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 the 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, setting 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 1043–1048 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.
Fast X-Ray Transients in NuSTAR Data
Abstract
Copyright and License
© 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
We thank the anonymous referee for their insightful and valuable input into our manuscript, which improved its quality.
NuSTAR is a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA).
This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC.
Facilities
NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission.
Software References
HEASoft (Nasa High Energy Astrophysics Science Archive Research Center (Heasarc) 2014), NuSTARDAS, XSPEC (K. A. Arnaud 1996).
Files
Brightman_2026_ApJ_999_75.pdf
Additional details
Related works
- Is new version of
- Discussion Paper: arXiv:2601.14375 (arXiv)
Funding
- National Aeronautics and Space Administration
Dates
- Submitted
-
2025-10-22
- Accepted
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2026-01-13
- Available
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2026-02-25Published
Caltech Custom Metadata
- Caltech groups
- Space Radiation Laboratory , Physics Department , NuSTAR , Division of Physics, Mathematics and Astronomy (PMA)
- Publication Status
- Published