TITLE: GCN CIRCULAR NUMBER: 17415 SUBJECT: GRB 150206B: iPTF Optical Observations DATE: 15/02/06 21:06:32 GMT FROM: Leo Singer at GSFC/iPTF L. P. Singer (NASA/GSFC), M. M. Kasliwal (Carnegie Observatories/Princeton), and S. B. Cenko (NASA/GSFC) report on behalf of the intermediate Palomar Transient Factory (iPTF) collaboration: Fermi detected GRB 150206B (Fermi trigger 444908790 / bn150206407) at 2015-02-06 09:46:27.48. At 2015-02-06 10:23:45, 37 minutes after the burst, we began searching for optical counterparts using the Palomar 48-inch Oschin telescope (P48). We imaged 20 fields covering an area of 144 deg^2 mostly inside the 1-sigma statistical+systematic region of the final Fermi GBM localization. We estimate a 51% prior probability that these fields contain the true location of the source. Sifting through candidate variable sources using image subtraction and standard iPTF vetting procedures, we detected the following optical transient candidates: iPTF15gz, at the coordinates: RA(J2000) = 13h 47m 18.31s (206.826292 deg) Dec(J2000) = +55d 38' 33.7" (+55.642694 deg) No source is visible at this position in archival SDSS and iPTF images. The following P48 photometry is suggestive of fading: +116.7 min: R = 20.25 +/- 0.13 +160.7 min: R = 20.54 +/- 0.19 iPTF15hb, at the coordinates: RA(J2000) = 14h 07m 09.34s (211.788915 deg) Dec(J2000) = +63d 02' 20.6" (+63.039047 deg) This position is near the galaxy SDSS J140709.19+630218.3, with photoZ ~ 0.2 suggesting an absolute magnitude of M_R ~ -21, consistent with typical optical afterglows. We report the following photometry: +98.20 min: R = 18.94 +/- 0.05 +142.2 min: R = 18.87 +/- 0.04 +186.3 min: R = 19.00 +/- 0.04 iPTF15gv, at the coordinates: RA(J2000) = 14h 15m 28.31s (213.867960 deg) Dec(J2000) = +58d 24' 47.8" (+58.413284 deg) This coincides with the galaxy SDSS J141527.96+582447.9, with photoZ ~ 0.085, suggesting M_R ~ -18. Although this is fainter than a typical optical afterglow, the source is rapidly fading: +125.3 min: 19.89 +/- 0.07 +169.3 min: 20.33 +/- 0.11 which fits a power law relative to the time of the burst, F_nu ~ t^-alpha, with alpha = 1.3 +/- 0.4. Times are relative to the GBM trigger. Magnitudes 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). There were several other optical candidates in these fields; all of the candidates listed below are coincident with galaxies of known redshift in SDSS, by which association we infer absolute magnitudes of M_R ~ -19, typical of type Ia supernovae but not long GRB optical afterglows (see Section 2.4 of Singer et al. 2015, http://arxiv.org/abs/1501.00495, and references therein). These are: iPTF15ha, at the coordinates: RA(J2000) = 14h 03m 11.73s (210.798855 deg) Dec(J2000) = +64d 22' 19.3" (+64.372029 deg) iPTF15gu, at the coordinates: RA(J2000) = 14h 29m 11.95s (217.299810 deg) Dec(J2000) = +56d 59' 21.4" (+56.989264 deg) iPTF15gy, at the coordinates: RA(J2000) = 14h 47m 18.26s (221.826083 deg) Dec(J2000) = +54d 36' 30.6" (+54.608492 deg) iPTF15gx, at the coordinates: RA(J2000) = 15h 03m 42.64s (225.927684 deg) Dec(J2000) = +54d 48' 03.1" (+54.800849 deg) Further observations of iPTF15gz, iPTF15hb, and iPTF15gv, are encouraged to determine if one of the three is associated with the GRB. The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi444908790.pdf shows the locations of our candidates and the P48 fields in relation to the Fermi GBM 1- and 2-sigma statistical+systematic contours.