TITLE: GCN CIRCULAR NUMBER: 20801 SUBJECT: LIGO/Virgo G275404: Additional iPTF Optical Transient Candidates DATE: 17/03/03 04:21:33 GMT FROM: Mansi M. Kasliwal at Caltech R. Lunnan (Caltech), S. Papadogiannakis (OKC), N. Blagorodnova (Caltech), A. A. Miller (Northwestern/Adler), L. P. Singer (NASA/GSFC), S. M. Adams (Caltech), C. Cannella (Caltech), S. B. Cenko (NASA/GSFC), R. Walters (Caltech), T. Barlow (Caltech), J. Rana (IUCAA), V. Bhalerao (IIT-B), Y. Cao (UW), R. Laher (IPAC), F. Masci (IPAC) and M.M. Kasliwal (Caltech) report on behalf of the iPTF (intermediate Palomar Transient Factory) and GROWTH (Global Relay of Observatories Watching Transients Happen) collaborations: We have continued tiled observations of LIGO/Virgo G275404 (LVC, GCN 20738) and LIGO/Virgo G275697 (LVC, GCN 20763) using the Palomar 48-inch Oschin telescope (P48). Including our first night of observations on 2017-03-01 UTC (GCN 20791) and a second night on 2017-03-02, we have observed a total of 256 fields spanning 1747 square degrees. We estimate a 20% chance that these fields contain the true location of G275404 and a 25% chance that they containing the true location of G275697. During preliminary sifting through candidate variable sources using image subtraction by our IPAC (Masci et al. 2016) and NERSC (Cao et al. 2016) pipelines, a total of 115 candidates were saved in the fields imaged. Applying standard iPTF vetting procedures and removing transients with a history of previous variability, we flagged 9 additional optical transient candidates in the 90% localization contour of G275404, listed below, for further follow-up. iPTF17buf 98.583906 76.596292 05:24 19.59 pstar=0.962; nuclear/stellar? iPTF17bug 97.67324 77.159053 05:24 18.6 0.0166 specz; nuclear iPTF17bur 295.1181 58.707877 11:38 19.14 pstar=0.016; nuclear/stellar? iPTF17buu 114.100254 74.505262 05:33 19.92 Hostless in iPTF reference but faint counterpart seen in PanSTARRS. iPTF17buw 127.499818 73.935927 06:08 20.23 pstar=0.948; nuclear/stellar? iPTF17bvo 133.140294 86.061384 06:11 19.55 nuclear; pstar=0.042 iPTF17bvt 115.702547 74.553167 05:33 19.57 nuclear/stellar?; looks like three-star blend in PS1 iPTF17bwl 150.611516 8.822758 08:23 18.77 0.114 specz; also detected 2017-03-01; nuclear; SDSS QSO iPTF17bxk 149.643558 6.963137 08:23 19.99 0.077 specz; offset 13.5" from the center of 2MASX J09583513+0657389 Two known transients detected by Pan-STARRS on 2017-02-16 also fall in this localization region and were recovered with iPTF : PS17bjk (iPTF17bez) and PS17bjd (iPTF17bfa). Positions are stated in the ICRS. Discovery times are noted in UTC hh:mm on 2017-03-02. Magnitudes are based on image subtraction and in the Mould R filter, calibrated with respect to point sources in SDSS as described in Ofek et al. 2012. We caution that many candidates are outside the SDSS footprint and lack a secure star/galaxy classification for the underlying source. We flag these as "nuclear/stellar?". Where available, we provide machine-learning probability scores on whether the underlying source is a galaxy/star (0/1) (Miller et al. 2016). We encourage spectroscopic classification of these candidates, and highlight iPTF17bug as a local candidate (d < 200 Mpc). We are grateful to the Palomar crew (John Henning, Jeff Zolkower, Carolyn Heffner, Jamey Eriksen, Nick Ganciu) for their hard work in reviving a faulty declination encoder essential to collecting this dataset during the last two days of iPTF survey operations.