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

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  534
SUBJECT: VLT Detection of Fading Optical Source in the GRB 000131 Error Box
DATE:    00/02/06 22:46:47 GMT

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, #530), as follows:

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

1.54-m Danish:

2000 February 4.19; 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 object, which has faded significantly between the two observing nights. The variation is noticed on both 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."

 

 

(click for larger images)

 

FORS1: 480 s in R, taken 2000 Feb 4.04-4.22.

The spike to the left of the OT is due to a bright (V~9) star to the south.
(North - up, East - left)

 

FORS1: 480 s in R, taken 2000 Feb 6.10-6.22.

The spike to the left of the OT is due to a bright (V~9) star to the south.
(North - up, East - left)

 

FORS1: 480 s in V, taken 2000 Feb 4.04-4.22.

The brightening of the field is due to a bright (V~9) star to the south.
(North - up, East - left)

 

FORS1: 480 s in V, taken 2000 Feb 6.10-6.22.

The brightening of the field is due to a bright (V~9) star to the south.
(North - up, East - left)

 

DFOSC: 3600 s in gunn-i, taken 2000 Feb 4.19.

The spikes are due to a bright (V~9) star to the south.

 

DSS-2, quoted limit: R~22.0

The circle indicates the approximate position of the OT, with the surrounding box covering the sections shown above.

 

DSS-2, quoted limit: R~22.0

The diamond represents the IPN+BATSE error-box, with the two squares being the 6.8x6.8' pointings of FORS1. The DFOSC-pointing covers the whole image.
The OT is situated just to the North (up) and West (right) of the bright star at the Position 2-pointing.

 

Last updated on Feb 8 2000, by Brian Lindgren Jensen