GCN Circular 534

VLT Detection of Fading Optical Source in the GRB 000131 Error Box


H. Pedersen, B. L. Jensen, J. Hjorth, L. Fogh Olsen (U. of Copenhagen),
M. I. Andersen (U. of Oulo), J. Gorosabel (DSRI, Copenhagen),
M. Vestergaard (Ohio State U.), L. Kaper (U. of Amsterdam), and
A. O. Jaunsen (U. of Oslo), on behalf of a European GRB consortium, report:

"Using the 8.2-m Antu telescope, Paranal, and the 1.54-m Danish telescope, 
La Silla, we have performed optical observations, covering the entire error 
box of GRB 000131 (GCN #529
Not found
 
 
, #530
Not found
 
 
), as follows:

Antu:

2000 February 4.04 UT; B, V, R ; 240 s each filter ; FWHM = 0.9" 
2000 February 4.22 UT; B, V, R ; 240 s each filter ; FWHM = 0.9" 
2000 February 6.10 UT; B, V, R ; 240 s each filter ; FWHM = 1.2" 
2000 February 6.22 UT; B, V, R ; 240 s each filter ; FWHM = 1.2" 

1.54-m Danish:

2000 February 4.19 UT; I ; 3600 s ; FWHM = 1.3"

For Antu, two separate pointings were obtained using FORS1; the exposure
times apply to each of these. For the 1.54-m, the error box was covered by 
a single pointing with DFOSC.

Intercomparison of the Antu images reveals a single, unresolved object, which 
has faded significantly between the two observing nights. The variation is 
noticed on the V and R exposures, while the B exposures do not show the object 
at any epoch. The object is detected in the combined 1.54-m Danish I-band 
image.

Its position is RA(J2000) = 6:13:31.0, Dec(J2000) = -51:56:40, with an
uncertainty of about 2".

The object has R ~ 24 in the Feb 4 images and has faded 0.6+-0.1 mag
on Feb 6, corresponding to a power law decay index of -1.2+-0.2.

Sections of the images can be seen at

http://www.astro.ku.dk/~brian_j/grb/grb000131/

jointly with a DSS finding chart.

We appreciate the kind assistance given by the VLT Service Mode team."