AstroNote 2026-174

The Follow-up X-ray Telescope on board the Einstein Probe conducted a 7.3 ks Target of Opportunity observation (co-PIs: Yin, Jayaraman, Quirola-Vasquez) at the position of AT2026nik (Wise et al.; AstroNote 2026-152) beginning at UTC 2026-05-29T18:45:25.914.

We use the full 7.3 ks of data available for our analysis. We stacked observations from both the A and B modules using the reproject_image_grid tool in CIAO, and detected an uncatalogued X-ray source at RA = 11:22:51.7, Dec = -06:13:19.8 (J2000), with an uncertainty of ~15” (radius, 90% C.L. statistical and systematic), consistent with the optical position of this transient. This source was detected at a significance of 4.7-sigma. 

We analyzed the FXT spectrum in the 0.5-10 keV energy range. We fit this using an absorbed power-law with a line-of-sight hydrogen column density of 3.76 x 10^20 cm^-2 (calculated using the nh utility). We find a photon index of 2.45 ± 0.65, with an unabsorbed flux of 2.9 (+0.8/-0.6) x 10^-14 erg/s/cm^2. Uncertainties in these parameters are reported at the 90% C.L. Our measured X-ray flux is fainter than that reported by Quirola-Vasquez et al. in GCN 44749. This discrepancy could possibly arise from our setting the photon index as a free parameter (leading to a steeper PL index), while they fixed it at 1.5. 

Adopting the tentative redshift of z = 0.182 from Wise et al., our calculated flux would correspond to an X-ray luminosity of roughly 3 x 10^42 erg/s. As also pointed out by Quirola-Vasquez et al., this is broadly in line with the early X-ray luminosity observed for similar LFBOTs, including AT2018cow (Rivera Sandoval et al. 2018, Margutti et al. 2019) and AT2026dbl (Jayaraman et al., AstroNote 2026-38). 

We encourage additional multiwavelength follow-up.

We thank Hui Sun and the Einstein Probe staff for approving and rapidly executing our requested observations. The Einstein Probe is a space mission supported by the Strategic Priority Program on Space Sciénce of the Chinese Academy of Sciences, in collaboration with the European Space Agency, the Max-Planck-Institute for Extraterrestrial Physics (Germany), and the Centre National d'études Spatiales (France).