GRB 970508

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
Loading...
 
 )
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."

HST Observations of the Host Galaxy of GRB970508: 

A. Fruchter (STScI), E. Pian (ITESRE) and an international
collaboration report:

The field of GRB970508 was imaged by HST with the STIS CCD in open
filter mode (50CCD) on 1998 August 5.78-6.03 for a total exposure time
of 11,568 seconds.  An extended object, which we believe to be the host
galaxy of GRB970508, was detected at the astrometric position of the
optical transient of GRB970508.  The galaxy has high signal-to-noise in
our data and is clearly resolved, with a major axis of approximately 
0."5 .  Including an uncertain contribution from the optical transient, 
the galaxy has a magnitude of V= 25.25 +/- 0.20, where the primary 
source of uncertainty is due to the wide bandpass (and thus poor color 
resolution) of the detector.   This agrees reasonably well with previous 
photometric estimates (Bloom et al. 1998, Zharikov et al. 1998).

The image of the galaxy is well described by an exponential disk with a
scale length of approximately 0."07, and an ellipticity of 0.3.
However, the FWHM of the PSF is comparable in size to the observed
scale length; thus an accurate measurement of the surface brightness
profile of the galaxy will require modelling and/or deconvolution.

We have obtained an astrometric solution with an r.m.s. uncertainty of
approximatgely 0."01 which maps the previous STIS image (Fruchter et
al. 1997, Pian et al 1998) onto the new data.  The position of the OT
in the June 1997 STIS image, and the center of the galaxy on the August
1998 image, agree to within the astrometric error.

Sections of the images are available on the web from 
http://www.stsci.edu/~fruchter/GRB/970508.html

Bloom, J.S., Djorgovski, S.G., Kulkarni, S.R. and Frail, D.A., 1998,
	      Ap. J. (Lett.), submitted, astro-ph/9807315.
Fruchter, A., Bergeron, E., and Pian, E., 1997, IAUC 6674.  
Pian, E., et al., 1998, Ap. J. (Lett.), 492, L103.  
Zharikov, S.V., Sokolov, V.V.  and Baryshev, Yu.V., 1998, GCN 31
Loading...
 
 .