TITLE: GCN GRB OBSERVATION REPORT NUMBER: 144 SUBJECT: GRB980703 optical photometry DATE: 98/07/12 06:03:58 GMT FROM: James Rhoads at KPNO James Rhoads, Ron Downes, and Jennifer Christensen report on behalf of the Kitt Peak Gamma Ray Burst followup team: Using images we obtained on July 4.433 (see GCN Circ. 123), we have measured the R band magnitude of the proposed optical counterpart to GRB 980703 (GCN Circ. 128, 130). We have also measured R band magnitudes of local comparison stars to facilitate photometric calibration of other data. We converted instrumental to standard magnitudes using observations of Landolt standard stars in the fields SA 109 and Mark A. We fitted for the zero point and color term of the photometric transformation. Our standard star fields and the first GRB frame were all taken at essentially the same airmass, so we did not fit for an airmass correction term. All magnitudes for standard stars were measured in 14 arcsecond diameter apertures, to match the procedure of Landolt (1992 AJ 104, 340). Magnitudes for objects in the GRB frame were measured in several apertures and corrected to the equivalent 14 arcsecond aperture magnitude using a curve of growth derived from stars near the GRB. The GRB measurements themselves were based on 2.7 to 4.1 arcsecond diameter apertures. Our results, based on our combined final image (60 minutes integration with the KPNO 0.9m telescope), are summarized below: ID x y d(RA) d(Dec) R dR (1998 July 04.433 UT) OT 896.4 1051.6 0.0 0.0 21.287 0.083 V 1004.6 1119.4 -46.1 -73.6 21.185 0.079 1 891.5 1248.5 -133.9 3.4 18.815 0.015 2 837.5 1195.1 -97.6 40.1 20.132 0.035 3 939.3 1157.7 -72.2 -29.2 19.807 0.026 4 889.9 1077.4 -17.6 4.4 20.392 0.040 5 909.6 1068.6 -11.6 -9.0 16.642 0.015 6 927.0 1064.1 -8.5 -20.8 20.717 0.049 7 887.4 1043.7 5.3 6.1 22.662 0.219 8 919.9 977.6 50.3 -16.0 19.109 0.017 9 857.0 974.5 52.4 26.8 16.492 0.010 10 248.5 1246.8 -132.8 440.5 15.842 0.016 11 748.8 1718.7 -453.7 100.3 16.045 0.004 12 395.5 1731.0 -462.0 340.6 18.174 0.021 13 375.8 1728.4 -460.3 354.0 17.824 0.024 14 1633.4 1604.2 -375.8 -501.1 17.069 0.006 15 1683.8 941.1 75.1 -535.5 17.175 0.005 16 1691.3 1273.4 -150.9 -540.5 16.843 0.012 17 1565.7 1440.8 -264.7 -455.1 15.617 0.005 18 1500.5 1245.5 -131.9 -410.8 17.774 0.015 19 1495.4 448.3 410.2 -407.3 15.787 0.009 20 504.0 529.4 355.0 266.8 15.579 0.010 (x,y) are coordinates in our image. d(RA) and d(Dec) are approximate offsets in arcseconds from the location of the transient, uncorrected for field rotation. R and dR are the measured R band magnitude and the statistical uncertainty in that magnitude (determined using the IRAF tasks "phot" and "mkapfile"). The uncertainty in the photometric zero point is an additional 0.010 magnitudes. The color term was (-0.001 +- 0.018)(V-R) and so will add a small additional uncertainty ( < 0.02 mag for objects of normal color) to the R band magnitudes since we have no color information for most objects in the field. The ID entries are mostly arbitrary numbers. "OT" refers to the optical transient. "V" refers to the variable star reported by Pedersen et al (GCN Circ. 142). None of the objects tabulated above shows photometric variations substantially above 1 sigma random errors in a comparison of our three exposures (each 20 minutes long, beginning on 980704 UT 09:51:16, 10:13:40, and 10:35:40). Individual magnitude measurements for the optical transient and for Pedersen et al's variable are as follows: UT R(OT) dR(OT) R(V) dR(V) 1998 July 04.418 21.168 0.118 21.010 0.098 1998 July 04.433 21.353 0.114 21.114 0.093 1998 July 04.448 21.218 0.101 21.170 0.099 (Here "UT" is the UT of mid-exposure.) Our star number 5 is the USNO catalog entry used by Frail et al (GCN Circ 128) to calibrate their optical photometry. Applying a correction of +0.142 magnitudes to their photometry based on our calibration implies magnitudes for the transient of 21.34 on July 4.48 and 22.04 on July 6.60. This agrees with the zero point of Bloom et al (GCN Circ 135) to much better than their reported accuracy of 0.2 mag. Comparing our measurement to the July 4.458 H band magnitude from Henden et al, we find R-H = 3.75 +- 0.22 . Comparing to the July 4.37 I band measurement of Vreeswijk et al (GCN Circ 132) yields R-I = 0.64 +- 0.22 . In both cases we have corrected the colors for differences between the observation epochs by assuming an achromatic t^(-0.75) decay of the flux. James Rhoads rhoads@noao.edu Ron Downes downes@stsci.edu Jennifer Christensen christen@stsci.edu