Gas absorption of soft X-rays strongly impacts the redshift distribution of dark Fast X-ray Transients
Abstract
The progenitors of many Fast X-ray Transients (FXTs) and those of γ-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 γ-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 restframe hence at a redshift z≳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 ∼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 ≳ 4. Additionally, we conduct simulations of FXTs observed with EP-WXT, showing that the detected counts for absorbed events (NH≳1022~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.
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BibTeX
@misc{sanchezsierras2026absorption,
title = {{Gas absorption of soft X-rays strongly impacts the redshift distribution of dark Fast X-ray Transients}},
author = {Javier Sánchez-Sierras and Peter G. Jonker and Jonathan Quirola-Vásquez and Agnes P. C. van Hoof and Andrew J. Levan and Maria E. Ravasio and Joyce N. D. van Dalen and Franz E. Bauer and Jia-Ying Cao and Francesco Carotenuto and Jennifer A. Chacón and Ashley A. Chrimes and Laura Cotter and Gregory Corcoran and Rob A. J. Eyles-Ferris and Guoli Huang and Shuai-Qing Jiang and Zexi Li and Yifang Liang and He-Yang Liu and Daniele B. Malesani and Antonio Martin-Carrillo and Daniel Mata Sánchez and Nikhil Sarin and Manuel A. P. Torres and Qinyu Wu and Tong Zhao and Wen-Da Zhang},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.13385},
eprint = {2609.13385},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.13385},
}
RIS
TY - UNPB
TI - Gas absorption of soft X-rays strongly impacts the redshift distribution of dark Fast X-ray Transients
AU - Javier Sánchez-Sierras
AU - Peter G. Jonker
AU - Jonathan Quirola-Vásquez
AU - Agnes P. C. van Hoof
AU - Andrew J. Levan
AU - Maria E. Ravasio
AU - Joyce N. D. van Dalen
AU - Franz E. Bauer
AU - Jia-Ying Cao
AU - Francesco Carotenuto
AU - Jennifer A. Chacón
AU - Ashley A. Chrimes
AU - Laura Cotter
AU - Gregory Corcoran
AU - Rob A. J. Eyles-Ferris
AU - Guoli Huang
AU - Shuai-Qing Jiang
AU - Zexi Li
AU - Yifang Liang
AU - He-Yang Liu
AU - Daniele B. Malesani
AU - Antonio Martin-Carrillo
AU - Daniel Mata Sánchez
AU - Nikhil Sarin
AU - Manuel A. P. Torres
AU - Qinyu Wu
AU - Tong Zhao
AU - Wen-Da Zhang
PY - 2026
DA - 2026/09/11
PB - arXiv
DO - 10.48550/arxiv.2609.13385
UR - https://arxiv.org/abs/2609.13385
AB - The progenitors of many Fast X-ray Transients (FXTs) and those of γ-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 γ-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 restframe hence at a redshift z≳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 ∼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 ≳ 4. Additionally, we conduct simulations of FXTs observed with EP-WXT, showing that the detected counts for absorbed events (N_H≳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.
ER -