EP260507b: LCO optical upper limits

ATel #17789; R. A. J. Eyles-Ferris (Leicester), G. Corcoran (UCD), P. G. Jonker (Radboud), J. A. Chácon (PUC) and F. E. Bauer (SSI and UTA) report on behalf of a larger collaboration
on 10 May 2026; 08:54 UT
Credential Certification: Rob Eyles-Ferris (raje1@leicester.ac.uk)

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We observed the field of the X-ray transient EP260507b (ATel #17783) using LCO 1m telescopes located at the Cerro Tololo Inter-American Observatory (Chile) and Siding Spring Observatory (Australia) equipped with SINISTRO instruments. A series of 6x300 s exposures were taken in each of the SDSS-r and SDSS-z filters, starting at 2026-05-09 18:32:48 UT and 2026-05-09 09:43:21 UT respectively (~47.5 and 38.7 hr after the first X-ray detection).

No source, new or otherwise, is detected within or at the border of the EP/FXT uncertainty region (ATel #17783). From the stacked images, we obtain the following 3-sigma upper limits:

r > 22.3
z > 21.2

These upper limits are in AB magnitudes, calibrated using nearby stars from the SkyMapper catalogue, and are not corrected for Galactic extinction.