Einstein Probe observations of the transient gamma-ray source Fermi J0346.0+5405
ATel #17687; J. W. Hu (NAO, CAS), Y.-H. I. Yin (HKU), Y.-H. Chi (NJU), C. C. Jin (NAO, CAS), H. Feng (IHEP, CAS), P. Zhou (NJU), J. Yang (ZZU), R.-Z. Li (YNAO, CAS), X. Tian (GXU), W. D. Zhang, W. Yuan (NAO, CAS)
on 18 Feb 2026; 11:59 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jingwei Hu (hujingwei@nao.cas.cn)
Subjects: X-ray, Transient
We report on follow-up observations of the new GeV transient Fermi J0346.0+5405 (ATel #17679) performed with the Einstein Probe (EP).
The Wide-field X-ray Telescope (WXT) onboard EP covered the field of Fermi J0346.0+5405 three times on 2026 February 9 and once on February 13. There was no WXT coverage during the reported GeV flare on February 10. No X-ray source was detected within the 95% LAT error circle in any of the four observations. The derived 90% confidence upper limit on the 0.5-4 keV flux for the four observations are 1.6 x 10^-11, 1.1 x 10^-11, 2.6 x 10^-11, and 0.8 x 10^-11 ergs/s/cm^2, respectively.
A target-of-opportunity follow-up observation was performed with the Follow-up X-ray Telescope (FXT) onboard EP starting at 2026-02-14T08:12:35 (UTC), with an exposure of 2.2 ks. Within the LAT 95% confidence region, only one X-ray source was detected: EP J034634.2+540101, located at R.A. = 56.6421 deg, DEC = 54.0160 deg (J2000), with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The FXT spectrum can be fitted by an absorbed power law with a photon index of 0.86(+0.45/-0.43), with the NH fixed at the Galactic value of 9e+21 cm^-2. The derived unabsorbed flux in 0.5-10 keV is 2.8 (+1.3, -0.8) e-12 ergs/s/cm^2. All quoted uncertainties are at the 90% confidence level. The FXT position is consistent with that of the VLBI source ICRF J034634.5+540059. At this position, a historical X-ray source, 2RXS J034636.3+540107 (count rate 1.2 x 10^-2 cts/s), has been reported. Assuming the FXT best-fit spectral model, its estimated unabsorbed 0.5-10 keV flux is 3.2 x 10^-12 ergs/s/cm^2, consistent with the flux measured with FXT.
No additional new X-ray sources were detected within the LAT error circle. We derived a 90% confidence upper limit on the 0.5-10 keV flux for any undetected source in the FXT observation of 6 x 10^-14 ergs/s/cm^2, assuming the same spectral model.
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). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.
