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." ////////////////////////////////////////////////////////////////////////////// Circular No. 6848 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions) BMARSDEN@CFA.HARVARD.EDU or DGREEN@CFA.HARVARD.EDU (science) URL http://cfa-www.harvard.edu/iau/cbat.html Phone 617-495-7244/7440/7444 (for emergency use only) GRB 970508 A. J. Castro-Tirado and J. Gorosabel, Laboratorio de Astrofisica Espacial y Fisica Fundamental, Madrid; T. Galama and P. Groot, University of Amsterdam; J. van Paradijs, University of Amsterdam and University of Alabama at Hunstville; and C. Kouveliotou, Universities Space Research Association, report: "R- band images have been obtained by J. H. Telting, B. Garcia-Lorenzo, and L. Jones (Isaac Newton Group Telescopes) with the 4.2-m William Herschel Telescope on La Palma on Mar. 20/21. At the same position of the optical counterpart to GRB 970508 reported by Bond (IAUC 6654), the co-added image (13700 s) reveals an object 5.4 mag fainter: R = 25.20 +/- 0.25 (based on the secondary standards given by Sokolov et al. 1998, at http://xxx.lanl.gov/abs/astro-ph/?9802341). This is about 1.0 mag brighter than expected from the extrapolation of the power-law decline (R = 26.18 +/- 0.09), suggesting that the optical light is dominated by the host galaxy. Assuming its location at z = 0.8 (Metzger et al. 1997, Nature 387, 879), this corresponds to an absolute blue magnitude of -17.9 +/- 0.5, placing the object below the knee of the galaxy luminosity function." (C) Copyright 1998 CBAT 1998 March 24 (6848) Daniel W. E. Green ////////////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 151 SUBJECT: GRB970508 optical observations DATE: 98/08/21 17:21:17 GMT FROM: Andrew S. Fruchter at STScI 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.