Exploring the connection between compact object mergers and fast X-ray transients

The cases of LXT 240402A and EP250207b

¹ Department of Physics, University of Rome Tor Vergata Via della Ricerca Scientifica 1 00133 Rome, Italy
² Instituto de Astronomía, Universidad Nacional Autónoma de México Apartado Postal 70-264 04510 México CDMX, Mexico
³ Department of Physics, University of Rome La Sapienza P.le Aldo Moro 2 00185 Rome, Italy
⁴ Department of Astronomy and Astrophysics, The Pennsylvania State University 525 Davey Lab University Park PA 16802, USA
⁵ McWilliams Center for Cosmology and Astrophysics, Department of Physics, Carnegie Mellon University Pittsburgh PA 15213, USA
⁶ Los Alamos National Laboratory Los Alamos NM 87545, USA
⁷ National Astronomical Observatories, Chinese Academy of Sciences Beijing 100012, China
⁸ School of Astronomy and Space Sciences, University of Chinese Academy of Sciences Beijing 100049, People’s Republic of China
⁹ Department of Physics, University of Cagliari, SP Monserrato-Sestu km 0.7 09042 Monserrato, Italy
¹⁰ Ioffe Institute 26 Politekhnicheskaya St.Petersburg 194021, Russia
¹¹ Instituto de Astrofísica de Andalucía (IAA-CSIC) Glorieta de la Astronomía s/n 18008 Granada, Spain
¹² Ingeniería de Sistemas y Automática, Universidad de Málaga, Unidad Asociada al CSIC por el IAA, Escuela de Ingenierías Industriales, Arquitecto Francisco Peñalosa 6 Campanillas 29071 Málaga, Spain
¹³ Space Science Division, Naval Research Laboratory Washington DC 20375, USA
¹⁴ Faculty of Science, University of Helsinki Gustaf Hallströmin katu 2 FI-00014 Helsinki, Finland
¹⁵ Institute of Physics and Technology, Ural Federal University Mira str. 19 620002 Ekaterinburg, Russia
¹⁶ Instituto de Astronomía, Universidad Nacional Autónoma de México km 107 Carretera Tijuana-Ensenada 22860 Ensenada Baja California, México
¹⁷ Nevada Center for Astrophysics, University of Nevada Las Vegas NV 89154, USA
¹⁸ The Hong Kong Institute for Astronomy and Astrophysics, University of Hong Kong Pokfulam Road Hong Kong, China
¹⁹ Department of Physics, University of Hong Kong Pokfulam Road Hong Kong, China
²⁰ Department of Physics and Astronomy, University of Nevada Las Vegas NV 89154, USA

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Received: 9 October 2025
Accepted: 30 November 2025

Abstract

Context. The connection between compact object mergers and some extragalactic fast X-ray transients (FXRTs) has long been hypothesised but never ultimately established.

Aims. In this work, we investigate two FXRTs, the LEIA X-ray Transient LXT 240402A and the Einstein Probe EP 250207b, whose precise positions lie close to nearby (z ≲ 0.1) quiescent galaxies with a negligible probability of chance coincidence, identifying them as particularly promising cases of merger-driven explosions in the local Universe.

Methods. We used Chandra to derive accurate localisations for both events and secure otherwise ambiguous associations with their optical counterparts. Deep optical and near-infrared observations with VLT, GTC, and LBT were performed to characterise the surrounding environment and search for kilonova emission, the hallmark of neutron star mergers. Complementary early-time X-ray monitoring with Swift and Einstein Probe was used to constrain the non-thermal afterglow.

Results. We find that both FXRTs remain compatible with a compact binary merger progenitor, which produced low-mass ejecta and kilonova emission subdominant to the afterglow. However, alternative explanations such as a distant (z ≳ 1) core-collapse supernova cannot be conclusively ruled out.