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GCN Circular 1382

GRB020331: Third Epoch and Revised Second Epoch Optical
2002-04-24T21:11:54Z (22 years ago)
George Ricker at MIT <>
GRB020331: Third Epoch and Revised Second Epoch Optical Observations 
with Magellan

A. Dullighan, G. Monnelly, N. Butler, R. Vanderspek, G. Ricker, Z. 
Wang (MIT); P. Challis, R. Kirshner (CfA)


We obtained third epoch observations of the HETE error circle of 
GRB020331 (Ricker et al., GCN #1315) with the Baade 6.5m telescope at 
Magellan on 2002 April 13.21 UT using MagIC. The observations took 
place 12.53 days after the burst, and 12.00 days after our first 
epoch observations (Monnelly et al., GCN #1339). One 300s and three 
600s R-band exposures were stacked to image a 2 arcmin square region 
centered on the coordinates of the afterglow candidate first reported 
by Fox et. al. (GCN #1334).

Further calibration of the data from our second epoch observations 
has led to a revised magnitude estimate of R = 23.1 +/- 0.2 at 8.37 
days after the burst (cf. R = 22.9 initially reported in GCN #1346). 
We still place the magnitude of the first epoch observations at R = 
21.1 +/- 0.2 at 0.53 days after the burst.  This leads to a nominal, 
revised power law decay index of alpha = 0.67 +/- 0.1. However, the 
decay is likely to be significantly steeper than would be indicated 
by this nominal index, for the reasons given below.

In our third epoch observations, we measure an R magnitude of 23.2 
+/- 0.3 using aperture photometry at the position of the counterpart, 
consistent with the brightness at epoch 2. As in our first epoch 
observations, magnitudes are calibrated against star G020331 (R = 
18.945) from the photometry of Henden et. al. (GCN #1319).

Furthermore, we find that the emission centroid of the third epoch 
image is offset by  0.8" +/- 0.1" to the east. Our image reveals a 
source extended E-W through the position of the counterpart. We 
interpret the extended source to be the host galaxy of the optical 
counterpart: the GRB optical counterpart has faded below the 
directly-detectable limit, and our aperture photometry at all epochs 
includes the centroid of the host galaxy. In the third epoch image, 
we estimate that almost all of the detected light originates in the 
putative host galaxy. PSF subtraction of a 23.2 magnitude point 
source at the centroid of the galaxy leaves a faint residual source 
at the position of the counterpart, with an R magnitude of ~25. Due 
to the contamination by the putative host galaxy at the earlier 
epochs, the power law decay of the counterpart is likely to be 
steeper than we reported in GCN #1339 and GCN #1346.

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