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GCN Circular 1385

Subject
GRB020321 : Optical observations.
Date
2002-05-06T10:43:33Z (22 years ago)
From
Isabel Salamanca at U. of Amsterdam <isabel@science.uva.nl>
Isabel Salamanca, Evert Rol, L. Kaper (Univ. of Amsterdam), 
A. J. Castro-Tirado (IAA-CSIC Granada), A. Fruchter (STScI, Baltimore),  
J. Greiner (MPE Garching  and AI  Potsdam), J. Hjorth, (Univ. of Copenhagen),
E. Pian (INAF, OA Trieste), E. van  den  Heuvel  (Univ. of Amsterdam), 
on behalf of GRACE report:


Deep broad-band images of the NFI error box (GCN 1285 & 1348) and WFC
error box (GCN 1281 & 1284) of GRB 020321 were obtained with EFOSC at
the 3.6m telescope at ESO, La Silla.

The log of observations follows below:

Date (2000)	exposure	Filter	lim mag (3 sigma)
==========================================================
March 21/22	900 sec/each	UBVR	U=22.4 B=24.1 V=23.6 R=23.8
March 23/24	900 sec/each	  VR	V=23.7 R=23.6
April 18/19	1200 sec	   R	R=23.6

Magnitudes were calibrated by using the standard star WOLF 629 of the
Landolt catalogue, observed during the first night, which was photometric.

Very close to the position of the X-ray counterpart reported by in 't
Zand et al. (GCN 1348) we found two objects which are not present in the DSS,
with coordinates: 

Source	 RA(J2000)		 DEC(J000)	    error	
 A  	 16h 12m 44.1s, 	 -83d 43' 09.2"    0.5 arcsec
 B   	 16h 12m 44.4s, 	 -83d 43' 15.5"    2 arcsec 
X-ray    16h 12m 43.7s, 	 -83d 43' 13.9"    4 arcsec

Their magnitudes at the three epochs are:

(March 21.18102 BURST TRIGGER in BeppoSAX)

Epoch UT(2002)	Filter	 Source A	Source B
===========================================================
March 22.2814	R	20.81 +/-0.03	22.84 +/- 0.11
March 22.3179	V	21.43 +/-0.03	22.98 +/- 0.14
March 22.4260	B	22.79 +/-0.08	23.84 +/- 0.21
March 22.4378	U	> 23.6 		> 23.6 
March 24.3958	R	20.78 +/-0.04	23.30 +/- 0.22
March 24.4076	V	21.47 +/-0.05	23.54 +/- 0.30
April 19.2908	R	20.93 +/-0.04	23.68 +/- 0.26

-------
Note: UTs are mid-exposure.

Both sources are extended. Source "A" does not appear in the DSS 2
(red), although it has a magnitude similar to other stars
that are barely visible in the DSS image. However, it does not show
any clear fading behaviour as would be expected from a GRB optical
afterglow.

Source "B" consists of two 'blobs', one of which disappears at epochs
2 and 3. The decrease in magnitude is too shallow for the usual
power-law behaviour of an afterglow: the index alpha would be ~0.2,
while for gamma-ray afterglows alpha is usually around 1. However,
this could be consistent with an OT superposed on its host-galaxy.

Images in V and R for the three epochs, as well as a light curve can
be found at http://zon.wins.uva.nl/~isabel/GRBs/GRB020321/grb020321.html


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