Einstein Probe detection of the brightening of the Galaxy LEDA 735555
ATel #17775; J. W. Hu, X. Mao (NAO, CAS), Y. F. Liang (PMO, CAS), Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe team
on 6 May 2026; 10:31 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)
We report on the detection of the brightening of Galaxy LEDA 735555 by the Einstein Probe (EP). The source was first detected by the Wide-field X-ray Telescope (WXT) on board EP in a single pointing observation (with an exposure time of 4.7 ks) at 2026-05-01T21:53:57 (UTC). The position of the source is R.A. = 222.744 deg, Dec. = -28.965 deg, with an uncertainty of 2.8 arcmin in radius (90% C.L. statistical and systematic). A backwards search by data stacking finds that the source has brightened since 22nd April.
The WXT spectrum can be fitted by an absorbed power law with a photon index of 2.07(+2.08/-1.77), with the NH fixed at the Galactic value of 1.3e+21 cm^-2. The derived unabsorbed flux in 0.5-4 keV is 1.09(+1.26, -0.60) x 10^-11 ergs/s/cm^2.
With the observation of the Follow-up X-ray Telescope (FXT) on board EP performed at 2026-05-05T12:00:58, an X-ray source was detected at R.A.= 222.7393 deg, Dec.= -28.9394 deg (with an uncertainty of 10 arcsec), consistent with the WXT source position. This position is consistent with that of the Galaxy LEDA 735555, a previously known X-ray source, with an angular separation of 3.2 arcsec. Preliminary analysis shows that the FXT spectrum can be modeled by an absorbed power law with a photon index of 1.88 (+/-0.22) and an absorption column density of NH=1.72(+0.84, -0.8) e+21 cm^-2. The derived unabsorbed 0.5-10 keV flux is 9.06 (+0.87, -0.75) x 10^-12 ergs/s/cm^2, which is more than one order of magnitude higher than that from the historical eROSITA detection (1eRASS J145057.1-285620; ~ 2e-13 ergs/s/cm^2). Given the photometric redshift of z_ph = 0.093047, this corresponds to a luminosity of ~2 x 10^44 erg/s.
Recent optical data also indicate contemporaneous brightening of the Galaxy LEDA 735555. We note that a previous optical brightening event from this source was reported in 2021 and announced as AT 2021jbt via the Transient Name Server.
Multi-wavelength follow-up observations are encouraged.
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.
