GCN 44203: GRB 260330B: EP-FXT follow-up and identification of candidate afterglow

2026-04-03T10:26:00.527Z | rev 0 | event: GRB 260330B
S. Guillot, G. X. Lan (IRAP), D. Turpin, A. Saccardi (CEA/Irfu), report on behalf of the SVOM and EP collaborations:


EP-FXT performed two follow-up observations of the SVOM/ECLAIRs-detected burst GRB 260330B (SVOM/sb26033003; Lan et al., GCN 44157), also detected as a sub-threshold event by Fermi/GBM (Trigg et al., GCN 44159). The first epoch started on 2026-03-31 at 16:39:42 UT, approximately 26.3 hours after the SVOM trigger, with a total exposure time of 3174 s. The second exposure started on 2026-04-02 at 19:48:54 UT, i.e., 77.5 hours after the SVOM trigger, with a total exposure time of 3200 s.

Three uncataloged sources are detected within the ECLAIRs error circle in the first epoch:

Source 1: EPF_J165032.7+365028 at RA, Dec (J2000) = 252.6363, 36.8413 with Fx=(1.20+/-0.32)e-13 erg/sec/cm2 from the FXT-B telescope.

Source 2: EPF_J165010.3+364729 at RA, Dec (J2000) = 252.5430, 36.7914 with Fx=(2.40+/-0.43)e-13 erg/sec/cm2 from the FXT-B telescope.

Source 3: EPF_J164951.2+365709 at RA, Dec (J2000) = 252.4635 36.9525 with Fx=(1.96+/-0.41)e-13 erg/sec/cm2 from the FXT-B telescope.

All three sources detected by EP/FXT during the first visit are detected in the second epoch data set. However, in the second epoch EPF_J165032.7+365028 has faded by a factor ~4, down to a flux of (4.0 +/- 2.3)e-14 erg/sec/cm2, while the flux of the other sources remained constant within their uncertainties.

The fading behavior of this X-ray source confirms it as the afterglow of GRB 260330B, localized at RA, Dec (J2000) = 252.6363, 36.8413 with an error radius of 10 arcsec.

All fluxes are calculated in the 0.5-10 keV range with 1-sigma uncertainties.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).