GCN Circular 191

GRB 981220 Optical Observations, BVRI

M. R. Metzger, C. Martin, and B. Kern (Caltech) report:

Images of the GRB 981220 joint localization errorbox (GCN 159
Not found
 
 
,160
Not found
 
 
)
were obtained on UT Dec 23.15 (VRI, GCN 163
Not found
 
 
) , UT Dec 24.2 (BVRI), and
UT Dec 27.2 (VRI) at the Palomar 5m telescope using COSMIC.  The
seeing varied between 1.4 and 2.0 arcsec (FWHM).  Photometric
calibration was perfomed on UT Dec 24 using Landolt standards, and
astrometry was computed using a solution of 70 nearby stars from USNO
A2.0.  For reference, on this system we find the following magnitudes
for calibrating objects near the Galama et al (GCN 168
Not found
 
 
) radio source:

                                   mag(+-) 
      Pos (J2000)           B         V         R         Ic 
03 42 28.69 +17 09 26.1  23.81(14) 22.98(13) 22.34(09) 22.00(15) 
03 42 28.90 +17 09 33.0  21.61(03) 20.75(04) 20.12(03) 19.56(04) 
03 42 30.51 +17 09 20.9  22.12(06) 20.58(06) 19.65(04) 18.61(05) 
03 42 28.33 +17 09 42.8  19.46(02) 18.47(04) 17.97(02) 17.36(02) 

At the radio position of Frail and Kulkarni (GCN 170
Not found
 
 
), no source was
detected on Dec 23.15 to the limits (2-sigma) I > 23.0, R > 23.8, V >
24.2.  The object marked "1" on the APO image referred to by Vrba et
al on GCN 171
Not found
 
 
 has I = 19.6 +- 0.1 on Dec 23.15 and Dec 24.2 at rough
position 03h42m31.7 +17:10:04; it is fairly red, with (V-I) ~= 2.
This disagrees with the photometry reported on GCN 171
Not found
 
 
, and could be
due to a difference in photometric zero point or different object.

We can also place B-band upper limits at the radio position.  On Dec
24.2, there is no significant detection (though possibly a very faint
peak), which puts a limit of B >= 25.4 (1-sigma).  Any host is less
blue and fainter than implied on GCN 179
Not found
 
 
 given the Echelberger et
al. (GCN 176
Not found
 
 
) magnitude.  The detection of flux by Masetti et al. (GCN
179
Not found
 
 
) with B ~= 24 on Dec 23 would imply variability, and thus may be
the optical afterglow of GRB 981220.  The V limit on Dec 23 is
consistent with a blue spectrum, and the power law time decay in B
would be steeper than alpha = -1.7 (though uncertainties are large).

No significant variability of objects (other than asteroids) is
detected over the Dec 23 to Dec 27 interval in V, R, or I, consistent
with other reports.  A B-band image from Dec 24 may be found at
http://astro.caltech.edu/~mrm/grb981220.html.

This report may be cited in publications.