[vsnet-grb-info 16837] Fermi GBM GRB Trigger 460608875: iPTF P48 Observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18118
SUBJECT: Fermi GBM GRB Trigger 460608875: iPTF P48 Observations
DATE:    15/08/10 14:29:14 GMT
FROM:    S. Bradley Cenko at NASA/GSFC  <brad.cenko at nasa.gov>

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

We conducted follow-up observations of Fermi trigger 460408875 (t0 =
2015-08-04 19:21:11.47) with the Palomar 48-inch Oschin telescope (P48).
Observations began at 4:33 UT on 2015-08-05, ~ 9.2 hours after the GBM
trigger time.

We imaged 12 fields covering an area of 89 deg^2 in and around the 1-sigma
statistical+systematic uncertainty region of the final Fermi GBM
localization.  We estimate an 59% 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:

iPTF15bwd, at the coordinates:
 RA(J2000) = 22h 32m 14.92s (338.062183 deg)
 Dec(J2000) = +22d 48' 00.1" (+22.800041 deg)
The source is located near the resolved galaxy SDSS J223214.84+224756.8,
with a photometric redshift of 0.12 +/- 0.04.
The following P48 photometry indicates no evidence for significant
intra-night variability:
 +9.16 hr: R = 19.75 +/- 0.08
 +10.50 hr: R = 19.79 +/- 0.07

iPTF15bwe, at the coordinates:
 RA(J2000) = 21h 55m 15.97s (328.816535 deg)
 Dec(J2000) = +31d 20' 14.9" (+31.337481 deg)
The source is located near the resolved galaxy 2MASX J21551552+3120178.
The following P48 photometry indicates no evidence for significant
intra-night variability:
 +9.35 hr: R = 19.59 +/- 0.06
 +10.69 hr: R = 19.61 +/- 0.06
A low resolution spectrum obtained with the Double Spectrograph (DBSP) on the 
Palomar 200-inch Hale Telescope at 9:59 UT on 2015-08-07 suggest that the source is 
a Type Ia SN at a redshift of z = 0.07 (from SN features).

iPTF15bwf, at the coordinates:
 RA(J2000) = 22h 39m 12.66s (339.802770 deg)
 Dec(J2000) = +28d 18' 05.3" (+28.301464 deg)
The source is located near the nucleus of the resolved galaxy
SDSS J223912.63+281805.2, with a photometric redshift of z = 0.18 +/- 0.03.
The following P48 photometry indicates no evidence for significant
intra-night variability:
 +9.33 hr: R = 20.35 +/- 0.18
 +10.66 hr: R = 20.25 +/- 0.10

PTF15bws, at the coordinates:
 RA(J2000) = 22h 07m 02.95s (331.762304 deg)
 Dec(J2000) = +31d 07' 22.5" (+31.122922 deg)
The source is located near the nucleus of the resolved galaxy
SDSS J220702.90+310722.4, with a photometric redshift of z = 0.10 +/- 0.03.
The following P48 photometry indicates modest evidence for significant
intra-night variability:
 +9.35 hr: R = 20.04 +/- 0.07
 +10.69 hr: R = 20.34 +/- 0.09

PTF15bwt, at the coordinates:
 RA(J2000) = 22h 40m 26.82s (340.111732 deg)
 Dec(J2000) = +28d 54' 28.1" (+28.907801 deg)
The source is located near the resolved galaxy SDSS J224026.88+285427.7,
with a photometric redshift = 0.20 +/- 0.04.
The following P48 photometry indicates modest evidence for significant
intra-night variability:
 +9.33 hr: R = 20.43 +/- 0.12
 +10.66 hr: R = 20.32 +/- 0.09

PTF15bwu, at the coordinates:
 RA(J2000) = 22h 35m 56.44s (338.985184 deg)
 Dec(J2000) = +23d 58' 42.7" (+23.978529 deg)
The source is located near the resolved galaxy SDSS J223556.44,+235842.3,
with a photometric redshift = 0.10 +/- 0.07.
The following P48 photometry indicates modest evidence for significant
intra-night variability:
 +9.16 hr: R = 20.61 +/- 0.20
 +10.50 hr: R = 20.46 +/- 0.15
A low resolution spectrum obtained with the Double Spectrograph (DBSP) on the 
Palomar 200-inch Hale Telescope at 11:05 UT on 2015-08-07 suggest that the source is 
a Type Ia SN at a redshift of z = 0.173 (from nebular host galaxy emission lines).

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).

We do not consider any of these sources to be likely candidates for
the optical afterglow of Fermi trigger 460608875.

The diagram http://asd.gsfc.nasa.gov/Brad.Cenko/Fermi460608875.pdf
shows the locations of our candidates and the P48 fields in relation
to the Fermi GBM 1- and 2-sigma statistical+systematic contours.