GCN Circular 19652

Y. Tachibana,  Y. Saito, Y. Muraki, Y. Ono, T. Fujiwara, T. Yoshii, K. Saisho,
H. Ohuchi, Y. Yano, S. Harita, Y. Yatsu, and N. Kawai (Tokyo Tech)
report on behalf of the MITSuME collaboration:

We report the optical follow-up observations of GRB 160703A (S. B. Cenko et al., GCN Circular #19645) with a tricolor camera (g', Rc, and Ic) attached to the MITSuME 50 cm telescope of Akeno Observatory, Yamanashi, Japan.

The observation started at 2016-07-03 12:10:54 UT (~30 sec after the burst).
Although the first eight frames (each exposure time is 30 sec) are covered by thick clouds, we detected a bright optical transient source in the refined BAT error circle (A. Y. Lien et al., GCN Circular #19648)  ~5 min after the trigger.
The position of the counterpart is
RA(J2000) = 19h 09m 40.1s
Dec(J2000) = +36d 55' 08.1"
with an uncertainty of ~0.5 arc sec. 

Nearby the optical transient, a known source N2JN062852 (RA, Dec = 19h 09m 40.212s, +36d 55' 06.9816$B!I(B)
catalogued in GSC2.3 is located. 
In the photometric analysis, we ignored the contamination from the nearby source because its 
flux is substantially lower (Fmag = 20.27) comparing with the optical counterpart.
The results were shown below:

  T0+[min]    MID-UT    T-EXP[sec]    g$B!G(B    g$B!G(B_err    Rc    Rc_err    Ic    Ic_err
----------------------------------------------------------------------------------------------------------------------
   5.82          12:15:54        30          15.46    0.06        14.59    0.06        14.25    0.09
   58.35        13:08:26      3900        18.18    0.05        17.08    0.04        16.87    0.06
   330.52      15:40:36      7680        19.13    0.06        17.96    0.05        17.61    0.06
----------------------------------------------------------------------------------------------------------------------
  T0+ : Elapsed time after the burst
  T-EXP: Total Exposure time

We used GSC2.3 catalog for flux calibration.
Further analysis is ongoing.