P. D'Avanzo, A. Melandri (INAF-OAB), S. Piranomonte (INAF-OAR), E. Palazzi (INAF-IAFS Bo), E. Cappellaro (INAF-OAPd), E. Brocato (INAF-OAR), M. Branchesi (Urbino University/INFN Firenze), L.Amati (INAF-IASF Bo), L. A. Antonelli, (INAF-OAR), P. Astone (INFN-Roma), S. Campana, S. Covino (INAF-OAB), V. D'Elia (INAF-ASDC), F. Getman (INAF-OAC), G. Giuffrida (INAF-ASDC), A.Grado (INAF-OAC), G.Greco (Urbino University/INFN Firenze), L. Limatola (INAF-OAC), M. Lisi (INAF-OAR), S. Marinoni, P. Marrese (INAF-ASDC), L. Nicastro, E. Pian (SNS-Pisa), L. Pulone (INAF-OAR), F. Ricci (Sapienza University), G. Stratta (Urbino University/INFN Firenze), G. Tagliaferri (INAF-OAB), V. Testa (INAF-OAR), L. Tomasella (INAF-OAPd), V. Lorenzi, A. Garcia de Gurtubai Escudero (INAF-TNG) on behalf of the INAF Gravitational Astronomy group report:
We observed the field of the optical transient MASTER OT J020906.21+013800.1 (Lipunov et al. GCN Circ. 18729) with the 3.6m Italian TNG telescope (Canary Islands, Spain), equipped with the DOLORES camera in imaging and spectroscopic mode starting on 2015 Dec 28.8247 UT.
We note that the position of the optical transient reported Lipunov et al. (GCN Circ. 18729) is superposed to the galaxy SDSS J020906.10+013759.9. This galaxy is clearly visible in our 30s acquisition image obtained with the r-sdss filter, while there is no evidence for the optical transient. From a preliminary analysis, an upper limit r=21.0 mag (AB) has been obtained by placing fake stars of different magnitudes at the reported transient position. Calibration was obtained with reference to SDSS stars in the field.
Two optical spectra, each one lasting 900s, were obtained at a mid time of 2015 Dec 28.8367 UT using the grism LR-B (wavelength range 4000 8000 AA). The slit was placed at the position of the optical transient. In the co-added spectrum we identify the emission lines O[III] and Halpha at a common redshift of z~0.034, superposed to the galaxy continuum. This redshift value is in agreement with the photometric redshift z=0.034 0.074 reported by the SDSS for this galaxy. At such redshift, the observed magnitude of the optical transient reported by Lipunov et al. (GCN Circ. 18729) corresponds to an absolute magnitude of -17.6.
Our results suggest that, at the epoch of our observations, the optical transient reported by Lipunov et al. (GCN Circ. 18729) already faded below its host galaxy level.