[vsnet-grb-info 16528] GRB 150512A: iPTF Optical Observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17809
SUBJECT: GRB 150512A: iPTF Optical Observations
DATE:    15/05/13 18:50:52 GMT
FROM:    Leo Singer at GSFC/iPTF  <leo.p.singer at nasa.gov>

L. P. Singer (NASA/GSFC), V. Bhalerao (IUCAA), 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 150512A (Fermi trigger 453118948 / bn150512432) at
2015-05-12 10:22:25.87. At 2015-05-13 03:58:15, 17.6 hours 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 covering the 1-sigma
statistical+systematic region of the final Fermi GBM localization. We
estimate a 76% 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:

iPTF15asm, at the coordinates:
  RA(J2000)  =  13h 40m 45.03s (205.187617 deg)
  Dec(J2000) = +63d 19' 53.3"  (+63.331461 deg)
This position is 6" from the galaxy SDSS J134044.09+631954.4 (z = 0.104).
We report the following P48 photometry:
  -95 days:     R > 20.27
  +18.01 hours: R = 19.49 +/- 0.06
  +18.93 hours: R = 19.42 +/- 0.06

iPTF15asn, at the coordinates:
  RA(J2000)  =  13h 57m 40.01s (209.416716 deg)
  Dec(J2000) = +65d 08' 01.6"  (+65.133789 deg)
We report the following P48 photometry:
  -95 days:     R > 20.42
  +18.01 hours: R = 19.82 +/- 0.07
  +18.93 hours: R = 19.98 +/- 0.07

No source was visible in archival P48 and SDSS images at either position.
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 encourage further observations of both candidates to determine if
either is fading and potentially related to the GRB.

The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi453118948.pdf
shows the locations of our candidates and the P48 fields in relation to
the Fermi GBM 1- and 2-sigma statistical+systematic contours.