TITLE: GCN CIRCULAR NUMBER: 20795 SUBJECT: LIGO/Virgo G275697: optical spectroscopy of Swift/XRT source 1 DATE: 17/03/02 18:39:28 GMT FROM: Daniele Malesani at DARK/NBI Johan P.U. Fynbo, Daniele Malesani, Lise Christensen (DARK/NBI), Giorgos Leloudas (WIS), and Elena Pian (SNS; INAF/IASF Bo) report on behalf of a larger collaboration: We observed the location of the X-ray "source 1" within the LVC error region for the GW trigger G275697 (LVC Circ. 20763) reported by Swift (Evans et al., LVC Circ. 20773) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera and spectrograph. Observations were carried out in the r band, and started on 2017 Mar 1.25 UT (in twilight, at high airmass). We clearly detect the 2MPZ galaxy mentioned by Evans et al. (LVC GCN 20773) and coincident with the X-ray source. We measure a magnitude r = 16.67 +- 0.01 AB (5" aperture radius, calibrated against nearby stars from the Pan-STARRS catalog). This value is consistent with the (Kron) magnitude r = 16.68 listed in the Pan-STARRS catalog, indicating no variability compared with the archival value. A 900-s spectrum was obtained covering the wavelength range 3300-9500 AA, under non-photometric conditions. Several emission lines are detected, corresponding to Halpha, Hbeta, the [O III] doublet, [N II] and [S II], all at a common redshift z = 0.0385, corresponding to a (luminosity) distance of 170 Mpc. The width of the Halpha and Hbeta lines is around 1700 km s^-1, indicating the presence of a broad-line region typical of an AGN. Also, the location of this object in the extinction-insensitive BPT diagram (e.g. Kewley et al. 2013, ApJ, 774, L10) is consistent with LINERs and AGN. We caution that the [N II] lines, used for the BPT diagram, falls at this redshift close to the telluric B band, but correcting for the flux loss should, if anything, move our target even more towards the AGN locus. Finally, we note that the Halpha/Hbeta ratio indicates significant extinction, even after correcting for the non-negligible Galactic reddening (A_V ~ 1 mag). Our data thus indicate that the XRT source 1 is an AGN, which would naturally explain the detected X-ray emission (at a level of ~10^43 erg s^-1). We acknowledge excellent support from the NOT staff, in particular Pere Blay and Amanda Djupvik.