GCN Circular 24090

F. Onori (INAF-IAPS Roma), M. Turatto (INAF-OAPd), S. Benetti (INAF-OAPd), L. Tomasella (INAF-OAPd), L. Nicastro (INAF-OAS), E. Palazzi (INAF-OAS), A. Melandri (INAF-OAB), S. Piranomonte (INAF-OAR), S. Yang (INAF-OAPd), P. D���Avanzo (INAF-OAB), M. Cecconi (INAF-TNG), A. Ghedina (INAF-TNG), H. Stoev (INAF-TNG), A. Brosio (Osservatorio Astronomico Lilio), S. Savaglio (UniCal) on behalf of GRAWITA report:

We carried out follow-up observations of the optical transient MASTER OT J231043.63+470956.4, discovered by Lipunov et al. (GCN Circ. #24084) during observations of the skymap of the LIGO/Virgo event S190408an (LVC, GCN Circ. #24069). 

Optical imaging observations were obtained with the 0.5m telescope of the Osservatorio Astronomico Lilio, located in Savelli (Italy) starting on 2019-04-10 at 02:08:51 UT with the g-sdss, r-sdss, i-sdss filter. The OT is clearly detected with r~15.59+/- 0.02 mag (AB).

We observed the transient with the 3.6m Italian TNG telescope (Canary Islands, Spain), equipped with the DOLORES camera in spectroscopic mode starting on 2019-04-10 at 04-57-48 UT. A total of 3 optical spectra, each one lasting 180 s, were obtained using the grism LR-B (wavelength range ~ 400-800 nm, resolution 1.05 nm). The spectrum shows a weak Halpha emission and other Balmer lines in absorption at z=0, superposed on a bright continuum.

These results are consistent with MASTER OT J231043.63+470956.4 being a Galactic transient (likely a CV). The transient is therefore unrelated with the LIGO/Virgo GW event S190804an (as already proposed by  Lipunov et al. GCN Circ 24084).