GCN Circular 157

James Rhoads, Arjun Dey, Buell Jannuzi, and Megan Sosey (on behalf of
the KPNO GRB followup team); Sylvio Klose; Daniel Reichart; Andrew
Fruchter; and Francisco Castander report:
 
We have used data obtained by Rhoads on the night of 980403 UT to
calibrate Klose's I band measurement of the optical transient
associated with GRB 980329 (GCNC #43).  We find an optical transient
magnitude I=20.8 +- 0.3 on March 29.8-30.0 (cf. Reichart et al, in
preparation).  The corresponding flux density is 11 +- 3 microJansky.
The data consisted of 1200 seconds integration per filter on the GRB
980329 field in R band (April 3.147) and I band (April 3.167) at
airmass 1.1, plus brief R and I band observations of the SA107 field
at airmass 1.4.  The data were taken with the 4 meter Mayall telescope
and Mosaic CCD Imager at Kitt Peak National Observatory.  Weather was
photometric with poor image quality (from 1.25 to 1.75 arcsec).
Details of the photometric calibration and a table of reference star
magnitudes in the GRB 980329 field are at the end of this circular.

We used the same data to measure the flux at the location of the
transient.  No obvious source is present at the location of the
transient (7:02:38.0 +38:50:44 J2000.0) in either filter.  R band
photometry using a 0.75 and 1.25 arcsecond radius apertures, corrected
for aperture losses using the curve of growth, yield flux estimates
of 0.40 +- 0.25 and 0.80 +- 0.25 microJansky, where the error bar
includes contributions from photon counting noise and from sky
subtraction uncertainties only.  Conservatively, this gives a 3 sigma
upper limit of 1.50 microJansky (for a one-sided confidence interval
with probability 99.73%) and R > 23.3 magnitudes.

For the I band, the aperture-corrected point source fluxes measured in 
0.75 and 1.25 arcsecond radius apertures are 0.06 +- 0.51 and -0.12 +- 0.44
microJansky.  The first implies a 3 sigma upper limit of 1.48 microJansky
and to I > 23.0, while the second gives an upper limit of 1.10 microJansky
and I > 23.3 magnitudes.

In addition, we report a K band upper limit using data obtained by
Dey, Jannuzi, and Sosey (on behalf of the NOAO Deep Widefield Survey
team) on 1998 April 3.205 using the Kitt Peak National Observatory 2.1
meter telescope and ONIS camera.  No point source is apparent at the
location of the transient.  The RMS counts in the image imply a 3
sigma upper limit of 13.4 microJansky for a point source (in a 1
arcsec radius aperture and corrected using the curve of growth),
which corresponds to K > 19.2 at the 3 sigma level.

 - Details of the photometric calibrations - 
Reliable optical fluxes were measured for the Landolt standard stars
SA107-212, 213, 357, 359, 351, 457, 456, 600, 599, 612, 626, and 627.
Standard star and GRB field photometry was done in a similar fashion,
with growth curves derived from multiaperture photometry used to
correct all magnitudes to an effective 7" radius.  The photometric
errors in the GRB frame were taken from the IRAF "mkapfile" task,
which applies the aperture corrections.  Additional errors were added
in quadrature to account for uncertainties in the photometric zero
point (+- 0.004 mag in both R and I), color correction terms (+-
0.07*[R-I - 0.42]), and airmass correction term (+- 0.01 mag for I
band and +- 0.024 mag for R band).
 
The table of measured magnitudes for objects near GRB 980329:
# RA (J2000) Dec        R     err(R)      I    err(I)
#
7:02:39.0 38:50:32.7  15.7     1.0      15.300  0.2 
7:02:37.5 38:50:33.5  15.85    1.0      15.450  0.12
7:02:35.1 38:50:23.2  16.30    0.3      15.988  0.036 
7:02:40.1 38:50:11.8  16.9664  0.0314   16.647  0.0155
7:02:39.4 38:50:03.1  18.4428  0.0275   18.093  0.0175
7:02:38.7 38:50:26.9  20.655   0.0646   19.473  0.0623
7:02:36.6 38:50:36.3  20.3862  0.0536   19.331  0.0478
7:02:36.3 38:50:19.8  20.5135  0.0464   19.625  0.0459 
7:02:38.4 38:50:50.7  21.2876  0.0688   20.026  0.0711 
7:02:51.0 38:49:31.0  17.6592  0.0456   16.668  0.0399 
7:02:50.4 38:49:57.1  17.9663  0.0258   17.539  0.0161
#
Users of this table should be aware that the magnitude uncertainties
for the different stars include some sources of systematic error
(airmass and color terms) that are not independent from star to star.
Also, the first two entries are substantially saturated in R and I;
the third is substantially saturated in R and perhaps marginally in I;
and the fourth may be marginally saturated in R.  Photometric errors
quoted for saturated stars are approximate guesses.
 
More detail, and sections of the processed R and I band images, are
available from  http://www.noao.edu/noao/grb/980329.html .
This report is citable.