[vsnet-grb-info 16824] GRB 150721A (Fermi GBM Trigger 459150552): iPTF P48 Observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18105
SUBJECT: GRB 150721A (Fermi GBM Trigger 459150552): iPTF P48 Observations
DATE:    15/08/02 01:20:21 GMT
FROM:    S. Bradley Cenko at NASA/GSFC  <brad.cenko at nasa.gov>

S. B. Cenko, L. P. Singer (NASA/GSFC), M. M. Kasliwal (Carnegie), and V. Bhalerao (IUCAA) report on behalf of a larger collaboration:

We imaged 12 fields covering an area of 87 deg^2 in and around the 1-sigma statistical+systematic uncertainty region of the final Fermi GBM localization.  We estimate a 72% a priori 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:

iPTF15bud, at the coordinates:
  RA(J2000) = 22h 20m 08.68s (335.036172 deg)
  Dec(J2000) = +08d 59' 01.9" (+8.983848 deg)
The source is coincident with the nucleus of the resolved source SDSS J222008.67+085901.7 (photo-z = 0.20 +/- 0.02)
The following P48 photometry indicates no evidence for intra-night variability:
  +5.25 hr: R = 19.88 +/- 0.04
  +5.97 hr: R = 19.91 +/- 0.08

iPTF15buc, at the coordinates:
  RA(J2000) = 22h 20m 26.17s (335.109057 deg)
  Dec(J2000) = +05d 59' 31.1" (+5.991972 deg)
The source is coincident with the nucleus of the resolved source SDSS J222026.17+055930.4 (photo-z = 0.08 +/- 0.01)
The following P48 photometry indicates no evidence for intra-night variability:
  +5.09 hr: R = 19.64 +/- 0.04
  +5.83 hr: R = 19.57 +/- 0.04

iPTF15bub, at the coordinates:
  RA(J2000) = 22h 21m 17.29s (335.322031 deg)
  Dec(J2000) = +06d 52' 24.7" (+6.873532 deg)
The source is coincident with the nucleus of the resolved source SDSS J222117.28+065224.4 (photo-z = 0.32 +/- 0.07)
The following P48 photometry indicates no evidence for intra-night variability:
  +5.26 hr: R = 20.40 +/- 0.07
  +6.00 hr: R = 20.43 +/- 0.20

iPTF15bua, at the coordinates:
  RA(J2000) = 22h 00m 00.79s (330.003312 deg)
  Dec(J2000) = +11d 25' 14.0" (+11.420547 deg)
The source is coincident with the nucleus of the resolved source SDSS J220000.78+112513.9 (photo-z = 0.25 +/- 0.06)
The following P48 photometry indicates no evidence for intra-night variability:
  +5.54 hr: R = 20.94 +/- 0.12
  +5.56 hr: R = 21.22 +/- 0.20

iPTF15btz, at the coordinates:
  RA(J2000) = 21h 57m 06.00s (329.274989 deg)
  Dec(J2000) = +12d 06' 45.8" (+12.112722 deg)
The source is near the nucleus of a resolved source in SDSS.
The following P48 photometry indicates marginal evidence for fading:
  +5.01 hr: R = 20.26 +/- 0.07
  +5.56 hr: R = 20.50 +/- 0.12

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).

Given the likely nuclear associations and the lack of fading, we do not consider any of these sources to be likely candidates for the optical afterglow of GRB 150721A.

The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi459150552.pdf shows the locations of our candidates and the P48 fields in relation to the Fermi GBM 1- and 2-sigma statistical+systematic contours.