TITLE: GCN CIRCULAR NUMBER: 41596 SUBJECT: EP250827a: very early ATLAS detections, starting 43 sec after the EP trigger DATE: 25/08/29 19:11:08 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias ipf@iac.es
I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), A. López-Oramas, and D.S. Aguado (IAC and ULL)
We report on very early ATLAS detections of the optical afterglow of EP250827a, a fast X-ray transient discovered by Einstein Probe WXT (ID: 01709200787) on 2025-08-27 07:40:43 UTC (Hua et al., GCN 41553; and Ni et al., GCN 41573), with UV-optical detections reported in Levan et al. (GCN 41554), Li et al. (GCN 41555), Levan et al. (GCN 41557), Lipunov et al. (GCN 41558), Gritsevich et al. (GCN 41559), An et al. (GCN 41560), Ghosh et al. (GCN 41564), O’Neill et al. (GCN 41567), Pérez-Fournon et al. (GCN 41568), Passaleva et al. (GCN 41570), Eyles-Ferris et al. (GCN 41576), Santos et al. (GCN 41580), and Siegel et al. (GCN 41585), spectroscopic redshift of z = 1.613-1.61 (Levan et al., GCN 41557; and Passaleva et al., GCN 41570) and Swift-XRT follow-up observations (Evans et al., GCN 41563).
Using the ATLAS forced-photometry server (Shingles et al. 2021, Transient Name Server AstroNote 2021-7), we found that there were four ATLAS observations that covered the EP250827a position on 2025-08-27, two of them before the EP trigger and two after it. As expected, the source was not detected in the ATLAS pre-trigger observations but it is well detected in the two post-trigger observations, the first of them at a start time of only about 43 sec (about 16 sec in the source rest frame) after the EP trigger. There is a flux rise between the two observations. The source is well detected in both the original (reduced) and difference (reduced - reference) ATLAS images. For the forced photometry measurements, we used the coordinates of the optical counterpart given by Levan et al. (GCN 41554) from the analysis of Gemini-South GMOS observations.
In the following, we give the ATLAS limiting magnitudes and forced-photometry magnitudes (using difference images), not corrected for Galactic extinction.
MJD | 5-sigma limit | filter | ATLAS observation | 60914.306552 | 20.14 | orange | 04a60914o0247o | 60914.313502 | 20.18 | orange | 04a60914o0252o |
MJD | mag | error | filter | ATLAS observation | 60914.320437 | 18.707 | 0.070 | orange | 04a60914o0257o | 60914.327372 | 18.568 | 0.065 | orange | 04a60914o0262o |
The reported ATLAS observations MJD times are the start times of the individual 30-sec exposures.
The first detection is likely the earliest one after the trigger of an optical afterglow of EP-discovered fast X-ray transients.
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