emergence of an additional component. Although broad-lined absorption features are absent, comparisons with Type Ic-BL supernovae suggest an SN contribution at late times, suggesting a collapsar origin for EP241021a. The likely SN in EP241021a appears to require an additional energy source beyond $^{56}$Ni decay. These results support the view that some fast X-ray transients detected by the Einstein Probe arise from massive stellar explosions.

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Journal
Monthly Notices of the Royal Astronomical Society
Published
2025-11-20
DOI
https://doi.org/10.1093/mnras/staf2064
Primary Topic
Gamma-ray bursts and supernovae
Type
article
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article

Unveiling the nature of the Einstein Probe transient EP241021a

S. P. Littlefair, M. Fraser, Jesús Chacón, Rob A. J. Eyles-Ferris et al.
Monthly Notices of the Royal Astronomical Society
Gamma-ray bursts and supernovae
article

Unveiling the nature of the Einstein Probe transient EP241021a

S. P. Littlefair, M. Fraser, Jesús Chacón, Rob A. J. Eyles-Ferris, F. E. Bauer, A. J. Levan, D. Stern, A. Martín-Carrillo, P. T. O’Brien, P. G. Jonker, D. Malesani, Luciano Izzo, Laura Cotter, J. Sánchez-Sierras, Gavin P. Lamb, D. H. Hartmann, D. Mata Sánchez, J. Quirola-Vásquez, M. A. P. Torres, V. S. Dhillon, Fiona Harrison, M. E. Ravasio, Nikhil Sarin, Joyce N. D. van Dalen, Agnes P. C. van Hoof, G Corcoran, V. D’Elia
article en

Abstract

ABSTRACT We present a multiwavelength analysis of the fast X-ray transient EP241021a, discovered by the Wide-field X-ray Telescope aboard the Einstein Probe satellite on 2024 October 21. The event was not detected in gamma-rays. Follow-up observations from $\sim$1.5 to 100 d post-trigger were obtained across X-ray, ultraviolet, optical, near-infrared, and radio bands with ground- and space-based facilities. The redshift is constrained to $z = 0.7485$ from prominent optical spectral features. The optical light curve shows complex evolution: an initial $\sim t^{-0.7}$ decay, followed by a rapid re-brightening peaking at day 7.7 with $\sim t^{-1.7}$ decay, and a third phase peaking near day 19 with $\sim t^{-1.3}$ decay. The spectral energy distribution (SED) and its temporal evolution are consistent with a mix of non-thermal and thermal components. Early optical-to-X-ray spectral indices agree with optically thin synchrotron emission, while steepening of the optical SED after $\sim$20 d indicates either a shift in emission mechanism or the emergence of an additional component. Although broad-lined absorption features are absent, comparisons with Type Ic-BL supernovae suggest an SN contribution at late times, suggesting a collapsar origin for EP241021a. The likely SN in EP241021a appears to require an additional energy source beyond $^{56}$Ni decay. These results support the view that some fast X-ray transients detected by the Einstein Probe arise from massive stellar explosions.

Monthly Notices of the Royal Astronomical SocietyVol. 545(4)
University College Dublin (IE), California Institute of Technology (US), University of Copenhagen (DK), Jet Propulsion Laboratory (US), Radboud University Nijmegen (NL), University of Leicester (GB), Universidad de La Laguna (ES), Stockholm University (SE), Pontificia Universidad Católica de Chile (CL), University of Tarapacá (CL), Agenzia Spaziale Italiana (IT), Astronomical Observatory of Capodimonte (IT), University of Warwick (GB), Millennium Institute of Astrophysics (CL), Physical Sciences (United States) (US), Instituto de Astrofísica de Canarias (ES), AlbaNova (SE), Liverpool John Moores University (GB), Clemson University (US), University of Sheffield (GB)
National Science Foundation, National Aeronautics and Space Administration, California Institute of Technology, Gruber Foundation, UK Research and Innovation, European Commission, Korea Astronomy and Space Science Institute, Knut och Alice Wallenbergs Stiftelse, Vetenskapsrådet, Ministério da Ciência, Tecnologia, Inovações e Comunicações, Agencia Nacional de Investigación y Desarrollo, Jet Propulsion Laboratory
Openalex Percentile: Top 76%
Gamma-ray bursts and supernovae
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