[vsnet-grb-info 3061] GRB060218: Refined photometric calibration of comparison stars

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4898
SUBJECT: GRB060218:  Refined photometric calibration of comparison stars
DATE:    06/03/20 20:19:10 GMT
FROM:    Malcolm Hicken at Harvard/Physics  <mhicken at cfa.harvard.edu>

Malcolm Hicken (CFA), Maryam Modjaz (CFA), Peter Challis (CFA), Robert
Kirshner (CFA), Jose Luis Prieto (OSU), Krzystof Stanek (OSU) and Richard
Cool (Arizona) report:

The CFA Supernova Group obtained UBVRr'i' photometry of 9 comparison stars
in the field of GRB060218/SN2006aj on March 4, 2006 UT.  Data was taken
using the FLWO 1.2m telescope at Mt. Hopkins, Arizona.  We present our
V-band light curve in Modjaz, et. al. (2006), astro-ph/0603377, submitted
to ApJL.  A finding chart, coordinates and photometry can be found at the
following website:

http://cfa-www.harvard.edu/oir/Research/supernova/sn2006aj_compstars.html

It should be noted that our V magnitudes are approximately 0.27 mag
fainter than those derived in GCN 4777 by Cool, et. al. at

http://mizar.as.arizona.edu/~grb/public/GRB060218.

There is also an offset in the other bands.  In investigating this
photometric offset, Richard Cool has discovered calibration offsets
between the two SDSS photometric reductions of this field indicating
possible non-photometric conditions or other calibration problems in the
SDSS photometry. The problem is being investigated to ensure data with
suspicious photometric quality will be flagged as such in future SDSS GRB
releases.