TITLE:   GCN CIRCULAR
NUMBER:  37087
SUBJECT: EP240806a: Gemini North-GMOS spectroscopic redshift of z  =  2.818
DATE:    24/08/07 09:21:34 GMT
FROM:    Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>

J. Quirola-Vásquez (Radboud Univ.), P. G. Jonker (Radboud Univ), A. J. Levan (Radboud Univ. & Warwick Univ.), D. B. Malesani (DAWN/NBI and Radboud Univ.), F. E. Bauer (PUC), J. van Dalen (Radboud Univ.), M. E. Ravasio (Radboud Univ.), D. Mata Sanchez (IAC), M. A. P. Torres (IAC), report on behalf of a larger collaboration:

We observed the optical counterpart of EP240806a (Wu et al., GCN 37063) using the Gemini North-GMOS (imaging and spectroscopy mode) located on Mauna Kea.
Photometric observations started at 2024-08-06 UT 10:46:16.4 (i.e., ~6 hours after the EP detection), consisted of 4 exposures of 300 s each, and were carried out using filter r'. An uncatalogued source is detected inside the FXT position at the coordinates:

RA (J2000) = 00:45:56.67
Dec (J2000)= 05:05:39.37

The counterpart magnitude was r = 22.17 +/- 0.02 (AB) calibrated against the Pan-STARRS sources in the field. The transient is detected at a position consistent with the counterpart reported by Li et al. (GCN 37064), Izzo & Malesani (GCN 37065) and Pérez-Fournon et al. (GCN 37080).

Moreover, the target was observed by GMOS in spectroscopy mode. Observations started at 2024-08-06 UT 12:37:00.9 (i.e., ~7.8 hours after the EP detection), with 4 exposures of 900 s each and using the grating R150 covering a wavelength between ~400 and 1,000 nm. In a preliminary reduction of the spectra, we clearly detect a continuum over the entire wavelength range. From the detection of the Damped Lyman-alpha at ~4670 AA and multiple absorption features (e.g., S II, Si II, C II, Al II), we infer a redshift of z = 2.818, which means an average X-ray luminosity of ~1.4e50 erg/s.

We acknowledge excellent support from the Gemini North staff, especially Adam Smith.