TITLE: GCN CIRCULAR NUMBER: 20419 SUBJECT: LIGO/Virgo G268556: Spectroscopic observations of ATLAS17aeu and iPTF17cw DATE: 17/01/10 16:58:02 GMT FROM: Mansi M. Kasliwal at Caltech T. Kupfer (Caltech), R. Quimby (SDSU), S. M. Adams (Caltech), H. Vedantham (Caltech), M. M. Kasliwal (Caltech) report on behalf of the GROWTH (Global Relay of Observatories Watching Transients Happen) and iPTF (intermediate Palomar Transient Factory) collaborations We report the spectroscopic follow-up observations on ATLAS17aeu (Tonry et al. LVC GCN #20382) and iPTF17cw (Kasliwal et al. LVC GCN #20398) using the Double Beam spectrograph (DBSP) on the Palomar 200-inch telescope. The position of both targets are within the probability contour of LIGO/Virgo G268556 (LVC, GCN #20364). All observations were obtained on 2017 Jan 7 UT under poor weather conditions with a partly cloudy sky. Classifications were performed using Superfit (Howell et al. 2005, ApJ, 634, 1190). ATLAS17aeu was observed on 2017-01-07.5 UT with a total exposure time of 60 min resulting in a signal to noise ratio ~1.5 in the continuum. No obvious emission or absorption lines are detected in the spectrum. To search for features from a Galactic source at z=0, we set a 3-sigma flux limit around 6563 Angstrom of 3.3e-17 ergs/s/cm^2/A. This result is consistent with Chambers et al. (LVC GCN #20407) who reported a featureless spectrum of ATLAS17aeu obtained with Gemini. iPTF17cw was observed on 2017-01-07.55 with a total exposure time of 20 min which results in a signal to noise ratio ~11 in the continuum. The spectrum shows a blue continuum peaking around 3850 Angstroem (rest frame) with weak, broad features. We found a good match of the spectrum with the SNIc-BL SN1998bw pre-maximum at z=0.093. The redshift is consistent with the photoz=0.101±0.0279 derived by SDSS for the host Galaxy.