GRB 970508

Optical Observations of GRB 970508:                               #030

J. S. Bloom, S. R. Kulkarni, S. G. Djorgovski, Caltech and
D. A. Frail, NARO report on behalf of the Caltech GRB effort:

"Optical follow-up of GRB 970508 on the Keck-II 10m was conducted on
the nights of Nov 28-29, 1997 and Feb 22-23, 1998. In both cases,
imaging observations were obtained with the Low Resolution Imaging
Spectrograph (LRIS).  Coincident with the position of the optical
transient, a faint source is easily detected in both B- and R-bands.
Differencing photometry with a number of field objects provided by V.
Sokolov (see their paper at http://xxx.lanl.gov/astro-ph/9802341),
suggest that the light curve has levelled off from the initial
power-law decline seen from 2-100 days from the time of the burst
(Sokolov et al. ibid).

We find:
B (UT Nov 29.6, 1997) = 26.32 +/- 0.26 mag (integ., 2400s),
B (UT Feb 23.4, 1998) = 26.27 +/- 0.14 mag (integ., 2400s),
R (UT Nov 28.6-29.5, 1997) =  25.09 +/- 0.14 mag (integ., 5400s), and
R (UT Feb 22.4, 1998) =  25.29 +/ 0.16 mag. (integ., 3600 s).
The systematic uncertainty of the Sokolov photometric zero-point (0.05
mag) is not included in the error analysis.  Had the power-law decline
continued the predicted brightness at the time of observations would be
[B,R] 26.65,25.63 (Nov) and 27.11,26.07 (Feb).  The 1-sigma errors for
the power-law light curve are estimated at 0.25 and 0.2 for B and R,
respectively, at each epoch. At the second epoch, then, the source is
significantly brighter then predicted by ~0.8 mags in each band.

Thus, although the object does not appear extended (consistent
with the HST findings of Fruchter et al. IAUC 6674) beyond that which
is expected from the seeing (~0.8"), we interpret the level-off as due
to the faint host galaxy--itself responsible for the persistent [OII]
emission. 

In fact, the lack of extent of the galaxy may be connected with its
previously inferred underluminous nature (Fruchter et al., and
Natarajan et al. 1997).  Furthermore, the B-R color (+1) is consistent
with an irregular galaxy at z=0.8 (Fukugita et al. 1995).

We thank V. Sokolov for collegial help in providing photometry of
secondary stars fainter than those reported in their Huntsville
poster.

This circular is a citable reference."

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
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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
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