Gas absorption of soft X-rays strongly impacts the redshift distribution of dark fast X-ray transients
Year: 2026
Type: article
Abstract: ABSTRACT The progenitors of many fast X-ray transients (FXTs) and those of $\gamma$-ray bursts (GRBs) are strongly linked. Given that ‘dark’ GRBs are typically found to be events suffering from enhanced extinction in the host galaxy, we investigate the nature of ‘dark’ FXTs. However, unlike $\gamma$-rays, soft X-rays are strongly affected by absorption, implying that dark FXTs discovered by Einstein Probe’s Wide-field X-ray Telescope (EP-WXT) have to be emitted at energies >2–3 keV rest frame hence at a redshift $z\gtrsim 2$. To illustrate this, we present two dark FXTs, EP241103a and EP260409a, discovered by EP-WXT. For EP241103a, the high extinction precluded the detection of the optical counterpart, while for a probable source redshift of $\approx 2.5$, the rest-frame X-ray photons are not severely affected. For EP260409a no counterpart is detected in the optical, but we do detect a near-infrared counterpart. A plausible scenario for EP260409a is that it lies at a redshift $z \gtrsim 4$. Additionally, we conduct simulations of FXTs observed with EP-WXT, showing that the detected counts for absorbed events ($N_{\mathrm{H}}\gtrsim 10^{22}$ cm$^{-2}$) strongly decrease at low redshifts, hindering their detection. These results support the theoretical prediction that dark FXTs have an intermediate to high redshift and a different selection function from those of dark GRBs.
Authors: Javier Sánchez-Sierras, Peter G Jonker, Jonathan Quirola-Vásquez, Agnes P C van Hoof, Andrew J Levan, Maria E Ravasio, Joyce N D van Dalen, Franz E Bauer, Jia-Ying Cao, Francesco Carotenuto, Jennifer A Chacón, Ashley A Chrimes, Laura Cotter, Gregory Corcoran, Rob A J Eyles-Ferris, Guoli Huang, Shuai-Qing Jiang, Zexi Li, Yifang Liang, He-Yang Liu, Daniele B Malesani, Antonio Martin-Carrillo, Daniel Mata Sánchez, Nikhil Sarin, Manuel A P Torres, Qinyu Wu, Tong Zhao, Wen-Da Zhang
Institutions: Radboud University Nijmegen, University of Warwick, Institute of Space Sciences, Institut d'Estudis Espacials de Catalunya, Universitat Politècnica de Catalunya, University of Tarapacá, Chinese Academy of Sciences, Institute of High Energy Physics, Astronomical Observatory of Rome, National Institute for Astrophysics, Pontificia Universidad Católica de Chile, European Space Agency, European Space Research and Technology Centre, University College Dublin, University of Leicester, University of Chinese Academy of Sciences, National Astronomical Observatories, University of Science and Technology of China, Purple Mountain Observatory, University of Copenhagen, Universidad de La Laguna, Instituto de Astrofísica de Canarias, University of Cambridge
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $3,046
Funders: National Science Foundation, National Aeronautics and Space Administration, California Institute of Technology, Fondation Sanofi Espoir, “la Caixa” Foundation, Ministerio de Ciencia, Innovación y Universidades, University of Leicester, University of Warwick, European Commission, Chinese Academy of Sciences, Centre National d’Etudes Spatiales, Korea Astronomy and Space Science Institute, Ministério da Ciência, Tecnologia, Inovações e Comunicações, Agencia Nacional de Investigación y Desarrollo, Instituto de Astrofísica de Canarias, Science and Technology Facilities Council, Agencia Estatal de Investigación, Fondo Nacional de Desarrollo Científico y Tecnológico
Awards: G1105844351, G1175115578, G1255325603, G210041235, G2262748287, G2428672771, G2469010403, G2601397997, G2636502879, G2650114919, G2690211755, G2724668986, G300979063, G342036044, G34346518, G3452655474, G3480869486, G3681454997, G3715634343, G3803561910, G4008337681, G4042783231, G425668148, G4526231891, G465191129, G4655338613, G491712853, G4919915994, G4993383890, G5080475149, G5099673792, G5178665745, G5180915604, G5195188938, G5404395142, G5601225551, G5635532810, G5806957892, G5834573078, G617610489, G624877701, G7084143925, G7731791180, G7805626310, G7863371703, SPECTRO-TEMPORAL STUDIES OF AGN POPULATIONS, G8257951253, G8284171658, G8497050386, CENTRO DE ASTROFISICA Y TECNOLOGIAS AFINES (CATA)
OpenAlex ID: W7212218480