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Astron. Astrophys. 337, 356-362 (1998)


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BVRcIc light curves of GRB970508 optical remnant and colors of underlying host galaxy

S.V. Zharikov 1, V.V. Sokolov 1 and Yu.V. Baryshev 2

1 Special Astrophysical Observatory of R.A.S, Karachai-Cherkessia, 357147 Nizhnij Arkhyz, Russia (zhar,sokolov@sao.ru)
2 Astronomical Institute of St. Petersburg University, 198904 St. Petersburg, Russia (yuba@aispbu.spb.su)

Received 14 May 1998 / Accepted 17 June 1998

Abstract

Optical observations of the GRB970508 optical remnant were continued with the 6-m telescope of SAO RAS in standard the [FORMULA] bands in Oct.-Dec. 1997 and in Jan. 1998. The results of photometry of the GRB970508 remnant and of three nearby galaxies are presented. The [FORMULA] light curves of the GRB970508 remnant can be described by a power law plus a constant [FORMULA]. In determination of parameters of the faint host galaxy we used the results of our [FORMULA] photometry of May-August, 1997, the data of recent observations with Keck-II and WHT telescopes and also the data of photometry of [FORMULA] and B bands obtained by other authors based on our secondary standards. The level-off from the initial power law decline seen in the first months after the burst was observed in all bands. The effect is the strongest in the [FORMULA] band: the difference on the last date of the [FORMULA] observations reaches [FORMULA]1.3 mags. The best [FORMULA]-fits for [FORMULA], [FORMULA], C parameters of the data in each of 4 bands point to the presence of a constant faint source with [FORMULA], [FORMULA], [FORMULA], [FORMULA]. The average [FORMULA] is [FORMULA]. The optical remnant has a power law spectrum with a spectral slope equal to [FORMULA] and does not change after the optical curve maximum. The [FORMULA] spectrum together with the absolute magnitude of the constant component [FORMULA], and the linear size of the underlying host galaxy [FORMULA] conform to a host galaxy, such as a starburst dwarf, a red starburst dwarf, a irregular dwarf, a HII galaxy, or a blue compact dwarf galaxy. All these types of dwarf galaxies show evidence of starburst or post starburst activity. The galaxy G2 has a spectrum similar to that of the host GRB galaxy and lies at the projected distance of [FORMULA] kpc from GRB.

Key words: gamma-rays: bursts – galaxies: dwarf

Send offprint requests to: S. Zharikov, (zhar@sao.ru)

© European Southern Observatory (ESO) 1998

Online publication: August 17, 1998

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