GCN Circular 233

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
Not found
 
 
 for the B image, to GCN 204
Not found
 
 

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
Not found
 
 
) 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
significant. 

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
Not found
 
 
), 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

http://tonno.tesre.bo.cnr.it/~masetti/grb990123.html

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


This message can be cited."