GCN Circular 144
Subject
GRB980703 optical photometry
Event
Date
1998-07-12T06:03:58Z (28 years ago)
From
James Rhoads at KPNO <rhoads@noao.edu>
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