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GCN Circular 233

GRB990123, Optical BVRI Observations
1999-01-27T19:24:25Z (25 years ago)
Elena Pian at ITESRE-CNR,Bologna <>
GRB 990123 BVRI Photometry

N. Masetti, E. Palazzi, E. Pian, F. Frontera (ITESRE-CNR, Bologna), C.
Bartolini, A. Guarnieri, A. Piccioni (Astronomy Dept., Univ. Bologna), 
G. Valentini (Teramo Astr. Obs.) and E. Costa (IAS-CNR) report:

"We have imaged the field of GRB990123 with the Bologna Astronomical
Observatory 1.5m telescope equipped with BFOSC, using Johnson B and V, and
Cousins R and I filters in January 24-26. 

Calibrations were done referring to GCN 206 for the B image, to GCN 204
for the R image, and using the standard star BD+252534 (Taylor et al.
1989, AJ 97, 1798) for the V and I images. 

In the appended table the preliminary magnitudes of the optical transient
(GCN 201) are reported along with the observation dates.

Power-law fits to the R and V band data yield temporal decay indices of
alpha = 1.20 +- 0.08 and alpha = 1.25 +- 0.16, respectively (errors are 1
sigma), with satisfactory chi square values.  No indication is found of a
flattening of the light curve due to an underlying galaxy. 

Suggestion of a slight flux increase is seen on January 25 in both R and V
bands, as opposed to the overall fading trend.  This is however not

The BVRI spectral continuum of the optical transient in the first night
(average time of Jan 24.16), dereddened with E(B-V) = 0.016 (GCN 207), has
a power-law shape (f_nu = k*nu^{-beta}) with best-fit slope 
beta = 1.1 +- 0.2 (1 sigma). 

The BVRI light curves and the radio-to-X-ray spectral energy distribution
of the transient are also shown at the Web site

Multiwavelength observations, particularly in the near-infrared and
millimetric bands, are needed and urged.

      Date (UT)    mag    error  band
  1998 Jan 24.122  19.36  0.20    I
           24.142  19.79  0.10    R
           24.164  19.97  0.16    V
           24.194  20.64  0.07    B
           24.216  19.92  0.10    R
           25.138  20.91  0.10    R
           25.159  20.77  0.10    R
           25.181  21.01  0.20    V   
           25.203  20.93  0.20    V
           26.154  21.77  0.16    V
           26.154  21.56  0.10    R

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