GCN Circular 16

GRB971214 Optical Observations and Calibrations                     #016

A.A. Henden, C.B. Luginbuhl and F.J. Vrba, U.S. Naval Observatory
Flagstaff Station, report a single measure of the OT reported by Halpern
et al. (IAUC 6788), obtained from a composite R-Band image with 80min
total exposure spread over Dec 15.4535-15.5593 UT (midpoint Dec 15.5064) as
R=21.67 +/- 0.10.  This is in effect a differential magnitude with
respect to stars 1,5-9 listed in the table below.  We caution that this
measure is approximate since we as yet have no calibrated color
information for the OT to permit accurate transformation to the
standard system; the error of +/- 0.10 mag includes only the photon
statistical contribution and does not include any error introduced by
the lack of the color correction.  We would like to here acknowledge
the assistance of Jan van Paradijs at UAH/University of Amsterdam and
J. Heise at SRON Utrecht, whose prompt dissemination of the SAX-WFC
position allowed us to make this early observation.

Local Standards in the GRB971214 Field

We have further obtained calibrated BVR photometry for several objects
near the OT position, and offer them as local "standards" for
calibrating measures of the OT.  The "standards" are listed here with
RA and DEC positions based on the GSC; we omit the RA hours (all 11)
and Declination degrees (all +65).

 Obj     RA (2000) DEC      B     err     V     err     R     err    I     err
  1   56 29.896 12 16.29  17.651 0.024  16.835 0.011  16.370 0.009
  2   56 25.832 11 36.29                              20.1   0.1
  3   56 20.239 12 34.64
  4   56 13.434 11 16.00
  5   56 15.102  9 47.35  19.378 0.079  18.039 0.022  17.175 0.009
  6   56  6.309 10  5.77  15.412 0.018  14.816 0.090  14.445 0.008
  7   56 22.332  8 26.01                18.261 0.027  17.249 0.009
  8   56 33.250  9 17.90  16.449 0.019  15.861 0.009  15.471 0.021
  9   55 52.290 10 15.61                19.964 0.120  18.919 0.019

NOTES: The object referred to as Ref1 by Dierks et al. (GCN Message 17
Dec 1997 and IAUC 6791) is our #2; our measure of R~20.1 is an
independent measure from the APM measure and is approximate because of
a poor measure at V and therefore uncertain color correction, but
nonetheless verifies their measure. We caution that their object Ref2
is slightly non-stellar in our images and should probably be avoided as
a local calibration source.  Castander et al. (IAUC 6791) report
R=20.14 +/- 0.02 for our #2.  The source located approximately 26" SSW
as noted by Rhodes et al. (GCN Message 17 Dec 1997 and IAUC 6793) is
also our #2; their object 26" NE of the OT is our #1 (they report
I=15.93 and 18.46 for #1 and #2, respectively); their object 55" NW is
our #3.

We plan to obtain further calibrated VR and I measures in this field,
weather permitting, in the coming months.  A finding chart showing
these local "standards" is posted on our ftp site at

        ftp.nofs.navy.mil  (192.68.148.67)

as file /pub/outgoing/fjv/grb971214/locstds.ps.gz.  We will update the
photometric measures as we obtain and reduce more data, and will keep
the latest data in the file /pub/outgoing/fjv/grb971214/locstds.dat.  We
encourage observers to consult this file to put their measures on a
standard system.

This message is quotable in publications.

C.B. Luginbuhl; cbl@nofs.navy.mil