Detectability of Gravitational-wave counterparts of EP-FXTs observed during the O4b LIGO-Virgo-KAGRA Observing Run

Abstract

Fast X-ray transients (FXTs) detected by the Einstein Probe mission have emerged as a rapidly growing class of extragalactic transients, whose physical origin remains uncertain. Compact binary coalescence (CBC) systems have been proposed as one possible progenitor for at least a subset of these events, making FXTs promising targets for multi-messenger studies with gravitational-wave (GW) observations. This work presents the first systematic investigation of GW counterparts to FXTs observed by Einstein Probe and assesses the detectability of associated CBC. We focus on FXTs detected during the second half of the fourth observing run (O4b) of the Advanced LIGO-Virgo-KAGRA detector network by searching for temporal coincidences with GW candidates from the fifth Gravitational-Wave Transient Catalog (GWTC-5). We analyze a sample of 47 FXTs, including 11 with measured redshifts, and assess the significance of the association between FXTs and GW candidates using a ranking statistic. We find no significant GW counterpart associated with any FXT in our sample. In the absence of a detection, we place 90% exclusion-distance constraints under the assumptions of binary neutron star and neutron star-black hole progenitor scenarios. For observations with the full LIGO and Virgo detector network, the typical median exclusion distances are $\sim$178 Mpc and $\sim$349 Mpc, respectively. These constraints disfavor a nearby compact-binary merger origin. Longer periods of joint observations by the LVK and Einstein Probe, combined with improved GW detector sensitivity, will enhance the prospects for identifying genuine GW-FXT associations and place tighter constraints on the progenitor scenarios of these events.