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GCN Circular 31

Subject
GRB970508 SAO RAS optical observations
Date
1998-03-26T22:08:00Z (26 years ago)
From
V. V. Sokolov at SAO RAS <sokolov@relay.sao.ru>
S. V. Zharikov, V. V. Sokolov, SAO RAS and Yu. V. Baryshev,
St.Petersburg University report on behalf of the SAO RAS optical 
follow-up team:

"Optical observations of the GRB 970508 optical remnant were continued
with the 6-m telescope of SAO RAS in the standard B, V, Rc, Ic bands
in October-December 1997 and in January 1998.

We found:
Rc (UT Oct.09.94, 1997) = 24.30+/-0.20  (integ., 2100sec.)
Rc (UT Nov.10.04, 1997) = 24.70+/-0.15  (integ., 2400sec.)
V  (UT Nov.10.04, 1997) = 25.10+/-0.17  (integ., 2700sec.)
Rc (UT Nov.25.97, 1997) = 24.70+/-0.14  (integ., 2500sec.)
Ic (UT Nov.25.99, 1997) = 23.90+/-0.14  (integ., 4800sec.)
V  (UT Nov.26.06, 1997) = 25.50+/-0.35  (integ., 2100sec.)
B  (UT Dec.01.91, 1997) = 25.75+/-0.30  (integ., 2400sec.)
Rc (UT Jan.24.87, 1998) = 24.96+/-0.17  (integ., 2580sec.)
V  (UT Jan.24.87, 1998) = 25.44+/-0.25  (integ., 2400sec.)
B band images have been obtained by A. I. Kopylov.
In determination of parameters of the faint host galaxy
we used the results of our BVRcIc photometry of May-August, 1997
(see at http://xxx.lanl.gov/astro-ph/9802341),
the data of recent observations with Keck-II
(Bloom et al., 1998, GCN Circular #30)
and WHT telescopes (Castro-Tirado et al., IAU Circular No. 6848),
and also the data of Rc and B bands photometry obtained
by other authors based on our secondary standards:
Pedersen H.  et al., 1997, astro-ph/9710322;
Garsia M.R., et al., 1997, astro-ph/9710346;
Galama T.J.  et al., 1998, astro-ph/9802160.

The level-off from the initial power-law decline seen
in the first months after the burst is observed in all bands.
The effect is the most strong in the Ic band: the difference at
the last date of Ic observations achieves ~1.3 mags. The best
X^2-fits of the data in each of 4 bands point to the presence of a faint
constant source with 
Ic = 24.13 +/- 0.28, Rc = 25.55 +/- 0.19, V = 25.80 +/- 0.14,
B = 26.68 +/- 0.14. 

The fitted magnitudes of the constant component
and small angular size of underlying host galaxy
(Pian et al. 1998, ApJ 492, L103)
correspond to a dwarf elliptical galaxy with an absolute magnitude
of M_B = -16.76 +/- 0.14 mags for z = 0.835
(Metzger et al. 1997, Nature 387, 879).

Corresponding figures for all 4 photemetrical bands can be seen in
http://www.sao.ru/~zhar/home/GRB/970508.html

This message is quotable in publications."
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