GCN Circular 45478

Murray Brightman (Caltech) on behalf of the NuSTAR Science Operations Center:

We report on the detection of NuSTAR 260828, a 1100s hard X-ray transient found at 2026-08-28 18:39:29.6 (UTC, MJD=61280.7) with a position of RA=91.9004 deg, -8.49441 deg (~10 arcsec uncertainty).

The spectrum can be described by a power-law with spectral index, Gamma=1.7^+2.0_-1.9 with a 3--79 keV flux of 2.9 +/- 2.2 x10^-12 erg cm^-2 s^-1, which extrapolates to 1.1^+3.1_-0.9 x10^-12 erg cm^-2 s^-1 in the 0.2--10 keV band.

No X-ray source has been catalogued at the position of the NuSTAR source, despite multiple Swift observations of the region. With the soft, albeit unconstrained spectrum, and position close to the Galactic plane, a flare star cannot be ruled out. We encourage follow-up observations at other wavelengths in order to determine the nature of this X-ray transient.

More information about previously detected fast X-ray transients in the NuSTAR archive can be found in this paper: [https://ui.adsabs.harvard.edu/#abs/2026ApJ...999...75B/abstract]

NuSTAR is a NASA small explorer mission operated at Caltech. It was launched in June 2012 into low-Earth orbit and operates in the 3--79 keV energy band. It was the first focusing hard X-ray (>10 keV) observatory in orbit and is currently the most sensitive X-ray telescope above 10 keV of any operating mission.