TITLE: GCN CIRCULAR NUMBER: 18497 SUBJECT: LIGO/Virgo G194575: iPTF Optical Transient Candidates DATE: 15/10/27 20:45:32 GMT FROM: Leo Singer at NASA/GSFC L. P. Singer (NASA/GSFC), M. M. Kasliwal (Caltech), R. Ferretti (Stockholm), S. B. Cenko (NASA/GSFC), T. Barlow (Caltech), Y. Cao Caltech), R. Laher (IPAC), and J. Rana (IUCAA) report on behalf of the intermediate Palomar Transient Factory (iPTF) collaboration: We have performed tiled observations of LIGO/Virgo G194575 using the Palomar 48-inch Oschin telescope (P48). The anti-sun probability island from the BAYESTAR localization was observable from Palomar for most of the night and weather conditions were good. Starting at 2015-10-23 03:26 UTC, about 14 hours after the GW trigger, we imaged 160 fields spanning 1114 deg2. We estimate a 17% prior probability that these fields contain the true location of the source. Sifting through candidate variable sources using image subtraction by both our NERSC and IPAC pipelines, and applying standard iPTF vetting procedures, we flagged the following optical transient candidates for further follow-up: name RA Dec time mag --------- --------- --------- ----- ----- iPTF15dkm 354.32375 -3.53254 03:39 18.66 iPTF15dlc 16.14909 -2.54011 08:15 16.17 iPTF15dld 14.55532 -3.66397 08:15 18.50 iPTF15dlj 19.76198 +10.00134 09:22 18.14 iPTF15dln 14.58195 +7.23471 09:15 18.81 iPTF15dmk 21.22653 +0.61874 10:58 19.60 iPTF15dmn 7.23638 -11.40550 09:07 18.37 iPTF15dmq 353.87809 +6.45032 04:36 18.64 iPTF15dni 21.26675 -4.70845 10:23 17.72 iPTF15dkv 7.77118 -2.65228 07:08 18.84 iPTF15dkn 357.54623 +0.09660 05:34 19.17 iPTF15dkk 357.57171 -3.16660 05:38 19.43 iPTF15dnh 14.90946 -14.19912 10:01 19.45 iPTF15dlb 16.18149 +2.06164 08:25 19.21 = (PS15byt) iPTF15dmv 21.02204 -10.50270 10:20 19.85 iPTF15dkt 7.20239 -2.70993 07:08 18.05 = (PS15coc) iPTF15dmt 97.71723 +39.31086 13:01 19.17 iPTF15dmu 22.59586 -4.41018 10:25 19.35 iPTF15dms 98.41981 +37.30400 13:00 19.97 iPTF15dmp 16.28538 -9.74387 09:59 20.05 iPTF15dmf 15.40890 -3.94111 08:15 18.48 iPTF15dmb 18.31767 -6.51451 10:09 19.45 iPTF15dlz 22.25290 -4.36629 10:25 19.22 iPTF15dlv 20.54769 -2.00612 10:26 19.18 iPTF15dlu 22.85842 -8.89533 10:21 19.40 iPTF15dlr 18.56416 +3.32916 10:03 19.14 (= PS1-14aen) iPTF15dlp 15.49983 +9.50920 09:14 19.11 iPTF15dlg 3.68611 -11.70795 09:05 19.10 iPTF15dla 14.73492 +1.74302 08:25 18.28 iPTF15dkj 359.67964 -7.68101 05:41 19.47 iPTF15dku 2.86523 -6.42730 07:11 17.74 (= LSQ15bjb, PS15cog) iPTF15dmr 6.96306 -11.47793 09:05 18.82 (= PS15bww) iPTF15dmj 21.09391 +3.58754 11:02 16.10 (= PS15cku) iPTF15dls 19.93774 +4.08302 10:03 19.34 iPTF15dlo 18.11055 +5.07488 09:26 19.17 iPTF15dlm 15.73540 +5.54822 09:17 18.90 iPTF15dll 16.22272 +4.76531 09:17 17.63 (= PS15byu) iPTF15dli 18.13482 +10.49597 09:22 19.68 (= PS15buq) iPTF15dlk 17.44493 +13.30794 09:21 16.34 iPTF15dko 3.41903 +8.90473 06:51 19.34 iPTF15dma 22.39782 -15.50700 10:16 19.93 iPTF15dlx 22.44977 -15.14902 10:16 19.39 Where relevant, we have given equivalent designations by Pan-STARRS (Smartt et al., GCN 18451, GCN 18484) and La Silla-QUEST (Rabinowitz et al., GCN 18473). The following sources are coincident with galaxies with redshifts (spectroscopic or photometric) < 0.1: iPTF15dkm, iPTF15dlc, iPTF15dld, iPTF15dlj, iPTF15dln, iPTF15dmn, iPTF15dni, iPTF15dkv, iPTF15dkn, iPTF15dkk, iPTF15dnh. iPTF15dlc may be a variable star superimposed on a nearby galaxy. iPTF15dni appeared to fade by 0.5 mag in one day, but its position in the nucleus of its host galaxy makes the host-subtracted photometry questionable. We will obtain further follow-up to confirm the fading. iPTF15dmk and iPTF15dmq lacked any plausible host galaxy or quiescent counterpart in SDSS or archival iPTF images, but showed no significant intra-night photometric evolution. iPTF15dlb (our designation of PS15byt) is a known nova in the galaxy IC 1613 (Williams & Darnley, ATel #8061). The following candidates coincided with galaxies with redshifts > 0.1: iPTF15dmv, iPTF15dkt. The following sources had no known redshift but showed unremarkable photometric evolution: iPTF15dmt, iPTF15dmu, iPTF15dms, iPTF15dmp, iPTF15dmf, iPTF15dmb, iPTF15dlz, iPTF15dlv, iPTF15dlu, iPTF15dlr, iPTF15dlp, iPTF15dlg, iPTF15dla, iPTF15dkj. The following sources were previously known SNe or AGNs: iPTF15dku, iPTF15dmr, iPTF15dmj, iPTF15dls, iPTF15dlo, iPTF15dlm, iPTF15dll, iPTF15dli, iPTF15dlk, iPTF15dko. iPTF15dku is our designation for LSQ15bjb. We report the following additional photometry: -377.25 days: R < 20.21 +0.71 days: R = 18.62 +/- 0.13 +1.71 days: R = 18.15 +/- 0.15 In summary, we encourage further follow-up of iPTF15dkm, iPTF15dld, iPTF15dlj, iPTF15dln, iPTF15dmn, iPTF15dni, iPTF15dkv, iPTF15dkn, iPTF15dkk, iPTF15dnh, iPTF15dmk, iPTF15dmq. Positions are stated in the ICRS. Times are in UTC. Magnitudes are based on image subtraction; they are in the Mould R filter and in the AB system, calibrated with respect to point sources in SDSS as described in Ofek et al. (2012, http://dx.doi.org/10.1086/664065). The diagram https://gracedb.ligo.org/apiweb/events/G194575/files/iptf.pdf shows the locations of our candidates and the P48 fields in relation to the LIGO/Virgo localization.