TITLE: GCN CIRCULAR NUMBER: 20902 SUBJECT: LIGO/Virgo G277583: Asiago spectroscopic classification of ATLAS17byo and ATLAS17cgg DATE: 17/03/18 10:01:28 GMT FROM: Daniele Malesani at DARK/NBI L. Tomasella, E. Cappellaro, S. Benetti (INAF OAPd), D. Malesani (DARK/NBI, Denmark), G. Greco (Urbino University/INFN Firenze), A. Rossi, E. Palazzi (INAF-IASF Bo), M. Branchesi (Urbino University/INFN Firenze), L. Amati (INAF-IASF Bo), L. A. Antonelli, S. Ascenzi (INAF-OAR), M.T. Botticella (INAF-OAC), S. Campana, S. Covino, P. D'Avanzo (INAF-OAB), V. D'Elia (INAF-ASDC), F. Getman, A. Grado, L. Limatola (INAF-OAC), M. Lisi (INAF-OAR), A. Melandri (INAF-OAB), L. Nicastro (INAF-IASF Bo), E. Pian (SNS-Pisa), S. Piranomonte, L. Pulone (INAF-OAR), G. Stratta (Urbino University/INFN Firenze), G. Tagliaferri (INAF-OAB), V. Testa (INAF-OAR), S. Yang (INAF-OAPD), E. Brocato (INAF-OAR) on behalf of GRAvitational Wave Inaf TeAm (GRAWITA) report: We report spectroscopic observations of the ATLAS transients ATLAS17byo and ATLAS17cgg (Smartt et al., LVC Circ. 20881), discovered in the skymap (cWB_plus_LIB.fits) of G277583 (LVC Circ. 20860), with event time 2017-03-13 22:40:09.593 UT (MJD 57825.945). The observations were performed with the Asiago 1.82 m Copernico Telescope equipped with AFOSC (range 340-820 nm; resolution 1.4 nm). Name RA DEC z Host Galaxy ATLAS17byo 16:33:19.87 +23:43:52.4 0.033 SDSS J163319.94+234356.4 ATLAS17cgg 08:03:55.23 +26:31:12.3 0.0217 IC 0491 1) ATLAS17byo was observed at MJD=57830.05 under bad sky conditions. However, the noisy spectrum shows that this transient is a normal type Ia SN, around maximum light. 2) ATLAS17cgg was observed starting from MJD=57829.88. We took 3 spectra showing that this transient is a young type IIn SN. The spectra display a blue continuum with over-imposed Balmer emission lines with composite profiles (FWHM ~ 1300-1700 km/s). The best match is found with SN 1998S (Fassia et al. 2000 MNRAS 318) before maximum. Multiband photometry was also collected and a preliminary analysis shows that: u = 17.66[0.014], g = 17.12[0.016] AB mag; B = 17.20[0.014], V = 17.13[0.018] Vega mag Our spectral classifications are consistent with those provided by Blagorodnova et al. (LVC Circ. 20897).