TITLE: GCN CIRCULAR NUMBER: 24459 SUBJECT: LIGO/Virgo S190425z: CAHA-GRANDMA Observation of the Swift UVOT Source DATE: 19/05/10 19:16:15 GMT FROM: Alexander Kann at IAA-CSIC D. A. Kann (HETH/IAA-CSIC), C. Thoene (HETH/IAA-CSIC), C. Stachie (Artemis), X. Zhang (THU), Z. Vidadi (Shao), M. Blazek (HETH/IAA-CSIC), K. Bensch (HETH/IAA-CSIC), L. Izzo (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), S. Antier (APC), S. Basa (LAM), M. Boer (Artemis), D. Corre (LAL), M. Coughlin (Caltech), D. Coward (OzGrav-UWA), J.G. Ducoin (LAL), B. Gendre (OzGrav-UWA), P. Hello (LAL), A. Klotz (IRAP), C. Lachaud (APC), N. Leroy (LAL), D. Turpin (NAOC), and X. Wang (THU) report on behalf of the HETH group and GRANDMA collaboration: We observed the location of the Swift UVOT transient (Breeveld et al., GCN #24296) discovered in the error region of the aLIGO/Virgo Binary Neutron Star merger event S190425z (Ligo Scientific Collaboration and VIRGO Collaboration, GCN #24168, #24228), with the 2.2m CAHA telescope and CAFOS. We obtained 3 x 240 s each in U/B and 6 x 120 s in V/Rc/Ic. The observations were performed on 2019-04-28 from 02:11:50 to 03:52:47 UT. At the position (Palmese et al., GCN #24312) of the likely counterpart (thought to be an M2 dwarf that had a powerful flare which UVOT detected, Lipunov et al., GCN #24326; Bloom et al., GCN #24337), we detect a source in Rc and Ic and derive the following upper limits and magnitudes: U > 22.9 mag; B > 23.5 mag; V > 23.5 mag; Rc = 22.51 +/- 0.15 mag (depth Rc > 23.8 mag); Ic = 21.53 +/- 0.15 mag (depth Ic > 22.9 mag). Magnitudes are given in the AB system and were obtained vs. a comparison star from PanSTARRS and the transformations of Lupton (2005). Our detections are in reasonable agreement with other reports (Palmese et al., GCN #24312; Bloom et al., GCN #24337; Hu et al., GCN #24324) and indicate the source was quiescent during our observations. GRANDMA (Global Rapid Advanced Network Devoted to the Multi-messenger Addicts) is a network of robotic telescopes connected all over the world with both photometry and spectrometry capabilities for Time-domain Astronomy (https://grandma.lal.in2p3.fr/). This message may be cited. [GCN OPS NOTE(12jun19), The second reference in the first paragraph was changed from "Breeveld et al., #24296" to "Ligo Scientific Collaboration and VIRGO Collaboration, GCN #24168, #24228".]