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Astron. Astrophys. 334, 117-123 (1998)

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BVRCIC photometry of GRB 970508 optical remnant: May-August, 1997

V.V. Sokolov 1, A.I. Kopylov 1, S.V. Zharikov 1, M. Feroci 2, L. Nicastro 3 and E. Palazzi 3

1 Special Astrophysical Observatory of RAS, Karachai-Cherkessia, 357147 Nizhnij Arkhyz Russia (sokolov,akop,zhar@sao.ru)
2 Istituto di Astrofisica Spaziale, CNR, Via Fosso del Cavaliere, I-00131 Roma, Italy
3 Istituto Tecnologie e Studio Radiazioni Extraterrestri CNR, Via P. Gobetti 101, I-40129 Bologna, Italy

Received 2 February 1998 / Accepted 24 February 1998


We present the results of photometric observations of the variable optical source associated to the remnant of the gamma-ray burst GRB 970508 performed at the Special Astrophysical Observatory of the Russian Academy of Science (SAO RAS) from May to August 1997. The observations were carried out with the standard (Johnson-Kron-Cousins) photometric [FORMULA] system using the 1-meter and 6-meter telescopes. The brightness of the optical source increased from [FORMULA] (May 9.75 UT) to [FORMULA] (May 10.77 UT), whereupon it was decaying in all the four [FORMULA] bands during about one month after the burst. Between the 3rd and the 6th night after the GRB 970508 event, the flux decrease of the optical source follows an exponential law in all the four bands. In that period the broad-band spectrum of the object does not change and can be approximately described by the power law [FORMULA]. The subsequent observations in [FORMULA] bands have shown a reduction of the source brightness decrease rate. The source flux decay after the maximum in the [FORMULA] band is well described by a single power-law [FORMULA] for the whole set of observations during 86 days. In the other bands the decay seems to slow down from the 31th day onwards. In particular, in the [FORMULA] band the source magnitude is about 23.1 from the 36th day after the GRB to the end of the observations (August 4, 1997).

Key words: gamma-rays: bursts

Send offprint requests to: nicastro@ifcai.pa.cnr.it

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© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998