Fast X-ray transients (FXTs) are flashes of X-rays that last for a few hundreds of seconds to a few hours. An enigmatic population of these transients that did not have a clear origin has been known for several decades, mostly found serendipitously in soft X-ray imaging observations. Recent progress in this field by Einstein Probe has found that many FXTs are associated with gamma-ray bursts and the collapse of massive stars. Motivated by this, we searched the NuSTAR archive in the harder 3--79 keV band for ∼1000 s duration transients. From 204 Ms of exposure we present five candidate FXTs, four of which are spectrally hard, with power-law indices −3<Γ<0, standing them apart from FXTs discovered in the soft band. Three have potential associations with galaxies at z=0.1−2, implying 3--79 keV luminosities of 1043 to 1048 erg s−1 and volumetric event rates of 125--2900 Gpc−3 yr−1. The properties of these NuSTAR FXTs most resemble low-luminosity gamma-ray bursts, and would be much more common than their higher-luminosity counterparts in this redshift range.
@article{arxiv.2601.14375,
title = {Fast X-ray transients in NuSTAR data},
author = {Murray Brightman and Joahan Castañeda Jaimes and Daniel Stern and Brian Grefenstette},
journal= {arXiv preprint arXiv:2601.14375},
year = {2026}
}