Einstein Probe detects X-ray transient EP260221a with no optical counterpart detected.

AI Summary

An X-ray transient was detected by Einstein Probe's WXT, followed by an uncatalogued X-ray source confirmation by FXT. Multiple optical follow-ups failed to detect a counterpart.

Notices (1)

Einstein Probe WXT — GRB · Sat, 21 Feb 2026 11:43:14 GMT

GCN Circulars (10)

GCN-43813 — EP260221a · Mon, 23 Feb 2026 09:42:06 GMT
A team of astronomers led by Amar Aryan from National Central University reported non-detection of the optical counterpart of the fast X-ray transient EP260221a using the 40cm SLT at the Lulin observatory as part of the Kinder collaboration. The team utilized various packages and algorithms to align, stack, and perform template subtraction and PSF photometry. The non-detection is consistent with previous reports and is likely due to shallower detection limits in their observations. The team did not detect the optical counterpart proposed by other teams and did not provide a magnitude measurement, but reported a 3-sigma upper limit of >20.7 in the AB system.

GCN-43807 — EP260221a · Sun, 22 Feb 2026 19:53:08 GMT
The University of Leicester team using the 2m Liverpool Telescope on La Palma detected a potential counterpart of the recently discovered X-ray source EP260221a, located at i’ = 22.21 ± 0.12 and z’ = 22.29 ± 0.18 magnitudes. The measured magnitudes are significantly brighter than previous catalogued values, suggesting this could indeed be the counterpart. The team derived 3-sigma upper limits of i’ > 23.4 and z’ > 23.0 for their stacked images.

GCN-43806 — EP260221a · Sun, 22 Feb 2026 17:22:21 GMT
A team of astronomers led by Alan Watson from UNAM and including researchers from UNAM, LAM, CPPM, IRAP, UAR Pytheas, ASU, IJCLAB, and NYUAD, report the detection of a potential optical counterpart for the transient event EP260221a using the DDRAGO imager on the COLIBRÍ telescope. The source, located at RA (J2000) = 11:55:01.22, Dec (J2000) = -20:58:57.6, is consistent with the position of a galaxy in the Legacy Survey catalog and exhibits significant fading in both r and z bands between two epochs. The team suggests that this galaxy might be the host of the transient event.

GCN-43805 — EP260221a · Sun, 22 Feb 2026 11:42:57 GMT
A team of astronomers led by Antonio Martin-Carrillo from University of Dublin, Ireland, reported non-detection of a new source associated with the fast X-ray transient EP260221a in a z-band observation using the GMOS-S instrument on the Gemini South telescope. The limiting magnitude reached was z = 24.8, and no new sources were detected within the error circle. The results are consistent with other non-detection reports and support the hypothesis that EP260221a is a transient X-ray source that may have faded or moved away from its initial position.

GCN-43803 — EP260221a · Sun, 22 Feb 2026 08:54:25 GMT
Two robotic telescopes, MASTER-Kislovodsk in Russia and MASTER-OAFA in Argentina, have observed the transient event EP260221a, with upper limits of 13.5 mag for MASTER-Kislovodsk and 20.7 mag for MASTER-OAFA. The observations began shortly after the alert was issued, with MASTER-Kislovodsk observing at a zenith distance of 76 degrees and MASTER-OAFA observing at 27 degrees.

GCN-43802 — EP260221a · Sun, 22 Feb 2026 05:03:51 GMT
The Einstein Probe mission detected a fast X-ray transient, EP260221a, with the Wide-field X-ray Telescope (WXT) on February 21, 2026. The event started at 11:39:48 UTC and lasted for 550 seconds. The Follow-up X-ray Telescope (FXT) on board the EP mission observed the source an hour later and detected an uncatalogued X-ray source consistent with the WXT position. The derived average unabsorbed X-ray fluxes for the WXT and FXT observations were 3.1x10^-10 erg/s/cm^2 and 6.9x10^-13 erg/s/cm^2, respectively.

GCN-43798 — EP260221a · Sat, 21 Feb 2026 20:39:37 GMT
The Xinglong Observatory at NAOC reported no detection of an optical transient in the field of EP260221a, a gamma-ray burst trigger, using their 0.8-m and 2.16-m telescopes. The team obtained 3x600s g-band and clear-band images, but no uncatalogued source was detected within the error circle, down to limiting magnitudes of g-band ~15.48 mag and clear-band 20.71 mag.

GCN-43794 — EP260221a · Sat, 21 Feb 2026 14:41:25 GMT
A team of astronomers using the Gravitational-wave Optical Transient Observer (GOTO) reported no detection of a new optical transient associated with the gravitational wave event EP260221a down to a 3-sigma depth of 20.9 mag (AB). The observations were taken using GOTO-South at Roque de los Muchachos Observatory and Siding Spring Observatory.

GCN-43793 — EP260221a · Sat, 21 Feb 2026 14:25:23 GMT
A team of astronomers led by Nikos Mandarakas reported no detection of a new source at the position of EP260221a using the DDRAGO imager on the COLIBRÍ telescope. The team obtained 32 minutes of simultaneous exposure in the r and z filters and set 5-sigma upper limits for the magnitudes: r > 23.1 and z > 21.8.

GCN-43791 — EP260221a · Sat, 21 Feb 2026 13:34:01 GMT
The Einstein Probe mission, a space X-ray observatory, has detected an X-ray transient designated EP260221a using its Wide-field X-ray Telescope (WXT). The transient, located at R.A. = 178.749 deg, DEC = -20.978 deg (J2000), triggered the WXT on February 21, 2026. A follow-up observation with the Follow-up X-ray Telescope (FXT) identified an uncatalogued X-ray source within the WXT error circle, with a position uncertainty of 20 arcsec. The discovery marks a significant finding for X-ray astronomy with the new capabilities of the Einstein Probe mission.