GCN Circular 39573
Subject
EP250225a: the candidate optical counterpart is an AGN
Event
Date
2025-03-03T15:25:46Z (4 months ago)
From
Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud Univ.), D. Mata-Sanchez (IAC and ULL), A. Martin-Carrillo (UCD), A. J. Levan (Radboud Univ.), P. G. Jonker (Radboud Univ.), report on behalf of a larger collaboration:
We observed again the variable object (Malesani et al., GCN 39516) located inside the X-ray localization of the fast X-ray transient EP250225a (Jiang et al., GCN 39475; Jiang et al., GCN 39529) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. The observation mid time was 2025 Feb 27.93 UT (2.42 days after the trigger) and a total of 5x300 s was secured in the r band.
The object is detected with a flux consistent with our previous observation (Malesani et al., GCN 39516) to within ~0.1 mag, and still brighter than the archival value from the Legacy Survey. Image subtraction confirms no significant variability between the two NOT epochs.
A spectrum of this source was secured with the Gran Telescopio Canarias (GTC). Observations started on 2025 Feb 28.98 UT (3.47 days after the trigger), and consisted of a single 1800 s exposure with grism R1000R, yielding usable data in the wavelength range 5000-9000 AA.
Detection of two broad features (FWHM ~ 3500 km s^-1), identified as Hbeta and Hgamma, as well as of narrow [O III] 5008, allows us to measure a redshift z = 0.587. The large velocities and the point-like PSF indicate that this is an active galactic nucleus.
Given the slow variability and the AGN nature of this source, its relationship to EP250225b is unclear, and the two may well be unrelated.
We acknowledge significant support from the observing staff at the NOT, in particular, Alejandra Diaz Teodori (NOT and Turku Univ.), Roar Rasmussen (NOT and Aarhus Univ.), and Saskia Schlagenhauf (QUB), and at the GTC, in particular Antonio Cabrera-Lavers (GTC).