GCN Circular 142

H.Pedersen, Copenhagen University Observatory,
H.Korhonen, Nordic Optical Telescope, La Palma, 
A.O.Jaunsen, Institute for Theoretical Astrophysics, Oslo, 
I.Ilyin & R.Duemmler, University of Oulu, and
F.Frontera, Inst.TeSTRE, Bologna (on behalf of the BeppoSAX team), report:

On July 4, 5, 6, and 7 we obtained R band images (600 sec each, seeing 
0.75"-1.0" FWHM) of the GRB980703-field using the 2.56-m Nordic Optical 
Telescope. The field-of-view was 3' x 3' and all images were acquired under 
good photometric conditions.

Images taken during the first three nights were centered on the position 
initially reported from RXTE by Levine et al. (IAUC 6966) and missed the 
radio- and optical transient (Frail et al., GCN #128) by ~8 arcsec.

Intercomparing these images we found one variable star. Its position is 
R.A. 23:59:03.7, Dec. +08:33:51.3 (J2000). An approximate R-band zero-point 
is determined from the USNO catalogue entry U0975_21757527 (a star 15" SW 
of the OT), which has R=16.5. This gives the following photometry for the 
new variable:

   UT                Var  

   1998 July 04.101  R= 21.0 +/- 0.2
             04.149  R= 21.1 +/- 0.2
             04.194  R= 21.6 +/- 0.2
             05.086  R= 21.5 +/- 0.2
             05.146  R= 21.2 +/- 0.2
             05.195  R= 21.1 +/- 0.2
             06.188  R= 21.1 +/- 0.2

An image centered on the RT/OT was taken on July 7. For the OT we find the 
following R-band magnitude (using the already mentioned standard star):

   1998 July 07.163  R= 22.45 +/- 0.2

The errors reflect a conservative estimate of the uncertainty in the
determination of the zero-point. 

Our result is consistent with a power-law decline as reported by Bloom et al. 
(GCN #136) when comparing to other reported R-band values.

Further information can be found on http://vega.uio.no/~ajaunsen/grb980703

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